Resonantly Enhanced Infrared Up-Conversion in Double-Step Asymmetric Subwavelength Grating Structure

The design and experimental demonstration of double-step, 1D amorphous germanium grating structures supporting quasi bound-states-in-continuum (quasi-BIC) resonance at 3.2 µm wavelength and its application to third-order sum-frequency generation-based up-conversion are reported. Linear transmission measurements on the fabricated metasurface with loosely focussed excitation spanning 0–3° angles show very good agreement with ideal plane-wave excitation of the periodic photonic structure. TSFG measurements performed on the same structures with tightly focusing mid-infrared signal and pump beams using a reflective-type objective with 15–40° angular excitation show ≈375 times enhancement with significant blue-shift in the resonance feature by ≈300 nm. To understand this excitation angle dependence of the resonance characteristics, a generalized plane-wave expansion (PWE) model is developed by considering varying excitation angle plane-waves incident on the metasurface with a discretized angular spectrum representation used to coherently combine the resultant electric and magnetic fields to obtain the linear transmission characteristics and nonlinear TSFG spectra. The PWE method is found to be particularly effective in modeling linear and nonlinear responses under realistic illumination conditions while ensuring optimal utilization of computational resources. Good agreement is obtained between the PWE simulations, linear transmission, and nonlinear TSFG measurements by considering appropriate angular excitation.