Heart‐Like Mode Profile in a High Depth‐to‐Width Ratio Non‐Suspended Nano‐Trapezoidal Silicon Waveguide for Near‐Infrared Gas Sensing

The mature near‐infrared on‐chip integration technology and low‐noise detector contribute to the development of optical waveguide gas sensors. The external confinement factor (ECF) governs light‐gas interaction efficiency in on‐chip infrared waveguide gas sensors. While suspended waveguides achieve ECFs exceeding 100% (surpassing free‐space sensors), their wet‐etch fabrication raises complexity and collapse risks. Conversely, typical non‐suspended rectangular waveguides suffer from limited ECFs (< 30%) and constrain performance. This work introduces a non‐suspended nano‐trapezoidal silicon waveguide design, featuring a high depth‐to‐width ratio and heart‐like mode profile, to overcome these limitations. Compatible with fabrication processes for standard non‐suspended rectangular waveguides, this approach achieves a significantly improved ECF of 121% and a low propagation loss of 2.03 dB/cm. We characterize the C 2 H 2 sensing performance at 1532 nm using both direct absorption spectroscopy (DAS) and wavelength modulation spectroscopy (WMS). The limit of detection at the optimal averaging time is 23.19 ppm (DAS) and 5.70 ppm (WMS). This design overcomes fundamental limitations in waveguide‐based evanescent field sensing, paving the way for more sensitive and manufacturable on‐chip gas sensors.