(198ad) Massive Enhancement of Optical Transmission across a Thin Metal Film Via Wave Vector Matching in Grating-Coupled Surface Plasmon Resonance

Investigations in grating coupled surface plasmon resonance often use only single mode resonances. In this study, we demonstrate the coupling of distinct surface plasmon resonance modes on opposite sides of a thin film metallic grating by varying the refractive index (RI) on either side of the grating. This coupling occurs by matching the resonance wavelengths, a function of the refractive index, across metal layer. In the first case, we deposited a high refractive index (RI) SnO₂ of increasing thickness to red shift the front side surface plasmon until it couples with the backside surface plasmon across a semi-transparent ~45 nm thin silver grating. Over an order of magnitude increase in transmission enhancement is shown, from 5-fold transmission enhancement through a bare silver grating referenced with flat silver to 100-fold enhancement after deposition of a tin oxide layer of appropriate thickness (~310 nm). This optical transmission enhancement is then probed through analytical experiments with liquids of various refractive indexes and computational studies. This matched system shows an increase in intensity sensitivity and decrease in wavelength sensitivity with respect to RI change. The deposited SnO₂ on the front side avert access of analytes to the enhanced electric field on the silver surface. Therefore, we also demonstrated an alternative way to couple the two plasmon modes. In the second case, a lower refractive index substrate was used to blue shift the back side surface plasmon and couple it to the front side surface plasmon by index matching using aque-ous solution of various RIs. For aqueous based matched system, the transmission enhancement increases from 3-fold for a bare silver grating to 13-fold for matched system.