CLEO BLOG by Frank Kuo

Espresso for the Brain – CLEO: 2013

2013-05-21T14:38:00.001-07:00

Being an active OSA young professional comes with additional bonuses once in a while. This time, I was happily summoned as “scientific paparazzi” to sneak into one of the committee meetings for CLEO: 2013 happening in the DC metropolitan area. Digging for insider info on CLEO’s hot topics and from CLEO: 2013 committee chairs and members – as they reviewed, scored and sessioned all the CLEO papers was the top mission.

My first impression about this conference is the vibrant energy. All the chairs and committee members were holding such high spirits. I don’t feel they came to the conference as referees to select the best papers. I feel they came to learn more and look for new inspiration. While it can be difficult to make decisions on which papers represent the best in the field – they are there to do their job – accepting only the highest-quality papers for the CLEO: 2013 program.

My first personal encounter with one of the Chairs was a short conversation with professor James C. Wyant, who also served as President of OSA in 2010. As program co-chair of “CLEO: Applications & Technology,” he is very happy to see CLEO is creating a trend of applying its strength in core science into applications. This, of course, will foster more interaction between academia and industry. He is especially keen on the topics about “metrology” and “sustainable energy – laser-driven inertial fusion energy”. If you are still not aware of these two topics, I strongly advise you to check out the short course on metrology, and the tour of the National Ignition Facility (NIF) to learn more and gain a first hand experience. All of these sound very exciting. Joining the tour allows you to have the chance to see one of the most powerful lasers in the world, and how to use it to mimic the core of the sun. And, the metrology course will introduce you to the tabletop X-ray light source that is one of the prominent rising stars in optical science. You better grab your opportunity to attend by checking out the CLEO website now.

Professor Wyant also shared the concern about the impact of U.S. federal government’s sequester on optical science too. Although we all feel sorry about the cuts on financial support, he is cautiously optimistic. Optical science has found its applications in many aspects of our society, and many more will come. With all of humanity benefiting from optical science applications, we shall look for more that originate from optical science to accompany our future. Thanks to him and many other researchers, we are striving toward this goal.

Then, I was lucky to catch a few humorous and witty scientists during the lunch break. Having a meal together with Professor Christian Wetzel, Professor Mark A. Zondlo, and Dr. Max Shatalov – manager of SETi. They all serve in the session of environment/energy. They were impressed by an increase of the number of the submitted papers. To me, it seems to make sense. With the population of Homo sapiens increasing, the Earth is barely breathing. Without our effort, we will definitely engage into an irreversible future. As a result, taking care of the environment must become our priority, and I am happy to see research that is helping to make this possible.

They also told me about some interesting topics you should not miss:

1. Using the quantum cascade lasers for the environmental sensing: We are all very excited that QC lasers are finally portable and can be brought to the field for various applications. For example, trace gases like SO₂, methane, or air pollutants are all targets under the scrutiny of QC lasers. If you are a green-oriented person, you should not miss this opportunity when you come to CLEO: 2013. In addition, we were discussing a very interesting paper in which a laser is used to probe the “particle size.” Again, if you feel intrigued about it, you just have to keep your eyes open for topics like these while wandering around in the conference center.

2. Using UV-LED, for sterilization and water purification: This is a perfect example of how optical science is helping the humanity. UV-LED, being more compact and consuming less energy compared with traditional light sources, will probably become the main light source for food sterilization (in our discussion, UV-LED shining on strawberries was the content). The environmental impact of adopting this new light source into the food processing chain is self-evident. Cool science with a mix of practical goals – I guess this is yet another reason why CLEO is awesome.

3. Solar energy harvesting: How to harvest solar energy in a more efficient way is always an attractive scientific challenge for the researchers. In our short break, we touched on the topic of multi-junction cells, patterned surface — either nano or micro scales to trap more light into the solar cells, and using organic media to harvest the solar energy. Checking out the talk presented by Rebecca Jones-Albertus is a good entry point for you to delve into this domain.

In order to please the crowds of hard-core scientists, I also had a short chat with professor Zhigang Chen, who is serving for the CLEO: QELS Fundamental Science session of Nonlinear Optics and Novel Phenomena. He mentioned with zeal to me the breakthrough in plasmonic resonance, arbitrary trajectory manipulation of light propagation, using photonic periodic structure to test the idea of super-symmetry, and so on. The depth of the fundamental science he was trying to convey blows me away. Topics like these will always find their places in CLEO, and I always feel this is one of CLEO’s strengths. In fact, the entire QELS program poses a mental stimulus to my brain. These courses are so stimulating they are like “ “espresso for the brain!”

The truth is what I mention here provides just a small glimpse into all the great content being featured at CLEO. To get a glimpse at the full conference program, visit the CLEO website here!

View exclusive interviews with the Chairs and get more personal insight on hot topics and trends at CLEO: 2013

There is no break in the review conference. Everyone is eager to share their ideas. People were shuffling around to maximize their precious time together.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Novel lasers shine and explore the new scientific frontiers – part I

2013-02-16T12:29:00.001-08:00

To probe new scientific frontier, we need new technology. On the other hand, the advance of the technology relies on the solid scientific foundation. Countless examples have shown us that science and technology evolves together to give us wonders and a better understanding of the universe and nature. Looking back in 2012, similar stories happened in laser and optics arena – Lasers are extending the working wavelengths into shorter (X-rays) and longer (THz) domain and probe new scientific frontiers. With this advance, we can have a better grasp of our nature.

Femtosecond X-ray free electron lasers, the most established source to generate “coherent (laser-like) X-ray”, relies on a gigantic synchrotron. In brief, a bunch of high-energy electrons from the synchrotron is sent into a long tunnel made of magnets. The tunnel, often more than 100 meters, is called undulator. The magnets are arranged in a way such that they create an alternate magnetic field to wiggle the electrons and force them into emitting X-rays. The wiggles are tuned to the wavelength of the X- ray and creating a feedback mechanism – this radiated X-ray acts on the electrons, concentrating them into smaller and tighter groups, and makes the electrons emit more X-ray coherently. Apparently, it is very similar to normal lasing scheme, in which the radiation in the cavity induces more radiations. The main difference is that in the case of X-ray, there is no cavity since no reflective mirrors are available in this wavelength region.

What excites us in 2012 is that this “new light” gives us a better way to elucidate the secret of our living nature. It is used to probe the structure of the proteins:

In the old days (well, even in nowadays), to peek at the precise structure of a protein with the atomic resolution, you need to grow a “sizable” protein crystal such that you can use crystallography to decipher its structure. The reason is straightforward – without a sizable crystal, it cannot withhold the constant bombard from incoherent X-ray interrogation. However, growing a crystal made of protein is not easy. People used to joke that if a graduate student grew a big crystal, then he bought his ticket to a Ph.D. degree. That sort of tells us the difficulty.

With the advance of the femtosecond X-ray FEL, it gives us a new window. We do not need monster size crystal anymore. This “new light” is coherent which means it is intense and produces high-quality diffraction. On the other hand, its pulse duration is short (within tens of femtoseconds) such that the entire process ends before the onset of substantial radiation damage.

Researchers from SLAC National Accelerator Laboratory demonstrate this new technology beautifully to us. They use a 430 m long undulator to create femtosecond X-ray, either 5 fs or 40 fs pulse, with the wavelength of about 9.4-keV. The target is a liquid jet filled with micro-crystal of lysozyme (each crystal is less than 1 micrometer by 1 micrometer by 3 micrometer). The experimental setup is shown conceptually in figure 1. This result is very satisfactory and fits well with data that are gathered from traditional X-ray crystallography (figure 2).

Figure 1. The experimental set up of new crystallography realized by femtosecond X-ray FEL. Courtesy of S. Boutet and et al. in Science 337 362 (2012).

Figure 2. Part of the protein structure (electron density map) at 1.9 Angstrom resolution of lysozyme elucidated by this new technique. Courtesy of S. Boutet and et al. in Science 337 362 (2012).

Trivia about X-ray optics:

X-ray, although belongs to the family of electromagnetic radiation, has some interesting property which amuses me quite a lot.

First of all, it has index of refraction very close but smaller to 1. Figure 3 shows how index of refractions varies with the frequency of light. This means that its phase velocity is faster than light! Shocking! For people who are interested in this topic in more details, check this link out.
Normal mechanism we use to focus visible or IR light simply does not work in X-ray regime. For the reflective optics, X-ray only reflects efficiently if the incident angle is very shallow (close to 90 degree). If the incident angle is too small, the x-ray penetrates into the optics without being reflected. This phenomenon creates the need of special reflective optics, such as Kirkpatrick-Baez mirror to focus the X-ray (figure 4).
Just like I mentioned in above, the index of refraction of X-ray is just slightly below 1, we have to re-think how to focus X-ray if we want to use refraction principle. First of all, you have to think air as a glass, since air has index of refraction equals to 1, and the material has something smaller than one. A collimated X-ray will focus when propagating from the material to the air. Secondly, one refraction is hardly enough, since the index of refraction in the material is just slightly smaller than 1. As a result, to create strong optical power, you need many air bubbles in tandem buried in the material. The Be lens used in figure 1 has the structure like the picture shown in figure 5. For more description on the X-rays lenses, check out this website.

Figure 3. The index of refraction vs. wavelength. Courtesy of D. Attwood, an online resource.

Figure 4. An Kirkpatrick-Baez mirror to focus the X-ray.

Figure 5. A beryllium lens with 20 lenslets in a 20 mm x 10 mm x 10 mm substrate (left). The lenslets are 5 mm deep. Courtesy of A. Khounsary and et al. at Beryllium and lithium X-ray lenses at the APS, an SPIE 2006 paper.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

The shiniest berry in the world – what biophotonics in nature has taught us.

2012-10-22T10:50:00.000-07:00

It is an understatement when we describe nature as the most talented painter. In fact, she is not only the greatest artist, but also the most renowned scientist, in essentially all aspects. Her scientific achievements are found everywhere. For example, today, much of our knowledge in the field of bio-photonics is just a re-discovery of what she has done (Another interesting topic which relates the evolution to optical science can be found here).

Many of the astonishing color patterns we found in the insect kingdoms are manifestations of nanometre-scale architectures. These architectures are collaborative works of cells. Those cells cooperate together to create optical effects we widely apply in modern photonic science. For instance, butterflies have cells structures that look like multi-layer reflective coatings on their wings. Depending on the thickness of each layer, different colors present vividly. Same tricks have been adopted and perfected by many shiny beetles. As shown in figure 1, enchanting colors on the surface of the insects are precisely the magic of multilayer structures. The layers are mostly composed of thin parallel sheets of chitin (secreted by the epidermis and often interspersed with other organic components). These layers differ in refractive index. And again, depending on the spacing between these layers and their indices of refractions, different colors can be reflected. Furthermore, some insects have arrays of very fine elements, known as nipple arrays, which look like micro lenses with subtle variation of index of refraction, to reduce reflectivity in their compound eyes and enhance collecting the light from the environment. Nature did create optical science way before mankind stole fire from Prometheus!

Figure 1. (a) A presentation of simple cuticular multilayer reflector. (b) The cross section of a cuticular reflector. (c) A colorful buprestid. (d)-(f) Different structures of cuticular multilayer reflectors commonly seen in insects. Courtesy of A. E. Seago, P. Brady, J-P. Vignerson, and T. D. Schultz in J. R. Soc. Interface 6(supp2) S165–S184 (2008).

Recently, researchers have discovered another beautiful example. A marble berry (Pollia condensata) originates from Africa produces colors like jewels (Figure 2). Looking at the photos of it, I actually feel it looks more like a star sapphire. Maybe we could nickname it “star-sapphire-berry”? Or “jewel berry”? Each berry is about 5 mm in diameter, and according to the authors, there are several wonders of these berries: (1) its charming metallic color is rare in nature. (2) If you look carefully, it has a brilliant pixelated appearance with green, purple, and red speckles (The authors call it “pointillism”, apparently they also think nature is an artist). (3) The color, if well preserved, can last more than 40 years, and counting. (4) It reflects about 30% of the sunlight, which makes it the shiniest organism we have found in nature so far. Colorful beetles can only take the 2^nd place. So how do all these happen?

Figure 2. (a) The charming marble berry (Pollia condensata). (b) A single bunch of the fruit. Courtesy of S. Vignolini, P. J. Rudall, A. V. Rowland, A. Reed, E. Moyroud, R. B. Faden, J. J. Baumberg, B. J. Glover, and U. Steinera in PNAS 109 39 15712 (2012).

The secret of this appealing demonstration lies on the structures of the cell walls of the cells that populate the surface of the fruit. Each cell has the cell wall composed of multi-layer cellulose micro-fibrils. The layers of micro-fibrils form an assembly called “helicoidal assembly” in which each layer has an orientation slightly shifted compared to the previous layer (Figure 3). The periodicity (p) of this helicoidal assembly defines the variation of the index of refraction in the cell wall, and this feature further defines what color the cell wall can reflect. The maximum reflectivity wavelength (λ) is found to be λ = p *2*n, where n is the average index of refraction of cellulose (n = 1.53). In the case of the marble berry, most of the cells have periodicity of 145 nm, which makes its blue coloration in general. Since each cell is a pixel on the surface of the berry and the value of p varies from cell to cell (from 125 to 200 nm), and, you see the fine pixilation of different colors (ranging from red, green, to purple). In addition, due to the handedness of the hellioidal assembly, the reflected light from each cell is either right-handed (RH) circular polarized, or left-handed (LH) circular polarized (Each cell has its own handedness, and on average, the total number of cells having RH equals to that having LH).

