Phase Singularity in Random Gold Metasurface for Refractive Index Sensing: Experimental Demonstration and Theoretical Analysis

Abstract

This study delves into the phenomenon of near-zero reflectance and phase singularity within a random array of gold nanoislands, employing a combination of theoretical and experimental approaches. The nanoislands are fabricated through the thermal dewetting of ultrathin gold films deposited via magnetron sputtering onto a glass substrate. Their morphology is characterized using scanning electron microscopy and atomic force microscopy. To unravel the plasmonic behavior, we conducted measurements of reflectance and phase spectra using a common-path interferometer in an attenuated total reflectance configuration. Our analysis reveals a partial state of topological darkness in the examined sample, characterized by near-zero reflection for the p polarization component. This discovery is further substantiated by the observation of phase singularity, as evidenced by abrupt ±π phase jumps in the differential phase spectra. Moreover, our study demonstrates the utility of measuring the phase and its derivative at resonance near the singularity for highly sensitive detection of changes in the bulk refractive index of the surrounding medium. The experimental findings are rigorously validated by comparing them with analytically calculated reflectance and phase spectra using the island film theory, yielding a good agreement.