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International audience; 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 the nanodisk thickness and to a much lesser extent on the 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.

International audience; 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 to be 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 postfabrication inspection of subwavelength nanodevice morphology.

Femtosecond time-resolved optical spectroscopy is emerging as the go-to technique to investigate the ultrafast thermal dynamics of nanometric objects. At this size scale, thermal interface resistance plays a major role, controlling the cooling dynamics of nanoparticles after initial impulsive excitation [1, 2]. In this context, connecting the ultrafast optical response of nano-objects to the correct thermal energy flow is of vital importance for quantitative retrieval of thermal parameters — such as the interfacial resistance and the local environment thermal conductivity — from experiments.

Fano resonances are central features in the responses of many systems including atoms, molecules, and nanomaterials. They arise as a consequence of interferences between two channels, most frequently associated with two system modes. In plasmonic materials, Fano interferences between optical modes have been shown, experimentally and theoretically, to induce narrow features in their scattering spectra. By investigating individual silver-gold heterodimers, we first experimentally demonstrate that Fano interference is also a key effect in the optical absorption of plasmonic nano-objects, in agreement with theoretical predictions. Conversely to previously investigated systems, the two interacting modes at the origin of absorptive Fano effect are mostly localized on either one or the other dimer component. Experimental results were obtained by selectively monitoring the optical absorption of one dimer component using a two-color nonlinear time-resolved technique. This also opens the way to full optical far-field noncontact investigations of charge or energy exchanges between nano-objects with a spatial resolution much smaller than the optical wavelength.

International audience; 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.

International audience; Tunable two-color ultrafast pump–probe experiments are performed on colloidal metal nanospheres in water and ethanol, and the results are interpreted through a model including thermal dynamics in both the nanoparticle and its environment. Transient optical signals measured for gold nanoparticles in ethanol display a complex temporal response strongly dependent on the wavelength of the probe beam. We demonstrate that its origin is related to a large contribution of liquid environment transient heating to the optical response, in contrast to the usual assumption that optical transmission changes reflect the evolution of nanoparticle temperature only. Accounting for this contribution enables an accurate measurement of thermal conductances at gold–ethanol and gold–water interfaces. Moreover, the thermal dynamics of the solvent can be selectively probed through a specific choice of probe wavelength.

The acoustic vibrations of single-metal and multi-material nanoparticles are studied by ultrafast pump–probe optical spectroscopy and described in the context of the continuous elastic model. The applicability of this model to the small size range, down to one nanometer, is discussed in the light of recent experimental data and ab initio calculations. Investigations of multi-material nano-objects stress the impact of the intra-particle interface on the characteristics of their vibrational modes, also yielding information on the composition and spatial distribution of the constituting materials.

International audience; Raspberry-like nano-objects made of large plasmonic satellites (>10 nm) covering a central dielectric particle have many potential applications as photonic materials, superlenses and (bio-) sensors, but their synthesis remains challenging. Herein, we show how to build stable and robust raspberry-like nano-systems with close-packed satellites, by combining monodisperse silica particles (80 or 100 nm diameter) and oppositely charged noble metal nanoparticles (Au or Ag) with well-defined sizes (10-50 nm). The spectral characteristics of their associated plasmonic resonances (wavelength, linewidth, extinction cross-section) and the electromagnetic coupling between satellites were observed using the spatial modulation spectroscopy technique and interpreted through a numerical model. The composite nano-objects exhibit numerous hot spots at satellite junctions, resulting in excellent surface-enhanced Raman scattering (SERS) performance. The SERS efficiency of the raspberry-like clusters is highly dependent on their structure.