Figure 3. Top left: The layers of the cellulose micro-fibrils form the helicoidal assembly. Top right: The simulation shows the structure of each layer and how cirlular polarized light is reflected. Bottom: How light is reflected when it hits different cells on the surface of the berry. There are 6 cells shown in the figure, each one has a different periodicity, so each one reflects different colors of light. Courtesy of the same authors in figure 2.

The surface of the berry is scrutinized under a powerful microscope and the result is shown in figure 4. In A/B/C of figure 4, only LH/RH/cross polarized light is selected for detection. In A and B (especially the insets) of figure 4, the reflection from each cell is either LH or RH circular polarized light (The cell reflects a shiny strip of light in either LH or RH detection, but not both). Overall, these microscopic images clearly depict that each cell is a building block of the alluring coloration on the surface of the berry. Amazingly, another organic bio-photonics is deciphered by the scientists and presented to us.

Figure 4. (a)-(c) The images of the berry under microscope. The microscope is set up in a way that it picks up only the desired polarization as specified in the text. (d) The experimental setup. (e) The reflective spectra of two different cells. Courtesy of the same authors in figure 2.

If you look carefully enough, wonders are hidden in nature everywhere. We as human are picking up very fast in adopting the gift given by nature into technology applications. I hope by now, you are intrigued by bio-photonics. If that is the case, mark your calendar and join us in bio-photonics section in CLEO 2013. It will be a great conference for you to immerse in this astounding field.

Already doing research in this direction? Submit your abstract before January 30^th. Share with us!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

A big step forward in lens design -- aberration-free ultrathin flat Lenses made of metamaterials of gold antennas.

2012-08-28T09:54:00.001-07:00

If you think metamaterial has only “invisible cloak” and/or “negative index of refraction” in her hat, think again. Researchers from the school of engineering and applied science at Harvard utilize a powerful feature of the metamaterial to create ultra-thin and flat lens that is diffraction limited. They also create a flat axicon as another example when pioneering in this field (A quick glimpse on the Axicon: A lens with a flat surface on one side, and a conical shape on the other, has the ability to focus a Gaussian beam into a Bessel beam at the focal region, and create hollow ring beam shape in the far field).

To focus light, we need to create a converging spherical wavefront, or at least, a wavefront that is converging. To do so, we need to introduce different phase retardation on different portion of the incident light. A spherical lens does so by letting the light pass through different amount of material. For example, when light is passing through the center of the lens, it lags behind compared with that passes through the edge of the lens. As a result, there is a phase difference between them. This phase difference, or phase retardation between them, produces a converging wavefront. However, when a plane wave like light passes through a spherical lens, it suffers from spherical aberration. That is to say the spherical lens does not produce a perfectly converging spherical wavefront which is required for the light to focus tightly. The light exiting from the edge of the lens suffers stronger deviation. The best way to solve this so far is to use an aspherical lens to correct this imperfection.

A puzzle like this can be solved beautifully by the use of the metamaterial. If we are able to create metameterials which can introduce different phase retardation, arrange them in a way such that they produce a converging spherical wavefront, we can have a flat, thin and aberration-free lens. This idea is realized and performed nicely finally. Researchers create gold antennas with different shapes. When impinged by the light, each shape is able to create different amount of phase retardation while the scattering intensity remains similar. From figure 1, we can see clearly that only the phase of the impinging light is shifted while the scattering amplitude remains close to each other. The lens is then realized by patterning silicon wafer with these gold nano-antennas using electron beam lithography. The antennas with different shapes are arranged in a ring pattern which would give the desired phase retardation. The spacing between the antenna arrays is covered with silver and titanium which completely reflects the fraction of the incident beam that is not impinging on the antennas. The wavelength of the incident light is 1.55 um, and the diameter of the lens is just merely 0.45 mm.

Figure 1. (a) The small antennas at work: different shapes of gold antennas are able to produce phase retardations. The sizes of the antennas are ranging from 85-180 nm with the width of 50 nm. (b) The experimental layout. (c) The arrangements of the antennas on a lens made of silicon wafer. The antennas are spaced ~ 500 nm to each other. Courtesy of F. Aieta, P. Genevet, M. Kats, N. Yu, R. Blanchard, Z. Gaburro, and F. Capasso in NANO letters.

The lens has one more special feature, it takes linear polarized light and focuses the light in a cross polarization way. In other words, if you put in x-polarized light, the resultant converging wavefront is y-polarized, and vice versa. In addition, to create a converging spherical wavefront, the phase retardation has to be a hyperbolic function vs. the radius of the lens. And to create an axicon, the phase retardation has to be conical. A contour plot of the phase retardation vs. the radius of the lens is shown in figure 2. The results compared to the calculation are very promising, as shown in figure 2, lower part.

Figure 2. The phase retardation introduced by the gold nano antennas. On the left is the phase retardation for a focus lens; on the right is that for an axicon. The result, shown in the bottom of the figure fit well with the simulation! Courtesy of F. Aieta, P. Genevet, M. Kats, N. Yu, R. Blanchard, Z. Gaburro, and F. Capasso in NANO letters.

By arranging the antennas differently, we can create virtually all kinds of lens. For example, if the array of the antennas has translational symmetry, we can have an elliptical lens. By tailoring the phase front to have great curvature, we can have a high NA objective. In addition, they are flat, thin and easy to assemble. There is a caveat to be aware of. At the moment, only 1% of light is transmitted, since the antennas only cover small amount of the area on the wafer. In order for the real application, the lens efficiency has to be decent, maybe around 80%. The researchers are trying diligently to increase the efficiency, presumably by increasing the density of the antennas.

There are quite a few other interesting flat lens designs using nanotechnology. A few examples are nano-holes and nano-slit lens. For the reflective optics, flat dielectric grating reflector with nano-scale patterns is also attractive. Feel excited? Just click on the links.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Applications of photonics in photochemistry, green energy, and more!

2012-05-09T19:22:00.001-07:00

I am pretty sure you have loaded up your crazy mind with a big chunk of knowledge on Wednesday. Plenary session, a whole day of exhibition, and enthusiastic poster session guarantee everyone finding its own corner. What excites me the most is to see the interplay between different research fields. Like all of us today, I am happy to learn that photonics also finds its applications at each corner of the science. Let’s encapsulate a couple of them:

! Harvesting green energy with the help of photonics fibers !

Converting the solar energy into chemical energy is not a new idea. One interesting way of doing so is to grow algae, such as cyanobacteria. After you grow tons of them, you essentially squeeze them to get Algae oil, which is used to fuel the world (I sincerely hope it smells like olive oil). However, just like everything in the practical world, it faces some challenges, especially in terms of efficiency. It turns out that cyanobacteria are very picky about where they live. The amount of sunlight has to be just right for them to prosper. Like figure 1 shows, the optimal condition is only about 10 cm thick somewhere below the surface of the pond (or pond reactor). Same situation applies for the tube reactor. As you can see from the figure, most of the space is wasted.

Figure 1. The optimal zone where the algae grown. Courtesy of D. Erickson at http://www.cctec.cornell.edu/events/ctvf11/Jung.pdf.

Thanks to the researchers from Cornell University, we seem to have a solution now (a PowerPoint presentation about this topic is also available online). By pumping the light into a photonic crystal fiber (PCF) or a waveguide, you create total internal reflection on the inner surface. However, some very small portion of the E field is leaking through, which is known as evanescent field. Interesting enough, the intensity of evanescent field is very sensitive to the input angle of the light, and it dies out about 1 um away from the surface of the PCF or the waveguide (1 um is just about the size of a single cyanobacteria, figure 2, upper left). Utilizing the phenomenon of evanescent field, you can control the light intensity very precisely. As a result, the 1 um layer outside the PCF or the waveguide can be adjusted to a sweet home for the bacteria. On the other hand, you can pack a lot of the PCFs or waveguides into a container of the media where the bacteria grow. This design also saves space (figure 2, right)!

Figure 2. (a) A cartoon representation of the waveguide, a bacterium, and the decay evanescent field. (b) An experimental setup for the growth of the bacteria. On the right, an idea proposed by the authors shows how to couple the sunlight into 4 waveguides. Courtesy of M. Ooms et al. in PCCP 14 4817 (2012).

! PCF as a nano chemical reactor !

Measuring the absorption of a solution tells us a lot about the microscopic world. Some time in the journey of our academic life, we all had the chance to measure the absorption of an unknown solution, and tried to figure out what is in it. Normally we used a standard 1cm cuvette to fulfill our mission. We learned from the conference that PCF can play a better role in this old task. Professor Russell’s group in Max Planck Institute utilized a hollow core kagome PCF to replace the traditional sample cuvette (kagome is a pattern constructed by interlaced triangles; kagome PCF’s cross section has this pattern, figure 3). There are a couple of reasons to support doing so: 1. The sample volume per optical path length is very small since the hollow core diameter is very small (2.8 nL cm^-1 in the fiber the researchers used). 2. You can have a very long optical path length that is extremely useful for very dilute sample (you need to have a long optical path in order to have enough absorption by the sample -- Beer’s Law). 3. Light travels in a diffractionless single mode within the fiber, which reduces a lot of complexity.

Using these strengths, they are able to monitor a real time reaction undergoing in the PCF. What you have to do is to pump the solution into the PCF and monitor the absorption over the course of time (figure 3). The photo-chemical conversion of vitamin B12 to hydroxoco-balamin [H2OCbl]⁺in aqueous solution was measured for several pH values from 2.5 to 7.5 by this way.

Figure 3. (a) The cross section of the kagome PCF. (b) The light distribution in the fiber. Middle: the photochemical reaction in the experiment. Bottom: The experimental setup where a single-syringe infusion pump is used to pump the solution into the fiber. Courtesy of J. Chen et al. in Chem. Eur. J. 16 5607 (2010).

Some other applications such as using 3D inverse-oval photonic crystal to detect different solvents are out there waiting for your exploration. Don’t stop here; stay thirsty as Jobs like to say.

Time flies by pretty fast. Enjoy the rest of the conference, and check out what you can do in San Jose (maybe drive to San Francisco after the conference, you deserve a nice trip after a full week of intellectual challenges)!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Nanolasers are the rising stars

2012-05-07T14:02:00.000-07:00

If you attend “QM1H • Spasers and Nanoemitters” today, you know exactly what I am talking about. Exciting new materials, including metamaterials, quantum wells, and quantum rods, are used for the realizations of the nanolasers. If you missed it (which is quite possible since there are many other outstanding technical sessions packed today), this short article is your second chance. Three examples of nanolasers are presented here (summarized from today’s speakers) to give you a taste of the flavors.

Nanoscale coaxial lasers:

This is a piece of artwork of nanofabrication. The researchers from UCSD are able to fabricate a nanoscale coaxial laser cavity on an InP substrate (figure 1). It is composed of a metallic rod with different coaxial disks. One of the disks, the gain medium shown in red, is made of 6 quantum wells (each one is made of Inx=0.734Ga1–xAsy=0.57P1–y / Inx=0.56Ga1–xAsy=0.938P1–y, with an overall height of 200 nm). They are sandwiched between SiO2 and air plugs. With the help of these two plugs, the entire device behaves like a cavity which supports a few sparse EM like modes (figure 2). If you pump the device in a right way, you can excite these modes and build them up. The lower air plug also allows pump energy into the cavity and couples out the light generated in the coaxial resonator. So once you build up the modes, you can couple the light out. In other words, you can make this device lase.

Figure 1. The structure of a coaxial laser cavity. The enitre thing is ~ 500 nm in all dimensions. (b) and (c) shows the SEM images of two different structures. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).

Figure 2. The EM like plasmonic modes that can be supported by the cavity. Two different structures support different modes. Some of the modes can be pump and excited by 1064 nm laser. Courtesy of M. Khajavikhan et al. in Nature 482 204 (2012).

The researchers pump this nanolaser with a 1064 nm laser and it will lase at 1.26 and 1.59 micron at room temperature depending on the overall structures of the nanodevice. I would like to have one of these as souvenir.

Lasing spasers:

SPASER stands for Surface Plasmons by Stimulated Emission of Radiation -- an idea proposed by Bergman and Stockman in 2003. They suggested that it is possible to construct a nanodevice in which a strong coherent ﬁeld is built up in a spatial region much smaller than the wavelength. Simply speaking with the help of figure 3, you can induce and build up surface plasmons – the oscillation of electrons on the surface of a nanostructure – by providing it energy. In the example of figure 3, the energy is coming from the excited nanocrystal quantum dots (NQD). Once this oscillation starts, it further drains energy from NQDs into it and builds up strongly. As an analogy, the nano-structure (nano silver shell, for example) confines the plasmons onto its surface, which behaves like a laser cavity confining photons. And the quanta of the plasmons are like photons in the cavity.

Figure 3. (a) A theoretical spaser made of a nano silver sphere coated with NQDs. (b) the energy diagram shows how the energy is transferred from the NQDs to the plamonic modes of the nano silver sphere. (c) and (d) show two different plasmonic modes of it. Courtesy of M. Stockman in Nature Photonics 2 327 (2008).

