Title: 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 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 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 (especially above 77 K), 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 a near-diffraction limited beam, 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. 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 which will spontaneously lase in multi-mode 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. About the Speaker: Benjamin Williams is Professor and Chair of the Department of Electrical and Computer Engineering at University of California Los Angeles (UCLA). 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.Virtual Location URL: https://ucf.zoom.us/j/92584023013?from=addon

Title: 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 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 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 (especially above 77 K), 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 a near-diffraction limited beam, 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. 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 which will spontaneously lase in multi-mode 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. About the Speaker: Benjamin Williams is Professor and Chair of the Department of Electrical and Computer Engineering at University of California Los Angeles (UCLA). 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.Virtual Location URL: https://ucf.zoom.us/j/92584023013?from=addon