High-frequency Laser Field Research Articles

Iterative treatment for atoms in high-frequency laser fields: Two-color interference effect

In this paper, we present a practical numerical technique for calculating both the ionization rate and the wave function of an atom in high-frequency laser fields. This method permits the replacement of the exact final-state wave function by its asymptotic form in the matrix elements, which is shown to extremely simplify the numerical calculations. This approach is compared with the high-frequency Floquet theory and an exact numercical Floquet calculation, and proved to be very efficient and reliable. Moreover, this method is successfully applied to investigate the two-color interference effect in the stabilization regime. We find a significant enhancement or suppression on the ionization rate and the high-order harmonic generation through adjusting the relative phase between the two frequency components.

Disappearance of the dressed bound states in photodetachment from a short-range potential by an intense high-frequency laser field

It is proved that in three dimensions, and contrary to what is usually found in one dimension, the number of bound Kramers-Hennebrger states always reduces to zero in strong fields if the range of the potential is short and its depth finite. Numerical results showing the disappearance of the dressed ground state of an exponential potential in an intense high-frequency field are also presented.

Atomic-electron excitation by a local phase shift of the wave function

A very simple analytical model is presented to compute the time evolution of an atomic electron interacting with an ultrastrong high-frequency laser field. The model takes into account the evolution of the electronic wave function in terms of the action and trajectory of a classical electron. It shows the possibility of atomic excitation by a space-dependent shift of the wave function. It is an extension of the well-known strong-field approximation and the dipole Volkov states to the case of space-dependent fields, in the very-high-frequency regime. The great advantage of the model lies in its applicability in situations where the dipole approximation does not hold and, therefore, ab initio numerical computation of the time-dependent Schr\"odinger equation would require extremely long run times.

Kinetic approach to the electrical conductivity of dense plasmas in strong laser fields

Based on quantum statistical theory a kinetic equation is presented which is valid for dense plasmas in time-dependent electromagnetic fields. It generalizes previous results to quantum systems. Starting from this equation the collision frequency and the electrical conductivity for quantum plasmas in strong high-frequency laser fields are calculated without using any external cutoff-procedure. The influence of nonlinear field as well as plasma density effects on the collision frequency and on the conductivity is discussed. Finally, the results are compared with that obtained from the well-known classical theories.

Molecular dichotomy within an intense high-frequency laser field

It is shown that, under an intense high-frequency laser field, electronic distributions in molecules exhibit a dichotomy effect just as previously found in atoms. The generalization of the formal demonstration of the dichotomy effect as given in M. Gavrila and J. Shertzer, Phys. Rev. A 41, 477 (1990) to many-electron, polyatomic molecules is considered and the validity of the α0−2/3 scaling law of the Floquet eigenvalues, with respect to the field intensity parameter α0 of the HFFT, is discussed. To test the molecular dichotomy effect, numerical calculations are performed using a quantum chemical package (Gaussian 94), modified appropriately to incorporate the cycle-averaged displacements of the nuclear–electron Coulomb potential as found in the HFFT hamiltonian. Results of calculations on the two-electron H2 molecule are presented with an emphasis placed on the character of the total and orbital charge distributions and on trends to be observed in the electronic correlation at high intensities.

Ellipticity and pulse shape dependence of localised wavepackets

Atoms in intense, high-frequency laser fields exhibit the remarkable property that they can be stable against ionization. We investigate the structure of stabilized wavepackets for a two-dimensional model hydrogen atom interacting with an intense, high frequency laser pulse as a function of the laser pulse ellipticity and laser pulse rise-time. The computed wavepackets are compared with the corresponding Kramers-Henneberger (K-H) ground states. Laser pulse turn-on effects are studied by contrasting the structure of the localized part of the wavepackets and the ionizing part of the wavepackets for three different ellipticities and for various pulse turn-on times.

Collisional amplification of test electromagnetic waves in a plasma subject to an intense high-frequency laser field

Recently, calculations based on Fokker–Planck and Landau collision operators have been used to demonstrate that test waves with electric fields in the plane of polarization of external superintense laser fields can get amplified by nonlinear collisional processes. The calculations are similar in spirit to parametric instability calculations and make use of an averaging process for the collision operator whose range of validity is not clear. The Dawson–Oberman model of high-frequency plasma conductivity is used to reinvestigate this problem. It is shown that in the limit of small test wave frequency, ω1/ω0=ε≪1, no amplification is possible. This result contradicts the one obtained by earlier workers with averaged collision operators. At higher test wave frequencies earlier results are recovered.

