The integration of metallic plasmonic nanoantennas with quantum emitters can dramatically enhance coherent harmonic generation, often resulting from the coupling of fundamental plasmonic fields to higher-energy, electronic or excitonic transitions of quantum emitters. The ultrafast optical dynamics of such hybrid plasmon-emitter systems have rarely been explored. Here, we study those dynamics by interferometrically probing nonlinear optical emission from individual porous gold nanosponges infiltrated with zinc oxide (ZnO) emitters. Few-femtosecond time-resolved photoelectron emission microscopy reveals multiple longlived localized plasmonic hot spot modes, at the surface of the randomly disordered nanosponges, that are resonant in a broad spectral range. The locally enhanced plasmonic near-field couples to the ZnO excitons, enhancing sum-frequency generation from individual hot spots and boosting resonant excitonic emission. The quantum pathways of the coupling are uncovered from a two-dimensional spectrum correlating fundamental plasmonic excitations to nonlinearly driven excitonic emissions. Our results offer new opportunities for enhancing and coherently controlling optical nonlinearities by exploiting nonlinear plasmonquantum emitter coupling.

The coherent exchange of optical near fields between two neighboring dipoles 1, 2, 3 plays an essential role for the optical properties, quantum dynamics and thus for the function of many naturally occurring 4, 5 and artificial 3,6,7,8,9 nanosystems. These interactions are inherently short-ranged, extending over a few nanometers only, and depend sensitively on relative orientation, detuning and dephasing, i.e., on the vectorial properties of the coupled dipolar near fields. This makes it challenging to analyze them experimentally.Here, we introduce plasmonic nanofocusing spectroscopy to record coherent light scattering spectra with 5-nm spatial resolution from a small dipole antenna, excited solely by evanescent fields 10,11,12 , and coupled to plasmon resonances in a single gold nanorod 13,14,15 . We resolve mode couplings, resonance energy shifts and Purcell effects as a function of dipole distance and relative orientation, and show how they arise from different vectorial components of the interacting optical near-fields.Our results pave the way for using dipolar alignment to control the optical properties and function of nanoscale systems.

We introduce zinc oxide (ZnO) functionalized porous gold nanoparticles that exhibit strong second harmonic (SH) emission due to an efficient coupling of localized surface plasmons to ZnO excitons. The nanosponges are perforated with a random network of 10 nm sized ligaments, localizing plasmons in a high density of hot spots. We use a broadband, few-cycle ultrafast laser to probe coherent nonlinear emission from individual bare gold and ZnO-functionalized sponges. While the third harmonic spectrum of the hybrid particles redshifts with respect to that of bare gold sponges, a distinct blueshift is seen in their SH spectra. SH emission around 390 nm, slightly below the ZnO band gap, is enhanced by 10×. We attribute this to doubly resonant plasmon−exciton interactions: the laser drives nanosponge plasmon hot spot resonances, and this locally enhanced field induces two-photon excitation of localized ZnO excitons. This opens a path toward the design of efficient coherent nonlinear optical sources by combining randomly disordered nanoantennas with semiconductor gain materials.

Porous nanosponges, percolated with a three-dimensional network of 10 nm sized ligaments, recently emerged as promising substrates for plasmon-enhanced spectroscopy and (photo)catalysis. Experimental and theoretical work suggests surface plasmon localization in some hot-spot modes as the physical origin of their unusual optical properties, but so far the existence of such hot-spots has not been proven. Here we use scattering-type scanning near-field nanospectroscopy on individual gold nanosponges to reveal spatially and spectrally confined modes at 10 nm scale by recording local near-field scattering spectra. High quality factors of individual hot-spots of more than 40 are demonstrated, predicting high Purcell factors up to 10. The observed field localization and enhancement make such nanosponges an appealing platform for a variety of applications ranging from nonlinear optics to strong-coupling physics.

We present and investigate a novel approach towards broad-bandwidth near-field scanning optical spectroscopy based on an in-line interferometer for homodyne mixing of the near field and a reference field. In scattering-type scanning near-field optical spectroscopy, the near-field signal is usually obscured by a large amount of unwanted background scattering from the probe shaft and the sample. Here we increase the light reflected from the sample by a semi-transparent gold layer and use it as a broad-bandwidth, phase-stable reference field to amplify the near-field signal in the visible and near-infrared spectral range. We experimentally demonstrate that this efficiently suppresses the unwanted background signal in monochromatic near-field measurements. For rapid acquisition of complete broad-bandwidth spectra we employ a monochromator and a fast line camera. Using this fast acquisition of spectra and the in-line interferometer we demonstrate the measurement of pure near-field spectra. The experimental observations are quantitatively explained by analytical expressions for the measured optical signals, based on Fourier decomposition of background and near field. The theoretical model and in-line interferometer together form an important step towards broad-bandwidth near-field scanning optical spectroscopy.