A quasi-quantitative demonstration of multi-mode refractive index sensors based on standing-wave plasmonic resonances in split ring resonators

We propose a multi-mode refractive index sensor based on split ring resonators (SRRs). By applying thin dielectric layer with varied refractive index on top of planar SRRs, we clarify the relationship among sensitivity, resonant modes and the size of SRRs based on standing-wave plasmonic resonances model. Significant peak shifts are observed in FTIR measurement spectrums, consisting with the simulation results which suggest impressive sensitivities closer to SPR or LSPR for different resonant modes. Next, the corresponding detection lengths of each mode were examined by varying the thickness of the overlaid dielectric layer. Lower modes include 1 parallel to, 2 perpendicular to and 3 parallel to show thickness saturation effect within 500 nm while higher modes such as 5 parallel to, 6 perpendicular to and 7 parallel to present longer detection length at micron scale, which namely, no saturation effect is observed when the thickness of dielectric layer increased to 2 um. This valuable merit enables the analysis of activation-dependent cellular interactions that other label-free techniques like surface plasmon resonance (SPR) are incapable of. In conclusion, the distinct sensing behavior including sensitivity and detection length of the multi resonant modes in SRRs was investigated, showing SRR-based sensors promise a real-time, operation frequency flexible and multi-mode solution for biological and chemical detection.