International audience; Using time-resolved ultrafast pump-probe spectroscopy we investigated the electron-lattice energy transfer in small copper nanospheres with diameters ranging from 3.2 to 23 nm, either embedded in a glass or dispersed in a solvent. Electron-lattice scattering rate is shown to increase with size reduction, in agreement with our previous results obtained on gold and silver nanoparticles in the low excitation regime. We attribute this effect to the reduction of the screening efficiency of electron–phonon interactions close to the nanoparticle surface. To understand the discrepancy between the results on the electron-lattice scattering in different metals reported in the literature (reduction, no dependence or increase with nanoparticle size), we discuss the experimental conditions required for the accurate determination of electron-lattice energy transfer time from time-resolved investigations in the weak and strong excitation regimes and present power-dependent experiments on gold nanospheres in solution. Our findings are derived from a theoretical analysis based on the two-temperature model predictions and on a complete modeling of the nanoparticle transient extinction cross-section through the resolution of Boltzmann equation in the presence of hot electrons.

International audience; Modification of the plasmonic response of a metal nano-object due to interaction with a substrate is experimentally investigated measuring the quantitative optical extinction spectra of individual nano-objects with various elongated shapes (bipyramids and rods) deposited on a dielectric (silica) or absorbing (carbon) membrane. Apart from the expected dependence of the nanoparticle surface plasmon resonance (SPR) frequency on the nature of the substrate, large substrate and particle shape dependent modifications of its SPR width are demonstrated. These dependencies are ascribed to strong localization of the electromagnetic field associated with the longitudinal SPR of an elongated nano-object around its tips, leading to different interaction with the substrate depending on the particle shape and 3D orientation relative to the substrate. Both parameters have been precisely determined by electron tomography, permitting excellent reproduction of the experimental data. Experiments performed on silver-encapsulated bipyramids, whose shape evolves from a pyramidal one towards a cylindrical one, further confirm this effect.

International audience; The acoustic vibrations of metal nanoparticles encapsulated in a dielectric shell (Ag@SiO2) were investigated using a time-resolved femtosecond technique. The measured vibration periods significantly differ from those predicted for the bare metal cores and, depending on the relative core and shell sizes, were found to be either larger or smaller than them. These results show that the vibration of the whole core-shell particle is excited and detected. Moreover, vibrational periods are in excellent agreement with the predictions of a model based on continuum thermoelasticity. However, such agreement is obtained only if a good mechanical contact of the metal and dielectric parts of the core-shell particle is assumed, providing a unique way to probe this contact in multimaterial or hybrid nano-objects.

The optical response of hybrid metal-semiconductor nanoparticles exhibits different behaviors due to the proximity between the disparate materials. For some hybrid systems, such as CdS-Au matchstick-shaped hybrids, the particles essentially retain the optical properties of their original components, with minor changes. Other systems, such as CdSe-Au dumbbell-shaped nanoparticles, exhibit significant change in the optical properties due to strong coupling between the two materials. Here, we study the absorption of these hybrids by comparing experimental results with simulations using the discrete dipole approximation method (DDA) employing dielectric functions of the bare components as inputs. For CdS-Au nanoparticles, the DDA simulation provides insights on the gold tip shape and its interface with the semiconductor, information that is difficult to acquire by experimental means alone. Furthermore, the qualitative agreement between DDA simulations and experimental data for CdS-Au implies that most effects influencing the absorption of this hybrid system are well described by local dielectric functions obtained separately for bare gold and CdS nanoparticles. For dumbbell shaped CdSe-Au, we find a shortcoming of the electrodynamic model, as it does not predict the "washing out" of the optical features of the semiconductor and the metal observed experimentally. The difference between experiment and theory is ascribed to strong interaction of the metal and semiconductor excitations, which spectrally overlap in the CdSe case. The present study exemplifies the employment of theoretical approaches used to describe the optical properties of semiconductors and metal nanoparticles, to achieve better understanding of the behavior of metal-semiconductor hybrid nanoparticles. © 2011 American Chemical Society.

The time-domain nonlinear spectroscopy of a single silver nano-object is investigated by combining the spatial modulation linear spectroscopy technique with a high-sensitivity femtosecond pump-probe setup. The investigated single nanoparticle is first optically characterized for size and shape via quantitative measurement of its linear absorption spectrum, and time-resolved nonlinear pump-probe measurements are subsequently performed on the same particle. The breathing mode period of a single nanoparticle measured in the time domain is found to be in good agreement with that inferred by the optically deduced particle characteristics, opening the way of full optical investigation of the acoustic response of the characterized single nanoparticles. Copyright © 2011 John Wiley & Sons, Ltd.

