Fig. 1. (a) Schematic of a multilayer stack (Au/Si₃N₄/VO₂/Si₃N₄) coated silicon gratings for on-chip gas sensing. (b) Sensing mechanism. The variation of gas concentration induces an increase/decrease of light absorbance of the microstructures and then causes a temperature increase/decrease, which generates an electrical signal via VO₂.

Fig. 2. (a) Simulated absorption spectra at different n_gas for the device structure in Fig. 1(a). The period of gratings P = 3.3 µm, the grating depth h_g = 160 nm, and the grating width w_g = (w₁+w₂)/2 = 3 µm. (b) Q factor and FoM at various n_gas. (c) The electric field distribution at the resonance peak (λ = 3.3 μm) at n_gas = 1. (d) Poynting vector at the resonance. (e) The calculated absorption spectra as functions of wavelength and incident angle. (f) Sensitivities of different order modes versus the incident angle.

Fig. 3. (a) Schematic of on-chip mid-IR gas sensor based on an SOI platform, where the silicon substrate in the area underneath the sensor is removed to reduce the thermal dissipation. The thickness of each layer in the Au/Si₃N₄/VO₂/Si₃N₄/Si stack is 200 nm, 50 nm, 100 nm, 500 nm and 4 μm, respectively. The thickness of the oxide layer is 1 µm. P = 3.3 µm, h_g = 220 nm, w_g = 3 µm. (b) Simulated temperature distribution across the sensor with the illumination on and off when the power density of light is 3.75 W/cm² at 3.348 μm and corresponding absorption efficiency is 58%. (c) Temperature maximum under different wavelength illumination at the same power density and the absorption spectra of the gratings. The period number of the grating is 100. (d) Absorption spectra for variation of different environmental RI. (e) Absorption at 3.345 µm illumination and the associated device temperature versus the variation of different environmental RI.

Fig. 4. (a) Schematic of on-chip mid-IR gas sensor array based on an air bridge structure. The grating region has a size of 330 µm × 50 µm. The Au/Si₃N₄/VO₂/Si₃N₄ stack is shown in the inset and the thicknesses are 200 nm/50 nm/100 nm/500 nm, respectively. The stack is supported by the Si₃N₄/W bridges on Si substrate. (b) Temperature and relative electrical resistance variation of the sensors versus the variation of different environment gas RI. 'PD' means the power density of the incident light. (c) Simulated temperature distribution across the sensor with and without illumination at a wavelength of 3.348 µm with a power density of 1 W/cm² and the absorption is 58%.

Fig. 5. (a) Absorption spectra of a similar sensor as shown in Fig. 4 with an additional MOF layer placed on the top surface in pure Ar₂ and a mixture of Ar(90%)/CO₂(10%). The result in a mixture of Ar(90%)/CO₂(10%) ignoring the absorbance of CO₂ is also shown for comparison. P = 1.99 µm, h_g = 100 nm, w_g = 1.25 µm. RIs of CO₂ and MOF refer to literatures^{57, 60}. For a same membrane size (330 μm×50 μm) with the one in Fig. 4, there are 165 gratings in this case. (b) Simulated temperature distribution across the sensor illuminated at a wavelength of 2.71 µm with a power density of 1 W/cm² in a mixture of Ar(90%)/CO₂(10%).

Fig. 6. Gas molecular fingerprint spectrum reconstruction with the monolithically integrated plasmonic gas sensor array Φ(λ). (a) Schematic of the reconstruction process for an unknown input signal X(λ), where the photoelectric signal Y of each sensor is recorded. (b) The recovered spectrum (red line) and the reference spectra by simulation based on HITRAN database (black and blue lines).