Field Of Intense Laser Pulse Research Articles

On the radiation losses during motion of an electron in the field of intense laser radiation

Motion of the relativistic electron in the field of intense laser pulse of the arbitrary shape is considered. The pulse dimension is supposed to be of the order of the Gaussian laser beam dimension in the focal plane. It is supposed that the pulse is propagating along the external constant magnetic field. In the paraxial approximation the corrections of the first order to the vectors of the field of radiation as well as the force of the radiation friction are taken into account. Averaged relativistic equations of motion of electron are obtained with the help of averaging over the fast oscillations of the laser radiation. It is shown that with taking into account corrections of the first order to the field vectors an averaged force arises. This force is defined by pulsed character of radiation and proportional to the intensity but not to gradient of intensity. It is shown that radiation losses are of little importance in the transverse plane but may considerably act on the longitudinal motion of electron.

Dynamic stabilization of Na atom in an intense pulsed laser field**Project supported by the National Natural Science Foundation of China (Grant Nos. 11465016, 11664035, and 11764038).

We investigate theoretically the ionization properties of the valence electron for the alkali metal atom Na in an intense pulsed laser field by solving numerically the time-dependent Schrödinger equation with an accurate l-dependent model potential. By calculating the variations of the ionization probabilities with laser peak intensity for wavelengths ranging from 200 nm to 600 nm, our results present a dynamic stabilization trend for the Na atom initially in its ground state (3s) and the excited states (3p and 4s) exposed to an intense pulsed laser field. Especially a clear “window” of dynamic stabilization at lower laser intensities and longer wavelengths for the initial state 4s (the second excited state) is found. By analyzing the time-dependent population distributions of the valence electron in the bound states with the different values of principal quantum number n and orbital quantum number l, we can attribute the dynamic stabilization to the periodic population in the low-excited states since the valence electron oscillates rapidly between the lowly excited states and the continuum states.

Emission Spectrum of an Atom in the Field of Intense Ultrashort Laser Pulse

The spectral probability densities of the spontaneous emission and the generation of harmonics by a hydrogen atom in the field of intense laser pulse are calculated. A previously proposed approach is used, which is based on the Kramers−Henneberger transformation for the wave function of time-dependent Schrodinger equation and expansion in the unperturbed atomic eigenstates and the photon states. The spectral range up to the tenth harmonic is investigated with the laser pulse duration ranging from 6 to 12 optical cycles and the peak intensity varied from 1 to 10 TWcm−2.

Induced transparencies in metamaterial waveguides doped with quantum dots

The light-mater interaction in quantum dots doped artificial electromagnetic materials such as metamaterial waveguides has been studied. The effect of surface plasmon polaritons (SPPs) on the absorption coefficient of quantum dots in metamaterial waveguides is investigated. The waveguides are made by sandwiching a metamaterial slab between two dielectric material layers. An ensemble of quantum dots are deposited near the waveguide interfaces. The transfer matrix method is used to calculate the SSPs in the waveguide and the density matrix method and Schrödinger equation method are used to calculate the absorption spectrum. It is found that when the thickness of the metamaterial slab is greater than the SPP wavelength the SPP energy is degenerate. However when the thickness of the slab is smaller than that of the SPP wavelength the degeneracy of SPP state splits into odd and even SPP modes due the surface mode interaction (SMI) of the waveguide. We also found that the absorption spectrum has a minima (transparent state) which is due to strong coupling between excitons in quantum dots and SPPs in the waveguide. This transparent state is called the SPP induced transparency. However when the thickness of the slab is smaller than that of the SPP wavelength one transparent state in the absorption spectrum split into two transparent states due to the surface mode interaction. This type of transparency is called the SMI induced transparency. Transparent states can be achieved by applying pulse stress field or an intense laser pulse field. Hence present findings can be used to fabricate the metamaterial optical sensors and switches.

Nonresonant Compton scattering in an intense pulsed laser field

The influence of an intense pulsed light wave on nonresonant scattering of a photon by an electron is investigated in the approximation when the pulse width is considerably greater than the characteristic time of wave oscillation. The partial differential cross-sections for different numbers of absorption (emission) wave photons are analyzed for a range of external field intensities I ∼ 1017 W cm−2. The general relativistic expression for the nonresonant cross-section of laser-modified Compton scattering summed over all possible partial processes is derived. It is shown that for nonrelativistic energies the pulse character of the external field significantly affects the process.

