We demonstrate enhanced second harmonic generation in a gallium phosphide photonic crystal waveguide with a measured external conversion efficiency of 5$times10^{-7}$/W. Our results are promising for frequency conversion of on-chip integrated emitter
s having broad spectra or large inhomogeneous broadening, as well as for frequency conversion of ultrashort pulses.
We demonstrate second harmonic generation in photonic crystal nanocavities fabricated in the semiconductor gallium phosphide. We observe second harmonic radiation at 750 nm with input powers of only nanowatts coupled to the cavity and conversion effi
ciency $P_{rm out}/P_{rm in, coupled}^2 = 430%/{rm W}$. The large electronic band gap of GaP minimizes absorption loss, allowing efficient conversion. Our results are promising for integrated, low-power light sources and on-chip reduction of input power in other nonlinear processes.