However, the ideal spaser does not emit light; it simply converts more energy into its in-phase plasmonic modes. One way to make it emit light is to create an array of nanostructures – a principle proposed by professor N. Zheludev et al. and shared at CLEO 2012:

In their “lasing spaser”, the nanostructures are a two dimensional array of metallic nanowires (figure 4). They are situated on the surface of the amplifying medium (gain medium). One of the possible amplifying media can be a substrate packed with quantum dots. A working principle is like this: a pump source is used to excite the amplifying medium (in figure 4, a pump laser is used to excite the quantum dots in the substrate), the nano wires drain the energy from the excited quantum dots into its plasmonic modes, and the currents start to oscillate back and forth within each nanowire (figure 4). Most of the currents cancel each other if viewing the device from the far field. Only the currents on the edge of the array survive the cancellation and behave like an oscillating dipole that emits light (figure 4, bottom). The light would be more intense if the plasmonic modes have more quanta. The entire device can be a few tens of microns while it emits near or mid-IR light (tunable by adjusting the structure of the nanowires).

Figure 4. The device is made of an array of nanowires, gain medium (shown in green), and pumped normally with a laser. At the bottom, it shows how the nanowires behave when viewing them from the far field: the currents in the center of the array look like cancelling each other out, while the currents at the edge can emit light. Courtesy of N. Sheludev in Nature Photonics 2 351 (2008).

Lasing in self-assembled microcavities of CdSe/CdS core/shell colloidal quantum rods:

This laser is a bit bigger. It is about a few hundred microns but it is something you might be able to do if you have the quantum rods provided by the researchers of Italy. What you have to do is very straightforward. Dissolve the quantum rods in toluene, put a droplet of it on the glass substrate, and wait until it dries. Then you get a microcavity that will lase. Pretty amazing, and this is how:

The building block of this microcavity is a CdS quantum rod (about a few tens of nanometers) with an embedded CdSe nanocrystal (figure 5). What special about this rod is that it is a strong fluorescent little guy with a QE of ~ 70%. In other words, if you pump it with light, it tends to give back its energy through fluorescence. Put a droplet of toluene containing many of the rods on the surface, it will dry out in a special way: The convection created inside the droplet due to the evaporation of it pushes the rods condensate at the border of the droplet. Not only so, the rods will pile up in a regular pattern. Microscopically, you have a wall (tens of microns wide and tens of nanometers thick) of rods piled regularly on the border of the droplet. This wall is very fluorescent (since they are made of rods), and can behave like a cavity (since the wall has very different refraction index compared to the surrounding, the fluorescent light can be bounced back and forth between the wall). If you pump the wall with laser (532 nm) normal to the glass surface, you are able to create a lasing phenomenon at ~ 610 nm (figure 5 bottom). You can proudly say: everyone is able to make a microlaser if the rods are available in the market.

Cheers! And look for more at CLEO 2012!

Figure 5. The building block of the microlaser: a CdSe embeds in a CdS quantuam rods. The lower plot shows how the device is lasing: You pump the dried droplet at its border normally and the device can lase at ~ 610 nm. Courtesy of M. Zavelani-Rossi in Nanoscale 2 931 (2010).

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Trying to get a job? Try the online job fair at CLEO!

2012-04-16T14:57:00.001-07:00

There are many things you do not want to miss in CLEO:2012 – A conference full of high quality technical sessions spiced by cutting edge presentations from invited speakers, not to mention the inspirational talks of renowned plenary speakers. For young graduate students, these are stimuli they want to boost their research. On the other hand, in the mind of senior graduate students, there is one more mission besides getting loaded with technical knowledge – Landing on a job after graduation. The good news is that you can get two birds with one stone since CLEO provides a nice channel for you to get connected with your potential future employers.

If you are interested in staying in the academia, your advisor(s) and the department may be the best resources for you. However, if you consider changing the tracks and exploring the industrial career, CLEO: 2012 is something you cannot miss. It brings employers from the entire US under one roof, and you get to meet them all. This year, you can try the online job fair by CLEO and WORKinOPTICS by OSA to get a head start. Unfortunately, not all the employers are actively involved in the online job fair. As a result, walking throughout the exhibition hall will be your next move.

Trying to get exposed in the exhibition hall is a must. To get you exposed in a right way is not that straightforward. For the past few years, I feel lucky to have the opportunity to look into these job-hunting games from both sides (as a senior graduate student trying to impress future employers in the conference, and a employee actively working in the tradeshow). Here are some tips I hope that help:

1. Get to know the companies you want to visit before hand – Even though your lab has the instruments from the company you want to drop by, it does not mean you know the company, not at all. Try to do the homework to learn the histories of the companies, including their competitors and their niche technologies. This is the appetizer (topic) for you and the people who work on the exhibition hall in the first encounter. You intrigue them with the right motive, and it will be the impression that lasts in their minds. Besides, by studying the companies, you will find out the photonics industry is a big intricate web and companies are related to each other in a very intimate way.

2. Set the right goal – Your goal is not to give the resume away. Instead, your goal should be building up a new connection/strengthen the existent ones with the companies through the representatives. Making a good impression, staying in touch with them, and updating them with your research progress are means to achieve those objectives. You will never know when there will be a vacancy in the company. And believe me, when there is a vacancy, the first thing they do is to request their colleagues to see if they know anyone who is qualified. You want to be the one that comes cross their minds.

3. Who will you encounter? – Most of the time, you will bump into a sales representative, but not always. There is a chance you will meet technical sales support people, product managers, directors of divisions in the company, CTOs, and marketing personals. If you are a Ph. D. student, try to talk to people who have strong technical backgrounds such that they appreciate your effort. If you are a master candidate with a minor/major in marketing, you may find yourself more comfortable to talk to the product managers.

Of course you cannot tell one person’s job title by face. What happens if you pick the wrong one at the first place? Don’t’ worry; just ask politely after a nice and warmed up conversation. People who work on the floors are nice, and their duty is to help, in all possible ways. They will not say no to you. That harms them in a bigger way.

4. Never just hand in your resume right away, and do not walk away immediately after you do so – when standing on the carpet of the company’s territory, do not just look for the representatives and hand in your resume. Spreading as many as you can does not guarantee you a job. In addition, by doing so, your resumes won’t reach the places they are supposed to.

Wrap your purpose in a delicate way! For example, start the conversation with your interest in the new lasers that are released by the company in 2012. Ask technical details to show your knowledge. Then, slowly express your expertise in this field, and ask if there is any opening in the company. If yes, trying to learn more, if not, stay motivated and talk about the instruments in further depth. Simply walk-in, drop the resume, and walk away, you basically leave no indentation at all.

5. Avoid rush hours when planning your visit – In almost all the conferences, there is a time period where no technical sessions are happening. I call it the rush hour on the floor. It is true that at this time window, there will be more representatives working on the floor, but there will be ten times more visitors. So do the math.

6. Remember to get the contact information from the people you talk to – Trust me, when you start job hunting, you will need it. And you will regret you did not get it before. Asking for their business cards should be a habit for you if you want to start your career in the industry. You should also stay in touch with them. Ask them if they are visiting your area, if they plan to release new products and so on. One day, they might be your future colleagues.

A few more things about job hunting you might want to think about. I find there are many people who are not aware of this, or neglect this.

1. Job-hunting requires a warming up time – It is rare to start getting a phone interview or response from the companies right away when you start to post your resumes on the websites. It takes about 1-3 months. Do not get frustrated. Keep polishing your resumes. Make this as a habit.

2. Resumes have to be tailored to the jobs – There is no such thing as a universal resume that you can use for all the jobs posted. You have to spend time on each and every single one when submitting your applications (at least for those jobs you feel your skills match seamlessly). Writing a cover letter for your dream job will help for sure.

3. Do not abuse online job-hunting websites – Choose no more than three job posting websites for yourself (for the optical science people, WORKinOPTICS is definitely a good one to have). All the websites nowadays have most of the jobs posted by other websites. Three job-hunting sites are enough to cover them all. You do not want to submit three copies of the resumes to a company, do you? In addition, try to apply for the jobs at the company’s website
(s). When a job hunting website gives you a job posting of a specific company, you go to that company to apply directly. Do not rely on the “middle man” of the Internet!

Finally, I want to use one of my mottos to encourage all of us – “When you started your graduate study, you have already started your career. Jobs are just methods to achieve your life long career.” Indeed, a job may seem to be the only thing that matters for you when the time is pressing. You might even feel desperate when you are in the process. But this is just a short period of time in your life. Once you really get one, spend some time to figure out what you want to achieve in your career and lay it out in brevity. This is what matters the most! Having an idea of your career path is a very proud thing to possess. It is the rudder of your professional life and it keeps you from being lost in the ocean of the diverse jobs.

Best of luck in job hunting!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Recording the data at the “ultrasfast” rate for your digital device.

2012-03-25T18:49:00.002-07:00

The principle of magnetic storage used by most hard drives is an important pillar in the evolution of modern digital world. Before the advent of flash memory, it dominated the way we saved our data. Simply speaking, binary information (0 or 1) is presented by small magnets pointing forward (0) or backward (1); let’s say the north is the head. Writing the data is by changing the pointing directions of these magnets, usually fulfilled by an electric coiled wrapped head (by applying the current into the head, you create a strong external magnetic field that realign the directions of the small magnets in the hard drive, one at a time). In addition, packing in as many magnets as possible in a limited volume will define the capacity of a hard drive, and this is improving ever since the first device available. Imaging the first computer I had came with a hard drive of 400 MB, and now a decent one has a few TB storage capacities. By comparing the number, you can realize how much effort and advances in the business of data storage. For a very nice introduction, you can find at hard drive 101: magnetic storage.

Figure 1. A generic ferrimagnet, composed of Fe and Gd, shows the alignment of magnetic moment. Courtesy of I.Radu et al., Nature 472 205 (2011).

A nice paper where ultrafast laser pulses (sub 100 fs) instead of external magnetic field are used to write data intrigued my curiosity. I know immediately that it is the heating effect that causes the change of the magnetization of the small magnets in the hard drive. But for me, the heat has no directionality, how it can tell the magnet to point forward or backward. It should just erase the information since an ultrafast laser pulse can easily create a hot environment above Curie temperature where the magnetization is destroyed. So in my mind, an ultrafast laser is a hard drive terminator, not a hard drive writer. Driven by this curiosity, I dug in to find out, and this is how:

Strong magnets are either ferromagnetic of ferrimagnetic (that is right, only one letter difference). Iron is ferromagnetic, since when it gains magnetization, all the molecules have the magnetic moments (or the moment generator – the atoms’ spins) aligned in one direction. On the other hand, ferrimagnetic materials contain different atoms or same atoms in different chemical forms. For example, the alloy of GdFeCo or magnetite is ferrimagnetic. When these materials gain magnetic power, the magnetic moments of different atoms (or same atoms with different chemical forms) are pointing in the opposite direction. However the magnitudes are different. As a result, the cancellation is not complete, and some magnetic power retains. This lengthy discussion has a purpose, since in order to use ultrafast laser to write on the device, we need ferrimagnetic materials as our small magnets. Figure 1 is an example of a ferrimagnetic material made of the alloy of GdFeCo. The small arrows are the direction of the spins or magnetic moments if you prefer.

The working principle is like this: When an ultrafast laser pulse hits the ferrimagnetic material (in this paper, a alloy where the active components are Gadolinium (Gd) and Iron (Fe)), the temperature shoots up and all spins are free to flip (or randomize) due to the energy shot. As a result, magnetic moments are decreasing toward zero. In other words, they start to demagnetize (I got this part right). Here is the catch: Fe demagnetize faster than Gd. Like figure 2 shows, when the overall magnet moment of Fe reaches 0 before 0.4 ps, Gd is still decreasing toward zero. At this instant, another principle plays a pivotal role – exchange interaction between Gd and Fe. This exchange interaction says that the flips of the spins of the Gd and Fe can undergo collaboratively, with the spin of a Gd atom flips from up to down, that of a Fe atom goes from down to up. Mother Nature likes the exchange interaction, since it is more energy efficient. Now incorporate this to our discussion. Right after the 0.4 ps, the overall magnetic moment of Gd is still decreasing; it means the spins are flipping down. This gives the spins of Fe atoms a direction, which favors to flip up against Gd’s to please Mother Nature. So the magnetic moment of Fe starts to build up, instead of staying randomized. After the overall magnetic moment of Gd hits zero, the overall magnetic moment of Fe has built up quite a bit. This built-up will guide the entire alloy to relax into an overall ferrimagnetic form. If you are still with me, you know the secret of this technology. With this principle at hands, you can use an ultrafast laser to write the data in an unprecedented speed -- for a few hundreds of picoseconds you can write a bit. And maybe scientists can push this limit even further.

Figure 2. The evolution of the overall magnetic moment after the sample is hit by an ultrafast laser pulse. As can be seen from the diagram (inset of (b)), Fe demagnetizes faster than Gd. The difference in demagnetization rate makes the reversal of the magnetization possible by an ultrafast laser pulse. Courtesy of I. Radu, et al., Nature 472 205 (2011).

Researchers have demonstrated this phenomenon theoretically and experimentally (this article). As can be seen from figure 3, the magnetic moment flips back and forth after the ultrafast pulses hit the sample.

Figure 3. The magneto-optical image of GdFeCo alloy obtained after the action of a sequence of ultrafast laser pulses. (a) and (b) shows the images of the film with magnetic moment pointing down and up, respectively. (c) and (d) show the magnetization reversal after interacting with ultrafast pulses. The boundary of the circles shows the spot size of the light, and the scale bar is 20 um. Courtesy of T.A Ostler and et al., Nature Communication DOI:10.1038/ncomms1666.

Amazingly, ultrafast lasers are finding their applications at the frontiers in so many fields. They shine and rise in the fundamental chemical physics, advanced spectroscopy, astronomy, machining (Jim has a nice article about it), and now, computer science. Who knows what their next stop will be?

