Mid-Infrared Off-Axis Resonant Mode Hybridization in Amorphous Silicon Sub-Wavelength Grating Structures

In this study, the off-axis excited guided-mode resonance (GMR) hybridization is experimentally investigated in one dimensional (1D) partially etched amorphous silicon (a-Si) subwavelength grating structures operating in the mid-infrared wavelength range. The 1D dielectric photonic lattice is a promising platform for studying resonance hybridization effects owing to its simple working principle and ease of electromagnetic design, fabrication, and experimental characterization. It is observed that with increasing duty-cycle of the gratings, the avoided crossing between the coupled GMR branches underwent band-closure and subsequent band-flip, resulting in the Friedrich-Wintgen type bound-states-in-the-continuum (FW-BICs) transitioning from the upper to the lower GMR branch. On either side of the band closure, the transmission spectra show an interesting electromagnetically induced transparency (EIT)-like resonance, which manifests as a transmission peak within a broad transmission dip with increased electric field concentration near the peak. As a conceptual application of coupled GMRs, differential enhancement of infrared absorption from a polymethyl methacrylate (PMMA) layer coated on the grating structure with one of the optical resonances aligned to the absorption feature and the other acting as a built-in reference is demonstrated. The differential transmission contrast is enhanced by more than two orders of magnitude when compared to that of the native PMMA layer.