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A photonic 'lab-on-a-chip'

Georgia Tech researchers have shrunk an optical device called wavelength demultipler by combining into one crystal three unique properties of photonics crystals. This discovery opens the way to sophisticated biosensors mounted on 'lab-on-a-chip' devices -- sensors to run blood tests, detect chemicals in water supplies or for drug testing.
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Written by Roland Piquepaille, Inactive on

Georgia Tech researchers have shrunk an optical device called wavelength demultipler (WD) by combining into one crystal three unique properties of photonics crystals. This optical discovery opens the way to sophisticated and cheap biosensors mounted on 'lab-on-a-chip' devices -- sensors to run blood tests, detect chemicals in water supplies or for drug testing. Their new WD is less than a millimeter in all dimensions rather than the several centimeters of other currently available WDs. And it should not cost more to produce. Read more...

But what exactly is a wavelength demultipler?

In compact communication, signal processing and sensing optics technologies, multiple wavelengths of light are combined as a space-saving measure as they carry information. The wavelengths must then be separated again when they reach their destinations. Wavelengths used for these sophisticated applications have very high spectral resolution, meaning the distance between wavelengths is very small. The device that sorts out these crowded wavelengths is called a wavelength-demultipler (WD).

And the team of Georgia Tech, led by Ali Adibi, with other members of the Photonics Research Group, designed a very small WD "able to function at very high resolution in much tighter confines (as small as 64 microns by 100 microns) by developing a new design for photonic crystals."

The Georgia Tech team was able to shrink its WD by combining into one crystal three unique properties of photonics crystals -- the superprism effect (separating wavelengths much more finely than a regular prism), negative diffraction or focusing (reversing the expansion of the light beam and focusing it back to its original size after interacting with the material being analyzed) and negative refraction (filtering wanted and unwanted wavelengths).

The basic ideas of the three effects that are combined in the Georgia Tech photonic crystals (PCs) are shown in the figure below: "(a) shows a typical planar PC fabricated by etching a two-dimensional (2D) array of air holes in Si; (b) shows the superprism effect, in which the PC separates wavelength channels by directing them into different angles; (c) shows the application of the negative diffraction property of PCs for diffraction compensation; (d) shows the negative refraction at the PC interface" (Credit: Ali Adibi and his colleagues, via Optics Express).

The Georgia Tech photonic crystals

As said Adibi, "We believe we have developed the most compact WD that has been reported to date. If you want to have many optical functions on a single micro- or nano-sized chip, you have to be able to practically integrate all those functions in the smallest amount of space possible. Our WD solves many problems associated with combining delicate optical functions in such a small space."

This research work has been published by two scientific journals. The first article appeared in Optics Express under the title "Compact wavelength demultiplexing using focusing negative index photonic crystal superprisms" (Volume 14, Issue 6, Pages 2413-2422, March 20, 2006). Here are two links to the abstract and to the full text (PDF format, 10 pages, 1.03 MB). The image above comes from this document.

The second article was published by Laser Focus World under the name "Demultiplexers harness photonic-crystal dispersion properties" (June 2006). Here is a link to this scientific paper.

Sources: Georgia Institute of Technology news release, August 2, 2006; and various web sites

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