Presence Of Intense Laser Field Research Articles

Laser-assisted in-flight muon-catalyzed deuteron-triton fusion

We investigate the deuteron-triton (DT) fusion in a three-body collision system of $\mathrm{T}\ensuremath{\mu}$ (i.e, a muon bound to a triton) impacted by a deuteron in the presence of intense laser fields with a semiclassical (SC) method. In this model, the initial positions and momenta of triton and muon are sampled from microcanonical distribution, and the dynamical process of a deuteron with a given incident kinetic energy colliding with the $\mathrm{T}\ensuremath{\mu}$ atom is simulated by tracing the classical trajectories in the combined Coulomb potentials and laser fields. At the minimum distance between the deuteron and triton, quantum tunneling through the Coulomb barrier emerges, and the penetrability can be estimated with the Wentzel-Kramers-Brillouin formula. DT nuclear fusion occurs after this quantum tunneling, and the total fusion cross section takes the Gamow form. Within the framework of the SC model, we investigate the charge shielding effect, demonstrating that this effect emerges in the low energy regime, where the impact velocity of the deuteron is smaller than the average velocity of muons in the bound state. Furthermore, the tunneling penetrability is considerably enhanced because the deuteron closely approaches the triton due to the quiver motions of charged nuclei driven by intense laser fields. As a result, the fusion cross section in this in-flight muon catalyzed fusion system can be enhanced by up to six orders of magnitude. Moreover, we calculated the DT fusion sections for a wide range of laser parameters and obtained phase diagrams demonstrating the enhanced DT fusion. Finally, important implications in achieving a large number of fusion reactions catalyzed by muons assisted by laser fields are discussed.

Real-space Green's function approach for x-ray spectra at high temperature

There has been considerable interest in properties of condensed matter at finite temperature, including non-equilibrium behavior and extreme conditions up to the warm dense matter regime. Such behavior is encountered, e.g., in experimental time resolved x-ray absorption spectroscopy (XAS) in the presence of intense laser fields. In an effort to simulate such behavior, we present an approach for calculations of finite-temperature x-ray absorption spectra in arbitrary materials, using a generalization of the real-space Green's function formalism. The method is incorporated as an option in the core-level x-ray spectroscopy code FEFF10. To illustrate the approach, we present calculations for several materials together with comparisons to experiment and with other methods.

Impurity-related optical properties of a laser-driven quantum dot

In this paper, we explore theoretically the effect of an on-center Gaussian impurity on the optical properties of a laser-driven parabolic quantum dot system under uniform magnetic field. Laser radiation with circular polarization has been treated within the framework of high-frequency Floquet approach. The linear and third-order nonlinear optical absorption coefficients and refractive index changes have been examined by means of the compact-density matrix approach. Numerical results reveal that the position and magnitude of the resonant peaks of absorption coefficients and refractive index changes are sensitively affected by magnetic field, the depth and the range of the impurity potential. Besides, the presence of intense laser field causes a red-shift in the positions and reduction in the amplitudes of the resonant peaks of optical characteristics.

Control of a resonant tunneling structure by intense laser fields

The intense laser field effects on a resonant tunneling structure were studied using computational methods. The considered structure was a GaAs/InxGa1-xAs/Al0.3Ga0.7As/InyGa1-yAs/AlAs/GaAs well-barrier system. In the presence of intense laser fields, the transmission coefficient and the dwell time of the structure were calculated depending on the depth and the width of InGaAs wells. It was shown that an intense laser field provides full control on the performance of the device as the geometrical restrictions on the resonant tunneling conditions overcome. Also, the choice of the resonant energy value becomes possible depending on the field strength.

