Dual-Tone Raman Study of Optical Picocavities

NanoParticles on mirror (NPoM) offer a well-controlled and self-assembled system to study light-matter interactions at the nanoscale enhanced by the localised surface plasmon resonances (LSPRs) supported by the nanostructures. We study NPoMs with a well-defined spacer formed by a monolayer of Biphenyl-4-thiol (BPT) molecules under simultaneous excitation with two continuous-wave lasers to gain further insight into the physics of picocavities forming within such nanostructures. Plasmonic picocavities are described as ångström-scale metal protrusions that enable confinement of light to single-molecule volumes. [1] The high field gradients formed by picocavities in the vicinity of the molecules lead to breaking of the Raman selection rules and appearance of additional lines in the Raman spectra. Since the various plasmonic quasi-normal modes each have their own distinct resonance frequency and near-field distribution [2] , the use of multiple laser wavelengths allows to sample the picocavity formation with very high spatial resolution. We implement this new method and excite a single NPoM simultaneously with two lasers (633 and 785 nm), and record anomalous picocavity events on the Stokes side of each laser ( Fig. 1 ). We find that the intensity of these events is strongly biased toward one laser or the other (e.g. in Fig. 1b ), consistent with their description as point-like perturbations triggered by the local field intensity. The same phenomena were seen with many different types of nanoparticles (including nanosphere, nanocubes and silica-coated nanospheres). This opens up a window of opportunities to fabricate and study novel structures which leverage the spectral properties of LSPRs to further enhance spatial sensitivity in plasmonic nanosystems.