News digest: brains addicted to light and space-age insect repellents

Last week saw plenty of reports covering various developments involving optical technology, including the use of light to activate the brains of mice and lasers to zap mosquitoes.

Optical control of reward-related behaviour. Reprinted by permission from Macmillan Publishers Ltd: Nature, advance online publication, 18 March 2009 (doi:10.1038/nature07926)

Starting with medical news, we learn that researchers at Stanford University have been lighting up the brains of mice in order to further our understanding of drug addiction^1,2. With a little genetic engineering, they produced light sensitive proteins which were implanted into the reward centres of mouse brains. In the experiments, the mice roamed freely about their habitat and when they entered a particular room, a pulse of light was fired along an optical fibre into the brain to activate the reward centre. This conditioned them to want to spend more time in that room, even after the optical fibre was removed. As well as providing an insight into drug addiction, these studies could also lead to improved therapies for addicted patients.

The technique just described was adapted by Duke University scientists, in collaboration with the Stanford researchers, in order to investigate the mechanisms underlying deep brain stimulation, which is used to treat brain disorders such as Parkinson's disease^3,4,5. Exactly why this therapy works is not well understood, so they used mice with disorders similar to Parkinson's and hooked their brains up to fibres, to deliver optical pulses to the brain and activate light-sensitive proteins implanted there. This new technique enabled them to target areas of the brain with far greater precision than had ever been possible before, thereby revealing what was going on. With this knowledge, researchers may be able to improve on the current therapies for patients suffering from Parkinson's disease and other neurological disorders.

One optical technique that is now ready for clinical use is the optical perfusion camera developed by researchers at the University of Twente in the Netherlands^6,7. Using a laser Doppler technique, it images the flow of blood by illuminating the blood vessels beneath the skin and detecting the movement of red blood cells from the back scattered light. What makes it such a hit is the fact that these measurements can be taken in real time, allowing doctors to instantly assess blood circulation in their patients.

In materials and manufacturing news, we learn that a collaboration of researchers in Japan and France aims to improve the light extraction efficiency from LED's^8,9 By fabricating them on substrates containing V-shaped grooves, they have demonstrated that they can increase the efficiency with which light is coupled from the semiconductor into the air. They report on an LED which had a light coupling efficiency of 50%, which they say is 20 times greater than if the same device was manufactured on a conventional flat substrate.

Deposited layer in hybrid nanostructure photo-conductor. Reprinted by permission from Macmillan Publishers Ltd: Nature Materials 8, 68 - 75 (2009), copyright 2009

Photo-conductors work in the opposite way to LED's, by converting photons into electricity, and researchers at Northwestern University are looking into ways to not only improve the efficiency of these materials, but also to make them more environmentally friendly^10,11. Their method involves building a highly ordered nanostructure using zinc oxide, which they say boasts a low environmental impact and high performance, making it a suitable material for a new generation of photo-voltaic solar cells.

We also had news last week which describes one further step in the march toward light-based computation, where light pulses race around the chips and circuitry of our computers and hand-held devices. Researchers at the University of Illinois at Urbana-Champaign, report the latest developments with their transistor laser technology^12,13. Their new device, which takes two electrical inputs and produces one optical output, is capable of operating at up to 22.7 gigahertz. Such a device could be used for high-speed processing in electronic-photonic integrated circuits of the future.

The realisation of photonic integrated circuits is also the goal of the European and US researchers who recently reported on their silicon-organic-hybrid device, which is capable of processing light signals with light^14,15. They demonstrated their silicon-organic waveguide technology performing high-speed, all-optical processing. By removing the need for any electronic conversion of an optical signal before it can be processed, communication and computation could see vast improvements in speed. Your laptop computer could also run a lot cooler without all those electrons buzzing around inside, obviating the need for a noisy fan and putting an end to that unsettling smell of burning plastic.

