Surface plasmon resonance (SPR) is an optical phenomena where impinging photons excite free electrons at metal surfaces near the interface with a dielectric. Increased accessibility to fabrication methods, optical characterization techniques and numerical modeling has allowed for advanced understanding of SPR and integration of SPR into applications (areas include solar energy conversion, high-resolution microscopy, analytical spectroscopy, drug design, diagnostic testing and more). SPR-based sensing methods have been shown to increase efficiency and sensitivity while decreasing response time as compared to non-SPR methods. For example, shifts in SPR peaks can be tracked in real time during adsorption and binding events due SPR sensitivity to local refractive index. This allows SPR to be used as a diagnostic tool or to find binding affinities, both of which are faster than a diagnostic ELISA test or pulldown assay for affinity testing. SPR can also couple to spectroscopic processes, such as Raman scattering, infrared (IR) absorption or fluorescence emission, to enhance these processes by orders of magnitude. Since SPR processes and enhancements are tied to the size, shape and wavelength dependence of the local electric field enhancement, there is a continual push to improve plasmonic responses. This study outlines a method of increasing the magnitude of the electric field enhancement through thin oxide film deposition.
For surface plasmons (SPs) excited on a thin metal film, SPR modes on either side of the metal couple and enhance when they occur at similar wavelengths. Since the wavelength of excitation is a function of the optical properties at the metal-dielectric interface, matching wavelengths can occurs when there are symmetric dielectric conditions across the thin layer (e.g. the same material is present on either side). The hypothesis of this study was that deposition of some thickness of a high refractive index (RI) material (n > 1.5), such as a high RI oxide, on the ambient side (n = 1) of the thin metal film will yield a matching dielectric condition with the substrate side (n = 1.5). To test our hypothesis initially, computational simulations were performed to find the optimum grating geometry and oxide thickness to achieve effective index matching. This exemplifies the use of analytical equations to find adequate periodicities for a desired purpose and subsequent computational simulation optimization of that plasmonic system.
For ease of fabrication, commercially available DVD-Rs served as the test bed for our experiments. A thin layer of silver and subsequently varying thicknesses of tin oxide (n = 1.7) were deposited onto the gratings. Optical transmission through the silver-on-grating (no tin oxide) schemes were seen to have a modest 6 fold transmission increase (compared to silver without the grating structure). After the deposition of tin oxide, an 80-100 fold transmission enhancement was observed. This enhancement exceeded the expectations set for this experiment.
Several means to validate the enhancement of our plasmonic system remains. Computationally, electric field profiles can be solved for our oxide enhanced sensors and compared to the electric field profile of no oxide film sensors. Experimentally, this system can be used to study the change in local refractive index or the enhancement in response for spectroscopic techniques discussed earlier. Both of these experimental schemes can be validated with computational simulations before experiments are conducted. Experimental applications aside, the order of magnitude spectral transmission enhancement shows validation of our original hypothesis and opens the realm of possibility for further advances in thin high refractive index oxide layer deposition.