Monday, November 16, 2020

A Terahertz QCL Without the Cryogenics


Patricia Daukantas

The development of the terahertz quantum cascade laser (QCL) nearly two decades ago held great promise for applications ranging from explosives detection to skin cancer screening. However, the need for bulky cryogenic cooling equipment to make the QCL work properly limited the use of such devices in the field.

Left: The tiny terahertz quantum cascade laser compared in size to two coins. Right: The laser chip with a thermoelectric cooler on a block. In the background is a cryocooler. [Image: Khalatpour et al., MIT and University of Waterloo]
Now, researchers at two North American universities have developed a high-power, compact terahertz QCL with a maximum operating temperature of 250 K (Nat. Photon., doi: 10.1038/s41566-020-00707-5). The millimeter-scale source of 4-THz radiation, paired with a portable thermoelectric cooler, can generate sufficient power to produce images with room-temperature cameras and detectors.

The terahertz allure

Scientists have long known that the spectral region with wavelengths between 1 and 10 THz is full of interesting chemical and biological fingerprints. Illegal drugs, protein structures and atomic oxygen in the Martian atmosphere all show up in terahertz spectroscopy. Unfortunately, electronic devices don’t yield much power at frequencies above 1 THz, and conventional semiconductor photonic devices cannot operate below roughly 10 THz. Researchers have various methods for generating either narrowband or broadband terahertz radiation, but QCLs generating high-power coherent radiation in this spectral region would eliminate the need for upconversion or downconversion from other frequencies.

For most of the past decade, the maximum operating temperature of terahertz QCLs has not crept above 200 or 210 K, despite numerous attempts to improve the technology. A few years ago, scientists at the Massachusetts Institute of Technology (MIT), USA, found that high temperatures exacerbate carrier leakage over the tiny aluminum gallium-arsenide barriers inside QCLs.

Breaking barriers by making them higher

University of Waterloo scientist Zbig R. Wasilewski with a student in the lab. [Image: University of Waterloo]

In the current set of experiments, Qing Hu of MIT and Zbig R. Wasilewski of the University of Waterloo, Canada, and their students heightened the semiconductor barriers to reduce carrier leakage inside their QCL system. The team observed that this design required high-precision molecular beam epitaxy to fabricate—an important concern when refining the technology and someday making it commercially available.

The researchers tested one of their QCLs, cooled by a single-stage thermoelectric cooler, and plotted its output power versus current. The team also captured beam pattern images with a room-temperature camera and with the QCL cooled with a three-stage thermoelectric module.

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