  Resonant metasurfaces are an attractive platform for enhancing the non-linear
optical processes, such as second harmonic generation (SHG), since they can
generate very large local electromagnetic fields while relaxing the
phase-matching requirements. Here, we take this platform a step closer to the
practical applications by demonstrating visible range, continuous wave (CW)
SHG. We do so by combining the attractive material properties of gallium
phosphide with engineered, high quality-factor photonic modes enabled by bound
states in the continuum. For the optimum case, we obtain efficiencies around
5e-5 % W$^{-1}$ when the system is pumped at 1200 nm wavelength with CW
intensities of 1 kW/cm$^2$. Moreover, we measure external efficiencies as high
as 0.1 % W$^{-1}$ with pump intensities of only 10 MW/cm$^2$ for pulsed
irradiation. This efficiency is higher than the values previously reported for
dielectric metasurfaces, but achieved here with pump intensities that are two
orders of magnitude lower.
