Crosstalk suppression in germanium-on-insulator platform using subwavelength gratings for mid-infrared photonics

Abstract

Mid-infrared (MIR) is a promising spectral regime for gas and biochemical sensing since each molecule exhibits a unique vibrational absorption spectrum at this band. While integrated photonics on silicon-on-insulator (SOI) has become a successful platform, it cannot cover a broad MIR range, mainly limited by oxide absorption. Recently, a germanium-on-insulator (GOI) has emerged as a promising integrated photonics platform with broadband transparency covering 2-14 μm wavelengths. Germanium (Ge) and yttrium oxide (Y2O3), which exhibit low loss at the MIR regime, are used as a core and box insulator, respectively. However, the prevailing crosstalk issue of integrated photonics becomes more problematic at MIR due to extended evanescent fields with a longer wavelength. To address this issue, we propose using subwavelength gratings (SWGs), which can be effectively represented by homogenized anisotropic metamaterials. We arranged the SWGs in the cladding and formed an extreme skin-depth (eskid) waveguide, whose skin-depth is suppressed for transverse-electric (TE) mode. We then optimized SWG parameters to achieve an exceptional coupling that can completely suppress the crosstalk, i.e., zero crosstalk. Anisotropic dielectric perturbation via SWG metamaterials allowed different field components to compensate for each other, making the overall coupling coefficient zero. We optimized our SWG-based eskid waveguide scheme near 4.2 μm wavelength, where we can directly apply it for CO2 sensing with its strong absorption. We expect our eskid scheme on the GOI platform to improve the overall performances of MIR photonic devices, especially for MIR molecular sensing applications.