Surface-emitting terahertz quantum cascade ring lasers

We have developed and fabricated a novel surface-emitting waveguide for terahertz quantum cascade lasers. The successfully employed double metal waveguide for such devices lacks of good far field pattern and low beam divergence. We have overcome these drawbacks by combining a second-order grating for surface emission with a ring waveguide geometry. Stable single mode emission has been observed over various operating conditions. We have measured circular beam profiles with a FWHM of 15° and achieved a grating-induced tuning range of about 300 GHz. By breaking the circular symmetry with two opposite π phase shifts in the grating, the far field pattern has changed into a tight center lobe with a FWHM of 5° and a preferred polarization direction is defined.