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

The world’s smallest Stirling engine is powered by laser!

2012-01-09T14:18:00.000-08:00

When people mention the word “laser” to you, what is the first thing coming to your mind? Most of us associate lasers to their scary and destructive power, just like how we are educated in the Star Wars movie series. In reality, lasers can be quite gentle and perform very accurate and precise assignments, like micro-machining (Jim has a nice article about it). In fact, laser can be so gentle that researchers have used it to power the world’s smallest Stirling engine, which is composed of single tiny melamine bead (~ 3 um in diameter) in the water bath.

To realize how this ingenious microscopic engine works, we have to step into the phenomenon of optical trapping/tweezers first. Thanks to the detailed illustration on wiki, I can just summarize it in a few sentences -- When the laser is tightly focused, or when it has the Gaussian beam intensity distribution, the tiny particle will be trapped in the focus or the center of the Gaussian beam, just like being trapped in a potential well. This is a result of momentum conservation. When the refracted light rays exit the particle, they exert momentum kicks to the particle, and the net result of these kicks is a force that traps the particle at the center of the focus. If the particle is in the focus, this force is zero. If the particle drifts away from the center, the kicks will be imbalanced and a net force will pull it back to the center. This particle behaves exactly like it is in a potential well. The steepness of the well depends on the laser intensity as you might guess it already. And our talented researchers use this technique to power the microscopic engine.

Here is how it goes. Figure 1 shows the comparison of a microscopic Stirling engine with a macroscopic one. As shown in step (1), the bead is trapped in a potential well by a focused laser beam. From step (1) to (2), the laser intensity is increased such that the bead would be confined in a smaller volume due to the steeper potential well. This is similar to moving a piston to squeeze the volume in the chamber. From (2) to (3), the water bath is heated by another NIR laser, and this step is similar to heating a macroscopic chamber. From step (3) to (4), the potential well is relaxed and the work is exerted from the bead to the surrounding, just like in macroscopic world, the gas is pushing the piston to exert work for useful application. From (4) to (1), the NIR laser is turned off, and the bead is cooled down, just like in the traditional Stirling engine, the gas is cooled back to the ambient temperature. Smart and elegant design, isn’t it?

Figure 1. The realization of the microscopic Stirling engine. Courtesy of V. Blickle and C. Bechinger in Nature Physics doi:10.1038/nphys2163 (2011).

Not only the realization of microscopic machines is presented, but also its power and efficiency are characterized. They have shown that if the entire cycle is working at a rate of 7.2 s, the power is at its maximum. So a micro-machine has a working ethic of macroscopic time scale. They also calculate the average work output, which is in the range of 10^-21 J! Since the machine is tiny, it is prone to the stochastic fluctuation. As a result, some of the cycles are exerting negative works. But fortunately, when average over many cycles, the machine works reliably.

Optical tweezers technique has been used for various applications. Beside the one we just present, Block’s group in Stanford is the true master in applying it to biophysical study. They have used this technique to investigating the transcription of the DNA and the motions of kinesin motors inside the cell, just to name a few. The basic principle is to attach the molecule of interest to the bead or beads, exert the force to the bead(s) through the laser, just like figure 2. By carefully controlling the laser intensity, the force can be finely tuned in a delicate way. So the step motion or the transcription of the DNA can undergo in a subtle and controllable way. In addition, by monitoring the location of the bead, we can extract the size of the step motion of the molecules (the molecules themselves are too small to see). This sophisticated setup has been perfected by Block’s group, and they are able to extract the step motion of kinesin and DNA transcription with a resolution of a few nanometers. This is another master piece of scientific work, because we are talking about a motion in a microscopic world through macroscopic technique. I strongly encourage people who are interested in this topic to take a look of their research website, since it contains lots of information, even some insightful literature!

Figure 2. Using the optical tweezers to probe microscopic world. The force, when tuned properly, can switch the reaction on and off since the reaction involves the change of the molecular length. Also by monitoring the bead location, we can know the step size of the motion. Courtesy of Block's research group website.

I keep wondering when the Nobel Prize was granted on the optical tweezers, did they envision its cool applications we described here already? Maybe...

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

“Metamaterial tiles” are hot in many applications – including invisibility cloak!

2011-11-15T09:42:00.001-08:00

Various forms of metamaterial have generated a lot of scientific attention in the past few decades. Some exciting “potential” applications include the well-publicized invisibility cloak (Thanks to Harry Potter). As you may know already, metamaterial gains its bizarre optical property (such as negative index of refraction) by its internal composition or structure, rather than its original physical property. Most metamaterial has its magic only in specific wavelength region and this wavelength region is correlated to how small you can make the internal structures of the metamaterial. This is exactly why almost all the research on metamaterial focuses on THz region since THz has very long wavelength and we do not need to make the structures awfully small to concoct the magic (I did read some articles about “universal metamaterials”, but it seems a long way to go. Let’s dream of that coming in CLEO 2012).

Digging into more details, you can have 2D or 3D metamaterial depending on your applications. 2D metamaterial – or so called metamaterial tiles (m-tiles) – seems to make a huge leap in guiding the advance in the invisibility cloak and sensing platform. And they are easier to make (through the help of photo-lithography, or micro-machining on the surface). With this powerful combination, a booming in this field seems inevitable. Let us take a peek of its potential application in invisibility cloak first:

It is realized that for a TE plane wave, it is possible to have a perfect invisibility cloak providing that the metamaterial has the right permittivity and permeability. However this cloak has to have circular inner and outer boundaries. As far as we know, it is very difficult to make exact circular cloak even with nowadays technology. To get around it, certain compromise has to be made. Instead of using hollow sphere or cylinder, we can use hollow polyhedral, each facet of which is made by m-tiles. Having this idea in mind, research group in Germany carried out a simulation study, and the result is really promising. Polyhedral made by m-tiles will actually give quite satisfactory results, and it can hide the structure within it very well (figure 1). In some circumstance, you can even rotate the polyhedral without losing its cloaking magic.

Figure 1. An invisibility cloak made by a faceted dodecahedral. This simulation shows that the plane wave can propagate through it without too much distortion and objects can be hidden inside the dodecahedral. Courtesy of Oliver Paul, Yaroslav Urzhumov, Christoffer Elsen, David Smith, and Marco Rahm.

The powerful units of m-tiles have actually simple internal structures. Described in great details in this article, you can easily change (tune) its optical property by changing its size and shape. As shown as an example on figure 2, three hexagonal m-tiles of slightly different structures have different absorption peaks in THz region. This “easy to fabricate and tune ability” makes m-tiles more and more popular in the research world. Considering making these tiles on a flexible film, you can actually fold them into a functional shape with even more interesting applications. And maybe one day we will have some advanced mosaics made of various m-tiles.

Figure 2. Different shapes of Hexagonal m-tiles. Each side of the structure is ranging from a few um to tens of um. By slightly modifying the structures, each of them absorbs different THz frequency. This flexibility makes m-tiles very versatile. Courtesy of Christopher M. Bingham, Hu Tao, Xianliang Liu, Richard D. Averitt, Xin Zhang, and Willie J. Padilla in Optics Express 16 23 18565 (2008).

How about getting a step further -- making these m-tiles on the paper and transforming them to biosensor platforms? In a nutshell, researchers from Tufts University and Boston University use micromachining to fabricate micro-stencils on silicon nitride film. These micro-stencils have many of m-tiles on it. With the help of micro-stencils, they then imprint the pattern of the m-tiles on the paper by spraying on the paper substrates using electron beam evaporation (figure 3). Using this way, you can make as many m-tiles as you want! Now, this sensor is ready to be radiated by THz radiation. Since paper is relatively transparent in THz region, it is a very good substrate (a disposal one). Once the molecules have attached to the m-tiles, they will change the electric capacity of each m-tile. This change of capacity will reflect on the absorption peak of the m-tiles. And this makes it a good sensor for various molecules. In fact, this is quite a new way to sense the molecules. It can achieve the sensitivity of ~ mmole/L concentration. Not bad as a paper-based sensor!

Figure 3. Using micro-stencils to imprint as many m-tiles as you want on the paper! The inset shows how the absorption spectra of m-tiles are modified when different amounts of the molecules (in this case, urea) are attached to them. Courtesy of Hu Tao , Logan R. Chieffo , Mark A. Brenckle , Sean M. Siebert , Mengkun Liu , Andrew C. Strikwerda , Kebin Fan , David L. Kaplan , Xin Zhang , Richard D. Averitt , and Fiorenzo G. Omenetto in Adv. Mater., 23, 3197–3201 (2011).

In the near future, more applications of m-tiles can be seen, indeed.

Trip note:

Luckily, I attended several conferences in China at the end of October. I spent two weeks visiting several cities (Beijing, Wuxi, and Wuhan). In Wuhan – Optics Valley of China, I met student chapter of OSA in POEM 2011. I felt awesome! OSA had a booth, an eye catching poster, and a bunch of energetic students in this conference. It made me feel like home away from home. Way to go, OSA!

Figure 4. The poster behind OSA booth in POEM 2011, Wuhan, China. Energetic student chapter of China promoted OSA nicely. Felt so warm when I saw this.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Zoo of super resolution microscopy.

2011-09-10T13:36:00.000-07:00

Microscope, one of the most popular optical instruments, has been paving the way of biological science for the past three hundred years. With the aid of the microscope, detailed observations of sub-cell size resolution were made possible. This, in turn, accelerated our understanding of the biology in an unprecedented way. Three hundred years have passed; we now arrived at a new cross road -- While triumphing on the universe of biology, a desire to develop microscopes with specificities and better resolutions is creating another revolution.

Specificities problems are less optical relevant. It is like painting different organelles of the cell with different colors. To do so, scientists use fluorescent dyes to attach to different organelles or encode them directly into the genetic codes of the proteins. So we can differentiate what they are and where they are. Scientists are quite good in doing so.

Resolution is another story. It is a barrier imposed by fundamental physics. In other words, the enemy of a microscope is diffraction, which prevents how well you can resolve two points on the focal plane. Same principle also applies to how tight you can focus a collimated beam. Using the traditional microscope, you cannot have resolution better than hundreds of nanometers if visible light is used. The axial resolution is not much better. As a result, no matter how small the particle in the focal plane is (in this case, the fluorescent dye), you would always observe a blob with some sizable volume. How do achieve better resolution? What kind of tricks scientists can play to break the diffraction limit?

For me, the first milestone in super resolution is called FIONA (Fluorescence Imaging with One Nanometer Accuracy). What a lovely name! In a nutshell, it fits the fluorescent signal with a Gaussian function. By doing so, it finds the center of the dye theoretically. Just like finding a center of the blob in the example we gave above. This method is generally adopted in modern microscopy since it localizes the location of the dye in the lateral plane quite well. There is a caveat though -- you cannot have too many dyes in focal point. This is just going to screw up your fitting.

Same mathematical manipulation does not work satisfactory in axial direction. In addition to multi-photon microscopy which aims on attacking this problem, there are other neat techniques existent. The way to get around it is modifying and mixing the experimental setup with other optical phenomena. The most eye-catching technique to me is the research led by professor H. Hess in HHMI. By putting a three-way beam splitter, the florescent signal from the dye in the focal plane would interfere with itself and generate different interference pattern depending on how far the dye is offset from the true focal point. This method achieved tens of nm of axial resolution. What impresses me the most is the feeling I have when trying to understand the diagram of the experimental layout. Suddenly, you realize, the imagination to advance optical science is unlimited.

Figure 1. The optical layout for interference microscopy. Courtesy of G. Shtengel, et al. in PNAS 106 9 3125 (2009).

Other neat ways emanate from bright minds also. One way to do so is to create bizarre spatial beam profiles at the focal plane. By putting an annular apodization mask, the work from the research group led by professor E. Betzig in HHMI created the Bessel beam profile at the focal plane. Combined with structural illumination, they created a focal spot, which has z resolution of less than 300 nm. Another research group led by professor W. Moerner in Stanford used spatial liquid modulator (SLM) to create helix beam profile at the focal plane. With the help of de-convolution algorithm, they could localize a fluorescent dye with ~ 20 nm z resolution in a total of 2 micron depth. This is truly amazing.

Focusing the light into a tiny spot is not the only solution. In the branch of developmental and embryonic biology, the speed of taking the image with decent resolution is of prime concern. To solve this problem, scientists used a century old technique (using a tube lens to focus the light into a light sheet) with the modern spice of fluorescent labeling in the genetic level. Generated light sheet at the focal point excites a plane of fluorescent proteins in one shot. Research led by professor P. Keller (now in HHMI also) in European Molecular Biology Laboratory (EMBL) used this technique to elucidate the developmental process of a zebra fish embryo in the first 24 hours. A full digital documentation of the embryo is resulted. Optical science is delving very deep to search the origin of nature, isn’t it! A good website of light sheet microscopy can be found here!

Figure 2. Light scanned microscopy. The laser beam illuminated the sample from the side and excited the fluorescent proteins in the plane. Courtesy of P. Keller et al. in Science 322 1065 (2008).

Coincidentally I found a very intuitive and interactive website for super resolution microscopy. It is like a power shot to everyone who is interested in knowing more in modern microscopy.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Evolution, the master of optical science!

2011-07-30T13:19:00.000-07:00

Do not get me wrong; evolution is an expert of all physical science. But it intimately links nature to optical science without doubt -- from cyanobacteria that have been converting solar energy to chemical energy for 3 billion years to human beings who rely on vision for surviving.