Optical tunability of magnetic polaron stability in single-Mn doped bulk GaAs and GaAs/AlGaAs quantum dots

Optical control of magnetic property of a magnetic polaron (MP) in Mn-doped bulk GaAs and GaAs/AlGaAs quantum dots (QDs) have been studied. We have developed basis optimization technique for the method of linear combination of atomic orbitals (LCAOs), which significantly improve the accuracy of the conventional LCAO calculation. We have demonstrated that a monochromatic, linearly polarized, intense pulsed laser field induces a collapse of the MP and an ionization of Mn-acceptor in Mn-doped GaAs materials due to a dichotomy of hole wave function. We find this optical tunability of MP stability can be adjusted by confinement introduced in GaAs QDs.

Intense laser field effects on p – d exchange interaction in single manganese doped GaAs

We have developed a comprehensive theory about optical control of p – d exchange interaction between spins of hole and Mn2+ in single-manganese doped GaAs material irradiated by a monochromatic, linearly polarized, intense pulsed laser field (PLF) under nonresonant conditions. The p – d exchange interaction leads to formation of magnetic polaron. While the PLF induces a dressed acceptor Coulomb potential, which transforms single center problem into the one with two virtual positively charged centers, resembling hydrogen molecule ion (H2+). The dichotomy of hole wave functions, determined by the laser-intensity, affects strongly the p – d exchange interaction as well as binding energy of magnetic polaron. Increasing the laser intensity reduces the magnetic polaron binding energy. At larger excitation intensity, the magnetic polaron can be completely dissolved.

Evolution of the H2 + electron wavepacket under magnetic and electric fields of ultrashort intense laser pulse

Magnetic interaction was included in the simulation of the evolution of the electron wave-packet of the hydrogen molecular ion H2+ in femtosecond intense pulsed laser fields applied along the molecular axis. This evolution was followed by solving 2-D time-dependent Schrodinger equation at some fixed inter-nuclear separations. Magnetic interaction effects at non-relativistic intensities induced a phase shift in the time evolution of the electron wave-packet, and an excess z-component angular momentum as compared with the results obtained in the absence of magnetic interaction. Furthermore, the H2+ electron WP displacement showed a drift and wiggling in the propagation direction which was different from that observed under pure electric field of the laser pulse. The local fluxes at different points of the 2-D space borders and the time-dependent induced angular momentum are calculated and analyzed.

HD+in a beam in intense pulsed laser fields: Dissociation and ionization with high-energy resolution of the fragments

The isotopically mixed hydrogen molecular ion ${\text{HD}}^{+}$ was investigated in intense pulsed laser fields with high, i.e., vibrational resolution of the fragments. An ion beam of ${\text{HD}}^{+}$ with a translational energy of 11.1 keV was exposed to femtosecond laser pulses (100 fs) of intensities in the range from ${10}^{13}--{10}^{15}\text{ }\text{W}/{\text{cm}}^{2}$. Fragments from the two dissociation channels ${\text{HD}}^{+}\ensuremath{\rightarrow}\text{H}+{\text{D}}^{+}$, and ${\text{HD}}^{+}\ensuremath{\rightarrow}\text{D}+{\text{H}}^{+}$, as well as the Coulomb explosion channel, ${\text{HD}}^{+}\ensuremath{\rightarrow}{\text{H}}^{+}+{\text{D}}^{+}+{e}^{\ensuremath{-}}$, were projected onto a two-dimensional multichannel plate detector to measure their velocity distributions. The fragments from the three channels were spatially clearly distinguished because of the different velocities of H $({\text{H}}^{+})$ and D $({\text{D}}^{+})$ fragments. Fragments from most of the populated vibrational states of ${\text{HD}}^{+}$ can be discerned as well-resolved peaks in the speed and angular distributions. The typical light induced potential (LIP) effects such as bond softening and level shifting already observed in the studies of ${\text{H}}_{2}^{+}$ and ${\text{D}}_{2}^{+}$ are here also observed. In addition to these one-photon peaks, also peaks due to three- (net two-) and finally also to direct two-photon absorption, typical of the asymmetric ${\text{HD}}^{+}$, were found in the vibrationally resolved fragment spectra of the dissociation channels. Besides the fragments from one-photon bond softening, also fragments formed by two- and three-photon processes were found to have narrow angular distributions, in contrast to the approximately ${\text{cos}}^{2}$ distribution of fragments from vibrational levels above the LIP crossing being fragmented by classical dissociation. The relative probability of the two dissociation channels was investigated, addressing the question of the higher electron affinity to the proton or the deuteron during the dissociation, but could not be decided because of too low accuracy here. The Coulomb explosion channel was also clearly discerned by its fragment velocities as well as the narrow angular distributions of ${\text{H}}^{+}$ and ${\text{D}}^{+}$ fragments, which is quite analogous to those found in the cases of ${\text{H}}_{2}^{+}$ and ${\text{D}}_{2}^{+}$.

