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— The photoacoustic effect in liquids, generated by nanotransducers excited withshort laser pulses, offers high contrast imaging and promising medical treatment techniques.Understanding the role of the thermal boundary resistance (TBR) and the laser pulse durationin the generation mechanism of acoustic waves is essential to implement efficient photoacousticnanotransducers [1–3]. This work theoretically investigates, for the paradigmatic case ofwater-immersed gold nanoplates, gold-nanocylinders [2] and carbon nanotubes [3], the role of theTBR [4] and laser pulse duration in the competition between the launching mechanisms: thethermophone and the mechanophone. In the thermophone, the nanoparticle acts as a nanoheaterand the wave is launched by water thermal expansion. In the mechanophone, the nanoparticledirectly acts as a nanopiston. Specifically, for a laser pulse duration far exceeding the heat transfertime from the nanostructure to the surronunding water, the thermophone prevails. For theopposite case the mechanophone dominates. Activation of the mechanophone effect is sought totrigger few nanometers wavelength sound waves in water, matching the nanotransducer acousticfrequencies. This is at variance with respect to the commonly addressed case of water-immersednanotransducer excited with ns laser pulses, where only the thermophone effect significantly contributes.The present findings might be of impact in fields ranging from nanoscale non-destructivetesting to water dynamics at the meso to nanoscale. Furthermore, the results point to high-TBR,liquid-immersed nanoparticles as potentially efficient photoacoustic nanogenerators [5], with theadvantage of keeping the liquid environment temperature unaltered.REFERENCES1. Prost, A., F. Poisson, and E. Bossy, “Photoacoustic generation by a gold nano-sphere: Fromlinear to nonlinear thermoelastics in the long-pulse illumination regime,” Phys. Rev. B, Vol. 92,115450, 2015.2. Diego, M., M. Gandolfi, S. Giordano, F. Vialla, A. Crut, F. Vall´ee, P. Maioli, N. Del Fatti,and F. Banfi, “Tuning photoacoustics with nanotransducers via thermal boundary resistanceand laser pulse duration,” Appl. Phys. Lett., Vol. 121, 252201, 2022.3. Diego, M., M. Gandolfi, A. Casto, F. M. Bellussi, F. Vialla, A. Crut, S. Roddaro, M. Fasano,F. Vall´ee, N. Del Fatti, P. Maioli, and F. Banfi, “Ultrafast nano generation of acoustic wavesin water via a single carbon nanotube,” Photoacoustics, Vol. 28, 100407, 2022.4. Casto, A., F. M. Bellussi, M. Diego, N. Del Fatti, F. Banfi, P. Maioli, and M. Fasano, “Waterfilling in carbon nanotubes with different wettability and implications on nanotube/water heattransfer via atomistic simulations,” Int. J. Heat Mass Transf., Vol. 205, 123868, 2023.5. Yu, K., Y. Yang, J. Wang, G. V. Hartland, and G. P. Wang, “Nanoparticle-fluid interactionsat ultrahigh acoustic vibration frequencies studied by femtosecond time-resolved microscopy,”ACS Nano, Vol. 15, 1833–1840, 2021.

The measurement of acoustic vibrational modes is fundamental to the full comprehension of the mechanical properties of nano-objects, since they are related to the intrinsic characteristics of the material, such as crystallinity, size, shape, and elasticity. Furthermore, it has been shown that modifications in local environment can induce the appearance of new features in the inelastic light scattering spectra of nanoparticles that do not follow the usual selection rules. In this sense, optical spectroscopy techniques represent a powerful tool to study the vibrational modes of nano-objects and address both their intrinsic properties and their interaction with light, which plays a key role for applications in nanophotonics and optomechanics. In this contribution, we report low-frequency inelastic light scattering measurements on single gold nanocrystals, combined with detailed characterizations of their morphology and environment by transmission electron microscopy and tomography, and numerical simulations of their optical and vibrational responses. We demonstrate that lowfrequency Raman spectroscopy of single nanoparticles, a simple to implement and nondestructive technique, can be used to study the coupling between light and vibration at the nanoscale, by monitoring the changes in the internal electric field of the nanoparticle. We show that shape anisotropy and electric field inhomogeneities within a particle, induced by a local change in the particle environment, lead to the appearance of inelastic light scattering by acoustic modes that are not predicted by usual Raman selection rules, which are based on the (here invalid) assumption of a symmetric internal electric field.

