Doubly Resonant c-GaP Nanocavity for Second Harmonic Generation

We present a doubly resonant nanocavity for second harmonic generation (SHG) in the visible range, consisting of a 590 nm crystalline gallium phosphide (c-GaP) layer, a gold mirror, and a silica spacer to achieve optimal overlap of pump and harmonic resonances. Experimental results show a conversion efficiency spanning 4 orders of magnitude, with a maximum η̃_max = 0.014% at λ_SHG = 600 nm, comparable to best-performing semiconductor metasurfaces, and surpassing a bulk 400 μm GaP crystal by over an order of magnitude. Using nonlinear scattering theory, we explain the spectral response and demonstrate wavelength tunability via angle of incidence or spacer thickness adjustments. We extend this result to a closed-cavity design with an ultrathin gold top layer which we predict can achieve efficiencies up to η̃_max ∼ 0.4% when the thickness of the c-GaP layer approaches the coherence length. This design can be scaled up to planar nanocavities with higher Q-factor and used to enhance the performance of metasurfaces, advancing nonlinear conversion efficiency toward practical applications.