International audience; The impact of quantum confinement on the width of the surface plasmon resonance of a metal nanoparticle is theoretically investigated in a model system formed by a silver nanosphere in different environments. Calculations are performed using the time-dependent local density approximation (TDLDA) for nanoparticle diameters up to about 11 nm, permitting precise quantification of the surface plasmon broadening due to size reduction. As expected, this is found to be inversely proportional to the particle diameter, but with an amplitude strongly depending on the environment (increasing by a factor of 4 when changing from vacuum to alumina). This is ascribed to the fact that damping is governed by the electronic surface spill-out (inherent in any quantum model) and thus strongly depends on the surface profile of the confining potential, that is, on the particle surface conditions.

We report the second harmonic response of non-centrosymmetric gold decahedra with four different sizes, (side length between 17 and 150 nm). We show that, for small sizes, symmetry cancellations occur, leading to quadratic hyperpolarizabilities similar to that of gold spherical nanoparticles despite the non-centrosymmetric shape of the particles. For larger sizes, the quadratic hyperpolarizabilities scale with the volume of the particles, as expected from the multipolar contribution arising from the field retardation effects.

The spectrally- and polarization-resolved absorption cross-sections of a single gold nanorod have been investigated using the spatial modulation spectroscopy technique. The ensemble of its optical features, that is, longitudinal and transverse surface plasmon resonances and interband absorption, has been quantitatively characterized. The results are compared with numerical simulations using the discrete dipole approximation and the finite element method, yielding information on the investigated nanorod size and shape.

The extinction spectrum of single silver nanoparticles with size ranging from 20 to 80 nm is investigated with the spatial modulation spectroscopy technique using either a tunable laser or a white lamp as the broadband source. Results are in good agreement with the prediction of the Mie theory, permitting to extract the nanoparticle size from the measured absolute value of the optical extinction cross-section. In contrast, the deduced refractive index of the nanoparticle environment and the reduction of the electron mean free path show a large dependence on the precise value of the bulk silver dielectric function.

The characterization of a single metal nanoobject by comparing its theoretical and experimental far-field spectra measured by a spatial modulation spectroscopy (SMS) technique is discussed in the case of gold and silver nanoparticles. Quantitative determination of the polarization dependent absorption cross-section spectrum of a single nanoparticle is shown to permit its optical identification, i.e., determination of its shape, size and orientation on a surface.

The ultrafast nonlinear optical response of a single metal nanoparticle is investigated by combining a high-sensitivity femtosecond pump-probe setup with a spatial modulation microscope. Experiments are performed on 20 and 30 nm silver nanospheres, in situ characterized via their optical linear extinction spectrum. The measured transient response permits investigation of the electron-phonon energy transfer time in a single nanoparticle. Its dependence on the electronic temperature is quantitatively interpreted using the two-temperature model.

The impact of the environment on the frequency and damping of the breathing acoustic mode of noble metal nanoparticle is discussed using the model of isotropic homogeneous elastic spheres embedded in an elastic medium. The results are compared to the experimental investigations performed in glass embedded silver nanoparticles and gold colloids using a time-resolved pump–probe technique.

