Detecting dangerous chemicals with lasers, exploring the brain's circuitry with light and more

Now Jiayi Zhang, Tolga Atay, and Arto Nurmikko at Brown University have created a new type of dye-free optical probe that can directly sense naturally occurring neural activity. They have imbedded gold nanoparticles into tissue culture and shown that they can measure the electrical activity of live neurons. The technique takes advantage of a phenomenon known as surface plasmon polariton resonance, a sharp spectroscopic resonance at visible/near-infrared wavelengths. Basically, the gold nanoparticles are used to optically sense the local electric fields produced when nearby neurons fire. The neuronal activity modulates the electron density at the surface of the nanoparticle, which causes an observable spectral shift that the researchers can monitor. (Talk CWM3, "Detection of Neural Cell Activity Using Plasmonic Gold Nanoparticles.")

TINY LASER ARRAYS FOR SENSITIVE CHEMICAL DETECTION

Early miners used to carry canaries into coal mines because the birds were sensitive to certain gasses. Modern chemical analysis does the same thing, though much more powerfully. For instance, infrared spectroscopy can detect even trace amounts of a wide range of chemicals, including toxic components of hazardous waste or chemical weapons, because many chemicals absorb light in the mid-infrared band.

Now Federico Capasso and his colleagues at Harvard University are developing a new type of infrared spectrometer that could be just as powerful as these bulky instruments yet fit inside a shoe box. Instead of using thermal sources for infrared rays, a team lead by Capasso, his student Benjamin G. Lee, and his postdoctoral fellow Mikhail A. Belkin, has built one of these instruments, which is powered by a tiny array of infrared quantum cascade lasers on a chip smaller than a dime. The chip holds an array of 32 lasers, each emitting a distinct wavelength and together covering a broad spectral range in the infrared region. The researchers’ new paper demonstrates that their device could identify common chemicals as well as a conventional tabletop instrument, which has a much larger footprint. It is the first time that a laser of this type, capable of such performance, has been reported.

The advantage of using a laser source is that lasers are much brighter than thermal sources thus providing a higher signal-to-noise ratio. The lasers can also be fine-tuned to provide wavelengths on demand to scan accurately for chemicals of interest—akin to having thousands of canaries, each capable of detecting a range of chemicals. (Talk CMH1, "Continuously Tunable Compact Single-Mode Quantum Cascade Laser Source for Chemical Sensing.")

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