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Optical frequency combs (FCs), that establish a rigid phase-coherent linkbetween the microwave and optical domains of the electromagnetic spectrum, areemerging as a key high-precision tools for the development of quantumtechnology platforms. These include potential applications for communication,computation, information, sensing and metrology, and can extend from thenear-infrared with micro-resonator combs, up to the technologically attractiveterahertz (THz) frequency range, with powerful and miniaturized quantum cascadelaser (QCL) FCs. The recently discovered ability of the QCLs to produce aharmonic frequency comb (HFC), a FC with large intermodal spacings, hasattracted new interest in these devices for both applications and fundamentalphysics, particularly for the generation of THz tones of high spectral purityfor high data rate wireless communication networks, for radiofrequencyarbitrary waveform synthesis, and for the development of quantum keydistributions. The controlled generation of harmonic states of a specific orderremains, however, elusive in THz QCLs. Here we devise a strategy to obtainbroadband HFC emission of a pre-defined order in QCL, by design. By patterningn regularly spaced defects on the top-surface of a double-metal Fabry-PerotQCL, we demonstrate harmonic comb emission with modes spaced by (n+1) freespectral range and with a record optical power/mode of ~270 $\mu W$.

Sculpting harmonic comb states in terahertz quantum cascade lasers by controlled engineering