Frequency‐Modulated Combs via Field‐Enhancing Tapered Waveguides

Frequency-modulated (FM) combs feature flat intensity spectra with a linearfrequency chirp, useful for metrology and sensing applications. GeneratingFM combs in semiconductor lasers generally requires a fast saturable gain,usually limited by the intrinsic gain medium properties. Here, it is shown howa spatial modulation of the laser gain medium can enhance the gainsaturation dynamics and nonlinearities to generate self-starting FM combs.This is demonstrated with tapered planarized terahertz (THz) quantumcascade lasers (QCLs). While simple ridge THz QCLs typically generate combspresenting a mixture of amplitude and frequency modulation, the on-chipfield enhancement resulting from extreme spatial confinement leads to anultrafast saturable gain regime, generating a pure FM comb with a flatterintensity spectrum and a clear linear frequency chirp. The observed linearfrequency chirp is reproduced using a spatially inhomogeneous mean-fieldtheory model, which confirms the crucial role of field enhancement. Inaddition, the modified spatial temperature distribution within the waveguideresults in an improved high-temperature comb operation, up to a heat sinktemperature of 115 K, with comb bandwidths of 600 GHz at 90 K. The spatialinhomogeneity leads as well to very intense radio frequency (RF) beatnotes upto -30 dBm and facilitates dynamic switching between various harmonicstates in the same device