On-chip near-infrared gas sensing based on slow light mode multiplexing in photonic crystal waveguides

Photonic crystal slow light waveguides present a breakthrough in the manipulation of optical signals and enhancing the interaction between light and matter. In particular, two-dimensional (2D) photonic crystal waveguides (PCWs) on silicon photonic chips hold promise in improving the sensitivity of on-chip gas sensors. However, the development of the gas sensors based on 2D PCWs suffers from a high propagation loss and a narrow slow light bandwidth. In this study, our focus was on designing a one-dimensional (1D) PCW with lower propagation loss and tailored group indices across dual distinct frequency bands. To achieve this, a mode converter was employed to effectively stimulate both odd and even modes of the 1D PCW with odd modes ranging from 1520 to 1555 nm and even modes spanning 1615-1665 nm. Remarkably, we pioneered the application of slow light mode multiplexing to demonstrate the potential of the 1D PCW as an on-chip multi-gas sensor, specifically targeting acetylene (C₂H₂) and methane (CH₄). At 1533 nm, the odd mode exhibited an impressive interaction factor of 0.836; while at 1654 nm, the even mode achieved an even higher interaction factor of 1.308, and both retain relatively low propagation losses. This research not only introduces innovative strategies for expanding slow light bandwidth, but also presents a promising avenue for on-chip multi-gas detection.