The ultrafast optical nonlinearity of an optically characterized single gold nanorod is investigated around its surface plasmon resonance, by combining a far-field spatial modulation technique with a high sensitivity pump-probe setup. The spectrally and temporally dependent response is quantitatively interpreted in terms of the bulklike optical nonlinearity enhanced by the plasmonic effect. The plasmon resonance dynamics is shown to be mostly governed by nonequilibrium electron and phonon processes. Their contributions to the nonlinear optical response of a single metal nano-object are elucidated, and the latter is connected to the nonlinearities of ensembles.

The optical extinction spectra of single silver nanoparticles coated with a silica shell were investigated in the size range 10-50 nm. Measurements were performed using the spatial modulation spectroscopy technique which permits independent determination of both the size of the metal nanoparticle under study and the width of its localized surface plasmon resonance (LSPR). These parameters can thus be directly correlated at a single particle level for the first time. The results show a linear increase of the width of the LSPR with the inverse diameter in the small size regime (less than 25 nm). For these nanoparticles of well-controlled environment, this can be ascribed to quantum confinement of electrons or, classically, to increase of the electron surface scattering processes. The impact of this effect was measured quantitatively and compared to the predictions by theoretical models.

Developments of optical detection and spectroscopy methods for single nano-objects are key advances for applications and fundamental understanding of the novel properties exhibited by nanosize systems. These methods are reviewed, focusing on far-field optical approaches based on light absorption and elastic scattering. The principles of the main linear and nonlinear methods are described and experimental results are illustrated in the case of metal nanoparticles, stressing the key role played by the object environment, such as the presence of a substrate, bound surface molecules or other nano-objects. Special attention is devoted to quantitative methods and correlation of the measured optical spectra of a nano-object with its morphology, characterized either optically or by electron microscopy, as this permits precise comparison with theoretical models. Application of these methods to optical detection and spectroscopy for single semiconductor nanowires and carbon nanotubes is also presented. Extension to ultrafast nonlinear extinction or scattering spectroscopies of single nano-objects is finally discussed in the context of investigation of their nonlinear optical response and their electronic, acoustic and thermal properties.

No abstract

The dependence of the spectral width of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanorods protected by a silica shell is investigated as a function of their size. Experiments were performed using the spatial modulation spectroscopy technique that permits determination of both the spectral characteristics of the LSPR of an individual nanoparticle and its morphology. The measured LSPR is shown to broaden with reduction of both the nanorod length and its diameter, which is in contrast with the predictions of existing classical and quantum theoretical models. This behavior can be reproduced assuming the LSPR width linearly depends on the inverse of an effective length proportional to the square root of the particle surface with the same slope as that recently determined for silica-coated silver nanospheres.

The lifetimes of the acoustic vibrations of metal nanostructures depend sensitively on the properties of the environment, such as the acoustic impedance and viscosity. In order to accurately study these effects, they have to be separated from the damping processes that are inherent to the nanostructure. Here we show that this can be done experimentally by investigating individual gold nanowires suspended over a trench in air and liquid environments. The experiments were done by ultrafast pump-probe microscopy, recording transient absorption traces at the same point on the nanowire in both environments. These first experiments were performed with water, and the measured vibrational quality factors due to the presence of water were compared to continuum mechanics calculations for a cylinder in a homogeneous environment. Good agreement was found between the experimental quality factors and the calculated values. The continuum mechanics analysis shows that damping is dominated by the acoustic impedance of the solvent rather than by its viscosity for the nanowires in the present experiments. This experimental technique opens up the possibility of studying the effect of viscosity on the high frequency vibrational motions of nanostructures for a variety of liquids.

No abstract

International audienceKnowledge of the vibrational spectrum of metal clusters and nanoparticles is of fundamental interest since it is a signature of their morphology, and it can be used to determine their mechanical, thermodynamical, and other physical properties. It is expected that such a vibrational spectrum depends on the material, size, and shape of clusters and nanoparticles. In this work, we report the vibrational spectra and density of states of Au, Pt, and Ag nanoparticles in the size range of 0.5-4 nm (13-2057 atoms), with icosahedral, Marks decahedral, and FCC morphologies. The vibrational spectra were calculated through atomistic simulations (molecular dynamics and a normal-mode analysis) using the many-body Gupta potential. A discussion on the dependence of the vibrational spectrum on the material, size, and shape of the nanoparticle is presented. Linear relations with the nanoparticle diameter were obtained for the periods of two characteristic oscillations: the quasi-breathing and the lowest frequency (acoustic gap) modes. These linear behaviors are consistent with the calculation of the periods corresponding to the breathing and acoustic gap modes of an isotropic, homogeneous metallic nanosphere, performed with continuous elastic theory using bulk properties. Additionally, experimental results on the period corresponding to isotropic volume oscillations of Au nanoparticles measured by time-resolved pump-probe spectroscopy are presented, indicating a linear variation with the mean diameter in the size range of 2-4 nm. These, and similar results previously obtained for Pt nanoparticles with size between 1.3 and 3 nm, are in good agreement with the calculated quasi-breathing mode periods of the metal nanoparticles, independently of their morphologies. On the other hand, the calculated period of the mode with the highest (cutoff) frequency displays weak size and shape dependencies up to 4 nm, for all nanoparticles under study. In contrast with the behavior of other physicochemical properties, the clear consistency between experiments with atomistic and continuous media approaches resulting from this work indicates the existence of simple relations with size and weak dependence with the material and shape, for vibrational properties of metal nanoparticles