  Frequency-modulated (FM) combs feature flat intensity spectra with a linear
frequency chirp, useful for metrology and sensing applications. Generating FM
combs in semiconductor lasers generally requires a fast saturable gain, usually
limited by the intrinsic gain medium properties. Here, we show how a spatial
modulation of the laser gain medium can enhance the gain saturation dynamics
and nonlinearities to generate self-starting FM combs. We demonstrate this with
tapered planarized THz quantum cascade lasers (QCLs). While simple ridge THz
QCLs typically generate combs which are a mixture of amplitude and frequency
modulation, the on-chip field enhancement resulting from extreme spatial
confinement leads to an ultrafast saturable gain regime, generating a pure FM
comb with a flatter intensity spectrum, a clear linear frequency chirp and very
intense beatnotes up to -30 dBm. The observed linear frequency chirp is
reproduced using a spatially inhomogeneous mean-field theory model which
confirms the crucial role of field enhancement. In addition, the modified
spatial temperature distribution within the waveguide results in an improved
hightemperature comb operation, up to a heat sink temperature of 115 K, with
comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity also leads to
dynamic switching between various harmonic states in the same device.
