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

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A terahertz vertical-external-cavity surface-emitting-laser (VECSEL) is demonstrated using an active focusing reflectarray metasurface based on quantum-cascade gain material. The focusing effect enables a hemispherical cavity with flat optics, which exhibits higher geometric stability than a plano-plano cavity and a directive and circular near-diffraction limited Gaussian beam with M² beam parameter as low as 1.3 and brightness of 1.86 × 10⁶ Wsr^-1m^-2. This work initiates the potential of leveraging inhomogeneous metasurface and reflectarray designs to achieve high-power and high-brightness terahertz quantum-cascade VECSELs.

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