Gallium phosphide is an attractive material for non-linear optics because of its broad transparency window (E_b = 2.26 eV) and large Kerr coefficient (n_2 = 6*10^-18 m^2/W). Though well-established in the semiconductor industry as a substrate for visible LEDs, its use for chip-scale photonics remains limited due to fabrication challenges. Here we demonstrate unprecedentedly low loss (Q > 10^5) GaP-on-SiO2 waveguide resonators which have been dispersion-engineered to support Kerr frequency comb generation in the C-band. Parametric threshold is observed with as little as 10 mW injected power, followed by 0.1 THz frequency comb generation over a bandwidth exceeding 30 THz, in addition to strong 2nd and 3rd harmonic generation. Building on this advance, we discuss the prospects for low-noise, sub-mW-threshold microresonator frequency combs with center frequencies tunable from mid-IR to the near-IR. Applications of such devices range from precision molecular spectroscopy to ultrafast pulse generation to massively parallel coherent optical communication.
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