Speedy terahertz-based system could detect explosives
symmetrical indentations in its sides that alter the medium’s refractive index and restore uniformity to the distribution of the emitted frequencies.
Yang; his advisor, Qing Hu, the Distinguished Professor in Electrical Engineering and Computer Science; and first author David Burghoff, who received his Ph.D. in electrical engineering and computer science from MIT in 2014 and is now a research scientist in Hu’s group, reported this design in Nature Photonics in 2014. But while their first prototype demonstrated the design’s feasibility, it in fact emitted two frequency combs, clustered around two different central frequencies, with a gap between them, which made it less than ideal for spectroscopy.
In the new work, Yang and Burghoff, who are joint first authors; Hu; Darren Hayton and Jian-Rong Gao of the Netherlands Institute for Space Research; and John Reno of Sandia National Laboratories developed a new gain medium that produces a single, unbroken frequency comb. Like the previous gain medium, the new one consists of hundreds of alternating layers of gallium arsenide and aluminum gallium arsenide, with different but precisely calibrated thicknesses.
Getting practical
As a proof of concept, the researchers used their system to measure the spectral signature of not a chemical sample but an optical device called an etalon, made from a wafer of gallium arsenide, whose spectral properties could be calculated theoretically in advance, providing a clear standard of comparison. The new system’s measurements were a very good fit for the etalon’s terahertz-transmission profile, suggesting that it could be useful for detecting chemicals.
Although terahertz quantum cascade lasers are of chip scale, they need to be cooled to very low temperatures, so they require refrigerated housings that can be inconveniently bulky. Hu’s group continues to work on the design of increasingly high-temperature quantum cascade lasers, but in the new paper, Yang and his colleagues demonstrated that they could extract a reliable spectroscopic signature from a target using only very short bursts of terahertz radiation. That could make terahertz spectroscopy practical even at low temperatures.
“We used to consume 10 watts, but my laser turns on only 1 percent of the time, which significantly reduces the refrigeration constraints,” Yang explains. “So we can use compact-sized cooling.”
“This paper is a breakthrough, because these kinds of sources were not available in terahertz,” says Gerard Wysocki, an assistant professor of electrical engineering at Princeton University. “Qing Hu is the first to actually present terahertz frequency combs that are semiconductor devices, all integrated, which promise very compact broadband terahertz spectrometers.”
“Because they used these very inventive phase correction techniques, they have demonstrated that even with pulsed sources you can extract data that is reasonably high resolution already,” Wysocki continues. “That’s a technique that they are pioneering, and this is a great first step toward chemical sensing in the terahertz region.”
— Read more in Yang Yang et al., “Terahertz multiheterodyne spectroscopy using laser frequency combs,” Optica 3, no. 5 (2016): 499-502 (doi: 10.1364/OPTICA.3.000499)
Reprinted with permission of MIT News
