Surface-normal photonic crystal waveguide arrays for ultra-sensitive mid-IR gas sensing

This paper presents a novel silicon-based hollow-core surface-normal photonic crystal waveguide with an air-core even mode and high group index in the meta structure. This mode is proposed for gas detection in the mid-infrared range. The structure’s bandgap map and guided modes were calculated through numerical simulations, allowing for design optimization to enable slow-light propagation of the guided modes. The design enhances light–matter interaction by leveraging large group indices and dispersion control. In experimental simulations of photonic crystal waveguide structures, an ultra-high group index of 1424 was achieved at a wavelength of 3.31 μm, matching with methane absorption. This high group index—i.e., the realization of extremely slow light—facilitates enhanced interaction with analytes of interest. The result reveals that 60.1% of the energy of the fundamental mode resides within the center air hole, providing a high overlap with analytes. The device geometry can be customized to introduce the absorption wavelengths of various analytes, enabling arrays of different device geometries on the same chip for simultaneous detection of multiple gas species.