Terahertz quantum-cascade lasers based upon active reflectarray metasurfaces are shown to be a viable platform for scalable power with high-quality beams. The “metasurface” is made up of sub-wavelength arrays of antenna-coupled sub-cavities loaded with quantum-cascade active material. One has the ability to spatially engineer the amplitude, phase, spectral, and polarization response of the metasurface. We present several recent results. By placing the metasurface as part of external cavity, focusing QC-VECSELs have been demonstrated with high slope efficiency, high cw power, and near diffraction-limited beam quality. Additionally, VECSELs with electrically switchable polarization have been demonstrated.
Achieving high power in combination with high quality beam pattern is a ubiquitous challenge for semiconductor lasers. The demonstration of vertical-external-cavity surface-emitting lasers (VECSELs) in 1997 for visible and near-infrared semiconductor lasers has been a very successful approach. Terahertz (THz) quantum-cascade (QC) lasers, also have the challenge of combining high power and good beam pattern into one device – even more so because they typically use sub-wavelength metallic waveguides. The concept of VECSEL has been impossible to implement for QC lasers, since the optical gain is based on intersubband transitions of electrons, which only interact with the electric field polarized perpendicular to the quantum wells plane according to the "intersubband selection rule". To address this issue, we have developed an amplifying metasurface reflector that can couple the incident THz wave with the QC gain medium via metal-metal micro-cavity antenna reflectarray. Pairing the active metasurface with an output coupler, we demonstrated the first VECSEL in the THz regime in 2015. Based upon the prototype design, we have achieved a number of improvements to the QC-VECSEL including designing an inhomogeneous focusing metasurface to achieve a near-diffraction limited beam pattern with M2 = 1.3 and high brightness of 1.86×106 Wsr-1m-2, designing compact cavities and optimizing metasurface bias area to achieve continuous-wave operation above 77 K, achieving record high slope efficiency of 745 mW/A, as well as extending the VECSEL concept to cover a broad frequency range from 2.5 - 4.4 THz.
Terahertz quantum-cascade vertical external cavity surface emitting laser (VECSELs) are made possible through the development of amplifying reflectarray metasurfaces. The metasurface is made up of sub-wavelength arrays of antenna coupled sub-cavities loaded with quantum-cascade active material. The QC-VECSEL approach allows scaling of laser power while maintaining a high quality, near diffraction limited beam - something which has been a long standing challenge for THz quantum-cascade lasers with sub-wavelength metallic waveguides. The latest results of cavity and metasurface engineering are presented, including the demonstration of a focusing reflectarray metasurface that enables a "flat-optics" hemispherical VECSEL cavity, with improved geometric stability and a Gaussian profile beam with beam quality parameter of M2=1.3.
Vertical-external-cavity surface-emitting lasers (VECSELs) have been successfully used in the visible and near-infrared to achieve high output power with excellent Gaussian beam quality. However, the concept of VECSEL has been impossible to implement for quantum-cascade (QC) lasers due to the "intersubband selection rule". We have recently demonstrated the first VECSEL in the terahertz range. The enabling component for the QC-VECSEL is an amplifying metasurface reflector composed of a sparse array of metallic sub-cavities, which allows the normally incident radiation to interact with the electrically pumped QC gain medium. In this work, we presented multiple design variations based on the first demonstrated THz QC-VECSEL, regarding the lasing frequencies, the output coupler and the intra-cavity aperture. Our work on THz QC-VECSEL initiates a new approach towards achieving scalable output power in combination with a diffraction-limited beam pattern for THz QC-lasers. The design variations presented in this work further demonstrate the practicality and potential of VECSEL approach to make ideal terahertz QC-laser sources.