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.
Simon Hönl, Katharina Schneider, Miles Anderson, Dalziel Wilson, Tobias J. Kippenberg, and Paul Seidler, "Gallium phosphide microresonator frequency combs (Conference Presentation)," Proc. SPIE 10518, Laser Resonators, Microresonators, and Beam Control XX, 105180F (Presented at SPIE LASE: January 29, 2018; Published: 14 March 2018); https://doi.org/10.1117/12.2290511.5751433522001.
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