Mode-Specific Coupling of Nanoparticle-on-Mirror Cavities with Cylindrical Vector Beams

Nanocavities formed by ultrathin metallic gaps permitthe reproducibleengineering and enhancement of light-matter interaction, withmode volumes reaching the smallest values allowed by quantum mechanics.While the enhanced vacuum field in metallic nanogaps has been firmlyevidenced, fewer experimental reports have examined the far-fieldto near-field input coupling under strongly focused laser beam. Here,we experimentally demonstrate selective excitation of nanocavity modescontrolled by the polarization and frequency of the laser beam. Wereveal mode selectivity by recording confocal maps of Raman scatteringexcited by cylindrical vector beams, which are compared to the knownexcitation near-field patterns. Our measurements reveal the transversevs longitudinal polarization of the excited antenna mode and how theinput coupling rate depends on laser wavelength. The method introducedhere is easily applicable to other experimental scenarios, and ourresults help connect far-field with near-field parameters in quantitativemodels of nanocavity-enhanced phenomena.