  The development of integrated photonic circuits utilizing gallium phosphide
requires a robust, scalable process for fabrication of GaP-on-insulator
devices. Here we present the first GaP photonic devices on SiO$_2$. The process
exploits direct wafer bonding of a GaP/Al$_x$Ga$_{1-x}$P/GaP heterostructure
onto a SiO$_2$-on-Si wafer followed by removal of the GaP substrate and the
Al$_x$Ga$_{1-x}$P stop layer. Photonic devices such as grating couplers,
waveguides, and ring resonators are patterned by inductively coupled-plasma
reactive-ion etching in the top GaP device layer. The peak coupling efficiency
of the fabricated grating couplers is as high as 4.8 dB. Optical quality
factors of 17000 as well as second- and third-harmonic generation are observed
with the ring resonators. Because the large bandgap of GaP provides for low
two-photon absorption at telecommunication wavelengths, the high-yield
fabrication of GaP-on-insulator photonic devices enabled by this work is
especially interesting for applications in nanophotonics, where high quality
factors or low mode volumes can produce high electric field intensities. The
large bandgap also enables integrated photonic devices operating at visible
wavelengths.
