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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.

Polarization switchable terahertz quantum-cascade lasers are candidate sources for polarimetric terahertz imaging and sensing as their output polarization can be switched electrically without moving parts. We report on the importance of the external cavity on the spectral properties and polarization purity for a metasurface quantum-cascade VECSEL.

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Dynamic control of a laser's output polarization state is desirable for applications in polarization sensitive imaging, spectroscopy, and ellipsometry. Using external elements to control the polarization state is a common approach. Less common and more challenging is directly switching the polarization state of a laser, which, however, has the potential to provide high switching speed, compactness and power efficiency. Here, we demonstrate a new approach to achieve direct and electrically-controlled polarization switching of a semiconductor laser. This is enabled by integrating a polarization-sensitive metasurface with semiconductor gain medium to selectively amplify a cavity mode with the designed polarization state, therefore leading to an output in the designed polarization. Here the demonstration is for a terahertz quantum-cascade laser, which exhibits electrically-controlled switching between two linear polarization separated by 80°, while maintaining an excellent beam with narrow divergence of ~3°×3°, single-mode operation fixed at ~3.4 THz, combined with a peak power as high as 93 mW at a temperature of 77 K. The polarization-sensitive metasurface is composed of two interleaved arrays of surface emitting antennas, all of which are loaded with quantum-cascade gain materials. Each array is designed to resonantly interact with one specific polarization; when electrical bias is selectively applied to the gain material in one array, selective amplification of one polarization occurs. The amplifying metasurface is used along with an output coupler reflector to build a vertical-external-cavity surface-emitting-laser (VECSEL), whose output polarization state can be switched solely electrically. This work demonstrates the potential of exploiting amplifying polarization-sensitive metasurfaces to create lasers with desirable polarization states -a concept which is applicable beyond the terahertz and can potentially be applied to shorter wavelengths.

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