Fig. 2: The simulated and experimental results of thickness-modulating for refractive index sensing.

a The SU-8 multi-square resonant structure with different thicknesses through guided-mode coupling. The arrow indicates the incident lightwave. (t,n) means the thickness and refractive index at resonance of SU-8 square. b Simulated reflectivity angular spectrum changing with the refractive index (i) and thickness (ii). R in the scale bar means reflectivity. c The measured reflectivity angular spectrum (i) of our sensing system with a four-square resonant structure. The gray area in the figure means the resonance area. The captured gray images (ii) show a unique pattern under a corresponding incident angle. The ‘A-D’ letters mean different resonant structures and the asterisk number indicates the resonant dip. d The resonant incident angle changes with the refractive index under different thicknesses (i) and changes with the thickness under different refractive indices (ii). The dark line crosses the two figures stands for a fixed resonant angle, which corresponds to the experimental conditions (with a fixed incident angle). e The one-to-one relationship between the refractive index and the resonant thickness for a fixed incident angle. The inset is the reflectivity when the thickness varies at different refractive indices. f The relationship between the thickness and the resonant angle near the cut-off area, calculated via TMM and guided wave model (phase matching condition) for n = 1.33. The gray area means the cut-off area. The inset is the zoom-in near the cut-off area.