A Large‐Area, Mushroom‐Capped Plasmonic Perfect Absorber: Refractive Index Sensing and Fabry–Perot Cavity Mechanism

In most plasmon resonance based sensor to date, only the surface of the sensor is accessible to the gas or liquid as the sensing target. In this work, an interferometric, lithographically fabricated, large‐area, mushroom‐capped plasmonic perfect absorber whose dielectric spacer is partially removed by a reactive‐ion‐etch process, thereby enabling the liquid to permeate into the sensitive region to a refractive index change, is demonstrated. Findings of this paper demonstrate experimentally and numerically that etching the spacer below the metamaterial resonator increases the spectral shift of the resonance wavelengths as the surrounding refractive index changes. The sensitivity and the figure of merit, as the measure of the sensor performance, are significantly improved. In this paper, it is shown that the plasmonic perfect absorber can be understood as a Fabry–Perot cavity bounded by a “resonator” mirror and metallic film, where the former exhibits a “quasi‐open” boundary condition and leads to the characteristic feature of subwavelength thickness.