Terahertz Laser for Sensing and Imaging

This tiny terahertz laser is the first to reach three key performance goals at once: high power, tight beam, and broad frequency tuning. (Source: MIT)

A terahertz laser designed by MIT researchers is the first to reach three key perfor­mance goals at once — high constant power, tight beam pattern, and broad electric frequency tuning — and could thus be valuable for a wide range of applications in chemical sensing and imaging. The opti­mized laser can be used to detect inter­stellar elements in an upcoming NASA mission that aims to learn more about our galaxy’s origins. Here on Earth, the high-power photonic wire laser could also be used for improved skin and breast cancer imaging, detecting drugs and explosives, and much more.

The laser’s novel design pairs multiple semi­conductor-based, efficient wire lasers and forces them to sync oscil­lations. Combining the output of the pairs along the array produces a single, high-power beam with minimal beam diver­gence. Adjust­ments to the individual coupled lasers allow for broad frequency tuning to improve reso­lution and fide­lity in the measure­ments. Achieving all three perfor­mance metrics means less noise and higher resolution, for more reliable and cost-effective chemical detection and medical imaging, the researchers say. “People have done frequency tuning in lasers, or made a laser with high beam quality, or with high continuous wave power. But each design lacks in the other two factors,” says Ali Khalatpour, a graduate student in electrical engi­neering and computer science. “This is the first time we’ve achieved all three metrics at the same time in chip-based terahertz lasers. It’s like ‘one ring to rule them all,’” Khalatpour adds.

Last year, NASA announced the Galactic/Extra­galactic ULDB Spectro­scopic Terahertz Obser­vatory (GUSTO), a 2021 mission to send a high-altitude balloon-based tele­scope carrying photonic wire lasers for detecting oxygen, carbon, and nitrogen emissions from the “inter­stellar medium,” the cosmic material between stars. Extensive data gathered over a few months will provide insight into star birth and evolution, and help map more of the Milky Way and nearby Large Magellanic Cloud galaxies. For a component of the GUSTO chemical detector, NASA selected a novel semi­conductor-based terahertz laser previously designed by the MIT researchers. It is currently the best-per­forming terahertz laser. Such lasers are uniquely suited for spectro­scopic measur­ement of oxygen concen­trations in terahertz radiation, the band of the electro­magnetic spectrum between micro­waves and visible light.

Terahertz lasers can send coherent radiation into a material to extract the material’s spectral finger­print. Different materials absorb terahertz radiation to different degrees, meaning each has a unique finger­print that appears as a spectral line. This is especially valuable in the 1-5 terahertz range: For contra­band detection, for example, heroin’s signature is seen around 1.42 and 3.94 terahertz, and cocaine’s at around 1.54 terahertz. For years, Hu’s lab has been deve­loping novel types of quantum cascade lasers, photonic wire lasers. Like many lasers, these are bidirectional, meaning they emit light in opposite directions, which makes them less powerful. In traditional lasers, that issue is easily remedied with care­fully posi­tioned mirrors inside the laser’s body. But it’s very diffi­cult to fix in terahertz lasers, because terahertz radiation is so long, and the laser so small, that most of the light travels outside the laser’s body.

In the laser selected for GUSTO, the researchers had developed a novel design for the wire lasers’ wave­guides – which control how the electro­magnetic wave travels along the laser – to emit unidirec­tionally. This achieved high effi­ciency and beam quality, but it didn’t allow frequency tuning, which NASA required. Building on their previous design, Khalatpour took inspiration from an unlikely source: organic chemistry. While taking an under­graduate class at MIT, Khalat­pour took note of a long polymer chain with atoms lined along two sides. They were pi-bonded, meaning their molecular orbitals overlapped to make the bond more stable. The researchers applied the concept of pi-bonding to their lasers, where they created close connec­tions between otherwise-inde­pendent wire lasers along an array. This novel coupling scheme allows phase-locking of two or multiple wire lasers.

To achieve frequency tuning, the researchers use tiny knobs to change the current of each wire laser, which slightly changes the refractive index. That refractive index change, when applied to coupled lasers, creates a continuous frequency shift to the pair’s center frequency. For experi­ments, the researchers fabri­cated an array of 10 pi-coupled wire lasers. The laser operated with continuous frequency tuning in a span of about 10 gigahertz, and a power output of roughly 50 to 90 milli­watts, depending on how many pi-coupled laser pairs are on the array. The beam has a low beam diver­gence of 10 degrees, which is a measure of how much the beam strays from its focus over distances.

The researchers are also cur­rently building a system for imaging with high dynamic range greater than 110 decibels which can be used in many appli­cations such as skin cancer imaging. Skin cancer cells absorb tera­hertz waves more strongly than healthy cells, so terahertz lasers could poten­tially detect them. The lasers previously used for the task, however, are massive and inefficient, and not frequency-tunable. The researchers’ chip-sized device matches or outstrips those lasers in output power, and offers tuning capa­bilities. “Having a platform with all those perfor­mance metrics together could significantly improve imaging capa­bilities and extend its appli­cations,” Khalatpour says.

“This is very nice work — in the THz [range] it has been very difficult to obtain high power levels from lasers simul­taneous with good beam patterns,” says Benjamin Williams, associate professor of physical and wave elect­ronics at the University of Cali­fornia at Los Angeles. “The innovation is the novel way they have used to couple the multiple wire lasers together. This is tricky, since if all of the lasers in the array don’t radiate in phase, then the beam pattern will be ruined. They have shown that by properly spacing adjacent wire lasers, they can be coaxed into ‘wanting’ to operate in a coherent symmetric supermode – all collectively radiating together in lockstep. As a bonus, the laser frequency can be tuned to the desired wave­length – an important feature for spectro­scopy and for astro­physics.” (Source: MIT)

Reference: A. Khalatpour et al.: Phase-locked photonic wire lasers by π coupling, Nat. Phot., online 10 December 2018; DOI: 10.1038/s41566-018-0307-0

Link: Millimeter-wave and Terahertz Devices Group, Dept. of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, USA

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