Numerical optimization for the plasmonic Raman sensor including periodic hole arrays and tapering directions

Abstract

A plasmonic Raman sensor using periodic hole arrays was investigated numerically and experimentally. In previous work, we fabricated a hole array in a thin metal film on a dielectric substrate using focused ion beam lithography and succeeded in observing surface plasmon resonance. We demonstrated the effectiveness of hole shape dependency (i.e., cylindrical or tapered hole structures) for electric field enhancement, transmittance, and reflectance spectra obtained by numerical simulation using the finite-difference time-domain method. Those simulation results for an array of tapered holes agreed well with experimental results. Moreover, we numerically determined the optimized structure in terms of metal film thickness, tapered hole diameter, and hole period. However, optimal structure of a tapered hole array provides insufficient sensitivity (i.e., electric field enhancement) for measuring surface-enhanced Raman scattering. Therefore, we enhanced the electric field by using further structural parameters such as differs from tapered and incident light direction, which we expected to would give us a larger electric field. When the incident light coming from Si 3 N 4 side, the electric field enhancement was increased markedly. The electric field enhancement was more likely to be uninvolved in metal film thicknesses using 300 nm and 500 nm.