Topological darkness in random gold metasurfaces for high sensitivity detection using phase interrogation

In this study, we investigate the phenomenon of nearly zero reflectance and associated phase singularity in a random array of gold nanoislands (Au NI) both theoretically and experimentally. The Au NI were produced via solid-state thermal dewetting of ultrathin gold films, which were magnetron sputtered onto a glass substrate. The morphology of the nanoislands was characterized using scanning electron microscopy and atomic force microscopy. To understand the plasmonic response of the random array of Au NI, we conducted reflectance measurements for both s and p polarized beams demonstrating the p. These measurements were performed using the attenuated total internal reflectance configuration. A partial state of topological darkness in a random array of Au NI was demonstrated by showing nearly zero reflection for the p polarization component. Additionally, we employed a common path spectral interferometer to measure the differential phase spectra. Our findings revealed that the differential phase spectra exhibited abrupt ±π phase jumps, indicating the presence of a phase singularity regime. Moreover, we demonstrated high bulk reflectance index sensitivity (RIS) within this regime. To validate our experimental results, we compared them with analytical reflectance and phase spectra obtained through the application of island film theory. The agreement between the experimental and theoretical predictions provided strong confirmation of our measurement technique.