The acoustic response of surface-controlled metal (Pt) nanoparticles is investigated in the small size range, between 1.3 and 3 nm (i.e., 75−950 atoms), using time-resolved spectroscopy. Acoustic ...vibration of the nanoparticles is demonstrated, with frequencies ranging from 1.1 to 2.6 THz, opening the way to the development of THz acoustic resonators. The frequencies, measured with a noncontact optical method, are in excellent agreement with the prediction of a macroscopic approach based on the continuous elastic model, together with the bulk material elastic constants. This demonstrates the validity of this model at the nanoscale and the weak impact of size reduction on the elastic properties of a material, even for nanoparticles formed by less than 100 atoms.
The dependence of the spectral width of the longitudinal localized surface plasmon resonance (LSPR) of individual gold nanorods protected by a silica shell is investigated as a function of their ...size. Experiments were performed using the spatial modulation spectroscopy technique that permits determination of both the spectral characteristics of the LSPR of an individual nanoparticle and its morphology. The measured LSPR is shown to broaden with reduction of both the nanorod length and its diameter, which is in contrast with the predictions of existing classical and quantum theoretical models. This behavior can be reproduced assuming the LSPR width linearly depends on the inverse of an effective length proportional to the square root of the particle surface with the same slope as that recently determined for silica-coated silver nanospheres.
The 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.
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 interaction of intense femtosecond pulses with metals allows for generating ultrashort hard x-rays. In contrast to plasma theories, tunneling from the target into vacuum is introduced as electron ...generation step, followed by vacuum acceleration in the laser field and re-entrance into the target to generate characteristic x-rays and Bremsstrahlung. For negligible space charge in vacuum, the Kα flux is proportional to the incident intensity and the wavelength squared, suggesting a strong enhancement of the x-ray flux by mid-infrared driving pulses. This prediction is in quantitative agreement with experiments on femtosecond Cu Kα generation.
Ultrafast structural dynamics in the condensed phase represents a key topic of current physics, chemistry and materials science. Femtosecond hard X-ray pulses are important structure probes that have ...been applied in time-resolved X-ray absorption and diffraction. Optical pump/X-ray probe schemes with compact laser-driven table-top sources have allowed for tiny changes of diffracted intensity to be measured with X-ray photon statistics, which has set the ultimate sensitivity limit. To address the strong quest for a higher X-ray flux, here we present the first hard X-ray plasma source driven by intense mid-infrared sub-100-fs pulses at 3.9 μm. The comparably long optical period allows for accelerating electrons from the Cu target to very high kinetic energies and for generating a characteristic Kα flux of 109 photons per pulse, 25 times more than with our 800 nm driver. Theoretical simulations account for the experimental results in a wide range of driving fields and predict a further enhancement of X-ray flux.
We study the quasi-instantaneous change of electron density in the unit cells of LiH and NaBH4 in response to a nonresonant strong optical field. We determine for the first time the related transient ...electron density maps, applying femtosecond x-ray powder diffraction as a structure probe. The light-induced charge relocation in NaBH4 exhibits an electron transfer from the anion (BH(4)(-)) to the Na(+) cation. In contrast, LiH displays the opposite behavior, i.e., an increase of the ionicity of LiH in the presence of the strong electric field. This behavior originates from strong electron correlations in LiH, as is evident from a comparison with quasiparticle band structures calculated within the Coulomb-hole-plus-screened-exchange formalism.
Linear and nonlinear magnetophotonic properties of periodic arrays of nickel nanodimers are governed by the interplay of the (local) optical response of individual nanoparticles and (nonlocal) ...diffraction phenomena. The redistribution of light intensity between diffracted beams when a diffraction order onsets or disappears is known as Wood's anomaly. Here, angular and magnetic-field-dependent near-infrared spectroscopic measurements, performed for different optical wavelengths and grating constants, discriminate between the linear and nonlinear excitation mechanisms of Wood's anomalies. In the nonlinear regime, evidenced by the magnetic second-harmonic generation, Wood's anomaly is characterized by an order-of-magnitude larger effect in intensity redistribution between the diffracted beams as compared to the linear case. The nonlinear Wood's anomaly manifests itself also in the nonlinear magnetic contrast highlighting the prospects of nonlinear magnetophotonics.
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