Hybrid Metamaterial Absorber Platform for Sensing of CO₂ Gas at Mid-IR

Application of two major classes of CO₂ gas sensors, i.e., electrochemical and nondispersive infrared is predominantly impeded by the poor selectivity and large optical interaction length, respectively. Here, a novel "hybrid metamaterial" absorber platform is presented by integrating the state-of-the-art complementary metal-oxide-semiconductor compatible metamaterial with a smart, gas-selective-trapping polymer for highly selective and miniaturized optical sensing of CO₂ gas in the 5-8 µm mid-IR spectral window. The sensor offers a minimum of 40 ppm detection limit at ambient temperature on a small footprint (20 µm by 20 µm), fast response time (≈2 min), and low hysteresis. As a proof-of-concept, net absorption enhancement of 0.0282%/ppm and wavelength shift of 0.5319 nm ppm^-1 are reported. Furthermore, the gas- selective smart polymer is found to enable dual-mode multiplexed sensing for crosschecking and validation of gas concentration on a single platform. Additionally, unique sensing characteristics as determined by the operating wavelength and bandwidth are demonstrated. Also, large differential response of the metamaterial absorber platform for all-optical monitoring is explored. The results will pave the way for a physical understanding of metamaterial-based sensing when integrated with the mid-IR detector for readout and extending the mid-IR functionalities of selective polymers for the detection of technologically relevant gases.