Time-domain investigation of the acoustic vibrations of metal nanoparticles: size and encapsulation effects

Towards chiral acoustoplasmonics

The possibility of creating and manipulating nanostructured materials encouraged the exploration of new strategies to control electromagnetic properties. Among the most intriguing nanostructures are those that respond differently to helical polarization, i.e., exhibit chirality. Here, we present a simple structure based on crossed elongated bars where light-handedness defines the dominating cross-section absorption or scattering, with a 200 % difference from its counterpart (scattering or absorption). The proposed chiral system opens the way to enhanced coherent phonon excitation and detection. We theoretically propose a simple coherent phonon generation (time-resolved Brillouin scattering) experiment using circularly polarized light. In the reported structures, the generation of acoustic phonons is optimized by maximizing the absorption, while the detection is enhanced at the same wavelength and different helicity by engineering the scattering properties. The presented results constitute one of the first steps towards harvesting chirality effects in the design and optimization of efficient and versatile acoustoplasmonic transducers.

Characterizing core-shell nanostructures through photoacoustic response based on theoretical model in the frequency domain

Core-shell nanostructures are widely used, and their photoacoustic (PA) properties are important for applications. However, the relations between their structural parameters and the properties of the PA spectrum are indirect because most theoretical models have been reported for them in the time domain. In this study, we develop a complete model in the frequency domain to analyze the PA response of core-shell particles. As in the case of solid spheres, the core-shell particles have pronounced resonant modes. The PA mode varies with the thickness of the shell and the radius of the core. Under single-pulse irradiation, PA signals of gold-silica nanospheres obtained by our theory agreed with those of the theory in the time domain and experiments. Under multi-pulse irradiation, the magnitude of the PA signals peaked whether the repeated excitation itself or its harmonic was equal to the PA mode. The structure could thus be monitored by the PA signals. These findings enrich PA theory and may inspire new techniques for the noninvasive characterization of nanoparticles.

Radiative Relaxation of Gold Nanorods Coated with Mesoporous Silica with Different Porosities upon Nanosecond Photoexcitation Monitored by Time-Resolved Infrared Emission Spectroscopy

Gold nanorods (AuNRs) have been widely used in photothermal conversion, and a coating of silica (SiO₂) provides higher thermal stability, better biocompatibility, and versatile chemical functionalization. In this work, two gold nanorods coated with surfactant-templated mesoporous silica layers of the same thickness but different porosities, and thus different specific surface areas, were prepared. Upon irradiation with 1064 nm nanosecond pulsed laser, the transient infrared emissions of AuNR@SiO₂ enveloped the stretching mode of the Si-O-Si bridge (1000-1250 cm^-1), the bending mode of adsorbed H₂O (1600-1650 cm^-1) within the mesoporous silica layer, and blackbody radiation, in terms of an underlying broad band (1000-2000 cm^-1) probed with a step-scan Fourier transform spectrometer. The mesoporous silica shell and the adsorbed H₂O gained populations of their vibrationally excited states, and the whole AuNR@SiO₂ was heated up via the photothermal energy of the core AuNRs. An average temperature after 5-10 μs within 80% of the emission intensity was ca. 200 °C. The decay of the emission at 1000-1250 and 1500-1750 cm^-1 was both accelerated, and the blackbody radiation components were negatively correlated with the porosity of the mesoporous silica layer. Higher porosity of the mesoporous silica layer was associated with more effective depopulation of the vibrationally excited states of the silica layers on the AuNRs via the nonradiative thermal conduction of the adsorbed H₂O, since H₂O has a larger thermal conduction coefficient than that of silica, in concomitance with the accelerated emission kinetics. This work unveils the roles of the porosity, capping materials, and entrapping molecules of a core-shell nanostructure during the thermalization after photoexcitation.

