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A vertical-external-cavity surface-emitting-laser is demonstrated in the terahertz range, which is based upon an amplifying metasurface reflector composed of a sub-wavelength array of antennacoupled quantum-cascade sub-cavities. Lasing is possible when the metasurface reflector is placed into a low-loss external cavity such that the external cavity-not the sub-cavities-determines the beam properties. A near-Gaussian beam of 4.3 Â 5.1 divergence is observed and an output power level >5 mW is achieved. The polarized response of the metasurface allows the use of a wire-grid polarizer as an output coupler that is continuously tunable. V

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) whose output power is scaled up to watt-level by using an amplifying metasurface designed for increased power density. The metasurface is composed of a subwavelength array of metal-metal waveguide antenna-coupled sub-cavities loaded with a terahertz quantum-cascade gain material. Unlike previously demonstrated THz QC-VECSELs, the sub-cavities operate on their third-order lateral modal resonance (TM03), instead of their first-order (TM01) resonance. This results in a metasurface with a higher spatial density of the gain material, leading to an increased output power per metasurface area. In pulsed mode operation, peak THz output powers up to 830 mW at 77 K and 1.35 W at 6 K are observed, while a single-mode spectrum and a low divergence beam pattern are maintained. In addition, piezoelectric control of the cavity length allows approximately 50 GHz of continuous, single-mode tuning without a significant effect on output power or beam quality.

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A terahertz quantum-cascade (QC) vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated with over 5 mW power in continuous-wave and single-mode operation above 77 K, in combination with a near-Gaussian beam pattern with a full-width half-max divergence as narrow as ∼5° × 5°, with no evidence of thermal lensing. This is realized by creating an intra-cryostat VECSEL cavity to reduce the cavity loss and designing an active focusing metasurface reflector with low power dissipation for efficient heat removal. Also, the intra-cryostat configuration allows the evaluation of QC-VECSEL operation vs. temperature, showing a maximum pulsed mode operating temperature of 129 K. While the threshold current density in the QC-VECSEL is higher compared to that in a conventional edge-emitting metal-metal waveguide QC-laser, the beam quality, slope efficiency, maximum power, and thermal resistance are all significantly improved.

We compare the performance of 10 and 5 μm thick metal–metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.

We report a terahertz quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) based upon a metasurface consisting of an array of gain-loaded resonant patch antennas. Compared with the typical ridge-based metasurfaces previously used for QC-VECSELs, the patch antenna surface can be designed with a much sparser fill factor of gain material, which allows for reduced heat dissipation and improved thermal performance. It also exhibits larger amplification thanks to enhanced interaction between the incident radiation and the QC-gain material. We demonstrate devices that produce several milliwatts of continuous-wave power in a single mode at ~4.6 THz, and dissipate less than 1 W of pump power. Use of different output couplers demonstrates the ability to optimize device performance for either high power, or high operating temperature. Maximum demonstrated power is 6.7 mW at 4 K (0.67% wall-plug efficiency (WPE)), and 0.8 mW at 77 K (0.06% WPE). Directive output beams are measured throughout with divergence angles of ~5°. THE MANUSCRIPT The terahertz (THz) quantum-cascade (QC) vertical-external-cavity surface-emitting laser (VECSEL) is a recently developed approach for designing THz QC-lasers with scalable output power and high-quality beam patterns. 1 The enabling component of the QC-VECSEL is an amplifying metasurfacea reflectarray of resonant antenna-coupled THz QC-gain elements with subwavelength spacing that exhibits a reflectance R greater than unity across some finite gain bandwidth. The antenna elements are unable to lase on their own due to high surface radiation losses, but instead, the metasurface is used as the amplifying element of an external cavity laser. The external cavity mode effectively phase locks the antenna elements, and because the metasurface can be made large relative to the lasing wavelength, near-diffraction-limited beams with high power have been observed. 2 The external-cavity allows one to more easily modify the outcoupling efficiency and cavity resonant frequency, 3 while the metasurface offers unique opportunities such as engineering of "flat-optics" focusing 2 and polarization switchability. 4 The typical metasurface design consists of a 1D sub-wavelength array of narrow metal-metal ridges of width w and period Λ that are coupled to surface radiation via the TM01 cutoff resonances of the ridges. 5,6 This design has been used to realize many high-performance VECSELs operating between 3-4 THz. 3,7,8