Transient Optical Picocavities Within Coupled Plasmonic Nanostructures
Plasmonic nanocavities, such as a nanometre scale gap between a gold nanoparticle and gold mirror (NPoM), confine light beyond the free space diffraction limit. While these enhanced field intensities allow resolvable measurements of vibrational scattering from only a few hundred molecules, ensemble averaging destroys all information on individual molecular local environments.
In this thesis, I first investigate the single molecule vibrational scattering from a molecule placed into NPoM using a DNA structure. The DNA complicates the response, which is time variant with transient features suggestive of possible picocavity formation. Picocavities are transient atomic scale features on the metal surfaces which further confine fields (effective volume <1 nm3) with strong field gradients that locally alter the rules for vibrational scattering efficiency. These can alter the spectral response of a single nearby molecule (isolating it spectrally) and were previously noted in NPoM at cryogenic temperatures.
I change the gap material to a molecular monolayer to simplify the system and explore room temperature picocavities. I use automated analysis of large experimental datasets to detect and isolate transient vibrational scattering. Picocavity generation is found to depend on the local chemical environment near the gold surface. Picocavities are observed to chemically interact with the molecule being optically probed. This perturbs bond strengths across the molecule with the strength and direction of this perturbation being highly sensitive to the relative picocavity location on a < 0.1Å scale. This single molecule – metal atom system is explored by comparing experimental data to a theoretical Density Function Theory model.
Next, I extract the spatial distribution of picocavity formation in the gap by comparing transient scattering at two simultaneous wavelengths of light. Picocavities are found to more likely form at regions of higher optical intensity within the NPoM gap. This suggests that light plays a direct role in the yet undetermined picocavity generation mechanism.
Engineering and Physical Sciences Research Council (1948687)