Hydrogenic Impurity States in n-Doped and Undoped Quantum Wells of GaAs–AlxGa1-xAs Embedded in Intense Laser Fields

The ground state binding energy of hydrogenic impurities in n-doped and undoped GaAs–Al x Ga 1- x As quantum wells, under intense high-frequency polarized laser field is investigated. The calculation has been performed by means of a nonperturbative theory and variational approach which takes into account the “dressed” confinement barrier potential, the “dressed” Coulomb potential, and the screening effect due to the two-dimensional electron gas. We found that the impurity binding energy in a n-doped quantum well is dramatically reduced by both, the screening effect and the intense high-frequency laser field, thus leading, under certain circumstances, to a metal-insulator transition.

Intense field effects on hydrogen impurities in quantum dots

Calculations of the binding energy of an on-center donor hydrogenic impurity in a quasizero-dimensional quantum-well system [quantum dot (QD)] placed in an intense, high-frequency laser field are presented. A nonperturbative theory and a variational approach are used as the framework for this calculation. The effect of the intense laser field is to “dress” the impurity potential making it dependent upon the laser field amplitude. A rapid decrease of the binding energy, for different values of the QD radius and for both infinite and finite potential barriers, with increasing field intensity is predicted. An application is made for a spherical QD made of GaAs/Ga1−xAlxAs heterostructures.

Hydrogenic impurities in a quantum well wire in intense, high-frequency laser fields.

Calculation of the binding energy of an axial donor hydrogenic impurity in an ideal, infinite, cylindrical quantum wire placed in an intense, high-frequency laser field is reported. By making use of a nonperturbative theory that ``dresses'' both the potential of the impurity and the confinement potential in the quantum wire, and the variational approach a rapid decrease of the binding energy for different values of the wire radius with increasing field intensity is predicted. \textcopyright{} 1996 The American Physical Society.

Transient dynamics of Ohmic dissipative two-level systems driven by dc-ac fields

Transient dynamics of a biased two-level system with Ohmic dissipation driven by a high-frequency laser field is studied within the noninteracting-blip approximation. It is found that in the low temperature limit, if the friction parameter a is between 1/2 and 1, the rate constant for the tunneling of a system particle will increase with the intensity and frequency of the field, and the lower frequencies will be in favor of localization of the particle in the metastable state. For 0 < α < 1/2, quantum coherence is observed, and the transition temperature from the coherent to incoherent motion is found to depend on the field intensity. When the laser field intensity parameter approaches the zeros of the Nth Bessel function, the coherence disappears. This transition from coherent to incoherent motion oscillating with the intensity of the field gives rise to a cascade of transitions at different temperatures.

Entropic measure of wave-packet spreading and ionization in laser-driven atoms.

We consider wave-packet spreading and ionization of a one-dimensional model hydrogen atom interacting with a strong high-frequency laser field in the stabilization domain. We demonstrate the effect of the pulse shape on the dynamics in phase space and visualize our results with both the Wigner and the Q function phase-space quasiprobability representation. The Wehrl entropy is introduced and utilized as a convenient one-parameter measure of wave-packet spreading, phase-space localization, and ionization. @S1050- 2947~96!02007-0# We therefore also consider a second phase-space distribution function, the Husimi or Q representation @10#, which is al- ways positive. The Husimi distribution can be used to calcu- late an information-theoretic measure of the number of par- ticipating quasiclassical Gaussian wave packets. This measure is the Wehrl entropy @11# and, apart from interfer- ence terms, is determined by the number of Gaussian coher- ent states to ''tile'' its phase-space contour. The Wehrl en- tropy has been investigated and utilized by a group of authors @12#. It has recently been studied within a wider class of entropies that are based on a comparison of the wave function with an arbitrary basis of states in phase space @13#. This Wehrl entropy is directly related to the uncertainty area of the Q function in phase space and it is appropriate as a measure of wave packet spreading and ionization. We will use the Wehrl entropy to study the effect of these phenomena in the stabilization domain using both an adiabatic sin 2 and a nonadiabatic trapezoidal pulse. In the next section we will describe our numerical model, based on the solution of the time-dependent Schrodinger equation in one dimension. We will then investigate the sta- bilization dynamics for two different pulse shapes ~one adia- batic, the other with a fast turn-on! using the Wigner func- tion, before turning our attention to the Q function and finally to the Wehrl entropy.