Dynamic characteristics of electrons in high-order corrected fields of ultrashort laser pulses

Highorder correction to the fields of ultrashort, tightlyfocused laser pulses expressed in power series of ε=1/(ω0t0) and s=1/(k0w0)（ω0=ck0 the central oscillatory frequency, t0 the pulse duration, w0 the beam waist radius), are derived. Based on paraxial approximation, the firstorder correction terms to the Gaussian pulses are explicitly given. Their corrections of amplitude and phase, are found to be related to the variable of ε. Applying them to the study of the electron dynamics in the intense laser pulse field, we found that as long as ω0t0>20, the zerothorder approximation (long pulse approximation) is adequate for describing the interaction. For ω0t0<20, higherorder corrections have to be taken into account.

Photon-absorption-induced intersubband optical-phonon scattering of electrons in quantum wells

In the presence of a normally incident intense mid-IR pulsed laser field, both photon-absorption-induced intrasubband and intersubband phonon-scattering of electrons are found by including the photon-assisted phonon-scattering process in a Boltzmann equation for phonon energies that are smaller than the energy separation between two electron subbands in a quantum well. The ultrafast dynamics of the electron distributions for different subbands is studied with various lattice temperatures, photon energies, field strengths, and quantum-well widths. Upward steps found in the differences between the electron distributions with/without the photon-assisted process are attributed to either photon-absorption-induced phonon scattering of electrons via intrasubband transitions or the photon-absorption-induced phonon scattering of electrons via intersubband transitions in quantum wells. The photon-absorption-induced phonon absorption by intersubband transitions of electrons from the first to the second subband is a unique feature in quantum-well systems and is found to have a significant effect on the electron populations in both subbands.

Optimization of high-order harmonic generation by genetic algorithm and wavelet time-frequency analysis of quantum dipole emission

We present an ab initio three-dimensional quantum study of the coherent control of high-order harmonic generation ~HHG! processes in intense pulsed laser fields by means of the genetic algorithm optimization of the laser-pulse amplitude and phase. Accurate time-dependent wavefunction and HHG power spectrum are obtained by the time-dependent generalized pseudospectral method and wavelet transform is used to obtain the dynamical phase associated with the dipole-emission time profile. It is shown that ‘‘intra-atomic’’ dynamical phase matching on the sub-optical cycle, attosecond, time scale can be achieved, leading to nearly perfect constructive interference between different returning electronic wave packets and marked improvement in both emission intensity and purity of a given harmonic order. The study of coherent control of atomic and molecular processes is a subject of much current interest in science and technology @1#. In particular, temporally shaped ultrashort laser pulses have been successfully used to design welldefined wave packets@2#, and to control multiphoton absorption @3# and chemical reactions @4#, etc. In the area of the interaction of atoms with intense-laser-pulse, high-order harmonic generation ~HHG! of orders as high as 300 has been observed @5,6#, with photon energies in excess of 500 eV. A novel concept of ‘‘intra-atomic’’ phase matching has been recently introduced, allowing the enhancement of the intensity of a specific high harmonic @7#. It is shown that by carefully tailoring the shape of the temporal profile of the driving laser pulse, one can control the time evolution of the electron response to the intense laser field on a sub-opticalcycle, attosecond time scale @7#. A semiclassical model @8,9# is used to interpret these results in terms of the constructive interference in the frequency domain between different electron trajectories @10#, but the effect of atomic structure is not considered. In this Rapid Communication, we present a fully ab initio quantum treatment of the coherent control and enhancement of high-harmonic emission by means of the genetic algorithm ~GA! optimization @11# of the laser-pulse shape and intra-atomic phase matching. We show that by combining the GA search algorithm and accurate quantum solution of the

Multiphoton ionization and high-order harmonic generation of He, Ne, and Ar atoms in intense pulsed laser fields: Self-interaction-free time-dependent density-functional theoretical approach

