The mechanical vibrations of individual gold nanodisks nanopatterned on a sapphire substrate are investigated using ultrafast time-resolved optical spectroscopy. The number and characteristics of the detected acoustic modes are found to vary with nanodisk geometry. In particular, their quality factors strongly depend on nanodisk aspect ratio (i.e., diameter over height ratio), reaching a maximal value of ≈70, higher than those previously measured for substrate-supported nano-objects. The peculiarities of the detected acoustic vibrations are confirmed by finite-element simulations, and interpreted as the result of substrate-induced hybridization between the vibrational modes of a nanodisk. The present findings demonstrate novel possibilities for engineering the vibrational modes of nano-objects.

Plasmonic nanocathodes offer unique opportunities for optically driving, switching, and steering femtosecond photocurrents in nanoelectronic devices and pulsed electron sources. However, angular photocurrent distributions in nanoplasmonic systems remain poorly understood and are therefore difficult to anticipate and control. Here, we provide a direct momentum-space characterization of multiphoton photoemission from plasmonic gold nanostars and demonstrate all-optical control over these currents. Versatile angular control is achieved by selectively exciting different tips on single nanostars via laser frequency or linear polarization, thereby rotating the tip-aligned directional photoemission as observed with angle-resolved 2D velocity mapping and 3D reconstruction. Classical plasmonic field simulations combined with quantum photoemission theory elucidate the role of surface-mediated nonlinear excitation for plasmonic field enhancements highly concentrated at the sharp tips (Rtip = 3.4 nm). We thus establish a simple mechanism for femtosecond spatiotemporal current control in designer nanosystems.

Hot carrier spatial and momentum distributions in nanoplasmonic systems depend sensitively on the optical excitation parameters and nanoscale geometry, which therefore determine the efficiency and functionality of plasmon-enhanced catalysts, photovoltaics, and nanocathodes. A growing appreciation over the past decade for the distinction between volumeand surface-mediated photoexcitation and electron emission from such systems has underscored the need for direct mechanistic insight and quantification of these two processes. Toward this end, we use angle-resolved photoelectron velocity mapping to directly distinguish volume and surface contributions to nanoplasmonic hot electron emission from gold nanorods as a function of aspect ratio, down to the spherical limit. Nanorods excited along their longitudinal surface plasmon axis exhibit surprising transverse photoemission distributions due to the dominant volume excitation mechanisms, as reproduced via ballistic Monte Carlo modelling. We further demonstrate a screening-induced transition from volume (transverse) to surface (longitudinal) photoemission with red detuning of the excitation laser and determine the relative cross-sections of the two mechanisms via combined volume and surface multiphoton photoemission modelling.Based on these results, we are able to identify geometry-and material-specific contributions to the photoemission cross-sections and offer general principles for designing nanoplasmonic systems to control hot electron excitation and emission distributions.

Time-resolved thermoplasmonics is emerging as the go-to technique for nanoscale thermal metrology. In this context, connecting the ultrafast optical response of nanoobjects to the correct thermal pathways is of paramount importance. We developed full thermo-optical models relating transient spectroscopy measurements, performed on metal nanoobjects in dielectric environments, to the overall system thermal dynamics. The models are applicable to small spherical nanoparticles embedded in a homogeneous matrix, following an analytical approach, and are expanded to include the cases of arbitrarily complex geometries and sizes relying on the finite-element method. These approaches are then exploited to rationalize several observations made in the context of previous time-resolved thermo-optical studies at the nanoscale. The present tools open the path for accurate retrieval of thermal parameters, notably the Kapitza resistance and the local environment thermal conductivity, from experiments. They also allow identifying the optimal parameters for selectively probing thermal dynamics of either a nanoobject or its nanoscale environment.

