Dual Frequency Comb on a Chip

A compact, integrated, silicon-based chip used to generate dual combs for extremely fast molecular spectroscopy. (Source: A. Dutt, A. Mohanty, E. Shim, G. Patwardhan, Columbia Eng.)

Researchers under the direction of Columbia Engi­neering Pro­fessors Michal Lipson and Alexander Gaeta (Applied Physics and Applied Mathe­matics) have minia­turized dual-frequency combs by putting two frequency comb gene­rators on a single milli­meter-sized chip. “This is the first time a dual comb has been generated on a single chip using a single laser,” says Lipson, Higgins Professor of Elec­trical Engi­neering.

A frequency comb is a special kind of light beam with many different fre­quencies, all spaced from each other in an extremely precise way. When this many-color light is sent through a chemical specimen, some colors are absorbed by the specimen’s molecules. By looking at which colors have been absorbed, one can uniquely identify the molecules in the specimen with high precision. This frequency-comb spectro­scopy enables molecular finger­printing and can be used to detect toxic chemicals in industrial areas, to implement occu­pational safety controls, or to monitor the

“Dual-comb spectro­scopy is this technique put on steroids,” says Avik Dutt, former student in Lipson’s group. “By mixing two frequency combs instead of a single comb, we can increase the speed at which measure­ment are made by thousand­folds or more.” The work also demon­strated the broadest frequency span of any on-chip dual comb i.e., the diffe­rence between the colors on the low-frequency end and the high-fre­quency end is the largest. This span enables a larger variety of chemicals to be detected with the same device, and also makes it easier to uniquely identify the molecules.

Conven­tional dual-comb spectro­meters, which have been intro­duced over the last decade, are bulky tabletop instru­ments, and not portable due to their size, cost, and com­plexity. In contrast, the Columbia Engi­neering chip-scale dual comb can easily be carried around and used for sensing and spectro­scopy in field environ­ments in real time. “There is now a path for trying to integrate the entire device into a phone or a wearable device,” says Gaeta.

The researchers minia­turized the dual comb by putting both frequency comb gene­rators on a single milli­meter-sized chip. They also used a single laser to generate both the combs, rather than the two lasers used in conven­tional dual combs, which reduced the experi­mental complexity and removed the need for compli­cated electronics. To produce mini­scule rings, the team used silicon nitride, a glass-like material they have perfected speci­fically for this purpose. By combining the silicon nitride with platinum heaters, they were able to very finely tune the rings and make them work in tandem with the single input laser.

“Silicon nitride is a widely used material in the silicon-based semi­conductor industry that builds computer/smart­phone chips,” Lipson notes. “So, by leveraging the capa­bilities of this mature industry, we can foresee reliable fabri­cation of these dual comb chips on a massive scale at a low cost.” Using this dual comb, Lipson’s and Gaeta’s groups demonstrated real-time spectro­scopy of the chemical dichloro­methane at very high speeds, over a broad frequency range. A widely used organic solvent, dichloro­methane is abundant in industrial areas as well as in wetland emissions. Columbia Engi­neering’s compact, chip-scale dual comb spectro­meter was able to measure a broad spectrum of dichloro­methane in just 20 micro­seconds, a task that would have taken at least several seconds with conven­tional spectro­meters.

As opposed to most spectro­meters, which focus on gas detection, this new, minia­turized spectro­meter is espe­cially suited for liquids and solids, which have broader absorp­tion features than gases. “That’s what our device is so good at gene­rating,” Gaeta explains. “Our very broad dual combs have a moderate spacing between the successive lines of the frequency comb, as compared to gas spectro­meters which can get away with a less broad dual comb but need a fine spacing between the lines of the comb.”

The team is working on broa­dening the frequency span of the dual combs even further, and on increasing the reso­lution of the spectro­meter by tuning the lines of the comb. “One could also envision inte­grating the input laser into the chip for further minia­turizing the system, paving the way for commer­cializing this tech­nology in the future,” says Dutt. (Source: Columbia U.)

Reference: A. Dutt et al.: On-chip dual-comb source for spectroscopy, Sci. Adv. 4, e1701858 (2018); 
DOI: 10.1126/sciadv.1701858

Link: Dept. of Electrical Engineering, Columbia University, New York, USA

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