We present a detailed study of the multiphoton ionization and high-order harmonic generation (HHG) processes of rare-gas atoms (He, Ne, and Ar) in intense pulsed laser fields by means of a self-interaction-free time-dependent density-functional theory (TDDFT) recently developed. The time-dependent exchange-correlation potential with proper short- and long- range potential is constructed by means of the time-dependent optimized effective potential (TDOEP) method and the incorporation of an explicit self-interaction-correction (SIC) term. The TDOEP-SIC equations are solved accurately and efficiently by the time-dependent generalized pseudospectral technique. In this study, all the valence electrons are treated explicitly and nonperturbatively and their partial contributions to the ionization and HHG are analyzed. The results reveal instructive and qualitatively different behavior from each subshell orbital. Moreover, we found that the HHG yields from Ne and Ar atoms are considerably larger than that of the He atom in strong fields. Three main factors are identified for accounting the observed phenomena: (a) the binding energy of the subshell valence electron, (b) the orientation of the valence electron orbital (with respect to the electric-field polarization), and (c) the effect of multiphoton resonant excitation. In particular, we found that the ${\mathrm{np}}_{0}$ valence electrons (in Ne and Ar) with lowest binding energies and electron orbital orientation parallel to the electric-field direction, make the dominant contributions to both ionization and HHG processes in sufficiently strong fields.

Self-interaction-free time-dependent density-functional theory for molecular processes in strong fields: High-order harmonic generation ofH2in intense laser fields

We present a self-interaction-free time-dependent density-functional theory (TDDFT) for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. The time-dependent exchange-correlation (xc) energy potential with proper short- and long-range potential is constructed by means of the time-dependent optimized effective potential (OEP) method and the incorporation of an explicit self-interaction-correction (SIC) term. The resulting time-dependent OEP/SIC equations are structurally similar to the time-dependent Hartree-Fock equations, but include the many-body effects through an orbital-independent single-particle local time-dependent xc potential. A numerical time-propagation technique is introduced for accurate and efficient solution of the TDDFT/OEP-SIC equations for two-center diatomic molecular systems. This procedure involves the use of a generalized pseudospectral method for nonuniform optimal grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split-operator scheme in the energy representation for the time development of the electron orbital wave functions. High-precision time-dependent wave functions can be obtained by this procedure with the use of only a modest number of spatial grid points. The theory is applied to a detailed study of high-order harmonic generation (HHG) processes of ${\mathrm{H}}_{2}$ molecules in intense pulsed laser fields. Particular attention is paid to the exploration of the spectral and temporal structures of HHG by means of the wavelet time-frequency analysis. The results reveal striking details of the spectral and temporal fine structures of HHG, providing new insights regarding the detailed HHG mechanisms in different energy regimes.

Probing the spectral and temporal structures of high-order harmonic generation in intense laser pulses

We present an ab initio three-dimensional quantum study of high-order harmonic generation ~HHG! of atomic H in intense pulsed laser fields. Accurate time-dependent wave functions are obtained by means of the time-dependent generalized pseudospectral method recently developed and wavelet transform is used to perform time-frequency analysis of the resulting HHG power spectra. The results reveal striking details of the spectral and temporal fine structures of HHG, providing insights regarding HHG mechanisms in different energy regimes and benchmark data for testing the validity of existing HHG models. PACS number~s!: 42.65.Ky, 32.80.Wr, 42.50.Hz Recently a great deal of attention has been devoted to the study of multiple high-order harmonic generation ~HHG! processes in intense short laser pulses @1‐3#. Besides its fundamental interest for strong-field atomic and molecular physics, the HHG provides a potential tunable coherent light source in the extreme ultraviolet ~XUV! region, a so-called ‘‘tabletop synchrotron’’ @2#. Further, the HHG may lead to a promising way for generating subfemtosecond ~attosecond! pulses of radiation of high frequency @4,5#. For optimal control of the HHG processes, it is essential to have a thorough understanding of the spectral and temporal structures of the high-order harmonics and the detailed underlying mechanisms for HHG. The first experimental observation of the temporal coherence of HHG has recently been reported @6#. In this Rapid Communication, we present an ab initio threedimensional ~3D! precision quantum calculation of the time

Multiple high-order harmonic generation in the presence of intense laser and static magnetic fields

We perform a nonperturbative study of the effect of static magnetic fields on the multiple high-order harmonic generation (HHG) processes of atomic H in intense pulsed laser fields. The time-dependent Schrödinger equation in three spatial dimensions can be solved accurately and efficiently by means of the recently developed time-dependent generalized pseudospectral method (Tong X M and Chu S I 1997 Chem. Phys. 217 119). We found that the presence of a static magnetic field perpendicular to the laser field polarization direction can introduce a second HHG plateau, allowing significant extension of the cut-off. The high harmonics in the second plateau reach a maximum enhancement when the cyclotron frequency of the electron is about equal to one half of the laser frequency. Through a detailed time-frequency wavelet analysis of the HHG spectra, we have identified a `synchrotron radiation'-like new mechanism responsible for the production of the second plateau.