Neuroscience indicates that about 25%~ 50% of the brainpower and as many as 30 different areas of the brain are devoted to vision processing. This simply means that each human being is hard wired as an optical scientist, although we hardly recognize this. Over the past millions of years, evolution has perfected our imaging device in a subtle way. Recently, a report on Biomedical Optics Express shows for the first time the eyes’ imaging sensors -- cones and rods by using adaptive optics to minimize the aberration caused by the eye structure. As shown in the figure 1, cones, the round structures, create red, green, and blue perception of colors. There are about 6-7 millions of them, concentrated at the center of the retina -- forvea. A friendly and easy to digest article about this topic can be found here.

Figure 1. The cones in the fovea region of the eye. The retina is illuminated by 796 nm and 680 nm, respectively. The scale bar is 10 micron. Photo is courtesy of A. Dubra and Y. Sulai in Biomedical Optics Express Vol. 2 No. 6. 1757 (2011).

The other component of eye that always surprises me is the crystalline lens. It is 9 mm in diameter and 4 mm thick – a tiny optics. The structure of it is more or less like a transparent onion, formed by ~ 20000 very fine layers. Each layer is composed of cells of elongated shape (about a few micron thick and ~ 10 mm long). What makes it special is the variation of the index of refraction. In the inner core of the lens, the index of refraction is about 1.406, while it changes to 1.386 at the less dense cortex. This kind of design combined with the change of the lens shape makes us see things clearly whether they are far or close. In fact, evolution designs gradient index optics way before we even learned about it.

Changing the focus to animal kingdom, you can find more examples that not only make your eyes wide open, but also give us ideas to advance our knowledge of optical science. For example, Lobster uses reflective unit in the eye to focus much more light onto retina (figure 2a). This is definitely one of nature’s demonstrations on micro lens system. Mantis shrimp can detect circular polarized light thanks to intrinsic quarter wave plates made of cells in their eyes (figure 2b, for details about this work, here it is). You might wonder why we need to have polarized vision except 3D movie utilizing this principle to create stereo perception. Next time, when you buy sunglasses, get polarized ones. Wear them and observe the world! Asphalt road reflects light differently depending on light’s polarization. You can see that windshields are not that homogeneous in transmitting light any more. Due to its dichroism caused by tension when molding, it transmits one polarization better. Press a piece of thick plastic, and observe its change in transmission of light (you will induce dichroism by stress). LCD screens can only been seen clearly when you tilt your head in one direction. Turn your head in a different way, the screen become completely dark since most screens emit polarized light. After these daily life experiments, you might regret that we lose this feature during evolution.

Figure 2. (a) The reflective unit of a lobster's eye. (b) A cross section of the cells that work as quarter wave plate in the eye of Mantis shrimp. The scale bar is 10 micron. (c) The phase retardation introduced by the cells. They work nicely in the entire visible region of the light spectrum. The photo is courtesy of T.-H Chiou, S. Kleinlogel, T Cronin, and et al. in Current Biology 18 429 (2008).

We actually lose even more. Homo sapiens can only see three different colors, while birds, some mammals, and insects see the forth kind – Ultraviolet. For instance, Reindeer has UV eyesight. This special ability has evolution advantage. Lichen, on which the animal feeds, absorbs UV light, so it would appear black to reindeer eyes. The animal's traditional predator, wolves, would also appear darker against the snow, as their fur absorbs UV light. Apparently, under UV illumination, things are very different in polar area. I will suspect most of the animals living in Arctic or Antarctic area are endowed with this ability. How birds acquire UV vision is another fun story. Thanks to a beautiful article by Scientific American, bird’s UV vision is illustrated in details. A quick summary from this article: I realize our vertebrate ancestors had 4 types of cones in the eyes. While birds inherit this feature, our mammal ancestors actually lost two of them! Fortunately, nature is kind to us. Through mutation, we regained a third variation of cones when walking down the evolution road. Without that mutation, we will all be color blinded!

Figure 3. How our perception of the colorful world improved, or degraded down the evolution road? Courtesy of Scientific American Magazine.

It is time for some wild experiment and conjecture. An article published on Nature in 2004 seems to tell us that the migratory avian creatures use the interplay of light, electrons, and earth magnetic field to guide their navigation. In a nutshell, it is called “radical-pair mechanism”. The light creates a coupling between an unpaired electron spin and nuclear spin through a light induced electron transfer. The earth’s magnetic ﬁeld alters the dynamics of transitions between spin states. These transitions in turn affect reaction rates and products. Such effects can be ampliﬁed and used by the creatures. What does this tell us? Quantum mechanics and light matter interaction are working in a fine and elegant form. For a quick read, follow this link.

Story like this can be found at every corner on earth. After all, we are all offspring of nature, and nature relies on sun (the ultimate light source) in countless and ingenious ways.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Interested in optical rulers? Well, which kind!?

2011-06-28T15:47:00.000-07:00

Recently, three-dimensional plasmon rulers based on nano-rods are reported on Science. Hopefully, it will be cool weaponry for measuring the structures of the molecules in the near future. Over the past few decades, optical rulers based on different principles emerged from various branches of optical science. At the same time, researchers are trying hard to push each methodology to the limit. Optical Rulers, as a result, attract an army of researchers and spin off fruitful results. A quick summary of them seems to be a fair amount of content for everyone.

First thing first, what does an optical ruler do? Quite straightforward, it measures the dimensions of the molecular structures. For example, what is the distance between two subunits of a hemoglobin protein? What is the height of a membrane protein when measured from the membrane surface? Put into one sentence – optical rulers are aimed to map out the 3D structures of the molecules such that we can use this information to figure out the functionality of the molecules.

First ruler comes to your mind, I guess, will be the technique of X-ray diffraction. It is a true powerful optical ruler. After all, the DNA structure is solved by it, and Nobel Prize acclaims this technique for more than once. It has great resolution ~ 1Å, and you do not have put anything attached to the molecules. However, the pitfall is that, you have to crystallize the molecules which are merely impossible for some molecules, and the crystal forms of the molecules are in general, not the in vivo forms of the molecules. A report on C&EN and JACS beautifully illustrate the structure changes dramatically depending on the environment.

What is the king of in vivo optical ruler? I would say so far it is NMR. It has the resolutions of a few Å, and the algorithm is advanced so much that complex proteins are revealing their true forms (through more advanced multi-dimensional NMR). The principle is very similar to the trick we play with tuning forks. If you hit on one of the forks and bring the other replica close in, you feel the vibration on the second one and actually both will make the same tone without two touching each other. The energy (in terms of sound wave) resonates in these two forks. In NMR, intrinsic atomic spin plays the role of the tuning fork. By incorporate the isotopes (such as ¹H ¹³C and ¹⁵N, these atoms have nonzero spins) into the amino acids of the proteins, the spins of these atoms behave like tuning forks with different tones. Imaging if there are two or more isotope atoms close to each other, the energy (in this situation, the microwave qunta) will be transferred between the isotopes (let’s say between ¹H and ¹⁵N) assuming one of them is excited by microwave. NMR is specialized in measuring this energy transfer. The closer they are the more efficient energy transfer is. This efficiency drops proportional to 1/r^-6, where r is the distance between two spins. Now, if you have many of these isotope atoms located at different amino acids of a protein, you can figure out which isotopes (or more interestingly, which amino acids) are closer to each other. With the help of computer, you can infer the structures of the proteins, much like a complex trigonometry based on the relative positions of the spins.

Figure 1. A typical 2D NMR spectrum. Each blob can be thought as a sign of energy transferring between the spins of some specific hydrogen and nitrogen. By doing this kind of cross mapping, we can figure out the 3D structure of the molecules.

Two other techniques, Förster resonance energy transfer (FRET) & multi-dimensional IR spectroscopy, utilize similar principles we just described. In FRET, fluorescent dyes are attached the molecules of interest and lasers with optical frequencies are often used. If you excite one of the dye with the laser, and if the second dye is very close to the first dye, you can actually observe the light emitted from the second dye, much like the tuning fork instance we mentioned. It has the resolution of ~ nm and is widely used in biological society. On the other hand, multidimensional IR spectroscopy uses intrinsic vibrational modes of the molecules, such as the stretching mode of C=O. C=O is abundant in a protein which makes it very attractive. Through this technique, you can follow the energy is transferring from one C=O to another, and figure out the distances between two amino acids. The resolution is also in the Å scale. Another neat thing about it is that, multi-dimensional IR is able to monitor the structural change in fs to ps time scale, and this makes it very unique.

Last but not least, let’s touch the topic that initiates this short article – an optical ruler based on plasmon. As you may already learn, the plasmon is some electrons oscillating on the surface of a nano-rod. Lasers with optical frequencies are very effective in exciting these plasmon modes. Another thing you also need to know is that, plasmon modes are sensitive to the surrounding, especially when there are other nano-rods around. Again, just like the tuning fork, energy (in terms of plasmons) that locates on one nano-rod can hop onto another nano-rod. The efficiency of this hopping, and the resonant frequency of the plasmon mode are highly dependent on the overall geometry of the nano-rods and the distance among them. This is the principle we are applying. The ruler is composed of 5 nano-rods (figure 2), carefully spaced to each other. Depending on the overall geometry, the transmission spectra of the ruler is changing dramatically. This change serves as a “legend” for 3D mapping (figure 3).


Figure 2. 3D layout of the plasmon ruler. The nano-rod in the middle has a dimension of 4080260 nm and is directly excited by the light source. Courtesy of N. Liu, M. Hentschel, T. Weiss, A. Alivisatos, and H. Giessen in Science 332 1407 (2011).

Figure 3. Depending on the relative position of the middle rod with respect to the other four, the transmission at certain wavelengths (as pointed out as resonance I and II) changes dramatically. This is the optical legend that can be used to infer the relative positions of the rods. Courtesy of N. Liu, M. Hentschel, T. Weiss, A. Alivisatos, and H. Giessen in Science 332 1407 (2011).

The future goal will be attaching this genre of nano-rods onto different domains of the molecules and monitoring the spectra of them. By doing so, you can figure out the distance among the rods and then map out the distances among different domains of the molecules (figure 4).

Figure 4. By attaching the nano rods (shown in yellow) onto different domains of the molecule, we can infer the 3D structure of it by interpreting the spectra of the nano rods. Courtesy of A. Mastroianni, S. Claridge, and A. Alivisatos in JACS 131 8455 (2009).

Well, next time when people are interested in optical ruler, maybe you should just say: “which kind of them are you interested!?”

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

CLEO/Europe EQEC 2011 is ready to relay the success of CLEO US.

2011-05-10T10:27:00.001-07:00

Just like the movie slogan, “everything that has a beginning has an end” (I should reverse this to make it more suitable for this short blog). Everything that has a terrific end has a new exciting beginning. Indeed, something electrifying is happening across the Atlantic. Every two year, CLEO/Europe EQEC 2011 (22-26 May, Germany) is taking the heat to Europe and once again is looking forward to resonating what we have just completed in CLEO.

For people like me, who doesn’t have the luxurious time and funding to enjoy another wonderful trip, you can easily find the detailed programs in this link. Once you click the link and scrutinize the abstracts, I hope you don’t get trapped. For sure, the conference is loaded with crazy and smart ideas. Here are just some I found:

EE1.5 SUN 10:00 “The Size of the Proton” -- This rings the bell in my brain. I read about this in Scientific America a few months ago, and that sticks to my mind. On the one hand, I simply have no idea how laser spectroscopy can be used to measure the radius of the proton. There must be some original way, to me, a proton is almost dimensionless. On the other hand, they give a more precise but exotic value of it. Would this value forces the theoretical particle physicists to re-think what they have formulated for the past decades?

JSII1,2: “Low Dimensional Carbon Nano-Structures in Photonics I, II” -- Apparently, the Nobel prize simply marks the beginning explosion of the graphene related research. We are familiar with the novel material features of graphene and nanotubes. In addition to investigate their physical properties using lasers, how about using them to mode-lock a laser? Or using double wall carbon nanotubes to create broadband ultrafast pulses? Sit in these presentations, and you will find out.

CL/EB1-3 “Medical Imaging, Advanced Microscopy, Advanced Biophotonics: Sensing and Imaging” -- This is where you find yourself accessing the fresh. For people who are fond of applications, these are the harbors. It is simply a zoo of creative methodologies. I like the techniques that play around with the light polarizations. You might enjoy using nanoparticles to be the bio-markers. Just explore them and make your own list of preference.

OK, Time for you to take this journey to Germany, either physically or virtually. The destination is the same for all of us. Remember to check out the plenary sessions -- from attosecond lights, and insights of the cold atoms/molecules, to the advances of solid-state lasers. Feels like another feast to me. Finally, don’t get me wrong, almost everything that discussed in CLEO, you can find the continuation of them in CLEO/Europe EQEC 2011. I will leave you to connect the dots. Cheers!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Fully Charged, will be back for more next year!

2011-05-06T12:43:00.000-07:00

Just want to touch a few more fields before we wrap up this amazing CLEO 2011. The truth is, we all learn a lot and we will crave for more soon.

I guess we are by now all familiar with the metamaterials thanks to the powerful broadcasting media and online news. Metamaterials have some complex indices of refractions, which bend the light in a whole new way. Even nature utilizes it. The amazing colors on the butterflies, insects, are all originated from the nanostructures – some variations of metamaterials. However, I realized yesterday, this is OLD news.

Researchers now have something new called (well, new to me) “configurable metamaterials”. Unlike before, a specific metameterial is only suitable for one frequency; nowadays we can tune the properties of them by varying the temperature, through optical pumping, and more. If we use some materials that have strong thermal or optical responses to construct the metamaterials, these phoeneoma can be achieved. The concept seems to be there for quite a while, but it is just thrilling to see the real works have been done.

