Nonresonant-Type Refractive Index Sensor Based on an Ultrasmall Plasmonic Cavity

We propose a nonresonant-type refractive index sensor based on a plasmonic cavity, consisting of high/low-indexes dielectric rod surrounded by silver. Surface plasmon polaritons with a wavelength of 1550 nm are confined in an ultrasmall high-index region of 200 × 200 × 200 nm³ by mode gaps of two low-index mirrors. The reflectivity of one mirror decreases as the index of the mirror made by analyte increases due to the reduction of the mode gap. The difference in the emission efficiencies through the two mirrors caused by the change in reflectivity depends directly on the index of the mirror. Therefore, the measurement of the output powers of the plasmonic cavity enables the estimations of the index of an analyte without using an external spectrometer or tunable laser.