Terahertz waves can be used to analyse plastics, clothing, semiconductors and works of art without damaging the materials being examined; for chemical sensing and identification; and to investigate the formation of stars and composition of planetary atmospheres.
Researchers led by Benjamin Williams, a UCLA associate professor of electrical engineering, have created the first vertical-external-cavity surface-emitting laser, or VECSEL, that operates in the terahertz range. VECSELs that use visible light have been used to generate high-powered beams, but the technique has not previously been adapted for terahertz frequencies.
To make it possible to build an external cavity laser with a high-quality beam, the UCLA researchers created a VECSEL with a ‘reflectarray metasurface mirror’. The device is made up of an array of many small antenna-coupled laser cavities such that when a terahertz wave hits the array, it doesn’t ‘see’ the cavities, but is reflected as if it were being reflected from a simple, flat mirror. Unlike a mirror however, the device amplifies the terahertz waves as well as reflecting them.
“This is the first time a metasurface and a laser have been combined,” Williams said. “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 polarisation, 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.
Luyao Xu, a graduate researcher in Williams’ lab, said, “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. For example, by using a freestanding wire-grid polariser, or filter, as a second mirror, we could optimise the lasers’ output power and efficiency simply by rotating the polariser.”
The researchers are working on several new designs to further advance the technology.