Recent New Developments of Steady‐State and Time‐Dependent Density Functional Theories for the Treatment of Structure and Dynamics of Many‐Electron Atomic, Molecular, and Quantum Dot Systems

AbstractWe present a short account of recent new developments of density‐functional theory (DFT) for accurate and efficient treatments of the electronic structure and quantum dynamics of many‐electron systems. The conventional DFT calculations contain spurious self‐interaction energy and improper long‐range potential, preventing reliable description of the excited and resonance states. We present a new DFT with optimized effective potential (OEP) and self‐interaction‐correction (SIC) to overcome some of the major difficulties encountered in conventional DFT treatments using explicit energy functionals. The OEP‐SIC formalism uses only orbital‐independent single‐particle local potentials and is self‐interaction free, providing a theoretical framework for accurate description of the excited‐state properties and quantum dynamics. Several applications of the new procedure are presented, including: (a) the first successful DFT treatment of the atomic autoionizing resonances, (b) a relativistic extension of the OEP‐SIC formalism for the calculation of the atomic structure with results in good agreement with the experimental data across the periodic table (Z = 2–106), (c) electronic structure calculation of the ionization properties of molecules, and (d) the delicated “shell‐filling” electronic structure in quantum dots. Finally we present also new formulations of time‐dependent DFT for nonperturbative treatment of atomic and molecular multiphoton and nonlinear optical processes in intense and superintense laser fields. Both the time‐independent Floquet approach and the time‐dependent OEP‐SIC technique are introduced. Application of the time‐dependent DFT/OEP‐SIC procedure to the study of multiple high‐order harmonic generation processes in intense ultrashort pulsed laser fields is discussed in detail.

Theoretical study of multiple high-order harmonic generation by intense ultrashort pulsed laser fields: A new generalized pseudospectral time-dependent method

We present a new time-dependent method for accurate and efficient non-perturbative treatment of multiple high-order harmonic generation (HHG) in intense laser fields. The time development of the wavefunction is obtained by a new split-operator time-propagation technique in the energy representation. The generalized pseudospectral technique is extended to perform an optimal spatial grid discretization, leading to significant improvement of the quality of the wavefunction over that obtained by the equal-spacing spatial discretization techniques. The accuracy of the method is demonstrated by several benchmark calculations including the excellent agreement of the HHG power spectra in length and acceleration forms. The method is applied to a detailed study of the coherent control of HHG spectra of atomic hydrogen in intense ultrashort pulsed laser fields with or without the chirp. Several novel chirp-dependent and pulse-duration-dependent phenomena are predicted. Comparison with available experimental observation is made.

Comparison of quasi-classical and quasi-static quantum approaches to the ionization of helium by a circularly polarized laser field

Employing the quasi-classical Fermi molecular dynamics (FMD) method, we find no evidence for the existence of a nonsequential double-ionization mechanism for helium interacting with a pulse of circularly polarized, high-intensity laser radiation. This contrasts with our earlier research, for linearly polarized laser light [ Phys. Rev. A49, R12 ( 1994)], in which effects of the nonsequential double ionization of helium were noted. We also compute emitted-electron kinetic energy spectra by FMD and compare these with spectra derived from a quasi-static tunneling model calculation. In this context we discuss the applicability of the FMD approach to simulations of the photoionization of multielectron systems by intense pulsed laser fields.

Dynamical stabilization of atoms in intense laser pulses accessible to experiment.

We analyze the excitation of atoms by intense pulsed laser fields and describe a mechanism leading to effective dynamical stabilization at a photon energy below the unperturbed binding energy. At low and intermediate intensities, the atom is left in a coherent superposition of the initial state and a set of Rydberg states which is stable against ionization; this effect subsists for pulse durations presently accessible to experiment. At higher intensities, population is transferred through degenerate Raman coupling to Rydberg states of higher angular momentum.