Abstract
In this work, we demonstrate a multi-mode refractive index sensors based on split ring resonators (SRRs). The SRR structures show multiple reflectance peaks under normal incidence, which can be interpreted by the model of standing-wave plasmonic resonances, (1) where L represents the total length of SRR, λm the wavelength of the resonance mode m (λ0 depends to the geometric structure), and n the refractive index of the surrounding medium. The multiple reflectance peaks present two distinct sets of resonance modes: 1||, 3||, 5||, 7|| and 2┴, 4┴, 6┴, 8┴ corresponding to two orthogonal E-field excitations (E|| and E┴), respectively. Next, by applying analytes with different different refractive indices on the planar SRRs, there appear red-shifts in the reflectance spectra. According to the SWPR model, we further obtain the sensitivity of our SRR-based sensors as below: (2) Here the additional correction factor Xa depends on the thickness of the dielectric layer and the decay length of the localized E-filed. Eq. (2) manifests the relationship among sensitivity (to change of local refractive index), resonant modes and the size of SRRs, which are verified by both our simulated and measured results. For example, our simulation suggests an impressive sensitivity of the SRR-based sensor of ranging from around 1850 nm/RIU to 400 nm/RIU for different modes (from 1st to 5th) when 200 nm-thick analyte was applied. In addition, we measure different analytes including PMMA, air and water by micro-FTIR spectroscopy and observe significant peak shifts consistent with our simulation. Finally, we examined the detection length (i.e. thickness effect) for the SRR system. For lower modes, 1||, 2|| and 3|| modes, the shift of wavelengths saturates when the thickness increases from 200 to 500 nm, while no saturation effect is observed for higher modes ( 5||, 6||and 7||) even the thickness becomes 2 um. This valuable merit may be used for the analysis of activation-dependent cellular interactions that other label-free techniques like surface plasmon resonance (SPR) have not been used. For bio-sensing application, lower modes can be utilized to detect small molecules, adhesions or bio-events that happened on the cell membrane, in order to take advantage of the high sensitivity and also reduce noise from environment. Higher modes own micron scale detection region and can help us detect intracellular bio-events. In conclusion, the SRR-based optical sensor possesses various advantages beyond conventional optical sensors (e.g., fluorescent and Raman scattering techniques) such as label free, high sensitivity, real-time diagnosis. In addition, it also demonstrates further benefits compared with the-state-of-the-art techniques (e.g., surface plasmon polariton and localized surface plasmon resonance) such as coupler free for simple and inexpensive optical setup, great detection lengths (up to micron scale) to enable intracellular detection, and scalable working frequencies in particular in IR regimes to prevent photo damage to cells and to reserve the characteristic fingerprints of molecules) by choosing different resonant modes and sizes. As a consequence, the SRR-based optical sensors promise a real-time, operation frequency flexible and multi-mode solution for biological and chemical detection, drug delivery and other applications.