From Optical to Chemical Hot Spots in Plasmonics

In recent years, the possibility to induce chemical transformations by using tunable plasmonic modes has opened the question of whether we can control or create chemical hot spots in these systems. This can be rationalized as the reactive analogue of the well-established concept of optical hot spots, which have drawn a great deal of attention to plasmonic nanostructures for their ability to circumvent the far-field diffraction limit of conventional optical elements. Although optical hot spots can be mainly defined by the geometry and permittivity of the nanostructures, the degrees of freedom influencing their photocatalytic properties appear to be much more numerous. In fact, the reactivity of plasmonic systems are deeply influenced by the dynamics and interplay of photons, plasmon-polaritons, carriers, phonons, and molecular states. These degrees of freedom can affect the reaction rates, the product selectivity, or the spatial localization of a chemical reaction. In this Account, we discuss the oportunities to control chemical hot spots by tuning the cascade of events that follows the excitation and decay of plasmonic modes in nanostructures. We discuss a series of techniques to spatially map and image plasmonic nanoscale reactivity at the single photocatalyst level. We show how to optimize the reactivity of carriers by manipulating their excitation and decay mechanisms in plasmonic nanoparticles. In addition, the tailored generation of non-thermal phonons in metallic nanostructures and their dissipation is shown as a promise to understand and exploit thermal photocatalysis at the nanoscale. Understanding and controlling these processes is essential for the rational design of solar nanometric photocatalysts. Nevertheless, the ultimate capability of a plasmonic photocatalyst to trigger a chemical reaction is correlated to its ability to navigate through, or even modify, the potential energy surface of a given chemical reaction. Here we reunite both worlds, the plasmonic photocatalysts and the molecular ones, identifying different energy transfer pathways and their influence on selectivity and efficiency of chemical reactions. We foresee that the migration from optical to chemical hot spots will greatly assist the understanding of ongoing plasmonic chemistry.

Making waves: Radiation damping in metallic nanostructures

Metal nanostructures display several types of resonances. In the visible and near-IR spectral regions, there are localized surface plasmon resonances (LSPRs) that involve the coherent oscillation of the conduction electrons. Extended metal nanostructures, such as nanowires or nanoplates, also exhibit propagating surface plasmon polaritons (PSPPs), which are motions of the electrons at the surface of the structure that have a well-defined momentum. In addition, the vibrational normal modes of metal nanostructures give rise to low frequency resonances in the gigahertz to terahertz range. These different types of motions/resonances suffer energy losses from internal effects and from interactions with the environment. The goal of this perspective is to describe the part of the energy relaxation process due to the environment. Even though the plasmon resonances and acoustic vibrational modes arise from very different physics, it turns out that environmental damping is dominated by radiation of waves. The way the rates for radiation damping depend on the size of the nanostructure and the properties of the environment will be discussed for the different processes. For example, it is well known that for LSPRs, the rate of radiation damping increases with particle size. However, the radiation damping rate decreases with increasing dimensions for PSPPs and for the acoustic vibrational modes.

Crystal structure dependence of the breathing vibration of individual gold nanodisks induced by the ultrafast laser

The ultrafast laser-excited breathing vibrations of gold nanodisks with different crystal structures have been studied via atomistic simulations. The vibrational periods and damping rates of nanodisks are obtained by the analysis of the simulated transient responses of nanodisks. It is shown that the breathing period of nanodisks is considerably dependent on their crystal structure, which is contrary to the cases for the breathing vibration of metal nanospheres and nanorods. Furthermore, single-crystal nanodisks exhibit much lower intrinsic damping as compared with polycrystalline nanodisks, for which the additional energy dissipation by the grain boundaries in the polycrystalline nanodisks could be one major factor.

Ultrafast measurements of the dynamics of single nanostructures: a review

The ability to study single particles has revolutionized nanoscience. The advantage of single particle spectroscopy measurements compared to conventional ensemble studies is that they remove averaging effects from the different sizes and shapes that are present in the samples. In time-resolved experiments this is important for unraveling homogeneous and inhomogeneous broadening effects in lifetime measurements. In this report, recent progress in the development of ultrafast time-resolved spectroscopic techniques for interrogating single nanostructures will be discussed. The techniques include far-field experiments that utilize high numerical aperture (NA) microscope objectives, near-field scanning optical microscopy (NSOM) measurements, ultrafast electron microscopy (UEM), and time-resolved x-ray diffraction experiments. Examples will be given of the application of these techniques to studying energy relaxation processes in nanoparticles, and the motion of plasmons, excitons and/or charge carriers in different types of nanostructures.

Mechanical Vibrations of Atomically Defined Metal Clusters: From Nano- to Molecular-Size Oscillators

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 (<1-2 nm), where nanoparticles become molecular clusters made by few tens to few atoms, is still little explored. The question remains to which extent the transition from small continuous-mass solids to discrete-atom molecular clusters affects their specific low-frequency vibrational modes, whose period is classically expected to linearly scale with diameter. Here, we investigate experimentally by ultrafast time-resolved optical spectroscopy the acoustic response of atomically defined ligand-protected metal clusters Au _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.

