Numerical optimization of periodic hole arrays for plasmonic Raman sensor

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. Those experimental results agreed well with simulation results (for an array of cylindrical holes) obtained using the finite-difference time-domain method. However, a cylindrical hole array provides insufficient sensitivity (i.e., electric field enhancement) for measuring surface-enhanced Raman scattering (SERS). Therefore, we enhanced the electric field by using focusing holes (tapered structure), which we expected to would give us a larger electric field than the cylindrical holes. Furthermore, for a hole array, we optimized the structural design in terms of metal film thickness, hole diameter, and hole period on the basis of theoretical predictions. We successfully designed and fabricated an arbitrary localized surface plasmon resonance for the optimized array for the excitation wavelength (λ= 632.8 nm) for the target molecule rhodamine 6G for SERS.