This morning, Dr. John E. Bowers gave an amazing talk on silicon photonics. I feel like soon in the future, silicon will replace the metallic wires in the computer, become the light source of miniature sizes penetrating to our daily lives, and constitute the cores of our gadgets. Furthermore, the data transmission rate is much higher (with tens of GBs per second, more than enough to watch all channels of HDTV at once), and the heat generation is negligible compared with the computers of modern days.

Make sure you check the article in photonics spectra and this one in from Intel to peek the future. Think about it, if we have the silicon-based waveguide/lasers of tiny dimensions, combined with flexible LED panels, we can make the electronics so small and life will totally be awesome.

In addition to using silicon as data transmission media, a hybrid silicon ring laser, like the three shown here, could be used as an on-chip light source in future photonic circuits. The rings are just 12.5 µm in radius and consist of III-V compound semiconductors. Waveguides – the black lines running below the rings – traffic the light back and forth. (Courtesy of Di Liang, University of California, Santa Barbara).

Fiber lasers/amplifiers have drawn huge amount of attention in the past decade. The Holy Grail is to replace the free space lasers in many applications. If they succeed in doing so, I would imagine all the laser-based medical devices would use fiber lasers since they can be made compact, robust, and essentially free of aligning. They can even revolutionize the optomechanics’ market (apparently, a lot of free space optomechanics will be forced to retire). Like past few years, great advancement/or continuous improvement are seen in this conference, such as mode-locked fiber lasers, fiber amplifiers, fiber parametric devices, new wavelength fiber lasers, beam combining and stabilization of fiber amplifiers, and even fiber based sensors (browse your brochure one more time if you are too busy to notice these).

Advances in biological microscopy and nanophotonic sensors once again carry the light applications into another degrees of freedoms. There are numerous ways of doing microscopy, old and new, classical and bizarre. Some promise more while the other create intellectual and instrumental challenges. Each category of them suggests a new direction for laser manufacturing. I guess this is part of the fun in research pioneering! On the other hand, advances in nonophotonic sensors do make me realize we have to re-think the limitation of “instrumental sizes” almost every time you think about it. Indeed, researchers give you a new definition of instrumental size almost every year!

See you all in next CLEO or actually next conference!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Applaud with appreciation to all the poster session’s presenters!!!

2011-05-05T13:16:00.000-07:00

Attending poster sessions is energetic and adventurous. It is even a great social event. Compared to technical session, you never fall asleep and you can interrupt the presenters whenever you want (How great is this, you simply have a personal tutor at your disposal). In additions, you learn more in less time if your mind is a knowledge sponge.

Forgive me for sampling only today’s poster session. Actually I totally regret I didn’t spend enough time in the last few days for poster sessions. To me, these successfully poster sessions really mark one of the highlights in CLEO 2011. Here are some of them that I got a chance to interrogate the presenters (they were just busy, it was quite hard to squeeze in to ask even one question):

JThB8, Generation of a macroscopic singlet state in an atomic ensemble – I learned from this poster that you can create a spin 0 ensemble using weak optical pump coupled with active feedback. In other words, you start from cold atoms and squeeze the distribution of the spins in a way that it approaches zero expectation values in all directions through a tailored Hamiltonian. Quite amazing, but it has been realized beautifully by the researchers.

JThB25, Twin-photon correlated confocal microscopy – the lateral resolution of the microscopy is defined by the diffraction limit. A clever way to improve the resolution for more than 60% is proposed and performed by utilizing a phase plate right in front of the sample. In a nutshell, the phase plate encodes different phases for the points that are not in the vicinity on the sample. If the points on the sample were more separated, the imposed phase would be more different. A smart detection scheme then only picks up the signals that have no phase difference. By doing so, the light from one point on the sample is amplified and that from points nearby are suppressed. This greatly enhances the lateral resolution.

JThB28, Photon-phonon entanglement in a coupled optomechanical system – the entanglement of photon and phonon is studied thoroughly in this simulation work. A system with two coupled optomechanical cavities is the model system (imagining one side of the cavity is on a spring, so this cavity supports phonon modes). Two cavities are coupled by cavity-supported light modes. It is found that the light can couple the photon modes to the phonon modes of the cavity. And this entanglement lasts more than 500 seconds. This really blows my mind away; I used to think the entanglement doesn’t sustain itself for this long.

JThB42, Conical interaction dynamics in a rhodopsin analog: isorhodopsin – Ultrashort pulses (~ 10 fs) from NOPA are used to investigate the isomerization of rhodopsin (the first chemical reaction in the mechanism of “seeing things”) and isorhodopsin. By compared with the results of isorhodopsin, it is found that the isomerization of the rhodopsin molecule is actually optimized on the right chemical bond location. As a result, the efficiency is superb. Nature does her job, indeed!

JThB45, Unidirectional perfect transmission resonances in nonlinear asymmetric photonic multilayer – Combining theory and experiment, a photonic crystal multilayer, which transmits light in one direction but not the other, is realized. In one direction, the transmittance is more than 92% (the reverse direction, the transmittance is less than 20%). This is actually a one-way photonic crystal and I think applications based on this will come in the near future. The presenter is so nice and gives me some advice on the technical session -- His way of arranging the chairs will definitely increase the seating capacity by at least 2-fold. Your opinion is greatly appreciated.

JThB137, A comparative study of Raman enhancement in capillaries – OK, I have to admit, I love this one. The experiment is straightforward but so smart and neat. The laser light is guided through a hollow photonic crystal fiber by a high NA objective. The hollow fiber is filled with the solution of the chemicals. The light has very high photon density in the fiber and the interaction length of the chemicals with the light in the fiber is long. Subsequently, the Raman signal is found to increase by ~ 10-fold. 10-fold is an astronomical number to me actually, but the result confirmed this nicely done work.

Thanks again to all the poster session presenters! Bravo!!!

p.s: Just realize there is a Light Street right beside Baltimore convention center. Maybe that is why we have CLEO 2011 here!?

Light Street, Baltimore, MA!!!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

When light dances with sound, teases the molecules, and plays an important role in green energy!

2011-05-04T11:50:00.000-07:00

Shuffling among many eye-opening technical sessions and CLEO’s Market Focus & Technology Transfer Showcase, I simply realize adventures with light are everywhere:

An amazing imaging technique called photoacoustic imaging/microscopy (JTuG) caught my full attention on Tuesday. It is a perfect example of light collaborating with sound to achieve something fascinating. Think about it, light and sound are siblings. They are governed by similar natural laws. You might argue that light can propagate in vacuum while the sound needs media to do so. But again, they are siblings, not twins. So this argument does not really hold. Anyway, taking that into account, isn’t it fun to see them hold hands and work on something new together!?

The principle behind this new imaging technique is actually straightforward. The pulsed laser light (MHz repetition rates) is focused into the tissue; the tissue of interest absorbs the light and expands. This process is repeated with laser repetition rate. The pulsed expansion creates the ultrasonic sound wave, and we detect this by transducers. We then reconstruct the image of the tissue through some complex algorithm. We know that ultrasonic can penetrate deep tissue, while the resolution of ultrasonic is not as great as optical imaging. On the other hand, optical imaging can only go to a few millimeters deep. By endeavors of the researchers in improving this technique, photoacoustic imaging actually combines the strength of these two – it can do deep tissue imaging with optical imaging resolution – optical resolution photoacoustic microscopy.

An experimental layout for photoacoustic microscopy.

Fast-forward to Wednesday, a wonderful QELS session (QWB) focusing on laser cooling and its further applications on quantum computations and simulations are really hardcore stuffs. Using light to produce ultra cold molecules is definitely pushing the frontiers of science. We all heard of atoms cooled by lasers and a Nobel Prize was given to this achievement. But for molecules, things are more difficult. They have inertial structures and as a result, complicated processes are involved when cooling them by laser light. However, diatomic molecules are being cooled to sub micro Kevin through Sisyphus and Doppler cooling (check this article for more). If you missed today’s presentations, it is totally ok. On Thursday, sessions like QThJ, QThM, QThN, and QThO will feed your quantum hunger.

Talking about green energy, Laser Inertial Fusion Energy (LIFE) is discussed during CLEO’s Market Focus. Due to human mankind’s need in energy, we turn to fusion, and we intend to do so by using extremely high power lasers. LIFE utilizes 384 powerful lasers to create pulses with the energy of 3.1 mega Joule in IR and 2.2 mega Joule in UV per pulse. Each laser has the size of a truck and can be swapped in and out as a unit if the lifetime is reached or malfunction is found. This kind of gigantic project requires the state-of-art techniques and actually drives the development of the optical industry, such as glass productions. If you dig even further, the diode pumped helium cooled mercury amplifier inside each laser is just breathtaking. The helium is blown through the gain media with 0.1 Mach speed to cool down the laser. In other words, you even need aerospace technology to prevent the turbulence inside the laser.

Finally, also thanks to CLEO’s Market Focus & Technology Transfer Showcase, I just learned that an animated website created by JDSU called Photovoltaic for generating and measuring energy is a good starting point to know how light plays the role in green energy. Enjoy it and do not forget to check out CLEO’s Technology Transfer Showcase program tomorrow.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Behind every successful conference!!!

2011-05-03T12:10:00.000-07:00

An exciting conference is mainly composed of two parts – technical sessions and exhibition. We enjoy the technical sessions because that’s where we learn from our peers, get our brainstorms, and have a quick update on the scientific frontiers. These are addicted as you can tell me about it. However, ask yourself about the definition of a successful conference. Most of us will say a conference will not be complete without stopping by the exhibition hall to see the zoo of new products and technologies presented by numerous companies. Well, the souvenirs we gather from each booth are attractive too!

If we take a step back, we realize that convention center is a very busy host. Every week it is embracing a new show and new people with crazy ideas. This makes me wonder, what kind of preparation is required to host a welcoming conference? For technical sessions, things are easier to picture since most people had the opportunity to observe or organize a symposium either in the school or in a research institute. We need to multiply everything by at least a hundred. These are OK, we can have more rooms, chairs, projectors, laser pointers, and most importantly, more coffee and labor. So we can make this happen. Now, if we think about the preparation of an exhibition hall, a blank image usually emerges. I did not even know how an exhibition hall looks like before all the companies fight for the space and start to build their own territories, not even mention about how to set it up. Thanks to my job duty and helps from the colleagues, now I have observed the way it happens…^_^

The convention center is moving at a very fast pace as we mentioned. The first thing we have to realize is the time you have to build a booth. Normally, each company has a full day working with the labor of the union to set up everything. In other words, people are working under great pressure. The very first scene I saw is that everyone on the floor was tense since all they faced is a concrete floor marked by chalk to specify the territories of each company. Phones were ringing all the time because on the other side of the exhibition hall, hundreds of trucks were waiting to ship the equipments into the hall. In the meanwhile, experienced workers maneuvered loaded carts, and crates were shuffling among people. These would go on for several hours, and step by step, each big and small cargo reached the right destination, while small hassle was happening all over the place (such as some trucks got lost, according to the workers, this is quite normal).

Small carts moving around the cargos and magically they all arrive at the right destinations.

After this was all set, locating the power jackets and building tiny power grids on the floor were next. We can fairly say, without the electricity, the exhibition hall would be like a haunting house rather than a technology showcase. But for the beauty of the exhibition, we want to hide these power grids. So we make them lying comfortably on the floor and covered by carpet later on. By doing so, we will never spot them unless you come to the hall before the grand opening. At the same time, small hydraulic trucks were busy putting the overhead canvas slogans and signs above company’s booth. From this moment on, we would not get lost, since the flag (well, the slogan) was waving on each and every corner of the hall.

The power grids (shown in orange) on the floor are taped down nicely and distributed very efficiently for the booth use.

Time to do some makeup. The “base foundation” was the carpet. It covers all the power grids and jackets, all the chalk signs, and marks the lanes of the traffic. Putting the carpet represents an important step – “the equipments are ready to get some fresh air.” Knowledgeable technicians started to open the crates (always with complaining, since equipments had legs, they moved around after staying in the trucks for so long), tried to do preliminary assembling, and finalized the floor plans. They spent another several hours to put optical tables together, made the equipments up and running, and arranged and cleaned the surfaces of each components. These are tedious work, and work was again under great pressure.

This booth is about 50% done. Technicians have been working for hours, and there are still al of of crates need to be opened and arranged on the optical tables.

Just like LEGO we played when we were young, setting up the booth was like intense LEGO works, except they are much bigger and you cannot quit if you feel tired. It is not uncommon to see people work way beyond midnight because they want to present the best to the researchers and the students the following day. After technicians and the product line managers were satisfied with the setups, final cleanup was required. Final vacuuming on the floor, tearing open the plastic wrapping of the carpet, and covering the equipments and tables with blanket were essential works not to be omitted. Everyone wanted to keep every link neat and flawless.

After a detailed inspection on the booth, we can call it a day. Hmmm, time for bed or time for a drink?! I would like to thank Mr. Hoang Hung and John Carter, the men who are in charge of the booth setup. Without his help and explanation on the details, I will not be able to peek through this new window to see how to dress the conference!

Another successful show!!! After seeing so much traffic, all the efforts and sweats are just sweet!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

From the smallest lasers to the biggest ones!!!

2011-05-02T17:40:00.000-07:00

With so many different kinds of lasers play essential roles in modern researches and daily life, it is tempting to find out what are the extremes among them. Thanks to this conference, this question intrigues me once again during a talk (QMF3) where a gigantic free electron laser (FEL) was mentioned and used to probe the atomic structures. Searching with the conference program brochure and within my memory, here is what I can find.

