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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 Si3N4 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.

The extremely asymmetrical scattering (EAS) of bulk and guided electromagnetic waves in nonuniform periodic Bragg arrays with steplike variations of the grating amplitude is analyzed theoretically by means of a recently developed approach based on allowance for the diffractional divergence of the scattered wave. Arrays of finite and infinite widths are investigated. It is shown that, for thin nonuniform arrays, EAS has the same pattern as for uniform arrays with mean grating amplitude. On the contrary, for wide nonuniform arrays, the scattered wave amplitudes are well determined by local values of the grating amplitude. In this case, the energy of the scattered wave is shown to concentrate mainly in regions with smaller grating amplitude. The sensitivity of EAS to small imperfections of periodic arrays is investigated theoretically. The physical explanation of the observed effects is based on the diffractional divergence of the scattered wave.

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 (= 632.8 nm) for the target molecule rhodamine 6G for SERS.