Terahertz quantum-cascade vertical external cavity surface emitting lasers

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This research addresses the challenge of making terahertz semiconductor laser sources with high power and excellent beam quality in the 2-5 THz range. Compact sources of terahertz radiation that operate with both high output power (tens to hundreds of milliwatts or more) and excellent beam quality are sorely needed for a range of spectroscopy and imaging applications for example in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening and illicit material detection, and non-destructive evaluation. The intellectual merit lies in the development of a new type of terahertz quantum-cascade (QC) laser: a vertical-external-cavity-surface-emitting-laser (THz QC-VECSEL). This new approach addresses a challenging issue for THz QC-lasers: how to operate both with high power and an excellent beam pattern. The enabling technology for this proposed laser is a so-called "active metasurface reflector", which is composed of a sparse array of antenna-coupled THz QC-laser sub-cavities locked to an external cavity mode. Each sub-cavity is designed to be an efficient radiator and have a very favorable geometry for heat removal, which allows high-power continuous-wave operation. This research will integrate and unite complementary concepts from antenna engineering and laser engineering. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, and technology advancement. Outreach to underrepresented minorities (URM) will specifically occur through the PI's development of research projects for a course designed for the recruitment and retention of URM engineering freshmen. The intellectual merit lies in the development of a new type of terahertz quantum-cascade (QC) laser: a vertical-external-cavity-surface-emitting-laser (THz QC-VECSEL). The enabling component is an innovative THz active metasurface reflector, which is composed of a sparse array of antenna-coupled THz QC-laser sub-cavities. The metasurface reflector forms part of the laser cavity such that many THz QC-laser sub-cavities are locked to a high-quality factor cavity mode, allowing for scalable power combining. The sparse distribution of laser material on the metasurface reduces the average power dissipation density and offers a favorable geometry for heat extraction which in turn will allow excellent continuous-wave performance of large effective area emitters. The primary goal is to achieve THz QC-lasers with directive beams (single transverse mode with divergence angle < 6 degrees), with near-diffraction limited beam quality, that have scalable power of >1 W peak power in pulsed mode, and >200 mW power in continuous-wave mode. A secondary goal is the design of external THz laser cavities based upon engineered inhomogeneous metasurfaces with customized gain, spectral, phase, and polarization response for new capabilities such as beam shaping, wavefront engineering, and dynamic polarization modulation. An integrated theoretical, computational, and experimental effort is proposed to (a) design and implement low-loss and efficient amplifying reflective QC metasurfaces, (b) design and implement external cavity configurations for single-mode operation with excellent beam-quality, and (c) design and grow high-efficiency quantum-cascade active region material suitable for use in the QC-VECSEL, (d) perform thermal design and engineering