Laser field induced optical gain in a group III-V quantum wire

Effect of intense high frequency laser field on the electronic and optical properties of heavy hole exciton in an InAsP/InP quantum well wire is investigated taking into consideration of the spatial confinement. Laser field induced exciton binding energies, optical band gap, oscillator strength and the optical gain in the InAs0.8P0.2/InP quantum well wire are studied. The variational formulism is applied to find the respective energies. The laser field induced optical properties are studied. The optical gain as a function of photon energy, in the InAs0.8P0.2/InP quantum wire, is obtained in the presence of intense laser field. The compact density matrix method is employed to obtain the optical gain. The results show that the 1.55 μm wavelength for the fibre optic telecommunication applications is achieved for 45 A wire radius in the absence of laser field intensity whereas the 1.55 μm wavelength is obtained for 40 A if the amplitude of the laser field amplitude parameter is 50 A. The characterizing wavelength for telecommunication network is optimized when the intense laser field is applied for the system. It is hoped that the obtained optical gain in the group III-V narrow quantum wire can be applied for fabricating laser sources for achieving the preferred telecommunication wavelength.

Relativistic proton-impact excitation of hydrogen atom in the presence of intense laser field

A theoretical treatment, using the first Born approximation, is presented to analyse the results of relativistic laser-assisted proton – hydrogen atom scattering. Specific calculations are carried out for excitation of hydrogen atoms from 1s1/2 to 2s1/2 states by proton impact. We work in an approximation in which the incoming proton may be described by Dirac–Volkov states in the presence of a laser field. Semi-relativistic Darwin wave functions are used to describe the hydrogen atom in its initial and final states, while relativistic, spin, and laser interaction effects are also accounted for. The results presented in this paper show that the differential cross section for this process depends not only upon the energy of the incident proton, but also upon its interaction with the laser field through intensity and frequency.

Quenching of magnetization in (III, Mn)V magnetic semiconductor quantum wells under intense laser field assisted by the quasi-two-dimensional electron gas

Laser-induced quenching of ferromagnetism in ( III 1 − x , Mn x ) V quantum well magnetic semiconductor is investigated. We propose a mechanism in which an increase of the magnon population of the ferromagnetic sample can be achieved due to the spin-flip electron–magnon scattering of the quasi-two-dimensional electron gas inside the quantum well magnetic semiconductor in the presence of intense laser field. In this case, the laser field imposes a drift velocity to the quasi-two-dimensional electrons so that whenever this drift velocity exceeds the phase velocity of the spin waves, energy from the quasi-two-dimensional electrons gained at the expense of the laser field is transferred to the magnon system thereby increasing the number of magnons (magnon amplification) and as a consequence, a loss of magnetization is obtained. Application for typical ( III 1 − x , Mn x ) V ferromagnetic semiconductor quantum wells such as Ga 1 − x Mn x As / AlAs ( x ∼ 5 % ) provides a reasonable loss of magnetization up to 30 % for laser electric field strengths up to 4 × 10 5 V / cm which is below sample damage threshold field values.

Laser control of reaction paths in ion–molecule reactions

Ion–molecule reactions are characterized by long-range dipole moments which can be used to control and manipulate their reaction path in the presence of intense laser fields. We present here an analytic model for the control of such reaction paths, including the effect of an intense laser field on the scattering and tunneling probability. We illustrate such effects for the Li++CH4 and reverse reaction using ab initio methods to calculate the reaction path and the permanent dipole moment of this system.

Gases of exploding laser-heated cluster nanoplasmas as a nonlinear optical medium

The manner in which strongly heated nanoclusters explode in the presence of intense laser fields influences all applications of this interaction. By measuring, with femtosecond time resolution, the ensemble average polarizability in a gas of intense laser-heated clusters, we have inferred the cluster explosion dynamics. The time evolution of the polarizability is characteristic of competition in the optical response between supercritical and subcritical density regions of the exploding cluster. These results are consistent with complementary time-resolved Rayleigh scattering measurements and with the predictions of a near-field plasma hydrodynamic model of the laser–cluster interaction. A significant implication of this cluster evolution appears in its macroscopic effect on a laser beam: a gas of exploding cluster plasmas causes nonlinear beam propagation owing to the space and time dependence of the ensemble polarizability. A strong self-focusing effect is observed experimentally that strongly contrasts with the ionization-induced refraction and beam spreading usually observed in the interaction of intense femtosecond laser pulses with nonclustered gases. These results illustrate how high energy density femtosecond plasma physics at the nanoscale determines what at first appears to be a gas-phase interaction.

