EECS Seminar: Terahertz metasurface quantum-cascade vertical-external-cavity surface-emitting-lasers (VECSELs)

Abstract:  The terahertz frequency range lies at the junction of the electronic and photonic regimes, which means that it is fertile ground for exploring hybrid systems that combine laser gain with microwave electromagnetic design. The terahertz quantum-cascade (QC) laser is an excellent example, which produces gain via intersubband electronic transitions within heterostructure quantum wells, but typically uses sub-wavelength waveguides that have a strong resemblance to microstrip transmission lines and patch antennas. I will discuss the development and recent advances to do with amplifying reflectarray metasurfaces based upon sub-wavelength arrays of such antennas loaded with QC-gain material, and their use in implementing vertical-external-cavity surface-emitting-lasers (VECSELs).

In the QC-VECSEL scheme, the amplifying metasurface is used as one reflector in an external cavity, which supports a well-shaped circulating THz beam. The QC-VECSEL architecture offers a solution to many problems that have plagued THz QC-lasers – namely their low emission powers and efficiencies, their limited beam quality (especially at high powers), and their limited range of continuous single-mode tunability. I will discuss several advances in the field of VECSELs in the past few years. First is the development of QC-VECSELs as stable, single-moded local oscillators for heterodyne receivers. Such lasers are in demand for future astrophysical instruments for mapping of molecular and atomic species within the interstellar medium – particularly at high frequencies above 2 THz which are difficult to access using electronic sources. We have recently shown that QC-VECSELs can provide mW to tens of mW continuous-wave power, in near-diffraction limited beams, with large fractional tunability (up to 20% of the center wavelength). Second, is the extension of QCLs and QC-VECSELs to higher frequencies above 5 THz and approaching the lossy Reststrahlen band in GaAs where lasers are nominally impossible. We have recently demonstrated QC-laser gain material operating up to 6.5 THz (46 μm wavelength), and tunable QC-VECSELs operating between 5.2-5.7 THz. Third is the initial demonstration of QC-VECSELs as frequency combs. Since every longitudinal mode in a QC-VECSEL interacts with the gain medium through the same metasurface resonance, there is no-spatial hole burning per se. As a result QC-VECSELs prefer to lase in single-mode, unlike waveguide QC-lasers that will spontaneously lase in multimode and frequency comb operation. However, we have shown that THz QC-VECSELs can be forced into a fundamental or harmonic frequency-comb regime via strong microwave modulation of the metasurface gain. This development, combined with the high powers and excellent beam patterns, may enable practical realization of THz QCL dual-comb spectrometers.

Bio:  Benjamin Williams is professor and chair of the Department of Electrical and Computer Engineering at UCLA. He received his bachelor's degree in physics from Haverford College in 1996, and his master's degree in 1998 and doctorate in 2003 both from the Massachusetts Institute of Technology in electrical engineering. He is currently associate editor for IEEE Transactions in Terahertz Science and Technology. He has received the APS Apker Award (1996), DARPA Young Faculty Award (2008), NSF CAREER Award (2012), and Presidential Early Career Award for Scientists and Engineers (PECASE) (2016). His research interests lie in photonic materials, devices and applications for the terahertz and mid-infrared frequency ranges, including low-dimensional semiconductors, quantum-cascade lasers, and plasmonics and metamaterials.