The winners of “the biggest” prize go to the FELs. Taking the one in the U.S. soil as an example, a FEL powered by a two-mile-long linear accelerator (linac) in Stanford Linear Acceleration Center (SLAC) has a grand name associated with it -- Linac Coherent Light Source (LCLS). Technically speaking, it is a laser of more than two miles in length and many many tons in weight (I don’t think people actually weight this monster, figure 1). Basically, after SLAC’s linac accelerates very short pulses of electrons to 99.9999999 percent of the speed of light; the LCLS takes them through a 100-meter stretch of alternating magnets that force the electrons to undulate back and forth. This motion causes the electrons to emit X-rays. Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently. As many as 10 trillion X-ray photons can be produced and squeezed into a bunch that’s a mere 100 femtoseconds long. This giant laser has a sibling across the Atlantic. In Europe, an x-ray free electron laser (European XFEL) shared by 14 countries is powered by a 2.1 km long superconducting linear accelerator.

Figure 1. The aerial view of the monster FEL laser in SLAC.

The runner-up would be FELs powered by the synchrotrons. Although there are huge ones such as LHC, the ones that are used to pump FELs are smaller, such as Japan’s SPring-8 and France’s SOLEIL (on Wednesday, a presentation (CWG5) utilizing this instrument will be discussed). Well, maybe Synchrotron has a bigger surface area than linear accelerator, since I am naming the prizes, let’s say, the length is what we compare.

Just a bit digression, an instrument (or experiment) involved 192 high power lasers is an unusual contender for this prize. Apparently, researchers in Lawrence Livermore National Laboratory’s (LLNL) National Ignition Facility (NIF) are trying to fulfill the dream of fusion by focusing many lasers in a capsule of size equivalent to a peanut. By squeezing so much energy in so little space (with the existence of hydrogen), they are optimistic about that the fusion is bound to happen.

Let’s swing to the other extreme. Nano-lasers with many different designs can proclaim the winners of the “smallest” prizes. A quantum cascade laser embedded in a microcavity and emitting THz radiation is one of the smallest lasers available (with a size of a few tens of microns). But the 44-nanometer "spaser – surface plasmon laser" will be very hard to beat. The device is a hybrid -- A bluish-green laser beam is shined into a suspension of gold nanoparticles. A layer of sodium silicate and an outer silica shell containing dye molecules surrounds each of these particles. When the gold is excited by the laser photons, collective oscillations of electrons on the surface, known as surface plasmons, are excited. The plasmons then excite the dye molecules, and subsequently the photons released from the dyes stimulate more plasmons on the gold at the same wavelength, causing the device to emit green laser light. How amazing is that!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

CLEO/Laser Focus World Innovation Award endorses the recent triumph of Terahertz (THz) spectroscopy and applications.

2011-05-01T19:33:00.000-07:00

This year, the award goes to Applied Research and Photonics Inc. for its endeavor in THz device and applications. This is indeed another sign saying that THz will be a hot topic for the following few years thanks to many people’s efforts over the past two decades. Besides, we are very happy to see this field has grown into a vibrant society with its own conference – Optical THz Spectroscopy and Technology (OTST). I was there, learned many news things from researchers all over the world, and enjoyed the nice breeze from the Pacific sea in Santa Barbara.

Thinking about THz, most of us immediately connect it with a couple of concepts, including the wavelength of it is long compared with the familiar optical and even mid-IR wavelength (a wavelength of 1 micron is 300 THz while 1 THz is 300 um), the property of great penetration to soft materials like tissues, plastics and, card boards, and its application in security screening due to the sensitivity of many explosives. In addition, its low energy and non-invasive feature is perfect for authenticity test on artwork and biomedical imaging. With this field burgeoning like never before, it is worth to take a quick look on the methodologies of generating THz.

The most orthodox way to create THz laser is to find a suitable gain media and pumping source. Just like a dye laser in which an electronic transition of dye is directly related to the lasing frequency, the transitions between the rotational states of methanol gas falls right into the THz region. A very good white paper using methanol and pumped by a CO2 laser can be found in here. Of course, the gain media is not restricted to methanol; even water vapor is actually a good THz source.

Mixing two lasers with the frequency difference in the THz regime is another neat way. Considering mixing two lasers in the optical fiber, the beating that is produced by the frequency difference of these two lasers is just the source of the THz. If we can filter out the pumping lasers, then we have a useful THz radiation. A more efficient way of doing so will be mixing two lasers in the nonlinear crystals (GaAs, LiNbO3, … etc). Depending on the nonlinearity of the crystal, frequency conversion is achieved with different efficiencies. In this case, the efficiency of THz generation is proportional to how strong the nonlinearity of the crystal is. This principle is adopted by Applied Research and Photonics Inc. to the extreme. The core of its THz device is based on a polymeric nanomaterial that has very strong nonlinearity.

How about generating ultrashort THz pulses to cover a wide THz spectrum at once. A very exciting way of doing so is through air plasma. The principle is quite straightforward although there are several variations of it (figure 1). Basically, you create air plasma in the air (by focusing intense laser pulses in air), drift the electrons away from the nuclei through different bias methods, and then the nature law takes over. Since the electrons will recombine with the nuclei through coulombic force, this process creates a transient current (or a oscillatory dipole). An oscillatory dipole/transient current in this process creates radiation that covers THz regime. Before the advance of intense lasers, researchers focus laser beams onto photoconductive antenna chips to create transient current. Actually this is still the most popular way to generate ultrashort THz pulses.

Figure 1. THz generation based on air plasma. In all scenario, air plasma is generated by intense laser pulse (red). Electrons are accelerated in the direction of laser pulse . This process creates THz in a cone fashion. Electrons are drifted away by external DC source (b), by another laser pulse (c), and by the laser pulse itself (d). Courtesy of Thomson M., Kreb M., Loffler T., and Roskos H. in Laser & Photon Rev. 1. No. 4. 349 (2007).

Just a reminder, on Monday, CLEO has an entire session focusing on TH Sources. Besides, the award presentation will be during the Plenary Session in the same day evening. This is definitely another power boost for you to learn how it goes in this field. Hopefully, next time when we encounter THz research, we all know a bit more in their business. You can also explore THz quantum-cascade lasers, THz generation by pulse front tilting in the crystal, and more through the power of the Internet…^_^.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

From quantum Zeno effect to all optical switch, part II.

2011-04-16T18:13:00.000-07:00

In the last blog, we took a trip starting from quantum Zeno effect and reached to one of its applications -- all-optical switch -- at a quick pace. This time, we will look into more phenomena that researchers use in order to achieve this all-optical switch future.

We discussed about photonic crystals (PCs) and their versatility in a recent blog. We learned that by changing the patterns of the PCs, it is able to select which color of light that can travel within it or be rejected. While the patterns play the crucial role in PCs, we have to realize that it is the modulation of the refractive index produced by the patterns that give PCs their unique physical properties. With this being said, it is not difficult to understand that if the refractive index of the material that PCs are made of can be changed, we are able to affect (or tune) PCs’ properties. This is exactly what researchers are trying to do recently:

Considering the silicon PC shown in figure 1a, there are two colors of light allowed to propagate in it (mode c and mode s). Now, it is known that putting some free electrons in the conduction band of Si would change its refractive index. To use this feature, researchers shine this PC with some light (pump) such that a few electrons in the Si can be kicked to the conduction band. Changing the refractive index shifts the center frequencies of mode c and mode s directly. In addition, since PC is so sensitive to its refractive index, just a few hundred fJ of energy is required to tune the transmittance property of the PC. The all-optical switch is then realized by the following: Let’s input two colors of light into the PC -- one is very close to mode s and one is right at mode s (figure 1b). Without the additional pumping light, mode s is transmitted. With the pump, mode s is suppressed and the other color now is able to transmit since the transmittance property is shifted. So by pump-on/pump on, we will have different colors of light coming out -- an all-optical switch, as we expect.

Figure 1. an all-optical switch based on a silicon PC. (a) The structure and the transmittance curve of this specific PC. (b) with/without pump, the transmittance of the PC is shifted. Here we use mode s as an example. Courtesy of T. Tanabe, M. Notomi, S. Mitsugi, A. Shinya, and E. Kuramochi on APL 87 151112 (2005).

Another example is a PC made of polystyrene. The pump beam can also control the transmittance of it. The structure and the transmittance curve are shown in figure 2. By pumping this PC with femtosecond laser pulses of a few nJ, it is found that the transmittance can be changed by more than 60%. And this feature definitely makes it a strong candidate for all-optical switch application.

Figure 2. (a) A SEM image of a PC made of polystyrene. (b) The transmittance curve of this PC without being pumped by optical pulses. Courtesy of Y. Liu, F. Qin, Z. Wei, Q. Meng, D. Zhang, and Z. Li on APL 95 131116 (2009).

Let’s change the gear and look at something that will also be presented in CLEO 2011. A phenomenon called inverse Raman scattering (IRS) is utilized for all-optical switch application. We are all very familiar with Raman scattering, in which a material is pumped with a strong light, and you can detect some other colors of the light in the output due to inelastic scattering of the pump light in the material. If now we input two frequencies of light -- one is pump, the other has bluer color such that the frequency difference between these two are equal to the energy loss of the inelastic scattering, IRS would drain the energy from the high frequency light to the pump. So by putting pump or not in to the material, we can actually decide we want to drain the energy from the high frequency light out or not. This is actually realized by using a optical fiber or a silicon ring resonator. Excitingly, these will be presented during the conference. So, do not forget to check it out if you are interested.

There are more to say on this topic, such as using “four wave mixing on a silicon photonic chip” or “quantum dots coupled with a PC” to achieve the all-optical switch goal. The pool of exploration is open, just get ready and jump in!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

From quantum Zeno effect to all-optical switch, part I.

2011-04-10T17:49:00.000-07:00

Needless to say, scientists have been puzzled and fascinated by the quantum nature of the physical law for more than a century. The history of science is all over it and evolves with it. Having this in mind, it is very reasonable to see that the Science magazine has named the discovery of the quantum machine as the most significant scientific advance of 2010. It is the first quantum mechanical resonator that can actually be seen by bare eyes and deserves another detailed blog by itself.

How about in the optical world? Have we successfully implemented or utilized the quantum nature of materials for cool applications? The exciting answer is YES, and we will be looking at some of them in this short blog:

Let’s start from one of the most bizarre behavior that quantum mechanics can do – Quantum Zeno Effect. It states that if your observation of an event is frequently enough, its decay to the natural state of equilibrium will be affected significantly, either being slowed down, frozen, or accelerated. In fact, scientists call it anti-Quantum Zeno Effect, if the process is being accelerated (by the way, you will be able to hear the talk from its explorer -- Gershon Kurizki in CLEO 2011: QELS Fundamental Science).

The name “Quantum Zeno effect” adopts a broader meaning when it enters the optical world. We now use this term to describe manipulating the evolutions of the populations of different quantum states (or photons with different colors) by external perturbation.

If you feel the aforementioned is hard to digest, I promise the following will be not. We will be looking at some real examples and these are aiming for a high goal -- all-optical switches. If you wonder why all-optical switch is important, just think about how hot your CPU can get most of the time and how fast light can travel compared with electrons.

Take a look at figure 1. Two optical fibers connected by a microdisk made of GaAs. As shown in the figure, the signal light is shown in green. The disk couples the signal weakly. It comes in from the lower left (upper right) side, coupled to the disk, to the second fiber, and leaks to the upper left (lower right). Now if we carefully put in another pump light, marked as blue, it would perturb the property of the disk such that the ability to couple the signal light will be ceased completely. As a result, no signal light can be coupled to another fiber through the disk. In other words, you can detect signal from the upper left by putting it from the lower left with the pump-off. With pump-on, you detect no signal into the second fiber. This is a switch controlled by pump light.

Figure 1. one model of all-optical switch utilizing a microdisc. Courtesy of Y. Huang and and P. Kumar in Optics Letters Vol. 35 2376 (2010).

In terms of quantum mechanics, the pump light opens a new channel of interaction inside the microdisk. It affects the existence of signal photons in the disk by draining them into another wavelength of light (shown in red in the figure). So the populations of photons with different colors are rebalanced. Virtually no signal photons are found in the disk when pump is on, and as a result, no leakage of it on the second fiber can be found.

Yu-Ping Huang, Joseph B. Altepeter, and Prem Kumar also present another similar methodology utilizing second harmonic generation (SHG) principle, as shown in figure 2. When there is no pump in the waveguide (WG-I), SHF process dominates. You put in signal with frequency ws; you get 2ws in the output. If the pump light is in, then every moment you have 2ws in the waveguide, it will interact with the pump and be drained to another frequency. So light with 2ws never builds up, and intensity of ws is not affected too much. The net result is that you still have the ws as the output. In summary, you have 2ws (ws) as output with pump-off (on). This is indeed another neat way of doing optical switch.

Figure 2. an all-optical switch based on SHG principle. Courtesy of Y. Huang, J. Altepeter, and P. Kumar.

