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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

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A broadband active metasurface is designed around quantum-cascade (QC) gain material for use in a broadband tunable QC vertical external-cavity surface-emitting laser (VECSEL). The metasurface is based on a unit cell containing two resonant waveguide elements that couple with the periodicity of the metasurface to produce a multiresonant reflection spectrum. Simulated reflectance spectra exhibit full-width half-maximum bandwidths up to 50% of the centre frequency. The tested QC-VECSEL demonstrates up to 15 mW of peak pulse power at 77 K and supports multimode lasing from 2.85 to 3.9 THz (>30% fractional bandwidth around 3.37 THz). The frequency coverage is limited by how short the cavity can be made, rather than the metasurface bandwidth. Consistent multimoding results from nonuniform distribution of spectral energy across the surface.