Metal nanostructures provide extreme focusing of optical energy that is limited fundamentally by quantum tunneling. We directly probe the onset of the quantum tunneling regime observed by a sharp reduction in the local field intensity in subnanometer self-assembled monolayer gaps using third harmonic generation. Unlike past works that have inferred local limits from far-field spectra, this nonlinear measurement is sensitive to the near-field intensity as the third power. We calculate the local field intensity using a quantum corrected model and find good quantitative agreement with the measured third harmonic. The onset of the quantum regime occurs for double the gap size of past studies because of the reduced barrier height of the self-assembled monolayer, which will be critical for many applications of plasmonics, including nonlinear optics and surface enhanced Raman spectroscopy.

Recently, self-assembled monolayers (SAMs) have been used for plasmonic rulers to measure the nonlocal influence on the Au nanoparticle - metal film resonance wavelength shift and probe the ultimate field enhancement. Here we examine the influence of surface roughness on this plasmonic ruler in the nonlocal regime by comparing plasmonic resonance shifts for as-deposited and for ultra-flat Au films. It is shown that the resonance shift is larger for ultra-flat films, suggesting that there is not the saturation from nonlocal effects previously reported for the spacer range from 0.7 nm to 1.6 nm. We attribute the previously reported saturation to the planarization of the as-deposited films by thinner SAMs, as measured here by atomic-force microscopy. This work is of interest both in probing the ultimate limits of plasmonic enhancement with SAMs for applications in Raman and nonlinear optics, but also in the study of SAMs planarization as a function surface roughness.

We measure third harmonic generation (THG) as a function of infrared fundamental wavelength for gold nanoparticles over a gold film. An order of magnitude enhancement in THG is found at ~2.5 eV. We ensure that this enhancement is away from plasmonic resonances, so that it may be attributed directly to the ultrafast third-order susceptibility of gold. Using a simple relation between linear and third-order susceptibilities in conjunction with the linear response of gold, we show that the experimental results agree well with the enhancement from interband transitions. This result is interesting for potential applications leveraging telecom-band sources, such as third harmonic deep-tissue imaging with plasmonic nanoparticle markers.

Single-photon sources are required for quantum technologies and can be created from individual atoms and atom-like defects. Erbium ions produce single photons at low-loss fiber optic wavelengths, but they have low emission rates, making them challenging to isolate reliably. Here, we tune the size of gold double nanoholes (DNHs) to enhance the emission of single erbium emitters, achieving 50× enhancement over rectangular apertures previously demonstrated. This produces enough enhancement to show emission from single nanocrystals at wavelengths not seen in our previous work, i.e., 400 and 1550 nm. We observe discrete levels of emission for nanocrystals with low numbers of emitters and demonstrate isolating single emitters. We describe how the trapping time is proportional to the enhancement factor for a given DNH structure, giving us an independent way to measure the enhancement. This shows a promising path to achieving single emitter sources at 1550 nm.

A single step process of integrating a resonantly tuned silver nanoparticle into photonic crystal nanobeam cavities fabricated by focused ion beam milling is presented. Even though the quality factor of the cavities is reduced by a factor of 20, the emission peak at the cavity resonance is enhanced by 5-fold with respect to the cavities without the metal nanoparticle. The fluorescence is also compared before and after etching away the nanoparticle. Experimental quality factors and wavelength shifts are found to agree reasonably well with simulation values. These results are promising for future single photon emission studies involving the incorporation of quantum dot or NV center emitters into hybrid plasmonic/photonic crystal cavities for enhanced emission rates while retaining reasonably high quality factors.