Super-resolution imaging using nano-bells

In this paper we demonstrate a new scheme for optical super-resolution, inspired, in-part, by PALM and STORM. In this scheme each object in the field of view is tagged with a signal that allows them to be detected separately. By doing this we can identify and locate each object separately with significantly higher resolution than the diffraction limit. We demonstrate this by imaging nanoparticles significantly smaller than the optical resolution limit. In this case the "tag" we have used is the frequency of vibration of nanoscale "bells" made of metallic nanoparticles whose acoustic vibrational frequency is in the multi-GHz range. Since the vibration of the particles can be easily excited and detected and the frequency is directly related to the particle size, we can separate the signals from many particles of sufficiently different sizes even though they are smaller than, and separated by less than, the optical resolution limit. Using this scheme we have been able to localise the nanoparticle position with a precision of ~3 nm. This has many potential advantages - such nanoparticles are easily inserted into cells and well tolerated, the particles do not bleach and can be produced easily with very dispersed sizes. We estimate that 50 or more different particles (or frequency channels) can be accessed in each optical point spread function using the vibrational frequencies of gold nanospheres. However, many more channels may be accessed using more complex structures (such as nanorods) and detection techniques (for instance using polarization or wavelength selective detection) opening up this technique as a generalized method of achieving super-optical resolution imaging.

Controlling the Quality Factor of a Single Acoustic Nanoresonator by Tuning its Morphology

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.

Acoustic Mode Hybridization in a Single Dimer of Gold Nanoparticles

The acoustic vibrations of single monomers and dimers of gold nanoparticles were investigated by measuring for the first time their ultralow-frequency micro-Raman scattering. This experiment provides access not only to the frequency of the detected vibrational modes but also to their damping rate, which is obscured by inhomogeneous effects in measurements on ensembles of nano-objects. This allows a detailed analysis of the mechanical coupling occurring between two close nanoparticles (mediated by the polymer surrounding them) in the dimer case. Such coupling induces the hybridization of the vibrational modes of each nanoparticle, leading to the appearance in the Raman spectra of two ultralow-frequency modes corresponding to the out-of-phase longitudinal and transverse (with respect to the dimer axis) quasi-translations of the nanoparticles. Additionally, it is also shown to shift the frequency of the quadrupolar modes of the nanoparticles. Experimental results are interpreted using finite-element simulations, which enable the unambiguous identification of the detected modes and despite the simplifications made lead to a reasonable reproduction of their measured frequencies and quality factors. The demonstrated feasibility of low-frequency Raman scattering experiments on single nano-objects opens up new possibilities to improve the understanding of nanoscale vibrations with this technique being complementary with single nano-object time-resolved spectroscopy as it gives access to different vibrational modes.

Mechanisms of resonant low frequency Raman scattering from metallic nanoparticle Lamb modes

The low frequency Raman scattering from gold nanoparticle bimodal assemblies with controlled size distributions has been studied. Special care has been paid to determining the size dependence of the Raman intensity corresponding to the quadrupolar Lamb mode. Existing models based on a microscopic description of the scattering mechanism in small particles (bond polarizability, dipole induced dipole models) predict, for any Raman-active Lamb modes, an inelastic intensity scaling as the volume of the nanoparticle. Surprisingly experimental intensity ratios are found to be anomalously much greater than theoretical ones, calling into question this scaling law. To explain these discrepancies, a simple mechanism of Raman scattering, based on the density fluctuations in the nanoparticles induced by the Lamb modes, is introduced. This modeling, in which the nanoparticle is described as an elastic isotropic continuous medium-as in Lamb theory, successfully explains the major features exhibited by low frequency Raman modes. Moreover this model provides a unified picture for any material, suitable for handling both small and large size ranges, as well as non-resonant and resonant excitation conditions in the case of metallic species.

Acousto-Plasmonic Sensing Assisted by Nonlinear Optical Interactions in Bimetallic Au-Pt Nanoparticles

A strong influence of mechanical action in nonlinear optical transmittance experiments with bimetallic nanoparticles integrated by gold and platinum was observed. The nanostructured samples were synthesized by a sol-gel method and contained in an ethanol suspension. UV-VIS spectroscopy evaluations, Transmission electron microscopy studies and input-output laser experiments were characterized. A two-photon absorption effect was induced by nanosecond pulses at 532 nm wavelength with an important contribution from the plasmonic response of the nanomaterials. All-optical identification of acoustical waves was remarkably improved by optical nonlinearities. High sensitivity for instrumentation of mechano-optical signals sensing particular fluids was demonstrated by using a variable carbon dioxide incorporation to the system.

