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Tuesday, May 21, 2013

Espresso for the Brain – CLEO: 2013



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

Saturday, February 16, 2013

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

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.
  1. 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.
  2. 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).
  3. 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.

Monday, October 22, 2012

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

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 2nd 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 30th. Share with us!



Tuesday, August 28, 2012

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

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.



Wednesday, May 9, 2012

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

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
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)!



Monday, May 7, 2012

Nanolasers are the rising stars

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=0.734Ga1xAsy=0.57P1y / Inx=0.56Ga1xAsy=0.938P1y, 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).
