Optical Sensor For Explosives

Light traveling through a nanoscale waveguide on a chip spreads beyond the waveguide and can interact with molecules above the surface of the chip (Source: Cornell Univ.)

Light traveling through a nanoscale waveguide on a chip spreads beyond the waveguide and can interact with molecules above the surface of the chip (Source: Cornell Univ.)

By combining expertise in photonics, mani­pulating light beams in nanoscale wave­guides on a chip and materials science, Cornell researchers have laid the ground­work for a chemical sensor on a chip that could be used in small portable devices to analyze samples in a lab, monitor air and water quality in the field and perhaps even detect explosives. The researchers use Raman scat­tering: When a laser strikes a molecule it kicks back the energy as photons of light at a variety of wave­lengths that depend on the structure and com­position of the molecule.

If a laser is fired into a wave­guide set into a silicon chip the light bounces off the inside surfaces and is confined to the wave­guide. But when the waveguide is only a few nano­meters high, it’s smaller than the light waves, which spread out beyond he guide to create a evanescent field above the surface of the chip. The beam can induce Raman scat­tering in the air above the chip, or in a drop of liquid placed on its surface for analysis. Light emitted by the excited molecules also follows the wave­guide; a prism at the end of the waveguide can spread that light into a spectrum that is a finger­print identi­fying the molecule that produced it.

“If you need a chemical sensor in the lab, that is not a problem,” said Jin Suntivich, assis­tant professor of materials science and engi­neering. “But finding a chemical sensor that you can take with you outside is a challenge. We want to develop a techno­logy that is small enough to attach to a phone, so that your personal elec­tronics can constantly monitor the world around you, and the moment you see some­thing out of the ordinary, the sensor can tell you what it is.” Sensors based on Raman scat­tering have been made before, using silicon nitride wave­guides. With a few changes, the Cornell researchers have come up with a design that could make a sensor more sensi­tive and small enough to be used in the field. “We’re not the first but we’re the best,” said Christopher Evans, a Kavli Post­doctoral Fellow in the Labo­ratory of Atomic and Solid State Physics.

A circular waveg­uide tangent to a straight guide causes light to cir­culate around and around, giving it more time to interact with material above the chip. The ring is about the diameter of a human hair. The first important change was to use waveguides of titanium dioxide in place of silicon nitride. Titanium dioxide has a much higher re­fractive index, making a greater contrast with the space above the chip, which creates a stronger eva­nescent field. The material is also trans­parent to light at visible wavelengths where Raman scat­tering is more pronounced.  In silicon nitride, visible light generates an inter­fering lumi­nescence. In early expe­riments, the researchers used a green laser pointer as a light source. For a future device, a tiny laser can be built into a chip, as can a prism or some other mechanism to spread out the wave­lengths of the Raman spectrum, and a photo­sensitive device to read it. One pos­sibility is to read the spectrum with the camera in a phone.

Inter­action of the pumping laser with the material above the chip increases with the length of the wave­guide. To increase the inter­action without making the chip unaccep­tably large, the researchers incor­porated a ring resonator. When a circular waveguide is set tangent to a straight guide, some of the light will enter the ring and continue to circle around it, letting the light interact continually with the material above the chip. The circum­ference of the ring can be adjusted to resonate with the wave­length of the light, inten­sifying the effect. “We have shown that we can increase the amount of peak signal from our sensors by an order of magni­tude or more, while simul­taneously reducing the device foot­print down to the cross-section of a human hair,” Evans said. Potential appli­cations include portable sensors to monitor air and water quality or conduct labo­ratory tests in the field. Chemists could observe chemical reactions while they occur. (Source: Cornell Univ.)

Reference: C. C. Evans et al.: TiO2 Nanophotonic Sensors for Efficient Integrated Evanescent Raman Spectroscopy, ACS Photonics, online 14 July 2016; DOI: 10.1021/acsphotonics.6b00314

Link: Laboratory of Atomic and Solid State Physics, Cornell University, Ithaka, USA

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