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We present a planarized waveguide cavity with an integrated broadband output coupler that improves both the output power and far-field properties of THz quantum cascade laser frequency combs. The laser mirror reflectivity can be tuned by the shape of the end facet, which is obtained with an efficient inverse design algorithm, where the structure is iteratively updated to match the desired figure of merit. Surface emission is achieved with a broadband patch-array antenna, and all the components have been optimized for octave-spanning emission spectra (2-4 THz). Experimentally, we demonstrate a broadband surface-emitting THz quantum cascade laser frequency comb with optical bandwidths of up to 800 GHz, surface emission into a narrow beam with divergence below (20° x 20°) and a peak power of 13 mW. [ABSTRACT FROM AUTHOR]

We present a new planarized waveguide geometry for THz quantum cascade laser frequency combs with improved waveguide losses, RF and thermal dissipation properties. Ridge devices display broadband free-running comb states, and the THz emission can be further broadened by RF injection. Tapered waveguide devices feature a strong field-enhancement effect, which results in an improved comb performance. This includes free-running comb states with strong single beatnotes up to nearly -30 dBm at 90 K, almost three orders of magnitude stronger than for ridge devices. Improved comb operation is maintained also for high operating temperatures, up to 115 K. [ABSTRACT FROM AUTHOR]