D F Zaretsky

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

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?? 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 ?M 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.

Harmonic emission from cluster nanoplasmas subject to intense short laser pulses

Harmonic emission from cluster nanoplasmas subject to short intense infrared laser pulses is studied. In a previous publication [M. Kundu et al., Phys. Rev. A 76, 033201 (2007)] we reported particle-in-cell simulation results showing resonant enhancements of low-order harmonics when the Mie plasma frequency of the ionizing and expanding cluster resonates with the respective harmonic frequency. Simultaneously we found that high-order harmonics were barely present in the spectrum, even at high intensities. The current paper is focused on the analytical modeling of the process. We show that dynamical stochasticity owing to nonlinear resonance inhibits the emission of high-order harmonics.

Landau damping in thin films irradiated by a strong laser field

The rate of linear collisionless damping (Landau damping) in a classical electron gas confined to a heated ionized thin film is calculated. The general expression for the imaginary part of the dielectric tensor in terms of the parameters of the single-particle self-consistent electron potential is obtained. For the case of a deep rectangular well, it is explicitly calculated as a function of the electron temperature in the two limiting cases of specular and diffuse reflection of the electrons from the boundary of the self-consistent potential. For realistic experimental parameters, the contribution of Landau damping to the heating of the electron subsystem is estimated. It is shown that for films with a thickness below about 100 nm and for moderate laser intensities it may be comparable with or even dominate over electron–ion collisions and inner ionization.

High-order harmonic generation in clusters irradiated by an infrared laser field of moderate intensity

Extending the Lewenstein model of high-order harmonic generation (HHG) in a laser-irradiated atom, a model of HHG in a cluster is formulated. The constituent atoms of the cluster are assumed to be partly ionized. An electron freed through tunnelling may recombine either with its parent ion or with another ion in the vicinity. Harmonics due to the former process are coherent within the same cluster and may be coherent between different clusters, while harmonics due to the latter process are incoherent. Depending on the density of available ions, the incoherent mechanism may dominate the total harmonic yield, and the harmonic spectrum, which extends to higher energies, has a less distinct cutoff and an enhanced low-energy part.

Nonlinear excitation of the Mie resonance in a laser-irradiated cluster

The nonlinear collective electron dynamics inside a large heated cluster irradiated by a strong linearly polarized short laser pulse are considered in the approximation of an incompressible medium. When the incident radiation frequency is near three-photon resonance with the Mie frequency, the field inside the cluster exhibits a third harmonic with an amplitude comparable with that of the fundamental. In the same parameter range, due to shielding, the field inside the cluster at the fundamental frequency is strongly reduced with respect to the incident field. The presence of the third harmonic can lead to a strong enhancement of the production of multiply charged ions. Third-harmonic generation by a cluster under the same conditions is analyzed, too.

Third harmonic generation by small metal clusters in a dielectric medium

A simple analytical theory which treats the nonlinear properties of cold metal clusters embedded into a homogeneous isotropic dielectric medium is formulated. The theory is applied to the description of third-harmonic generation (THG) by such clusters irradiated by a laser field of moderate intensity. Competition between different mechanisms of THG and possible interference effects are discussed. A comparison with the recent experimental data shows that the size-dependent surface nonlinearity may dominate over the bulk effects only for small clusters with radii below 10 nm.

Classical modelling of the nonlinear properties of clusters in strong low-frequency laser fields

The nonlinear collective electron dynamics of a cluster irradiated by a strong near-infrared linearly polarized short laser pulse are studied by classical molecular-dynamics simulations for a small model cluster with ~103 particles. The model naturally yields all the features of a cluster exposed to a strong laser pulse, such as inner and outer ionization, expansion of the ion core, etc. When the frequency of the incident radiation is near three-photon resonance with the Mie frequency in the laser-ionized cluster, both the electron acceleration and the local electric field acting on the ions inside the cluster exhibit a third harmonic of the fundamental frequency. The time evolution during the laser pulse of the third harmonic of the total electron acceleration as well as of the electric field inside the cluster is discussed for different parameters of the laser?cluster interaction. The effect of ion motion is investigated. The presence of a strong non-resonant second harmonic in the local electric field at ion positions off the cluster centre is reported.

The gain of high harmonics in an atomic jet and in a hollow-core fiber

Amplification of high harmonics generated by atoms irradiated by a high-power laser is considered. The general expression for the gain is obtained as a function of the atomic target and the laser parameters. For high-harmonic generation in a fiber, the gain may be substantial. An experiment for the gain measurement is discussed.

Vlasov theory of Mie resonance broadening in metal clusters

Dynamical electromagnetic properties of metal clusters with radii R<10 nm have been theoretically considered in the dipole approximation in a jellium model taking into exact account the spatial dispersion of free electron gas. The Vlasov equation for the plasma electron distribution function inside a spherical cluster exposed to a low-intensity external electromagnetic field is solved analytically in the linear response approximation for the cases of both diffuse and mirror reflection of electrons from the cluster boundary. The solution found is used to obtain the non-local relation between the current density and the electric field inside the cluster, which is substituted into the Maxwell equations and boundary conditions for the electromagnetic field. In the diffuse reflection case the complete set of field equations is reduced to a single integral equation which has been solved numerically to obtain the distributions of both the electric field and the electron density inside the cluster, as well as the cluster dynamical electric polarizability and the photoabsorption and photoscattering cross sections. It has been found, from the cluster photoabsorption spectra calculated, that in the considered model the width of the dipole Mie resonance is mainly due to the electron reflection from the cluster boundary, it slightly depends on the permittivity 0 of the environment, and in the case of clusters in vacuum it is described with a good accuracy by the conventional expression (R) = +bvF/R with b1.0, vF being the electron Fermi velocity.

Harmonic modulation of impurity in gas flow as a method to study the semiconductor sensors

Abstract The method was developed, involving the harmonic modulation of impurity concentration in a gas flow, in order to characterize semiconductor sensor surfaces. The method is similar to any spectroscopic technique and consists of the analysis of the conductivity response of semiconductor sensor to the harmonic modulation of impurity introduced into the gas flow. The anomalous behavior of amplitude of the harmonic signal dependency on frequency of the driving concentration wave was found if the thick film tin dioxide sensors were used to analyze propane in air mixtures. The theoretical model of the process was suggested assuming two simultaneous reactions running concurrently with the time constants of the same order of magnitude, thus providing quasi-resonance dependence of conductivity in the frequency domain. The simulation was done based on the differential equation, obtained from a set of expressions, describing the tin dioxide surface reactions kinetics. The results of the simulation correspond to the experimental data obtained.