The nonlinear wave transformation in nanoclusters immersed in a dielectric medium is considered. In nanoscale dimensions the interaction potential of the electron cloud with the ion core has nonlinear corrections which become zero when the cluster radius is increasing. Due to this corrections harmonics of a pump wave may be generated. The effectiveness of the wave transformation as a function of cluster parameters and pump wave intensity is estimated. The phase matching in the case of the third harmonic emission is taken into account. The Mie resonance enhancement of this harmonic is shown to be important. The applications of this effect are discussed.

High-order harmonic emission by an atom subject to the superposition of a strong infrared (IR) and a weak ultraviolet (UV) laser pulse is studied for the case when the UV photon energy remains below the ionization threshold. A simple analytical treatment is developed on the basis of the Lewenstein approach modified to account for the contribution of excited bound states of the atom. It is found that an order-of-magnitude enhancement of the high-harmonic emission rate can be expected, particularly in IR fields of moderate intensity. The results are discussed in view of recent experimental data and numerical simulations of the problem.

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.

The collective collisionless dynamics of the electron gas in free-electron nanofilms irradiated by an obliquely incident p-polarized laser wave are considered in the classical hydrodynamic and jellium-model approximations. The two cases of cold metallic nanofilms and hot free-electron nanofilms laser ionized and laser heated by a pump-laser prepulse are investigated with proper electron statistics. Both linear and nonlinear properties of the plasma resonance excitation in the nanofilms are studied in detail for different film parameters (film thickness, thickness of the diffuse film boundary, outer-ionization degree for hot laser-ionized/heated films, etc.). The significant role of the diffuse film boundaries for both linear absorption of the laser field and third-harmonic generation is demonstrated. For this goal, we do not use the standard dielectric-permittivity approach with boundary conditions between two different media but solve continuously over all space the full set of hydrodynamic and electrodynamic equations in nonrelativistic one-dimensional approximation. It is shown that collisionless edge absorption may be dominant in thin nanofilms, while in cold metal nanofilms it results in the appearance of several linear-absorption resonances below the bulk-plasma resonance frequency. For hot nanofilms, drastic broadening of the linear-plasma-resonance profile is obtained in calculations when the film thickness is reduced. In our model, the third-harmonic generation is determined by the density gradient in the diffuse film edges. Additional resonances in third-harmonic generation as a function of laser frequency are obtained for cold metal nanofilms. They differ from the standard third-order nonlinear resonance, which is located at one third of the plasma resonance frequency. The important role of the outer-ionization degree in forming the third-order nonlinear response of the hot laser-ionized film is also analyzed and discussed.

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.

Experimental and theoretical achievements in studies of atomic and metal clusters interacting with short intense infrared laser pulses are reviewed. The focus is made on the theoretical concepts describing the energy transfer from the laser field to the cluster nanoplasma and emission of laser harmonics from it. Both effects are considered assuming the collisionless regime, where the interaction of nanoplasma electrons with the self-consistent field dominates the individual collisions. The pivotal role of nonlinear resonances is underlined and described in detail. Possible ways for a further development of the theory and experimental perspectives are briefly discussed.

A method is proposed for fast and deep polarization of the system of hyperfine sublevels of the ground state of an atom having an optical excited state by means of two-component microwave pulses. The pulse of the bichromatic optical field that induces the transitions between the ground state and excited state of the atom is supposed to provide coherence among the hyperfine sublevels of the atomic ground state via the effect of coherent population trapping. The subsequent resonance microwave pulses create the polarization of equally populated ground state sublevels of the atom. The proposed polarization technique may be used for designing the new schemes of quantum computers, for the pulse transformation in optical experiments when light passes through a resonant medium containing rear-earth ions, as well as for producing polarized nuclear targets.

The rate of linear collisionless absorption of an electromagnetic radiation in a nanoplasma — classical electron gas localised in a heated ionised nanosystem (thin film or cluster) irradiated by an intense femtosecond laser pulse — is calculated. The absorption is caused by the inelastic electron scattering from the self-consistent potential of the system in the presence of a laser field. The effect proves to be appreciable because of a small size of the systems. General expressions are obtained for the absorption rate as a function of the parameters of the single-particle self-consistent potential and electron distribution function in the regime linear in field. For the simplest cases, where the self-consistent field is created by an infinitely deep well or an infinite charged plane, closed analytic expressions are obtained for the absorption rate. Estimates presented in the paper demonstrate that, over a wide range of the parameters of laser pulses and nanostructures, the collisionless mechanism of heating electron subsystem can be dominant. The possibility of experimental observation of the collisionless absorption of intense laser radiation in nanoplasma is also discussed.

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.

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 $\ensuremath{\sim}{10}^{3}$ particles. The model brings forth almost all of 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- or five-photon resonance with the (time-dependent) frequency of the dipole Mie-plasmon excitation in the laser-ionized and expanding cluster, both the total electron acceleration and the local electric field acting on the ions inside the cluster exhibit a resonant enhancement at the odd harmonics of the fundamental frequency. The time evolution during the laser pulse of these odd harmonics is discussed for different parameters of the laser-cluster interaction. The presence of even low-order harmonics (in particular, of the second) in the local electric field at ion positions off the cluster center is also demonstrated. This indicates nonlinear laser excitation of the quadrupole surface plasmon.

The rate of linear collisionless damping (Landau damping) in a classical electron gas confined to a spherical ionized cluster irradiated by an intense laser pulse is calculated. The expression for the energy absorbed by the cluster from the laser pulse is obtained in terms of the parameters of the self-consistent single-electron potential. It is explicitly calculated as a function of the electron temperature for the case when the self-consistent potential is modelled by an infinitely deep spherical rectangular well. On this basis the rate of collisionless heating and the width of the linear Mie resonance are estimated for realistic experimental parameters. Recommendations for unambiguous observation of Landau damping in laser-irradiated clusters are formulated.

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.

A new method is proposed for injecting hot ions into a magnetic trap, which is based on the Coulomb explosion of clusters ionised by radiation from a high-power femtosecond laser. The parameters of the trap required for the confinement of the hot plasma produced after the explosion of deuterium clusters are estimated. It is shown that the neutron yield in the d — d reaction in the trap can substantially exceed this yield directly in the laser beam focus.

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.

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.

A collective electron dynamics of a large cluster irradiated by strong linearly polarized IR laser pulses is considered. The nonlinear equation of center-of-mass motion of an electron cloud driven by a laser field is investigated for the particular case of a neutral cluster. The analysis of this equation demonstrates an appreciable presence of the third harmonic of the fundamental frequency in the internal electric field inside a cluster. It is shown that, for clusters with radii R = 100 Angstrom irradiated by a field with a peak intensity of I = 10(16) W/cm(2), the electric field strength of tripled fundamental frequency can be of the same order as the fundamental field strength. This result is applied for the interpretation of recent experimental data. In particular, the yield enhancement of both highly charged ions and X rays from clusters irradiated by a strong laser field, with respect to simple atomic species, is discussed. The cross section of third harmonic generation versus both the cluster and laser field parameters is analyzed.