Broadband terahertz metasurface lasers

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

This research addresses the challenge of making terahertz semiconductor laser sources that emit electromagnetic waves with frequencies between 2 and 5 THz (i.e. wavelengths between 60 and 150 microns). Compact sources of terahertz radiation that operate with both high output power and excellent beam quality are needed for a range of spectroscopy and multi-spectral imaging applications (i.e. where different terahertz wavelengths/colors provide additional image contrast). For example, applications in the fields of astrophysics, atmospheric science, biological and medical sciences, security screening, illicit material detection, and non-destructive evaluation all can benefit from terahertz sources. Terahertz quantum-cascade lasers are one of the most promising sources of radiation in this spectral range, however several hurdles must still be overcome to make them more practical. In past research, this team has demonstrated a new laser architecture that addresses the challenge of how to get high power simultaneously with high beam quality, known as the THz quantum-cascade vertical-external-cavity surface-emitting-laser. The next challenge is to make this architecture work in a "broadband" configuration, i.e. how to make lasers that emit across a wide range of wavelengths, without sacrificing power or beam quality. Hence, the unifying theme of this research is to develop a set of approaches that allow terahertz lasers to emit either (a) a single wavelength of light that can be tuned across a wide range, or (b) many wavelengths of light simultaneously across a wide range. Much of this work focuses upon design of a key enabling component: the so-called "active metasurface", which can be considered an artificial mirror, which not only reflects the light, but amplifies it as well. As a part of the project, the research will train graduate and undergraduate students, and will support recruitment and retention of underrepresented minorities to engineering through participation in a targeted research project course. The research goal of this proposal is the development of broadband terahertz quantum-cascade lasers based upon amplifying reflectarray metasurfaces, and that operate both with scalable high power and excellent beam quality. Within this overarching theme, we propose two primary thrusts: (a) the development of widely-tunable single-mode THz external cavity lasers, (b) the development of broadband multi-mode THz lasers and frequency combs. The enabling component of the vertical-external-cavity surface-emitting-laser is a reflectarray metasurface made up of sub-wavelength antenna-coupled microcavities loaded with laser gain material; this creates an active amplifying mirror which serves as one mirror in an open cavity. The intellectual merit in the proposed work is present in the development of novel reflectarray metasurfaces that provide gain over a large fractional bandwidths. A secondary innovation lies in the novel laser cavity configurations that are newly enabled by the flexibility of metasurface design. The power of this approach lies in (a) the ability of the VECSEL cavity configuration to support scalable high powers in a diffraction limited beam, and (b) the flexibility to locally engineer the amplitude, phase, and polarization response of the metasurface in both the spatial and spectral domain. The impact for spectroscopy and multi-spectral imaging is potentially large, since many previous demonstrations of tunable single-mode THz quantum-cascade lasers and frequency combs are either limited in output power, have a poor beam pattern, or both. The broader impacts are addressed at several levels including undergraduate and graduate research experiences, dissemination of results, and technology advancement. Outreach will specifically occur through the development of research projects for a course designed for the recruitment and retention of underrepresented minority URM engineering freshmen.