First Semiconductor THz Laser Eyed for Sensing Applications

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LOS ANGELES, Dec. 21, 2015 — A semiconductor laser that emits at terahertz frequencies could drive development of a new class of lasers for materials analysis and threat detection. 

The first vertical-external-cavity surface-emitting laser (VECSEL) capable of operating in this range, the device achieves output power >5 mW. For amplification it uses a metasurface mirror composed of subwavelength antenna-coupled quantum-cascade subcavities.

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A metasurface used to amplify a vertical-external-cavity surface-emitting laser that works in the terahertz range. Images courtesy UCLA Engineering .

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VECSELs that use visible light have been used extensively to generate high-powered beams, but the technique has not previously been adapted for terahertz frequencies. 

"This is the first time a metasurface and a laser have been combined," said professor Benjamin Williams, whose team led the development. "The VECSEL approach provides a route to have higher output powers simultaneously with excellent beam quality in the terahertz range. The metasurface approach further allows one to engineer the beam to have the desired polarization, shape and spectral properties."

Creating a beam that is symmetrical and straight over large distances and changing thermal conditions is a challenge for many semiconductor lasers, but particularly for terahertz quantum cascade lasers, which usually use metal laser cavities with dimensions much smaller than the wavelength. 

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A schematic of the metasurface and polarizer used to tune the laser.

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"By using this amplifying metasurface as part of the external cavity, not only can we improve the beam pattern, but we can also introduce new functionality to this laser with different cavity designs," said graduate student Luyao Xu. "For example, by using a freestanding wire-grid polarizer, or filter, as a second mirror, we could optimize the lasers' output power and efficiency simply by rotating the polarizer." 

The terahertz range of frequencies occupies the space on the electromagnetic spectrum between the IR region and microwaves. Terahertz waves can be used to analyze plastics, clothing, semiconductors and works of art without damaging the materials being examined. They are also useful for chemical sensing and identification, and to investigate the formation of stars and the compositions of planetary atmospheres. 

Funding came from the National Science Foundation. The research was published in Applied Physics Letters (doi: 10.1063/1.4936887 [open access]).