The terahertz range of frequencies occupies the space on the electromagnetic spectrum between microwave and infrared. Terahertz waves can be used to analyze 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 extensively to generate high-powered beams, but the technique has not previously been adapted for terahertz frequencies.
Unlike a simple mirror, the metasurface developed in Benjamin Williams’ lab amplifies terahertz waves as well as reflecting them.
Applied Physics Letters - Metasurface external cavity laser
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 so named because it 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 rather is reflected as if it were being reflected from a simple, flat mirror. Unlike a simple mirror however, the mirror amplifies 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 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.
Luyao Xu, a graduate researcher in Williams’ lab and lead author of the study, 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 polarizer, or filter, as a second mirror, we could optimize the lasers’ output power and efficiency simply by rotating the polarizer.”
Abstract - Metasurface external cavity laser
A vertical-external-cavity surface-emitting-laser is demonstrated in the terahertz range, which is based upon an amplifying metasurface reflector composed of a sub-wavelength array of antenna-coupled quantum-cascade sub-cavities. Lasing is possible when the metasurface reflector is placed into a low-loss external cavity such that the external cavity—not the sub-cavities—determines the beam properties. A near-Gaussian beam of 4.3° × 5.1° divergence is observed and an output power level over 5 mW is achieved. The polarized response of the metasurface allows the use of a wire-grid polarizer as an output coupler that is continuously tunable.
The demonstration of a metasurface VECSEL initiates a promising approach towards achieving THz lasers with high-quality beam patterns and significant levels of power. This prototype exhibited a near-Gaussian output beam pattern with low divergence (4.3° × 5.1°). Furthermore, the polarized response of the metasurface allows one to use a wire-grid as a continuously adjustable output coupler to optimize output power and efficiency for a given operating condition. Considerable room exists for further improvement of THz QC-VECSELs to achieve higher output power and efficiency, cw operation, and an even narrower far-field beam patterns. The intra-cavity access of the VECSEL has the potential to provide tremendous versatility to QC-lasers, such as spectral tuning and filtering, spatial mode shaping, and polarization control. Furthermore, while we use a homogeneous active metasurface here, the VECSEL approach gives the ability to leverage ongoing advances in inhomogeneous metasurfaces and reflectarrays to design phase, spectral, polarization, and spatial properties of the intra-cavity field and emitted beams
SOURCE - Applied Physics Letters, UCLA