Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

Abstract

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.

Probing Extended Modes on Disordered Plasmonic Networks by Wavefront Shaping

“…[16][17][18] Disordered semicontinuous metal films, in particular, exhibit intrinsic optical properties that strongly differ from those of bulk metals or ensembles of single nanoparticles, such as broadband absorption [19][20][21] and giant field fluctuations. [22][23][24] Previous experimental work has tried to quantify the spatial extent of these modes in the near-field, [25][26][27][28][29][30][31] where non radiative components (dark modes) dominate over radiative ones (bright modes). 29,32,33 It is not yet clear, therefore, to which extent the plasmonic modes of these systems can be actively controlled from the far-field by means of coherent control techniques, thus allowing for the dynamical reconfigurations of their optical fields at the nanoscale.…”

Hot-Electron Intraband Luminescence from Single Hot Spots in Noble-Metal Nanoparticle Films

“…Upon ultraviolet irradiation, luminescence of gold and silver surfaces is linear in excitation power and arises from the radiative recombination of d-band holes with relaxed sp-band electrons [2]. Under infrared excitation, non-linear up-converted broadband emission has been reported as background in surface-enhanced harmonic generation on rough silver surfaces [3] and was later shown to be spatially localized to regions of sub-diffraction hot spots [4]. With the excitation power-dependence being close to quadratic, this phenomenon has been mostly discussed in the context of conduction-band electrons recombining with d-band holes following two-photon absorption [2,5].…”

“…However, the wide interest in the plasmonic properties of metal nanoparticles in general and surface-enhanced spectroscopies on nanoparticle-decorated surfaces in particular has led to renewed scrutiny of the origins of nonlinear luminescence. While surface non-linear mixing in the presence of strong plasmonenhanced electric fields has been discussed as a mechanism for the generation of up-converted broadband emission [6], it is difficult to reconcile with the observed similarity between individual hot spot emission spectra [7] and their insensitivity to excitation wavelength [4], dielectric coating [8] and photoinduced surface charging effects [9]. Moreover, emission 3 intensity does not depend on pulse length up to 1ps in duration, but decreases above 1ps [10], pointing to a sequential excitation process affected by electron-phonon coupling.…”

“…For silver, the observed luminescence is further removed from the interband transition than for gold (transitions occur at ~4.0eV and ~2.4eV, respectively [22]): we therefore discuss luminescence from silver first, extending the results to gold after the emission mechanism is identified. Densely packed random silver particle films formed by the Tollens reaction exhibit continuum emission hot spots at a spatial density low enough to enable single-site spectroscopy [4].…”

Localized Plasmon in Silver Nanoparticles Synthesized by Microwave Radiation

“…A nano-metric material has dimensions under 100 nm with varied shapes like spheres, cubes, pyramids, rods, etc. This morphology, sizes and electric field intensities determine the Nps properties, each one with a characteristic plasmonic D DAVID PUBLISHING resonance [5][6][7][8]. Various methods have been implemented to synthesize NPs Ag.…”