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 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

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.