Resonant tunnelling in the presence of intense laser fields

AbstractIt is shown, by solving numerically the time‐dependent Schrödinger equation, that the resonant transmission of electrons through double barrier structures can significantly be modified and controlled by intense laser fields. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Time-dependent density-functional theory for molecular processes in strong fields: Study of multiphoton processes and dynamical response of individual valence electrons ofN2in intense laser fields

We present a time-dependent density-functional theory (TDDFT) with proper asymptotic long-range potential for nonperturbative treatment of multiphoton processes of many-electron molecular systems in intense laser fields. A time-dependent generalized pseudospectral method is extended for precision solution of the TDDFT equations for two-center diatomic systems. The procedure allows nonuniform optimal spatial grid discretization of the Hamiltonian in prolate spheroidal coordinates and a split-operator scheme in energy representation is used for the time propagation of the individual molecular spin orbital in space and time. The theory is applied to the first detailed all-electron study of multiphoton ionization and high-order harmonic generation (HHG) processes of ${\mathrm{N}}_{2}$ in intense laser fields. The results reveal unexpected and intriguing nonlinear optical response behaviors of the individual valence spin orbital to strong fields. In particular, it is found that the dominant contribution to the total HHG power spectrum of ${\mathrm{N}}_{2}$ is due to the constructive and destructive interferences of the induced dipoles of the two highest-occupied bonding $(3{\ensuremath{\sigma}}_{g})$ and antibonding $(2{\ensuremath{\sigma}}_{u})$ molecular orbitals in the presence of intense laser fields.

Energy absorption by charge-resonance-enhanced ionization during atom-ion collisions in an intense laser field

$p+$H collisions in the presence of intense laser fields are investigated by the classical trajectory Monte Carlo method. A prominent feature of these processes is that, in contrast to the usual ionization scheme in field-free ion-atom collisions, the ionization of the target atom is always accompanied by an increase in the total kinetic energy of the colliding proton and the target nucleus. This result is explained in terms of the charge-resonance-enhanced-ionization effect taking place when the internuclear separations reach a range of 4--10 a.u. Also, the energy-gain cross section is evaluated, and its dependence on laser intensities is discussed.

Conservation and Density Wave Equations of Plasma in Presence of Intense Laser Field

The pressure tensor, energy density, and conservation equations of plasma in an intense laser field are derived by using a nonequilibrium distribution function. The results show that the effect of the intense laser field on the behavior of plasma cannot be neglected. As an application, the adiabatic equation of state, electronic density wave equation, and relevant dispersion relation are obtained for the electrostatic wave behavior. A substitution relation of the conservation equations from weak to intense field limits are presented.

Molecules in intense laser fields: Enhanced ionization in a one-dimensional model of H2.

A one-dimensional model of ${\mathrm{H}}_{2}$ is used to examine the nonlinear behavior of a pair of electrons in the presence of intense laser fields and fixed nuclei. We present ionization rates of ${\mathrm{H}}_{2}$ at different frequencies and intensities of short intense electromagnetic pulses by solving exactly the time-dependent Schr\odinger equation for the above system as a function of internuclear distance. Anomalously high ionization rates are found at large critical internuclear separations, akin to similar results found previously in the one-electron molecule ${\mathit{H}}_{2}$${\mathrm{}}^{+}$. Independent one-electron and simultaneous two-electron ionizations are identified from the numerical simulations to occur at different intensities and frequencies. Field-induced barrier suppression models are shown to explain qualitatively the numerical results. \textcopyright{} 1996 The American Physical Society.

