Laser-induced nonlinear excitation of collective electron motion in a cluster

Abstract

We consider the nonlinear collective electron dynamics inside a large cluster irradiated by a strong linearly polarized short (≲0.1 ps) laser pulse. The equation of the centre-of-mass motion of the electron cloud driven by the strong laser field is derived using the approximation of an incompressible medium. The analysis of this equation demonstrates the presence of odd harmonics of the fundamental frequency in the cluster dipole moment, and in both the internal and the scattered electric field. Both neutral and ionized clusters are considered. For clusters with radii R ≃ 100 Å irradiated by a femtosecond titanium–sapphire laser with a peak intensity of I ≃ 1016 W cm−2, the internal electric field strength near the tripled fundamental frequency is shown to be of the same order as the field of the fundamental. The reason is that both for metallic and for laser-ionized van der Waals clusters the Mie surface-plasmon energy ℏωM is around 5 eV, which is close to three times the energy of a titanium–sapphire laser-field quantum. On the other hand, the condition for first-order resonance with the Mie frequency is not met during the presence of the main laser pulse, but only temporarily, either at the first onset of inner ionization on the leading edge of the pulse (for van der Waals clusters) or during the subsequent Coulomb explosion. In both cases, the ion density is reduced. The presence of a strong third harmonic leads, in particular, to the enhanced production of multiply charged ions in clusters irradiated by a strong laser field, as compared with isolated atoms. This point is discussed in the light of recent experimental results on the production of multiply charged ions in laser–cluster experiments. Third-harmonic generation by a cluster in a strong laser field, as a function of both the cluster and the laser-field parameters, is also considered.

Landau damping in thin films irradiated by a strong laser field

“…The latter may become particularly important, when it is nonlinearly excited. This was recently proposed theoretically [19,20] and observed experimentally [21] in laser-irradiated clusters. In references [17,19], the width of the Mie resonance was estimated assuming that the frequency of electron collisions with the boundary is…”

“…Obviously, this energy is the incoherent sum of the contributions from the various halfperiods within the time τ [see equation (19) below]. In the linear approximation used in this paper, averaging over the initial conditions is equivalent with averaging over the phase α of the field (9) at the instant of time when the electron starts from the boundary, so that…”

“…For a spherical cluster, such equations are derived in references [25,19,26] ¶ Of course, the phase α has no physical relevance at this point. It could be removed by just resetting time.…”

Radiation forces between dust grains in a plasma

“…Finally it should be noted that the secondary radiation processes resulting from the nonlinear dynamical response of a dust particle and its neutralizing cloud to the incident radiation, such as harmonic generation [10,14], would give an additional contribution to the repulsive forces discussed here.…”

Quantum memory effects in the dynamics of electrons in gold clusters

“…This observable is the subject of many recent studies [44][45][46][47] . Experiments have shown 44,48,49 that the overall line width of the surface plasmon in clusters and nanospheres is large, on the order of 0.5-1eV.…”