Interrogating Nanojunctions Using Ultraconfined Acoustoplasmonic Coupling

Single nanoparticles are shown to develop a localized acoustic resonance, the bouncing mode, when placed on a substrate. If both substrate and nanoparticle are noble metals, plasmonic coupling of the nanoparticle to its image charges in the film induces tight light confinement in the nanogap. This yields ultrastrong "acoustoplasmonic" coupling with a figure of merit 7 orders of magnitude higher than conventional acousto-optic modulators. The plasmons thus act as a local vibrational probe of the contact geometry. A simple analytical mechanical model is found to describe the bouncing mode in terms of the nanoscale structure, allowing transient pump-probe spectroscopy to directly measure the contact area for individual nanoparticles.

Picosecond optomechanical oscillations in metal-polymer microcavities

We experimentally study mechanical vibrations in planar Fabry-Perot microcavities made of metallic mirrors and a polymer spacer, using broadband pump-probe spectroscopy. These acoustic waves oscillate at a picosecond time-scale and result in spectral oscillations of the cavity transmission spectrum. We find that the oscillations are initiated at the metal mirrors and that their temporal dynamics match the elastic modes of the polymer layer, indicating that mechanical momentum is transferred within the structure. Such structures combine the strong optical absorption of metals with the elasticity and the processability of polymers, which open the road to a new class of optomechanical devices.

Breathing mode vibrations and elastic properties of single-crystal and penta-twinned gold nanorods

The acoustic vibrations of individual single-crystal and penta-twinned gold nanorods with widths from ∼7 to ∼26 nm are studied using atomic-level simulations and finite element calculations. It is demonstrated that the continuum model in the limit of an infinite rod length could be used to describe the breathing periods of nanorods with an aspect ratio as small as ∼2.5, in combination with bulk material elastic constants. The elastic moduli of gold nanorods are determined via their atomistically simulated extensional periods and the dispersion relation based on long-wavelength approximation. The twinned nanorods become stiffer as the width is reduced, which is in contrast to the size dependence of the modulus in single-crystal nanorods. Further finite element calculations for the breathing periods of nanorods are performed using isotropic elastic constants of bulk gold. We find that the breathing vibrations of the penta-twinned nanorods are more affected by the crystal structure effect than those of single-crystal nanorods, because a smaller range of crystal directions perpendicular to the long axis is involved in the breathing vibrations of twinned nanorods.

Mechanical Coupling in Gold Nanoparticles Supermolecules Revealed by Plasmon-Enhanced Ultralow Frequency Raman Spectroscopy

Acoustic vibrations of assemblies of gold nanoparticles were investigated using ultralow frequency micro-Raman scattering and finite element simulations. When exciting the assemblies resonantly with the surface plasmon resonance of electromagnetically coupled nanoparticles, Raman spectra present an ultralow frequency band whose frequency lies below the lowest Raman active Lamb mode of single nanoparticles that was observed. This feature was ascribed to a Raman vibration mode of gold nanoparticle "supermolecules", that is, nanoparticles mechanically coupled by surrounding polymer molecules. Its measured frequency is inversely proportional to the nanoparticle diameter and sensitive to the elastic properties of the interstitial polymer. The latter dependence as well as finite element simulations suggest that this mode corresponds to the out-of-phase semirigid translation (l = 1 Lamb mode) of each nanoparticle of a dimer inside the matrix, activated by the mechanical coupling between the nanoparticles. These observations were permitted only thanks to the resonant excitation with the coupling plasmon excitation, leading to an enhancement up to 10(4) of the scattering by these vibrations. This enhanced ultralow frequency Raman scattering thus opens a new route to probe the local elastic properties of the surrounding medium.

Imaging nano-objects by linear and nonlinear optical absorption microscopies

Absorption based microscopy measurements are emerging as important tools for studying nanomaterials. This review discusses the three most common techniques for performing these experiments: transient absorption microscopy, photothermal heterodyne imaging, and spatial modulation spectroscopy. The focus is on the application of these techniques to imaging and detection, using examples taken from the authors' laboratory. The advantages and disadvantages of the three methods are discussed, with an emphasis on the unique information that can be obtained from these experiments, in comparison to conventional emission or scattering based microscopy experiments.

Compressible Viscoelastic Liquid Effects Generated by the Breathing Modes of Isolated Metal Nanowires

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.