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QCL-based system performs terahertz spectroscopy in seconds

26 May 2016

MIT development based on chip-scale lasers and THz frequency comb could have applications in rapid detection of explosives.

Researchers from MIT’s Research Laboratory of Electronics have developed a new terahertz spectroscopy system based on a quantum cascade laser the size of a computer chip as its source of terahertz radiation. The work is described in the latest issue of the journal Optica.

The compact system is being used as a spectrometer and can derive a material’s spectroscopic signature in 100 μs, promising rapid, economical and portable applications in explosives detection, for example.

The new device is also highly efficient because it emits terahertz radiation in a “frequency comb” – a range of frequencies that are useful because they are evenly spaced. The researchers say this is a significant breakthrough because conventional terahertz spectroscopy requires a radiation source that is heavy and about the size of a large suitcase, and typically takes 15 to 30 minutes to analyze a single sample.

“With this work, we can answer the question, ‘What is the real application of quantum-cascade laser frequency combs?’” said Yang Yang, a graduate student in electrical engineering and computer science and first author on the Optica paper. “Terahertz is such a unique region that spectroscopy is probably the best application. And QCL-based frequency combs are a great candidate for spectroscopy.”

Different materials absorb different frequencies of terahertz radiation to different degrees, giving each of them a characteristic terahertz-absorption profile. Traditionally, however, terahertz spectroscopy has required measuring a material’s response to each frequency separately, a process that involves mechanically readjusting the spectroscopic apparatus, making the method complicated and time-consuming. Because the frequencies in a frequency comb are evenly-spaced, however, it is now possible to mathematically reconstruct a material’s absorption profile from just a few measurements, without any mechanical adjustments.

Evening it out

The trick is evening out the spacing in the comb. Quantum cascade lasers bounce electromagnetic radiation back and forth through the gain medium until the radiation has enough energy to escape. They emit radiation at multiple frequencies that are determined by the length of the gain medium. But these frequencies also depend on the medium’s refractive index, which describes the speed at which electromagnetic radiation passes through it; the refractive index varies for different frequencies, so the gaps between frequencies in the comb also vary.

To even out their lasers’ frequencies, the MIT researchers employ an oddly-shaped gain medium, with regular, symmetrical indentations that alter the medium’s refractive index and restore uniformity to the distribution of the emitted frequencies.

The team of Yang, his advisor, Qing Hu, Professor in Electrical Engineering and Computer Science, and first author David Burghoff first reported this design in Nature Photonics in 2014. But while their first prototype demonstrated the design’s feasibility, it actually emitted two frequency combs, centered on two different central frequencies, which made it less suitable for spectroscopy.

In the latest work, the MIT team along with 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.

Proof of concept

The researchers used their system to measure the spectral signature of an optical device called an etalon, made from gallium arsenide, whose spectral properties could be calculated theoretically in advance, providing a clear standard of comparison. The new system’s measurements were a precise 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 is continuing to work on the design of increasingly high-temperature quantum cascade lasers, but in the Optica paper, the team demonstrated that they could extract a reliable spectroscopic signature from a target using short bursts of terahertz radiation. That could make terahertz spectroscopy practical even at low temperatures.

“We used to consume 10 watts, but my laser is switched on only 1% of the time, which significantly reduces the refrigeration constraints,” Yang said. “So we can use compact-sized cooling.

Of the work, Gerard Wysocki, assistant professor of electrical engineering at Princeton University, commented, “This is a breakthrough, because these kinds of sources were not available in terahertz. Qing Hu is the first scientist to actually present terahertz frequency combs that are semiconductor devices, all integrated, and which promise very compact broadband terahertz spectrometers.”

The Optica paper’s key technical conclusion reads, “We have demonstrated the first multiheterodyne spectroscopy using two terahertz quantum cascade laser combs. Over a spectral range of 250 GHz, we achieve average signal-to-noise ratios of 34dB using cryogenic detectors and 24 dB using room-temperature detectors, all in just 100 μs.”

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