International audience; Single-particle optical investigations have greatly improved our understanding of the fundamental properties of nano-objects, avoiding the spurious inhomogeneous effects that affect ensemble experiments. Correlation with high-resolution imaging techniques providing morphological information (e.g., electron microscopy) allows a quantitative interpretation of the optical measurements by means of analytical models and numerical simulations. In this topical review, we first briefly recall the principles underlying some of the most commonly used single-particle optical techniques: near-field, dark-field, spatial modulation and photothermal microscopies/spectroscopies. We then focus on the quantitative investigation of the surface plasmon resonance (SPR) of metallic nano-objects using linear and ultrafast optical techniques. While measured SPR positions and spectral areas are found in good agreement with predictions based on Maxwell's equations, SPR widths are strongly influenced by quantum confinement (or, from a classical standpoint, surface-induced electron scattering) and, for small nano-objects, cannot be reproduced using the dielectric functions of bulk materials. Linear measurements on single nano-objects (silver nanospheres and gold nanorods) allow a quantification of the size and geometry dependences of these effects in confined metals. Addressing the ultrafast response of an individual nano-object is also a powerful tool to elucidate the physical mechanisms at the origin of their optical nonlinearities, and their electronic, vibrational 2 and thermal relaxation processes. Experimental investigations of the dynamical response of gold nanorods are shown to be quantitatively modeled in terms of modifications of the metal dielectric function enhanced by plasmonic effects. Ultrafast spectroscopy can also be exploited to unveil hidden physical properties of more complex nanosystems. In this context, two-color femtosecond pump-probe experiments performed on individual bimetallic heterodimers are discussed in the last part of the review, demonstrating the existence of Fano interferences in the optical absorption of a gold nanoparticle under the influence of a nearby silver one.

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.

With the recent advances of experimental techniques, the nonlinear ultrafast optical response of metal nano-objects can now be investigated both on ensembles and on single nanoparticles. Its connection with the metal electronic and lattice kinetics is studied on the basis of a model describing the wavelength and time-dependent modifications of the object material dielectric function. Its application is illustrated in the case of single silver nanospheres and gold nanorods, as well as on ensembles of noble metal nanoparticles and metal-semiconductor nano-hybrids. This quantitative analysis also permits to elucidate the physical mechanisms at the origin of ultrafast nonlinearities in confined metals at different timescales.

Time resolved excitation and detection of the coherent vibrational motion of metal nanoparticles are discussed in the light of femtosecond pump-probe experiments performed in silver nanoparticles with radius ranging from 2.1 to 15.3 nm. Analysis of the phase of the observed sample absorption oscillations shows that coherent excitation is dominated by an indirect displacive mechanism due to subpicosecond heating of the lattice by fast electron-lattice energy transfer for large nanoparticles (R>10 nm). For smaller particles, the results suggest an additional contribution from direct coupling with the non-equilibrium electron gas. Both mechanisms, being related to an isotropic particle expansion, the fundamental radial mode is preferentially excited because of its better spatial matching with the excitation process. Optical control of the acoustic nanoparticles vibration is also demonstrated.

The ultrafast surface plasmon resonance nonlinearities and their connection with the conduction band electron dynamics are discussed in metal nanoparticles in the light of the results of high sensitivity femtosecond pump-probe experiments in silver nanoparticles embedded in a glass matrix. The optical response is interpreted in terms of frequency shift and broadening of the surface plasmon resonance and is related to the changes of the metal nanoparticle dielectric function induced by ultrafast perturbation of the electron distribution. Alteration of the interband absorption is found to be responsible for the observed red shift and very short time delay broadening of the surface plasmon resonance, in agreement with numerical simulations and with measurements in silver films. On a longer time scale, a new nonlinear mechanism due to increase of the electron scattering off the surfaces is demonstrated. This mechanism, specific to confined system, plays an important role in the ultrafast nonlinear optical response of small nanoparticles.

Using a femtosecond pump-probe technique, the fundamental mechanical radial mode of silver nanoparticles is coherently excited and probed via its interaction with the electron gas. The mechanical oscillations are launched by an indirect displacive process and are detected via the induced modulation of the surface plasmon resonance frequency. The measured fundamental radial mode period and damping time are found to be proportional to the nanoparticle radius in the range of 3–15 nm, in agreement with theoretical predictions.

Ultrafast relaxation dynamics of an athermal electron distribution is investigated in silver films using a femtosecond pump-probe technique with 18 fs pulses in off-resonant conditions. The results yield evidence for an increase with time of the electron-gas energy loss rate to the lattice and of the free electron damping during the early stages of the electron-gas thermalization. These effects are attributed to transient alterations of the electron average scattering processes due to the athermal nature of the electron gas, in agreement with numerical simulations.

International audience; 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.

