Broadband metasurface design for terahertz quantum-cascade VECSEL

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 multi-resonant 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 non-uniform distribution of spectral energy across the surface.