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Self-starting harmonic frequency comb generation in a quantum cascade laser

Optical frequency combs establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications. Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis and for generation of THz tones of high spectral purity in the future wireless communication networks. We demonstrate for the first time self-starting harmonic frequency comb generation with a THz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of the population inversion in the laser. The mode spacing of the harmonic comb is shown to be uniform to within $5\times 10^{-12}$ parts of the central frequency using multiheterodyne self-detection. This new harmonic comb state extends the range of applications of quantum cascade laser frequency combs.