Looking at longer range optical communications now, there have been some advances reported in Free-space optical communication, a technology which one might argue is perpetually in its infancy. Lasers provide a solution for line-of-sight communication where the bandwidth capabilities of light is required and where an optical fibre link is not possible. It's great for satellites communicating across the vacuum of space, but here on Earth it must contend with the adversities of the atmosphere, which causes attenuation of the light by absorption and scattering. Researchers at the Stevens Institute of Technology in the US have conducted a study^16,17 which shows some progress in the field thanks to the development of quantum cascade lasers (QCL's). Their tests, which were carried out in conditions that included rain and fog, showed that QCL's operating in the mid infra-red, could achieve far better performance than their customary counterparts which operate in the near infra-red portion of the spectrum.

Mosquito (trebol-a)

Finally, in a story reported in The Wall Street Journal, which portrays the scientists involved as hair-brained stereotypes in white coats, we learn about a new method to prevent the transmission of malaria¹⁸. The invention is the optical equivalent of pulling the wings off of flies: it detects the presence of mosquitoes from their signature buzzing sound and then uses a laser to disable or destroy them. In parts of the world where malaria is a prevalent killer, any solution that stops the spread of this disease is going to be a welcome one.

1. Mice Get Hooked on Light
2. Temporally precise in vivo control of intracellular signalling
   Raag D. Airan, Kimberly R. Thompson, Lief E. Fenno, Hannah Bernstein & Karl Deisseroth
   Nature advance online publication 18 March 2009
   doi:10.1038/nature07926
3. How deep brain stimulation works for Parkinson's
4. Scientists have flash of light over Parkinson's treatment
5. Optical Deconstruction of Parkinsonian Neural Circuitry Viviana Gradinaru, Murtaza Mogri, Kimberly R. Thompson, Jaimie M. Henderson, and Karl Deisseroth (19 March 2009) Science DOI: 10.1126/science.1167093
6. Variations in blood circulation immediately visible with fast camera
7. M. Draijer, E. Hondebrink, T. van Leeuwen, and W. Steenbergen, "Twente Optical Perfusion Camera: system overview and performance for video rate laser Doppler perfusion imaging," Opt. Express 17, 3211-3225 (2009) doi:10.1364/OE.17.003211
8. Evanescent waves boost LED brightness
9. Ultrahigh spontaneous emission extraction efficiency induced by evanescent wave coupling
   X.-L. Wang, S. Furue, M. Ogura, V. Voliotis, M. Ravaro, A. Enderlin, and R. Grousson, Appl. Phys. Lett. 94, 091102 (2009), DOI:10.1063/1.3086887
10. Zinc oxide gives green shine to new photoconductors
11. A synergistic assembly of nanoscale lamellar photoconductor hybrids
    Marina Sofos, Joshua Goldberger, David A. Stone, Jonathan E. Allen, Qing Ma, David J. Herman, Wei-Wen Tsai, Lincoln J. Lauhon & Samuel I. Stupp
    Nature Materials 8, 68 - 75 (2009) doi:10.1038/nmat2336
12. High-speed Signal Mixer Demonstrates Capabilities Of Transistor Laser
13. Electrical-optical signal mixing and multiplication (2 --> 22 GHz) with a tunnel junction transistor laser
    H. W. Then, C. H. Wu, G. Walter, M. Feng, and N. Holonyak, Jr., Appl. Phys. Lett. 94, 101114 (2009), DOI:10.1063/1.3100294
14. New organic material may speed Internet access
15. All-optical high-speed signal processing with silicon-organic hybrid slot waveguides
    C. Koos, P. Vorreau, T. Vallaitis, P. Dumon, W. Bogaerts, R. Baets, B. Esembeson, I. Biaggio, T. Michinobu, F. Diederich, W. Freude and J. Leuthold
    Nature Photonics doi:10.1038/nphoton.2009.25
16. Stevens faculty release study on free-space optical communication in Optics Express
17. P. Corrigan, R. Martini, E. Whittaker, and C. Bethea, "Quantum cascade lasers and the Kruse model in free space optical communication," Opt. Express 17, 4355-4359 (2009). doi:10.1364/OE.17.004355
18. Rocket Scientists Shoot Down Mosquitoes With Lasers