International audience; The optical extinction response of individual Au–Ag@SiO2 heterodimers whose individual morphologies are determined by transmission electron microscopy (TEM) is investigated using spatial modulation spectroscopy. The extinction spectra show two resonances spectrally close to the surface plasmon resonances of the constituting Au and Ag@SiO2 core–shell particles. The interparticle electromagnetic coupling is demonstrated to induce a large increase of the optical extinction of the dimer around its Au-like surface plasmon resonance for light polarized along its axis, as compared to that for perpendicular polarization and to that of an isolated Au nanoparticle. For spherical particles, this interaction also leads to comparable shifts with light polarization of the two dimer resonances, an effect masked or even reversed for particles significantly deviating from sphericity. Both amplitude and spectral effects are found to be in excellent quantitative agreement with numerical simulations when using the TEM-measured dimer morphology (i.e., size, shape, and orientation of the individual dimers), stressing the importance of individual morphology characterization for interpreting heterodimer optical response.

The mechanisms at the origin of the ultrafast third-order optical nonlinearity of metallic and plasmonic materials are described focusing on the dominant resonant processes associated to energy absorption. Optical nonlinearity is discussed in terms of ultrafast modifications of the dielectric function of the constituting metal as a consequence of electron and lattice heating, using a bulk-like model. Based on this description, the time- and spectral-dependent nonlinear changes of the optical response of plasmonic materials due to interaction with a femtosecond light pulse are modeled. The results are illustrated in the case of an individual gold nanoparticle for different shapes (sphere, ellipsoid, and rod), and for an ensemble of gold nanoparticles dispersed in a dielectric matrix. The same approach can be extended to more complex plasmonic materials or meta-materials.

The relaxation dynamics of coherent LO phonons interacting with a photoexcited electronhole plasma is investigated in InP using an infrared time-resolved CARS technique. The plasma is created by two-photon absorption of one of the pulses used in the CARS experiment and measurements are performed as a function of its density. The results are interpreted in terms of coherent decay of the phonon-like LO phonon-plasmon hybrid mode, yielding information on the plasmon relaxation and free electron collision time.

International audience; 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.

International audience; The effects of the dielectric environment on the optical extinction spectra of gold nanorods were quantitatively studied using individual bare and silica-coated nanorods. The dispersion and amplitude of their extinction cross-section, dominated by absorption for the investigated sizes, were measured using spatial modulation spectroscopy (SMS). The experimental results were compared to calculations from a numerical model that included environmental features present in the measurements and the morphology and size of the corresponding nanorods measured by transmission electron microscopy. The combination of these experimental and theoretical tools permits a detailed interpretation of the optical properties of the individual nanorods. The measured optical extinction spectra and the extinction cross-section amplitudes were well reproduced by the numerical model for silica-coated gold nanorods, for which the silica shell provides a controlled environment. In contrast, additional environmental factors had to be assumed in the model for bare nanorods, stressing the importance of controlling and characterizing the experimental conditions when measuring the optical response of bare surface-deposited single metal nanoparticles.

International audience; Hybrid nano-objects formed by two or more disparate materials are among the most promising and versatile nanosystems. A key parameter in their properties is interaction between their components. In this context we have investigated ultrafast charge separation in semiconductor-metal nanohybrids using a model system of gold-tipped CdS nanorods in a matchstick architecture. Experiments are performed using an optical time-resolved pump-probe technique, exciting either the semiconductor or the metal component of the particles, and probing the light-induced change of their optical response. Electron-hole pairs photoexcited in the semiconductor part of the nanohybrids are shown to undergo rapid charge separation with the electron transferred to the metal part on a sub-20 fs time scale. This ultrafast gold charging leads to a transient red-shift and broadening of the metal surface plasmon resonance, in agreement with results for free clusters but in contrast to observation for static charging of gold nanoparticles in liquid environments. Quantitative comparison with a theoretical model is in excellent agreement with the experimental results, confirming photoexcitation of one electron-hole pair per nanohybrid followed by ultrafast charge separation. The results also point to the utilization of such metal-semiconductor nanohybrids in light-harvesting applications and in photocatalysis.

