Broadband Antenna-coupled THz Quantum Cascade Laser Frequency Combs With Inverse-designed Waveguide Facets

We present a broadband, monolithic outcoupler based on planarized double metal waveguides that addresses both the reflectivity design and broadband narrow beam emission in THz quantum cascade lasers. The waveguide mirror reflectivity can be adjusted by shaping the end facet, obtained with an efficient inverse design algorithm. The THz laser radiation is coupled to a broadband patch-array antenna for surface emission. All the components are optimized for octave-spanning spectra between 24 THz. We implement these concepts to demonstrate a broadband surface-emitting THz quantum cascade laser frequency comb featuring output powers of 13 mW, optical bandwidths in excess of 800 GHz and a single-lobed far-field pattern with a beam divergence below $( 20 ^{\circ} \mathrm{x}20 ^{\circ})$.