Quasi-BIC resonances in amorphous germanium zero contrast gratings with dual asymmetric step profile for mid infrared frequency up-conversion

Resonant metasurfaces supporting quasi bound-states-in-continuum (BICs) resonances are particularly attractive for achieving high quality factor resonances which can be used to enhance nonlinear optical processes. In this work we first experimentally demonstrate quasi-BIC resonances in the mid infrared wavelength range using amorphous Germanium (a- Ge) based one-dimensional (1D) sub-wavelength grating structures with vertical asymmetry. The vertical asymmetric structures studied here consist of 1D a-Ge partially etched zero-contrast gratings (ZCG) on quartz substrate with the addition of a second asymmetric step-profile creating in-plane asymmetry. The vertical asymmetry added to the grating structure through the second etching step creates an open channel for accessing symmetry-protected guided mode resonances (GMR) which are otherwise experimentally inert to outgoing radiation. The fabricated device dimensions are: total height of 424 nm, ZCG etch depth of 190 nm, asymmetric step etch depth of 100 nm, pitch of 2.0 μm, and grating (asymmetric step) duty-cycles of 75% (32.5 %). FTIR measurements show clear resonance peak in the transmission spectra at ~3.2 μm wavelength for transverse magnetic (TM) polarized incidence with quality factor of ~50. We also characterize the incidence angle dependence of the measured resonance peak under classical and conical mounting condition and observe close to angle insensitive response for the latter. Next, we demonstrate third-order sum-frequency generation (TSFG) based up-conversion of the resonant mid infrared light to visible wavelengths in the presence of an additional 1040 nm pump excitation. Maximum TSFG enhancement of ~32 is obtained close to the mid infrared resonance.