Fig. 1. Device structure. (a) Schematic of the device structure. (b) Schematic of the xy cross-section of the device (d = 500 nm, a₁ = 180 nm, a₂ = 270 nm, h = 100 nm, t = 130 nm, and P = a₁ + 2a₂).

Fig. 2. Reflection spectrum of the device. (a) Reflection spectra of the device at incidence angles θ = 0° and 2°. (b) Energy band diagram of the device. Reflection spectra and electric-field intensity distributions of (c) the SPR mode and (d) the quasi-BIC mode at an incidence angle of 2°.

Fig. 3. Principle of the device for refractive index sensing. Schematic diagram of monitoring the centroid wavelength shift of the (a) SPR mode and (b) quasi-BIC mode for refractive index sensing (e.g.,  temperature detection).

Fig. 4. Principle of the device for intensity modulation sensing. Monitoring the intensity of the spectra with the (a) SPR mode and (b) quasi-BIC mode for optical intensity modulation sensing (e.g., acoustic pressure sensing). (c) Dependence of the device’s Q-factor on the angle of incidence.

Fig. 5. Refractive index sensitivity of the quasi-BIC (θ = 2°). (a) Relationship between the quasi-BIC spectra and PDMS refractive index. (b) The centroid wavelength shift of the quasi-BIC versus PDMS refractive index changes. The blue data points are the simulation results, and the red curve is the linear fitting result. (c) The electric field distributions at the resonant wavelength and 1 nm off the resonant wavelength for the SPR mode and the quasi-BIC mode, respectively. (d) The quasi-BIC spectra at different temperatures. (e) Centroid wavelength shifts at different temperatures. The blue data points are the simulation results, and the red curve is the linear fitting result (assuming refractive index = 1.426 for PDMS at 25°C).