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

Abstract

AbstractIn 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.