Quantum signatures in the stabilization dynamics.

We investigate the phase-space dynamics of a hydrogenic atom in a very intense, high-frequency laser field, by comparing the time-dependent quantum Wigner functions with the corresponding classical distributions calculated using the Monte Carlo method. In both cases we model the atom by a one-dimensional soft-core potential in order to simplify the calculation. We demonstrate the importance of nonclassical interferences in the Wigner function and show how negative parts of it observed at the peak of the pulse are associated with the formation of a coherent superposition of dressed ``Kramers-Henneberger'' (KH) eigenstates. These signatures of the quantum dynamics are eliminated towards the end of the pulse when the coherence between the KH eigenstates degrades so that the classical and the quantum mechanical phase-space distributions come into agreement.

Level Crossing in a Driven Double Quantum Well

The motion of electrons in a double quantum well under the action of a monochromatic driving laser field is investigated. Closed-form solutions of the quasienergy and Floquet states are obtained with the help of SU (2) symmetry, from which the occurrence of quasienergy crossing is demonstrated to coincide with the coherent destruction of tunneling if the high-frequency laser field is involved.

Electron scattering in a Yukawa potential in the presence of a high-frequency laser field

A numerical investigation of electron potential scattering in the presence of a high-frequency, high-intensity electromagnetic field has been carried out using classical mechanics. A Yukawa potential has been considered to simulate the electron-atom interaction. The high-frequency approximation, in which the electron moves in the so-called dressed potential has been employed. Both scattering angles and energy transfer from the field have been calculated. Strong fields lead to marked reductions in the scattering angle. Except at high frequencies large energy transfers are obtained, in the approximation that the energy transfer can be evaluated using trajectories in the dressed potential. Preliminary calculations on the validity of the high-frequency approximation show that it yields, within about 30%, the maximum energy transferred as the field phase is altered, but is poor for the detailed variation with phase.

Quantum dynamics of a circular Rydberg state in a microwave field.

We present the first complete quantum treatment of the ionization of a circular Rydberg state by a linearily polarized microwave field. Experimentally accessible ionization threshold fields as well as the dynamics of the resonance eigenfunctions are investigated, together with a comparison to the behavior of a quasi-one-dimensional state subject to a microwave, and of a circular state in an intense, high frequency laser field.

Erratum: Multiphoton ionization in superintense, high-frequency laser fields. I. General developments

Erratum: Multiphoton ionization in superintense, high-frequency laser fields. I. General developments

Erratum: Multiphoton ionization in superintense, high-frequency laser fields. II. Stabilization of atomic hydrogen in linearly polarized fields

Erratum: Multiphoton ionization in superintense, high-frequency laser fields. II. Stabilization of atomic hydrogen in linearly polarized fields

Harmonic generation in the Kramers-Henneberger stabilization regime.

We show how the Kramers-Henneberger (KH) frame can be used to interpret the generation of high-order harmonics in very strong high-frequency laser fields in which the ionization of the atom is suppressed. We show that although a single KH eigenstate produces little harmonic generation in a constant-amplitude laser field, a wave packet of bound KH states with a low ionization rate can result in a long comb of high-order harmonics. We verify that there is a close link between the production of strong even harmonics and the use of ultrafast laser-field turn-on times, and that the even-harmonic strength is reduced for larger and, therefore, perhaps more realistic rise times.

Photoionization of atomic hydrogen dressed by a circularly polarized CO2-laser field.

We calculate the two-color ionization profile of atomic hydrogen in the presence of both a weak tunable high-frequency laser field and a low-frequency laser field of moderate intensity. The high-frequency spectroscopic light is used to probe the structure of Rydberg states dressed by the low-frequency field. We construct the photoionization spectrum from the sum of the resonance contributions by calculating the transition dipole moments between the initial state and the dressed Rydberg states, analogous to the study of photoionization in the presence of a dc field carried out by Alvarez and Silverstone [Phys. Rev. Lett. 63, 1364 (1989)].