International audience; The optical properties of a series of gold nanoparticles (D ≈ 2.3−8 nm) grafted with a stabilizing polymer with a wide range of chain lengths (Mw ≈ 6.5K−29.5K) have been studied quantitatively in different dielectric solvents. Mie−Drude dipolar theory was applied to model the localized surface plasmon resonance (SPR) peak position, as well as the peak width of the absorption spectra, using the dielectric function of gold. The modeled spectra yielded information on the dielectric function of the polymer shell, ϵs, from which the average polymer concentration in the shell was deduced. Combining information from optical modeling and structural properties obtained from small-angle neutron scattering (SANS) on the polymer shell thickness and from transmission electron microscopy (TEM) on the gold core size, the SPR peak shifts and their attenuated sensitivity to solvent refractive index were characterized. The SPR behaviors for all of the gold colloids with different core sizes and graft chain lengths were thus expressed as a function of the effective polymer volume fraction, p,of the composite nanoparticle. It was found that sensitivity to the solvent dielectric property decreased until, for p > 0.9 (corresponding to an effective shell thickness on the order of the core radius), the SPR mode was "frozen" in by the polymer shell and lost sensitivity to the solvent.

International audience; The spectral characteristics (wavelength and line width) and the optical extinction cross-section of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanobipyramids have been quantitatively measured using the spatial modulation spectroscopy technique. The morphology of the same individual nanoparticles has been determined by transmission electron microscopy (TEM). The experimental results are thus interpreted with a numerical model using the TEM measured sizes of the particles as an input, and either including the substrate or assuming a mean homogeneous environment. Results are compared to those obtained for individual nanorods and also show the importance of the local environment of the particle on the detailed description of its spectral position and extinction amplitude.

International audience; 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.

International audience; The acoustic vibration modes of spherical composite nanoparticles formed by a core and an arbitrary number of shells are computed using a semianalytic approach based on elastic theory. The modes observed in time-resolved pump-probe experiments are identified in the case of metal-dielectric-based spherical composite nanoparticles, and results are illustrated in the case of Ag@SiO2 core-shell nanoparticles. The presence of a light dielectric shell is shown to only weakly shift the frequency of the dominant mode observed in time-resolved experiments. Moreover, a large impact of the mechanical contact between the different materials forming the particle on the vibrational mode frequency and damping is predicted, offering the possibility of experimentally addressing this parameter.

The dephasing time of the Z 3 exciton-polariton in CuCl is investigated using a new time resolved coherent technique. The measurements performed both as a function of polariton frequency and crystal temperature allow a precise determination of the redistribution channels of the initial energy between the various degrees of freedom of the crystal. The results show that relaxation is mediated by the acoustic phonons at low temperature and by the longitudinal optic phonons at high temperature ( T ⩾ 40 K ). The dephasing time of the longitudinal exciton is also measured and the results are compared to those obtained for the transverse mode.

International audience; The cooling dynamics of glass-embedded noble metal nanoparticles with diameters ranging from 4 to 26 nm were studied using ultrafast pump-probe spectroscopy. Measurements were performed probing away from the surface plasmon resonance of the nanoparticles to avoid spurious effects due to glass heating around the particle. In these conditions, the time-domain data reflect the cooling kinetics of the nanoparticle. Cooling dynamics are shown to be controlled by both thermal resistance at the nanoparticule–glass interface, and heat diffusion in the glass matrix. Moreover, the interface conductances are deduced from the experiments and found to be correlated to the acoustic impedance mismatch at the metal/glass interface.

International audience; 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.

International audience; The impact of spherical symmetry breaking and rystallinity on the response of the vibrational acoustic modes of a nanoobject in frequency- and time-domain experiments is investigated using the finite-element analysis method. The results show that introduction of shape anisotropy, i.e., evolution from a nanosphere to a nanorod, modifies the periods of the fundamental radial and quadrupolar modes and leads to activation of a quadrupolar-like mode in time-domain studies. In contrast, crystallinity is shown to have a negligible impact on the breathing mode frequency of a nanosphere formed by a cubic crystal and does not activate any quadrupolar mode. The effect of an anisotropic excitation in time-resolved measurements of a large nanosphere is also discussed.

