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THz quantum cascade lasers (QCLs) based on double metal waveguides featurebroadband and high-temperature devices for use in spectroscopy and sensing.However, their extreme field confinement produces poor output couplingefficiencies and divergent far-fields. Here, we present a planarized THz QCLwith an inverse-designed end facet reflector coupled to a surface-emittingpatch array antenna. All the components have been optimized for octave-spanningspectral bandwidths between 2-4 THz and monolithically integrated on the samephotonic chip. We demonstrate this experimentally on broadband THz QCLfrequency combs, with measured devices showing a seven-fold improvement inslope efficiency compared to devices with a cleaved facet. They feature peakpowers of up to 13.5 mW with surface emission into a narrow beam with adivergence of (17.0{\deg}x 18.5{\deg}), while broadband fundamental andharmonic comb states spanning up to 800 GHz are observed.

Broadband surface-emitting THz laser frequency combs with inverse-designed integrated reflectors