Using two-photon absorption for the optical switch is yet another exotic way. Considering the design in figure 3, a toroidal resonator couples two fibers. The input light E1A (E1B) on fiber 1 (2) has frequency wA (wB). The resonator has strong two-photon absorption of wA + wB but nearly no absorption for wA, wB, 2wA, or 2wB. It is found out that, with the existence of strong E1B, you cannot find any E1A in the second fiber. In other words, by inputting E1B or not, we can control the existence of E1A on the fiber 2. The principle behind it is very similar to what we just discussed. With strong E1B in the resonator, any light of E1A will be destroyed through two-photon absorption process, so only strong E1B remains in the resonator. With the non-existence of E1A in the resonator, none of it can be coupled to the fiber 2. In addition, we can also use the strength of E1A to control the existence E1B in the first fiber! So a multi-functional optical switch emerges.

Figure 3. An all-optical switch based on two-photon absorption resonator. iR and T are coupling coefficient and transmission coefficient of the system. Courtesy of B. Jacobs and J. Franson in Physical Review A 79 063830 (2009).

Since two-photon absorption plays a core role of optical switches, researchers like Seth R. Marder and Joseph W. Perry are trying very hard to synthesize new organic compounds with desired two-photon absorption. You can learn about it in the morning section of QELS Fundamental Science.

We will look into more different kinds of all optical switches in the next blog.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

The best resource I can find to learn your first “optical fiber” lesson!

2011-03-26T10:53:00.000-07:00

When trying to search for more information about CLEO 2011 plenary speaker – Dr. Donald Keck, a pioneer and veteran in optical fiber technology, I realized that the best online resource to learn about optical fiber is not Wikipedia this time. It is actually the website of Corning. Apparently, as a company, Corning has a different standard in educating his customers. It puts a lot of efforts and resources into the contents. What impresses me about this website is that – it does not just pile the information day after day for you to dig in. It organizes the information so well that you just absorb the knowledge without noticing it. It contains knowledge for the beginners, amateurs, and serious researchers. All you need is a cup of coffee/tea and a nice break to enjoy it.

I will start with the “Fiber 101”. Almost all the technical terms you need to know (such as core and cladding, single/multimode fibers, and mode field diameter) will be explained with beautiful illustrations here. I will call it “fiber for dummy” section.

If you are interested in the history of optical fiber, make a little digression and feel it through “The History of Corning's Optical Fiber Innovation”. Over the past 40 years, the technology of fiber has advanced so much that we now have an optical fiber of more than a few thousand kilometers transmitting the information at the rate of terabits/s! Intriguingly, you can also see Dr. Donald Keck’s video clips and his historic photos scattered in this timeline-type-of-presentation.

Let us shift back to more scientific and technological points of views. If you want to know what and how the optical fiber can carry the information around the world and what the emerging technologies are, you might not miss this link “Long-haul Networks”. In this short report, concerns and solutions about transmitting information through fibers, new technologies to advance optical fiber’s applications are well discussed. All of us are fascinated by the fiber to the home (FTTH) for sure. Learning about it is straightforward through the following two links – “Broadband Technology Comparison” and “FTTH benefits”. I really hope soon we can have this technology at each corner of the country.

Time for the hard cores! You can find numerous application notes and conference papers in “here”. You can easily get lost if you do not know what you are looking for. This section is like a database for serious researchers. It also proves that Corning is intimately connected with the scientific community. With this being mentioned, do not forget to check Wikipedia now! After a quick power boost from the Corning website, I believe everyone can have a fundamental ideas about what optical fibers are and can surf through the heavy materials presented by wiki much easier.

Of course, the devotion of this website raises my expectation on Dr. Donald Keck’s plenary talk. I really look forward to learning the stories of the advances of optical fibers through a person who dedicates himself into this field for more than thirty years!

Finally, I found a very amazing video clip online to end this blog. It is an inspiring one made by Corning – A day made of glass, made possible by Corning – shows you that optical fiber is just a link or part of the technologies made possible by glass. Combing glass with chemistry, material sciences, and physics, the life is with unlimited potential!!!

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Small structures, huge effects -- what a photonic crystal can do ?!

2011-02-26T18:32:00.000-08:00

Most people (well, let’s assume most people love physical science, especially light matter interaction) have a rough understanding about photonic crystals (PCs). To start with -- photonic crystals (PCs) are periodic optical nanostructures that are designed to manipulate light. In general, to affect light’s propagation, the length of the repeating unit cell of the nanostructures in PCs is compatible to the wavelength of light. If you reach this far, you know quite a lot about PCs.

The next thing in our mind would be “where are the examples”!? If you follow scientific news closely, you might say – “Invisible cloak is made by PCs according to a BBC report or an article on sceince.” This is definitely true. However, there are a lot more applications based on PCs that are exciting and deserve us to take a detailed look. Furthermore, these applications are close to real world applications than you think!

Let us start with two-dimensional(2D) PCs. 2D PCs mean the unit cell is extending in two dimensions, just like the silicon slab shown in figure 1. In this case, we can picture the unit cell of the nanostructure is the air hole which extends in a plane. The light will be launched into the waveguide in the middle of the slab. Pioneered by Professor Susumu Noda (plenary speaker of CLEO 2011), this kind of silicon slab is a “light master”. By fine-tuning the air hole patterns, sizes and the positions, light with different frequencies will choose completely different routes when propagating in the waveguide through the slab. In general, the air hole size decides which frequency can leave the waveguide while the missing air holes (point defect) in the nanostructure determines where the light would be located and emitted. For example, the slab shown in figure 2 is an amazing design. It is a slab composed of seven PCs with different air hole sizes. In each PC, there is a point defect where three air holes are missing. Light with different frequencies chooses to leave the main highway (waveguide) and emits at different locations (point defect) as shown on the left of figure 2. The air hole has the size of about 400nm and the difference in air hole diameter of PC1 and PC7 is just 7 nm! These small structures and the differences in structures have huge effect on the light propagating.

Figure 1. A silicon 2D PC.

Figure 2. A seven-component 2D PC. Each PC is capable of selecting one frequency and emits it into the space. A mini-optical circuit indeed. Courtesy and copyright of B. Song, S, Noda, and T. Asano, Science 300 1537 (2003).

In addition to using the “point defect”, changing the hole sizes proves to be another neat way to localize the light, As shown in figure 3 (upper panel), the upper part of the slab is composed of air holes of diameter 410nm and 420nm, respectively. The difference is so small such that you can not tell easily even by a TEM image. If we launch the light through the lower waveguide, and scan the frequency of the laser, we found there is only one frequency that will hop into upper waveguide and be localized in a nanoscale volume (figure 3, lower panel). Furthermore, this method can select a very narrow bandwidth compared with the “point defect” method. As a result, it proves to have a much higher Q-factor in the localized cavity.

Figure 3. A highly selective 2D PC based on different air hole sizes. The inset on the lower panel shows the near field image of the dashed rectangle. Courtesy and copyright of B. Song, S. Noda, T. Asano, and Y. Akahane, Nature Materials 4 207 (2005).

Same idea can be applied to three dimensions. An interesting example is to suppress the spontaneous emission (SE) of a device (quantum well, for example). Modern techniques allow us to create a 3D PC such that certain frequencies of light cannot propagate in it. The set of the frequencies that are forbidden to propagate is called photonic bandgap (PBG). Now, if we embed a device into a 3D PC where the SE has the frequency in the PBG of this 3D PC, interesting things will happen. Since none of the accepting mode of the SE can exist or propagate in all directions, the emission lifetime is found to increase by several folds. Considering SE is not desirable in a laser system since it sets the threshold for the laser when it tries to lase and we cannot harvest the energy that is produced by SE in most cases, controlling the SE is indeed a great achievement.

Another nanostructure that is fascinating is a low dispersion optical fiber that can transport femtosecond light without adding too much dispersion. Creating a near chaos and/or random nanostructure such that no light frequency is resonant or interact constructively to it except propagating through the guided mode is the key. One example is shown in figure 4. An experiment shows that a 13 fs pulse only stretches to 26 fs when propagates in this fiber for a distance of 1 meter. This is a substantial improvement if we consider that normally a meter of fiber would stretch the pulse into ps regime. Maybe soon in the near future, we can transport ultra short pulse through fiber without stretching it!

I hope by now you are already fascinated by PCs…^_^.

Figure 4. The fiber core of a low dispersion optical fiber. Notice the structure has no periodicity at all. Courtesy and copyright of J. Bethge, G. Steinmeyer, S. Burger, F. Lederer, R. Iliew, IEEE J. Lightwave Technol. 27, p. 1698 (2008).

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.

Controlling Light's Propagation by Light ?!

2011-01-17T20:07:00.000-08:00

Light is wave. Well, most of the time, it behaves like so. Due to its wave nature, it is not trivial to confine any light beam to its original shape when it is propagating except a plane wave. Putting in one sentence, light beams tend to diverge. Actually, even a plane wave has edges in reality, so plane waves diverge too. So, what if we can create a light beam that retains its diameter and energy distribution transverse to the direction of propagation through space, a “spatial soliton” as being called by scientific community, wouldn’t that be a great achievement? After all, human beings have been trying to manipulate light for thousands of years. In fact, we can do much more thanks to the efforts of researchers who are fascinated by making this happen.

The way to do so is through “light-matter interaction”. With the advance of laser, we can now make the strength of the electric field high enough to affect material's property (refractive index, most of the time). That simply means that when strong light propagates in the material, it does not simply pass through it -- it interacts with the material. This kind of interaction modulates the material’s property (the change of the refractive index, for example). And this modulation in turn affects how light is propagating. With careful design, it is possible to have an equilibrium in which light propagates without changing its spatial shape. In other words, the modulation of the material’s property compromises the diffraction effect imposed by nature. This is like a real time feedback loop which can be pictured as that in the figure 1.

Figure 1. The virtual feedback loop when strong light beam is propagating in the material.

The first and foremost phenomenon nowadays is probably the self-focusing of a laser beam in some crystals. For example, when a laser beam is propagating inside Ti:Sapphire crystal, it makes the refractive index higher along its beam path due to laser intensity. This inhomogeneity of refractive index in turn focuses the laser beam. Without going to too much detail, this phenomenon is actually the basis for modern mode-lock lasers which you might easily find one in most universities.

How about a few more fancy things!? When two light beams launched parallel inside the crystal (Sr0.6Ba0.4Nb2O6 crystal, for example), they can attract or repulse each other depending on their phase relationship. This is again due to that light beams are modifying the refractive index of the crystal, and this modification guides how light beams should propagate in real time. Light beams can even form a spiral pair if certain parameters are achieved. So they circle each other while propagating forward. Just like a DNA strand made by light beams (figure 2).

Figure 2. Light beam pairs propagate and spiral about each other in the crystal. This graph is at courtesy of George I. Stegeman and Mordechai Segev in Science 286 1518 (1999)

Researchers like Professor Mordechai (Moti) Segev have pushed the limit one step further by employing external light sources in the feedback loop (figure 3). In brief, pairs of plane waves are mixed inside the crystal to create standing waves (constructive interference). The distribution of the energy modifies the refractive index of the crystal. As a result, a transient 2D photonic lattice/waveguide is formed (figure 4). Then the light is launched from the proper crystal surface. The light propagates and interacts with the “structured refractive index” such that the diffraction is suppressed. That is, it becomes a “spatial soliton” by the cooperation among itself, external light sources, and the crystal.

Figure 3. The virtual feedback loop when external light sources are used.

Figure 4. (a) Pairs of plane waves form standing waves inside the crystal. Red arrow indicates the direction of propagating light beam (b) The refractive index of the XY cross section in the crystal. Due to the distribution of the energy of standing waves, refractive index of the crystal is modulated (refractive index is higher when color is brighter). The graph is at courtesy of Jason W. Fleischer, Mordechai Segev, Nikolaos K. Efremidis, and Demetrios N. Christodoulides in Nature 422 13 147 (2003).

The story gets better. Professor Segev utilized a phenomenon called “Anderson Localization” to confine light beam’s diameter. In a nutshell, “Anderson Localization” states that when the random disorder of the medium becomes substantial, the wave within it tends to be localized since all the outgoing diffractive waves interfere destructively. Consequently, the light does not diffract and remains spatially intact when propagating. Based on this principle, Professor Segev creates a photo-induced 2D photonic lattice (the way we mentioned above), superimposed with disorder (from another laser source), and launched the light through it. It is shown that the light is actually localized in 2D if certain disorder is reached (figure 5). Again, the interplay between light and material made this happen.

Figure 5. Anderson localization of light. The graph is courtesy of T. Schwartz, S. Fishman, G. Bartal, and M. Segev in Electronics Letter 443 3 165 (2008) and Nature 446 52 (2007).

Before this article gets too long and boring, we should get some nanotechnology involved in this business. An interesting phenomenon called “Thermophoresis” is utilized to localize the light too. Thermophoresis states that the “temperature gradient” in the solution would change the distribution of the particles within it. If the particles tend to move from high/low temperature to low/high temperature, it is called “positive/negative” thermodiffusion. This is great weaponry for us if we want to localize light. Here is how it goes -- 20 nm diameter polystyrene (PS) particles were suspended in water solution, and the laser beam is propagating through it. Since the laser dissipates heat in the solution, along the beam path the temperature is higher. PS particles have “negative” thermodiffusion, so they will be more concentrated in the beam path (figure 6). In addition, PS particles have higher refractive index, so over all, refractive index is higher along the beam path. Just like self-focusing in the crystal, this inhomogeneity in refractive index will focus the light beam! This work is shown beautifully in a recent paper -- PRL 105, 163906 (2010).

For details about all these, guess what!? Coming to CLEO for Professor Sergev’s planetary talk!

Figure 6. Thermopherisis can be used to localize light when light is propagating through the solution of PS particles.

DISCLAIMER

The opinions expressed herein are those of the author and do not represent the Optical Society of America (OSA) or any OSA affiliate.