Single-particle optical spectroscopy methods have enabled quantitative investigations of the optical, electronic and vibrational responses of nano-objects in the recent years. In this work, single-particle pump-probe optical spectroscopy was exploited to investigate the cooling dynamics of individual gold nanodisks supported on a sapphire substrate. The measured time-resolved signals are shown to directly reflect the temporal evolution of the nanodisk temperature following its sudden excitation. The single-particle character of the experiments enables a quantitative analysis of the amplitudes of the measured time-resolved signals, allowing to rationalize their large probe wavelength-dependence. The measured cooling kinetics mainly depends on nanodisk thickness and to a much lesser extent on diameter, in agreement with numerical simulations based on Fourier law of heat diffusion, also accounting for the presence of a thermal resistance at the interface between the nanodisks and their substrate. For the explored diameter range (60-190 nm), the nanodisk cooling rate is limited by heat transfer at the gold-sapphire interface, whose thermal conductance can be estimated for each investigated nanodisk.

The plasmonic and vibrational properties of single gold nanodisks patterned on a sapphire substrate are investigated via spatial modulation and pump-probe optical spectroscopies. The features of the measured extinction spectra and time-resolved signals are highly sensitive to minute deviations of the nanodisk morphology from a perfectly cylindrical one. An elliptical nanodisk section, as compared to a circular one, lifts the degeneracy of the two nanodisk in-plane dipolar surface plasmon resonances, which can be selectively excited by controlling the polarization of the incident light. This splitting effect, whose amplitude increases with nanodisk ellipticity, correlates with the detection of additional vibrational modes in the context of time-resolved spectroscopy. Analysis of the measurements is performed through the combination of optical and acoustic numerical models. This allows us first to estimate the dimensions of the investigated nanodisks from their plasmonic response, and then to compare the measured and computed frequencies of their detectable vibrational modes, which are found in excellent agreement. This study demonstrates that single-particle optical spectroscopies are able to provide access to fine morphological characteristics, representing in this case a valuable alternative to traditional techniques aimed at post-fabrication inspection of subwavelength nanodevice morphology.

Dielectric coatings offer a versatile means of manipulating hot carrier emission from nanoplasmonic systems for emerging nanocatalysis and photocathode applications, with uniform coatings acting as regulators and nonuniform coatings providing directional photocurrent control. However, the mechanisms for electron emission through dense and mesoporous silica (SiO2) coatings require further examination. Here, we present a systematic investigation of photoemission from single gold nanorods as a function of dense versus mesoporous silica coating thicknesses. Studies with dense coatings on gold nanostructures clarify the short (∼1 nm) attenuation length responsible for severely reduced transmission through the silica conduction band. By contrast, mesoporous silica is much more transmissive, and a simple geometric model quantitatively recapitulates the electron escape probability through nanoscopic porous channels. Finally, photoelectron velocity map imaging (VMI) studies of nanorods with coating defects verify that photoemission occurs preferentially through the thinner regions, illustrating new opportunities for designing photocurrent distributions on the nanoscale.

When reducing the size of a material from bulk down to nanoscale, the enhanced surface-to-volume ratio and the presence of interfaces make the properties of nano-objects very sensitive not only to confinement effects but also to their local environment. In the optical domain, the latter dependence can be exploited to tune the plasmonic response of metal nanoparticles by controlling their surroundings, notably applying high pressures. To date, only a few optical absorption experiments have demonstrated this feasibility, on ensembles of metal nanoparticles in a diamond anvil cell. Here, we report a nontrivial combination between a spatial modulation spectroscopy microscope and an ultraflat diamond anvil cell, allowing us to quantitatively investigate the high-pressure optical extinction spectrum of an individual nano-object. A large tuning of the surface plasmon resonance of a gold nanobipyramid is experimentally demonstrated up to 10 GPa, in quantitative agreement with finite-element simulations and an analytical model disentangling the impact of metal and local environment dielectric modifications. High-pressure optical characterizations of single nanoparticles allow for the accurate investigation and modeling of size, strain, and environment effects on physical properties of nano-objects and also enable fine-tuned applications in nanocomposites, nanoelectromechanical systems, or nanosensing devices.