Gallium Phosphide Nonlinear Photonics

Gallium phosphide (GaP) is an indirect bandgap semiconductor used widely in solid-state lighting. Despite numerous intriguing optical properties---including large $\chi^{(2)}$ and $\chi^{(3)}$ coefficients, a high refractive index ($>3$), and transparency from visible to long-infrared wavelengths ($0.55-11\,\mu$m)---its application as an integrated photonics material has been little studied. Here we explore GaP-on-insulator as a platform for nonlinear photonics, exploiting a direct wafer bonding approach to realize integrated waveguides with 1.2 dB/cm loss in the telecommunications C-band (on par with Si-on-insulator). High quality $(Q> 10^5)$, grating-coupled ring resonators are fabricated and studied. We directly observe the Kerr effect in response measurements, and obtain the first experimental estimate of the nonlinear index of GaP at telecommunication wavelengths: $n_2=1.2(5)\times 10^{-17}\,\text{m}^2/\text{W}$. We also observe Kerr frequency comb generation in resonators with engineered dispersion. Parametric threshold powers as low as 3 mW are realized, followed by broadband ($>100$ nm) frequency combs with sub-THz spacing, frequency-doubled combs and, in a separate device, efficient Raman lasing. These results signal the emergence of GaP-on-insulator as a novel platform for integrated nonlinear photonics.