Generation of ultra high frequency acoustic waves in water is key to nano resolution sensing, acoustic imaging and theranostics. In this context water immersed carbon nanotubes (CNTs) may act as an ideal optoacoustic source, due to their nanometric radial dimensions, peculiar thermal properties and broad band optical absorption. The generation mechanism of acoustic waves in water, upon excitation of both a single-wall (SW) and a multi-wall (MW) CNT with laser pulses of temporal width ranging from 5 ns down to ps, is theoretically investigated via a multiscale approach. We show that, depending on the combination of CNT size and laser pulse duration, the CNT can act as a thermophone or a mechanophone. As a thermophone, the CNT acts as a nanoheater for the surrounding water, which, upon thermal expansion, launches the pressure wave. As a mechanophone, the CNT acts as a nanopiston, its thermal expansion directly triggering the pressure wave in water. Activation of the mechanophone effect is sought to trigger few nanometers wavelength sound waves in water, matching the CNT acoustic frequencies. This is at variance with respect to the commonly addressed case of water-immersed single metallic nano-objects excited with ns laser pulses, where only the thermophone effect significantly contributes. The present findings might be of impact in fields ranging from nanoscale non-destructive testing to water dynamics at the meso to nanoscale.

International audience; To be Raman-active (or, more generally, detectable using optical spectroscopy techniques), a vibrational mode of a nanosystem has to modulate its optical response. For small, isolated nanospheres, this is the case for only two categories of vibrational modes, namely, quadrupolar and radial ones. However, assembling nanospheres as dimers makes additional modes Raman-active, as previously demonstrated by the detection in the ultralow frequency range of a hybridized quasi-translation mode in previous measurements on single and ensembles of gold nanosphere dimers. In the present work, we use our recently developed single-particle Raman spectroscopy setup to compare inelastic light scattering by single isolated and dimerized gold nanospheres in an extended frequency range (0–40 GHz). The Raman spectra acquired on isolated nanoparticles present a single peak associated with their fundamental quadrupolar mode, consistently with previous ensemble measurements. In contrast, the spectra measured on dimers are richer and display a number of peaks increasing with decreasing interparticle distance, with all l = 2–8 Lamb modes detected in the quasi-contact case. These observations are rationalized using a recently developed classical model of inelastic light scattering by nanospheres. Importantly, our modeling approach takes into account the real electric field within the nanoparticles (computed using standard or generalized Mie theories) instead of relying on the frequently used Born and quasistatic approximations. This ingredient appears decisive for reaching a qualitative understanding of the measured spectra, explaining in particular the dominance of the l = 2 quadrupolar mode for isolated spheres and the increasing contribution of higher-order modes for increasing electromagnetic interactions in nanosphere dimers.

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.

The coupling effects affecting the vibrations of two close nanostructures (e.g., two metal nanoplates or nanospheres separated by a thin dielectric layer) may considerably alter their vibrational eigenfrequencies, as demonstrated by several recent experimental studies. In this work, we present theoretical investigations of these coupling processes based on a continuum mechanics approach, considering various systems composed by two identical nanostructures mechanically coupled by a spacer made of a different material and computing their eigenfrequencies as a function of the spacer thickness. We first discuss the vibrations of stacked slabs, a one-dimensional problem which can be treated analytically. The more complex configurations of dimers of rods or spheres coupled by a finite cylindrical spacer are then treated numerically. In all cases, the frequency shifts occurring for thin spacers can be simply interpreted as a modification of the boundary conditions of the problem as compared to the single nanostructure case, while those predicted near specific spacer thicknesses are ascribed to an avoided crossing effect, happening when the individual building blocks of the dimers (nanostructures and spacer) present common eigenfrequencies.

