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 which 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 micro strip transmission lines and patch antennas. I will discuss the development of 2-dimensional reflect array metasurfaces based upon sub-wavelength arrays of such antennas loaded with QC-gain material that reflects and amplifies normally incident THz waves.
Amplifying metasurfaces have become the key enabling component for the demonstration of terahertz QC vertical-external-cavity surface-emitting lasers (QC-VECSELs). In such a scheme, the 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 (especially above 77 K), their limited beam quality (especially at high powers), and their limited range of continuous single-mode tunability. First, the output power is scalable by designing the active area of the metasurface. In this way record high powers of 20 mW continuous-wave and > 1 W pulsed have been demonstrated at 77 K heat sink temperature. Second, the metasurface phase and polarization response can be spatially engineered. This has been used to demonstrate focusing reflectarray metasurfaces for QC-VECSELs with near-diffraction limited beam quality, as well as QC-VECSELs with electrically switchable polarization of the output beam. Third, since the laser gain is localized in the metasurface itself, and not in a bulk gain medium, the length of the external cavity can be made extremely short to lase on low-order Fabry-Perot modes. This has allowed large continuous fractional tuning of a single mode over > 20% fractional bandwidth, while maintaining good beam quality and high power. This is a record for THz QC-lasers, and opens the door for THz QC-lasers as frequency-agile sources for spectroscopy and multi-spectral imaging. Finally, I will discuss the prospects of such THz QC-VECSELs for multimode lasers and frequency comb sources.
Benjamin Williams is an Associate Professor in the Department of Electrical and Computer Engineering at University of California Los Angeles. He received his B.S. in Physics from Haverford College in 1996, and his M.S. in 1998 and Ph.D. 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), the DARPA Young Faculty Award (2008), the NSF CAREER Award (2012), and the 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