Investigation of photodynamics in intense laser fields using the Fourier grid Floquet propagator with applications to H 2+ and LiH

Advantages of the Fourier grid Hamiltonian method and the Floquet theory have been combined to fabricate a Fourier grid Floquet (FGF) propagator for economical treatment of long time photodynamics in the presence of intense laser fields. Initial applications to photodissociation of LiH and multiphoton dynamics of H 2 + show that considerable computational economy is obtained with the use of the FGF propagator. A scheme for further economy through the use of a reduced FGF propagator is discussed and applied to multiphoton dissociation of H 2 +.

Complex quasivibrational energy formalism for intense-field multiphoton and above-threshold dissociation: Complex-scaling Fourier-grid Hamiltonian method

We present a new complex-scaling Fourier-grid Hamiltonian (CSFGH) method for accurate and efficient determination of laser-induced (multichannel) molecular resonance states without the use of basis set expansions. The method requires neither the computation of potential matrix elements nor the imposition of boundary conditions, and the eigenvectors provide directly the values of the resonance wave functions at the space grid points. The procedure is particularly valuable for excited-state problems where basis set expansion methods face the challenge. The simplicity and usefulness of the CSFGH method is demonstrated by a case study of the intensity-dependent complex quasivibrational energy eigenvalues (ER, −Γ/2) and eigenvectors associated with multiphoton and above-threshold dissociation of H+2 ions in the presence of intense laser fields (I=1012–1014 W/cm2 ).

Approximate solution of the hydrogenlike atoms in intense laser radiation.

The exact solution to the minimally coupled Dirac equation is called the Volkov solution. An operator R^ is developed, which when applied to the free-electron state function gives us the Volkov solution. We apply the operator to the exact solution of the hydrogen atom to find that it is the approximate solution of the hydrogen atom in the presence of intense laser field. The restriction on the laser intensity is that the Kibble parameter ${\ensuremath{\varepsilon}}_{K}$, which is the ratio of the electron's quiver energy ${E}_{q}$ to its rest mass energy ${E}_{0}$, is small compared to 1.

Spectrum of hydrogen atoms in intense laser fields

Using the Kramers-Henneberger transformation we discuss the description of the atomic states in the presence of intense laser fields. The theory is presented for the simple case of a hydrogen atom, and a discussion on the range of laser intensities in which the model is valid is also given.

Collisional ionization in the presence of intense laser radiation: Quantum-mechanical calculations for He* (3S)+Ar±h/ω→He+Ar+(2P)+e−

We present a quantum-mechanical coupled-channels approach for studying laser-assisted collisional processes leading to transitions between discrete and continuum electronic states. This coupled-channels approach is an approximate application of our more general quantum-mechanical formalism for treating bound-continuum collisional transitions assisted by intense laser radiation, and uses discretization procedures to deal with the continuum states, and so-called electronic-field representations to account for the presence of intense laser fields. We discuss in detail all approximations leading to our coupled-channels approach. We then use our approach to calculate laser-assisted Penning ionization of Ar by He* (1s2s,3S) for the specific cases of radiation from the 10.6 μm line of a CO2 laser of intensity near 1.6×108 W/cm2, and the 1.315 μm line of an iodine laser of intensity near 109 W/cm2. We obtain ionization cross sections (per unit emitted-electron energy ε) dσ/dε as a function of ε. dσ/dε reflects a spectrum of emitted-electron energies that exhibits pronounced laser-modified collisional effects. The usual field-free spectral region around ε0(?0.149 hartree) is augmented by new spectral regions governed by radiative coupling and located near ε+(−)?ε0+(−) h/ω, where h/ω is the laser’s photon energy. Compared to their field-free counterparts, these laser-modified spectral contributions are more sharply peaked and favor Penning ionization over associative ionization more strongly. In carrying out these calculations we treat the radiative coupling in the dipole approximation. By analyzing this radiative coupling for He* (1s2s,3S)+Ar Penning ionization, we show that it rigorously vanishes asymptotically. We use this fact to propose a physically reasonable functional form for such radiative coupling between bound and continuum electronic states.