Continuous wave operation of terahertz metasurface quantum-cascade VECSEL with a long intra-cryostat cavity

We report continuous wave (cw) operation of a terahertz quantum-cascade vertical-external-cavity surface-emitting laser with an external cavity length approximately 30 mm, benefited by intra-cryostat focusing cavity. Compared to previous plano–plano cavities, off-axis paraboloid mirror is introduced into the as element reduce diffraction loss and enable cw lasing using small-area metasurfaces long lengths. The device shows in mode up 111 K, output power 11.5 mW at 77 K (0.5% wall-plug efficiency). A circular, directive beam pattern collected, free-running linewidths on order tens kHz are measured over seconds.