International audience; Highly symmetrical gold nanocages can be produced with a controllable number of circular windows of either 2, 3, 4, 6 or 12 via an original fabrication route. The synthetic pathway includes three main stages: the synthesis of silica/polystyrene multipod templates, the regioselective seeded growth of a gold shell on the unmasked part of the silica surface and the development of gold nanocages by dissolving/etching the templates. Electron microscopy and tomography provide evidence of the symmetrical features of the as-obtained nanostructures. The optical properties of nanocages with 4 and 12 windows were measured at the single particle level by spatial modulation spectroscopy and correlated with numerical simulations based on finite-element modeling. The new multi-step synthesis approach reported here also allows the synthesis of rattle-like nanostructures through filling of the nanocages with a guest nano-object. With the potential to adjust the chemical composition, size and geometry of both the guest particle and the host cage, it opens new routes towards the fabrication of hollow nanostructures of high interest for a variety of applications including sensing devices, catalytic reactors and biomedicine. New concepts We demonstrate a new concept for making hollow nanoscale structures which are central to the advances in many current and emerging areas of technology. Nanocages are hollow and porous nanostructures. The ones made of metal are needed for optics, catalysis, biomedicine, and sensing. But, they are difficult to make. In particular, it is difficult to yield precise nanoscale control of the porosity as well as the composition. We address this challenge by combining inorganic colloidal synthesis and metal deposition on biphasic sacrificial templates. The single-particle spectroscopy and simulation confirm that our approach affords tight control over the morphology and porosity at the nanoscale. Previous approaches to making metal nanocages rely on galvanic replacement reactions and siteselective deposition. They offer control over morphology, but limited control over composition, porosity and scaleup. Our approach provides a simple and general strategy to circumvent these issues. It can be applied to a wide range of materials, and with further developement to any nanorattle-like nanostructures.

International audience; The sudden absorption of light by a metal nanoparticle launches a series of relaxation processes (internal thermalization, acoustic vibrations, and cooling) which induce a transient modification of its optical response. In this work, the transient optical response associated with the internal thermalization of a single gold nanodisk (occurring on a few picoseconds time scale) was quantitatively investigated by time-resolved spectroscopy experiments, and the measured signals were compared with a model accounting for the effects of both electron and ionic lattice heating. We show that experimental time-resolved signals at delays posterior to nanodisk excitation and electron gas thermalization can be simply interpreted as a combination of electron and lattice temperature evolutions, with probe wavelength-dependent weights. This demonstrates the possibility to selectively probe the electronic or lattice dynamics, through the choice of specific probe wavelengths. Additionally, the time-dependent spectral shape of transient extinction cross-section changes is shown to be successively dominated by the effects of electron and lattice heating, which present distinct spectral signatures.

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.

Two-pulse laser excitation of FePt nanolayers distinguishes ultrafast stresses of magnetic excitations from the Poisson effect., Invar-behavior occurring in many magnetic materials has long been of interest to materials science. Here, we show not only invar behavior of a continuous film of FePt but also even negative thermal expansion of FePt nanograins upon equilibrium heating. Yet, both samples exhibit pronounced transient expansion upon laser heating in femtosecond x-ray diffraction experiments. We show that the granular microstructure is essential to support the contractive out-of-plane stresses originating from in-plane expansion via the Poisson effect that add to the uniaxial contractive stress driven by spin disorder. We prove the spin contribution by saturating the magnetic excitations with a first laser pulse and then detecting the purely expansive response to a second pulse. The contractive spin stress is reestablished on the same 100-ps time scale that we observe for the recovery of the ferromagnetic order. Finite-element modeling of the mechanical response of FePt nanosystems confirms the morphology dependence of the dynamics.

International audience; 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.

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.

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. ispartof: Nano Letters vol:18 issue:8 pages:5159-5166 ispartof: location:United States status: published

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.

SSCI-VIDE+CDFA+ADM; International audience; Acoustic vibrations of small nanoparticles are still ruled by continuum mechanics laws down to diameters of a few nanometers. The elastic behavior at lower sizes (n(SR)m with a number n of atoms ranging from 10 to 102 (0.5–1.5 nm diameter range). Two periods, corresponding to fundamental breathing- and quadrupolar-like acoustic modes, are detected, with the latter scaling linearly with cluster diameters and the former taking a constant value. Theoretical calculations based on density functional theory (DFT) predict in the case of bare clusters vibrational periods scaling with size down to diatomic molecules. For ligand-protected clusters, they show a pronounced effect of the ligand molecules on the breathing-like mode vibrational period at the origin of its constant value. This deviation from classical elasticity predictions results from mechanical mass-loading effects due to the protecting layer. This study shows that clusters characteristic vibrational frequencies are compatible with extrapolation of continuum mechanics model down to few atoms, which is in agreement with DFT computations.

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.