Plasmonic Perfect Absorber for Refractive Index Sensing and SERS

A plasmonic perfect absorber is modeled, and its absorption/reflection properties are investigated in the visible frequency regime using finite-difference time-domain numerical simulation to show how refractive index can be measured accurately using dual interrogation technique (wavelength and intensity interrogations). Very high absorption (> 95 %) is observed in the spectral range below 625 nm. The reflection and absorption spectra show a crossover (at 50 %) in which red shifts depend upon the refractive index of the surrounding medium existing on the top of the structured surface. An equivalent of 530 nm red shift of the crossover point per refractive index unit (RIU) is calculated, which is a clear indication that the structure is suitable for refractive index sensing application. The spectral absorption and reflection are polarization insensitive and show perfect absorption (similar to 90 %) below 550 nm over a wide range of angle of incidence. The structure is proposed to be good for surface-enhanced Raman scattering (SERS) due to high near-field caused by perfect absorption.