Polarization Engineered Second Harmonic Generation Enhancement from Amorphous Silicon - Gallium Nitride Heterogeneous Resonant Metasurface

We experimentally demonstrate second harmonic generation (SHG) enhancement from a heterogeneous resonant metasurface consisting of amorphous silicon sub-wavelength gratings supporting resonances within an underlying gallium nitride (GaN) layer. Electromagnetic design studies used to choose the sub-wavelength grating dimensions, corresponding spectral resonances, its angular sensitivity, electric field and non-linear polarization profiles are discussed. The efficient excitation of second-order nonlinearities within the GaN layer requiring longitudinal fundamental field component and efficient extraction of on-axis SHG using in-plane nonlinear polarization components are simultaneously satisfied for fundamental excitation polarized at 45^o, resulting in excitation of TE and TM-like resonances. Experimental studies on the fabricated structures characterizing the linear and nonlinear SHG properties including power, polarization, and spectral dependence are also discussed. The SHG spectral measurements show 1300-times enhancement for 1460 nm fundamental excitation. The SHG enhancement observed for the higher order resonance at 1460 nm is attributed to the trade-off between the increase in the in-plane to out-of-plane nonlinear polarization ratio, resulting in relative increase in on-axis SHG radiation and the decrease in quality factor of the resonances, which lowers the resonance field strength with increasing guided-mode order.