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

TL;DR

This paper demonstrates how the polarization and frequency of cylindrical vector beams can selectively excite specific modes in nanoparticle-on-mirror nanocavities, linking far-field input parameters to near-field mode characteristics.

Contribution

It introduces an experimental method to control and identify nanocavity modes using cylindrical vector beams, advancing understanding of mode-specific excitation mechanisms.

Findings

Mode selectivity depends on laser polarization and wavelength.

Transverse and longitudinal modes can be distinguished via confocal Raman maps.

Input coupling rates vary with laser parameters, affecting mode excitation.

Abstract

Nanocavities formed by ultrathin metallic gaps, such as the nanoparticle-on-mirror geometry, permit the reproducible engineering and enhancement of light-matter interaction thanks to mode volumes reaching the smallest values allowed by quantum mechanics. Although a large body of experimental data has confirmed theoretical predictions regarding the dramatically enhanced vacuum field in metallic nanogaps, much fewer studies have examined the far-field to near-field input coupling. Estimates of this quantity usually rely on numerical simulations under a plane wave background field, whereas most experiments employ a strongly focused laser beam. Moreover, it is often assumed that tuning the laser frequency to that of a particular cavity mode is a sufficient condition to resonantly excite its near-field. Here, we experimentally demonstrate selective excitation of nanocavity modes controlled by the polarization and frequency of the laser beam. We reveal mode-selectivity by recording fine confocal maps of Raman scattering intensity excited by cylindrical vector beams, which are compared to the known excitation near-field patterns. Our measurements allow unambiguous identification of the transverse vs. longitudinal character of the excited cavity mode, and of their relative input coupling rates as a function of laser wavelength. The method introduced here is easily applicable to other experimental scenarios and our results are an important step to connect far-field with near-field parameters in quantitative models of nanocavity-enhanced phenomena such as molecular cavity optomechanics, polaritonics and surface-enhanced spectroscopies.