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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)!


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

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