Engineering mode coupling in hybrid plasmon-photonic cavity for dual-band infrared spectroscopic sensing

On-chip infrared spectroscopy has become one of the indispensable key technologies for miniature biochemical sensors, gas sensors, food quality control, and environmental monitoring systems. The most important requirement for on-chip spectroscopic sensors is to miniaturize spectroscopic functions to be integrated into thermal emitters and infrared detectors. In this work, we propose a hybrid plasmon-photonic system consisting of a plasmonic grating coupled to a distributed Bragg reflector (DBR)-dielectric-metal cavity for on-chip dual-band spectroscopic sensing applications. The strong coupling between surface-plasmon polaritons and the cavity resonance leads to the hybridization of the photonic states; the mode splitting, the photonic band folding, and the formation of new eigenstates including bound states in the continuum are observed in the system. It is shown that, by engineering the photonic coupling, a dual-band resonant near-perfect absorber is achievable and easily controllable. As a proof of concept, we numerically demonstrate a set of five different dual-band absorbers for CO2, N2O, CO, NO, and NO2 gas sensing applications. The dual-band absorbers can be used for on-chip spectroscopic thermal emitters or infrared detectors in gas sensors. The hybrid plasmon-photonic system can be an attractive photonic platform for applications in emitting and sensing photonic devices.