Freestanding\nGermanium Photonic Crystal Waveguide\nfor a Highly Sensitive and Compact Mid-Infrared On-Chip Gas Sensor

The performance of mid-infrared (MIR) on-chip gas sensors,\noperating\nvia laser absorption spectroscopy, hinges critically on light–matter\ninteraction dynamics, significantly influenced by external confinement\nand the effective light path length. Conventional on-chip sensors,\nhowever, face challenges in achieving the required limit of detection\nfor highly sensitive applications, primarily due to their intrinsically\nshort effective light path. Furthermore, these sensors are limited\nin their spectral range coverage within the MIR spectrum by the constraints\nof standard silicon-based platforms. To overcome these limitations,\nour research presents a novel approach to fabricate a freestanding\ngermanium (Ge) photonic crystal waveguide (PCW) on a germanium-on-insulator\n(Ge–OI) platform, utilizing yttrium oxide (Y<sub>2</sub>O<sub>3</sub>) as the buried oxide layer. This device leverages the broad\ntransparent windows of Ge and Y<sub>2</sub>O<sub>3</sub>, broadening\nthe spectral coverage across the MIR range. The introduction of the\nPCW and its slow light effect significantly elevate external confinement\nand light–matter interactions, enabling a notable reduction\nin waveguide length, which traditionally limits on-chip configurations.\nThe freestanding structure not only expands the sensing region and\nenhances external confinement but also prevents the emergence of leaky\nmodes within the PCW. As a result, our compact sensor achieves an\nexceptionally low LoD of 7.56 ppm for carbon dioxide (CO<sub>2</sub>) sensing at the operational wavelength of 4.23 μm, with a\ncompact waveguide length of only 800 μm.