Fig. 1. Structural model of mental grating/dielectric/metal hybrid waveguide

Fig. 2. Structure of metal/dielectric/metal

Fig. 3. Reflection spectrum of sensing structure

Fig. 4. Magnetic field distributions at different resonant wavelengths. (a) λres1=697.5 nm; (b) λres2=864.8 nm

Fig. 5. Effects of structural parameters on reflection spectrum. (a) External environment refractive index; (b) dielectric layer thickness; (c) period of metal grating; (d) thickness of metal grating; (e) slit width of metal grating

Fig. 6. Resonance wavelength shift with different refractive indexes of porous silicon sensing unit

Fig. 7. Resonance wavelength shift of formaldehyde gas with different concentrations

Fig. 8. Linear fitting relationship between formaldehyde gas concentration and resonance wavelength