Coherent acoustic oscillations in metallic nanoparticles generated with femtosecond optical pulses

Abstract

Nonthermal generation of coherent-acoustic phonons is observed in metallic nanoparticles of tin and gallium, which are solid and liquid, respectively, at room temperature, by applying femtosecond pump-and-probe spectroscopy. Oscillations in the photo-induced differential reflection are clearly detected in the picosecond time domain. The measured period increases approximately linearly with size over a wide range and extracted sound velocities are consistent with those of bulk. Transition to ballistic regime in carrier excitation is also discussed on the basis of the disappearance of oscillations for particle sizes below hot-carrier mean free paths.

Photophysics and spectroscopy of metal particles

“…These modes change the particle size, which shifts the plasmon band because the peak position depends on the free electron density [15]. This affects both the transmission and reflection of light at frequencies near the plasmon resonance, causing a modulation in transient absorption [15][16][17] or transient reflectivity measurements [18]. The measured frequencies for our samples closely follow the predictions of classical mechanics calculations [19].…”

Thermomechanical behavior of surface acoustic waves in ordered arrays of nanodisks studied by near-infrared pump-probe diffraction experiments

“…In this regime the thermal population of the phonons in silicon is quenched and the thermal conductance can exhibit the signature of the quantization of the modes of the nanostructures [20,21]. It is important to underline that time-resolved experiments have already been performed on disordered metallic nanoparticles in an amorphous dielectric [22,23]. However, in these systems, the development of a detailed model of the laser-induced impulsive heating and the following relaxation process is very difficult, as a consequence of the not well-defined thermodynamic boundary conditions at the nanoparticles/dielectric interface.…”

Ultrafast time resolved x-ray diffraction, extended x-ray absorption fine structure and x-ray absorption near edge structure

“…[43][44][45] For thin films with thickness comparable to the optical absorption depth, a near homogeneous hot electron plasma is generated after the absorption of a femtosecond optical pulse which places the lattice at a stressed state, within a time that is shorter than the lattice response time. Under these conditions, coherent lattice vibrations can be generated [46][47][48] and in fact have been experimentally observed recently by electron diffraction in thin Al films. 47,49 In films thicker than the light penetration depth, the electron gas temperature generated by femtosecond optical pulse is nonuniform and has been predicted 50 that before electron--electron thermal equilibrium is established, the hot electron gas will interact with the surface to generate a "blast" force that exerts a "pressure" on the lattice in addition to the FIG.…”

“…Coherent phonons may also be excited and induce lattice vibrations 46 during the propagation of the acoustic and blast waves, and subsequently lattice expansion and contraction. The lattice vibrations generated by the propagation of the acoustic wave have been experimentally observed recently by electron diffraction in 20-nm-thin Al film (8 nm optical length); 47 Tentative theoretical studies based on the twotemperature model (TTM) and the Fermi-Pasta-Ulam anharmonic chain model have been used to explain the experimental coherent lattice vibrations.…”