The intrinsic dephasing time of an exciton polariton is investigated in the time domain by use of a time-resolved coherent two-photon excitation and probe technique. This method allows discrimination of the elementary interaction processes of the exciton polariton in semiconductors. The results obtained in CuCl show the role of the different phonons in the exciton-polariton dephasing

Ultrafast Kerr-type nonlinearities and relaxation dynamics of photoexcited electrons in silver nanocrystals embedded in glass have been investigated by means of femtosecond pump and probe spectroscopy. The transient absorption spectrum induced by the surface plasmon excitation shows a redshift and broadening of the surface plasmon band. The additional broadening is ascribed to the increase of surface plasmon damping and the redshift originates from a change in the real part of the dielectric function of the silver nanocrystals due to nonequilibrium electron heating. The observed redshift yields the nonlinear refractive index n2 of +2.4×10−10 esu and its time response is 1.9 ps.

Using a femtosecond pump-probe technique, we show that the fundamental mechanical radial mode of metal nanoparticles can be coherently excited and probed via its coupling with the electrons. The results permit determination of the frequency and damping of this mode as a function of the nanoparticle radius.

Optical absorption spectroscopy of a single metal nanoparticle is used to characterize its properties and to obtain quantitative information on its local environment. Experiments were performed using the spatial modulation spectroscopy (SMS) technique on 16 nm mean diameter gold nanoparticles embedded in different medium (i.e., deposited on glass or embedded in a polymer layer). Extraction of the nanoparticle characteristics and determination of the dielectric constant of its environment are discussed, focusing on the impact of the particle shape assumption. The refractive index of the local environment deduced from these measurements shows large particle-to-particle variation, yielding information about fluctuations of the dielectric properties of the surrounding medium on a nanometric scale, inaccessible in ensemble measurements. The influence of the environment of a nanoparticle on its optically extracted geometry and its surface plasmon resonance broadening by surface effect are also studied at a single-particle level.

The optical extinction spectra of single gold nanorods are investigated using a spatial modulation spectroscopy technique. The experimental results are compared to the computed spectra of nanoellipsoids using the dipolar approximation or a generalization of the Mie theory, focussing on the width of the longitudinal surface plasmon resonance. This is shown to be consistent with that deduced from the theoretical models using different available sets of dielectric constant for bulk gold, without introducing surface broadening effect. Extension of this approach to investigation of the ultrafast nonlinear optical response of a single gold nanorod is also discussed.

The near-field coupling between a gold and a silver spherical nanoparticle is investigated theoretically. Fano profiles are observed in the absorption cross section of the gold nanoparticle due to the coupling between the spectrally localized surface plasmon resonance of the silver nanoparticle and the continuum of interband transitions of the gold one. The effect of dimer internal characteristics (particle sizes and distance), surrounding medium (through the refractive index), and external excitation (polarization and propagation directions) are addressed. In particular, it is shown that the near-field coupling can be tuned from the weak to the strong regime by rotating the polarization direction, and that the Fano profiles are sensitive to the shadowing effect even for small particle sizes.

The absolute extinction cross-section spectrum of a single nanoparticle measured using spatial modulation spectroscopy (SMS) is correlated with its transmission electron microscopy (TEM) image. The sizes of quasi-spherical particles obtained by the two methods are found to be in good quantitative agreement. The possibilities opened up by combination of these methods for precise correlation of the optical response of a nanoobject with its geometry are illustrated in the case of a pair of interacting nanoparticles (dimer).

Using time-resolved pump-probe spectroscopy we have performed the first investigation of the vibrational modes of gold nanoshells. The fundamental isotropic mode launched by a femtosecond pump pulse manifests itself in a pronounced time-domain modulation of the differential transmission probed at the frequency of nanoshell surface plasmon resonance. The modulation amplitude is significantly stronger and the period is longer than in a gold nanoparticle of the same overall size, in agreement with theoretical calculations. This distinct acoustical signature of nanoshells provides a new and efficient method for identifying these versatile nanostructures and for studying their mechanical and structural properties., Comment: 5 pages, 3 figures