  We designed, fabricated and tested gallium phosphide (GaP) nano-waveguides
for second harmonic generation (SHG). We demonstrate SHG in the visible range
around 655 nm using low power continuous-wave pump in the optical communication
O-band. Our structures utilize modal phase matching, such that lower order
eigenmodes of the pump are phase matched to higher order eigenmodes of the
second harmonic. We observe phase matched SHG for different combinations of
interacting modes by varying the widths of the waveguides and tuning the
wavelength of the pump. The presented results contribute to the development of
integrated photonic platforms with efficient nonlinear wave-mixing processes
for classical and quantum applications.
