Dynamics of multiphoton excitation and quantum diffusion in Rydberg atoms

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.39.1800.We present a detailed two-dimensional (2D) quantal study of the dynamical evolution of microwave-driven Rydberg H atoms. We examine the range of validity of the conventional one-dimensional (1D) models and explore the frequency- and intensity-dependent excitation and ionization mechanisms. The main findings of this paper can be summarized as follows: (i) The excitation spectra of Rydberg H atoms are strongly frequency dependent and can be roughly grouped into three characteristically different regions, each with a different excitation mechanism. In this paper, we emphasize the study of the two major excitation mechanisms: quantum diffusion and multiphoton resonant excitation. The region dominated by quantum diffusion lies in the frequency range ωc<ω0<ωd, where ω0 is the rescaled field frequency (ω0=ωn30; n0 is the principal quantum number of the initial state); ωc, the classical chaotic threshold; and ωd, the quantum delocalization border. In this region, quasienergy levels are strongly perturbed and mixed and excitation is efficient, leading to the so-called underthreshold photoelectric ionization phenomenon. On the other hand, we found a series of frequency regions (in ω0>ωd) where the ionization is mainly due to multiphoton resonant excitation through the more isolated quasienergy avoided crossing points. (ii) The excitation pathways (1D versus 2D) are strongly intensity dependent. For microwave (rescaled) field strength ɛ0 (≡ɛn40) in the range ɛc0 ladders rather than the n2=0 ladder, as often assumed in the 1D model. As field strength increases above ɛq, however, the 1D model improves significantly. (iii) The quantum localization phenomenon is observed in the classically chaotic region (ωcɛq. (iv) The stability of quantum diffusive motion is analyzed in terms of the quantal phase-space diagram and the autocorrelation function. The results lend support to the view that quantum mechanics can impose limitations on classical chaotic motion. (v) The way of turning on the field (sinωt or cosωt) does not affect significantly the dynamical evolution of the system. (vi) Finally, a computationally powerful new technique, invoking the use of artificial intelligence algorithms as well as the generalized Van Vleck perturbation theory for effectively reducing the dimensionality of the Floquet matrix, is introduced to facilitate the study of multiphoton resonant excitation of Rydberg atoms

Multiphoton detachment of H-. II. Intensity-dependent photodetachment rates and threshold behavior—complex-scaling generalized pseudospectral method

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.50.3208.We extend our previous perturbative study of the multiphoton detachment of H- [Phys. Rev. A 48, 4654 (1993)] to stronger fields by considering the intensity-dependent photodetachment rates and threshold behavior. An accurate one-electron model potential, which reproduces exactly the known H- binding energy and the low-energy e-H(1s) elastic-scattering phase shifts, is employed. A computational technique, the complex-scaling generalized pseudospectral method, is developed for accurate and efficient treatment of the time-independent non-Hermitian Floquet Hamiltonian H^F. The method is simple to implement, does not require the computation of potential matrix elements, and is computationally more efficient than the traditional basis-set-expansion–variational method. We present detailed nonperturbative results of the intensity- and frequency-dependent complex quasienergies (ER,-Γ/2), the complex eigenvalues of H^F, providing directly the ac Stark shifts and multiphoton detachment rates of H-. The laser intensity considered ranges from 1 to 40 GW/cm2 and the laser frequency covers 0.20–0.42 eV (in the c.m. frame). Finally we perform a simulation of intensity-averaged multiphoton detachment rates by considering the experimental conditions of the laser and H- beams. The results (without any free parameters) are in good agreement with experimental data, both in absolute magnitude and in the threshold behavior

Nonperturbative treatments of level shifts of excited states and high-order harmonic generation in strong fields

This is the published version, also available here: http://dx.doi.org/10.1364/JOSAB.7.000425.In this paper we accomplish three goals. First, we present new nonperturbative results of complex quasi-energies (shifts and widths) for several low-lying excited states of atomic H in strong fields, using the L2 non-Hermitian Floquet matrix technique. Second, we present a new nonperturbative L2 technique for the treatment of ac Stark shifts of arbitrary excited states. We found that all the Rydberg states in weak fields are upshifted and closely follow the quadratic field dependence described by the ponderomotive potential e2F2/4mω2. Large deviation from the ponderomotive shift and intricate level-shift behaviors, however, occur in strong fields. Finally, we present a classical nonperturbative treatment of the electronic motion in intense laser fields. We show that the spectral analysis of classical trajectories can provide detailed insights regarding the mechanisms responsible for the multiple-harmonic generation recently observed in high-intensity experiments

Floquet-Liouville supermatrix approach: Time development of density-matrix operator and multiphoton resonance fluorescence spectra in intense laser fields

This is the publisher's version, also available electronically from http://journals.aps.org/pra/abstract/10.1103/PhysRevA.33.1798.A Floquet-Liouville supermatrix (FLSM) approach is presented for nonperturbative treatment of the time development of the density-matrix operator of atoms and molecules exposed to intense polychromatic fields. By extending the many-mode Floquet theory (MMFT) recently developed, the time-dependent Liouville equation for the density matrix of quantum systems undergoing relaxations (due to radiative decays and collisional dampings, etc.) can be transformed into an equivalent time-independent non-Hermitian FLSM eigenvalue problem. This yields a numerically stable and computationally efficient approach for the unified treatment of nonresonant and resonant, one- and multiple-photon, steady-state and transient phenomena in nonlinear optical processes, much beyond the conventional rotating-wave-approximation (RWA) method. Connections of the FLSM approach to the MMFT in the limit of zero relaxations are also made, providing the understanding of the physical significance of FLSM supereigenvalues and eigenvectors. In addition to the exact FLSM formalism, we have also presented higher-order perturbative results, based on the extension of the generalized Van Vleck (GVV) nearly degenerate perturbation theory, appropriate for somewhat weaker fields and near-resonant processes, but beyond the RWA limit. The implementation of the GVV method in the time-independent Floquet-Liouvillian allows the reduction of the infinite-dimensional FLSM into a finite-dimensional GVV-Liouville matrix, from which essential analytical results are readily obtained. As an illustration of the usefulness of the new formalism, we extend both the FLSM and the GVV methods to a formal study of the multiphoton-induced resonance fluorescence spectra of two-level systems subject to purely radiation relaxations. Both the time-averaged power spectrum and the time-dependent physical spectrum are exploited in details, and novel new features in intense fields are pointed out

Above-threshold multiphoton detachment of H- by two-color laser fields: Angular distributions and partial rates

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.51.4797.We present a general nonperturbative formalism and an efficient and accurate numerical technique for the study of the angular distributions and partial widths for multiphoton above-threshold detachment in two-color fields. The procedure is based on an extension of our recent paper [D. A. Telnov and S.-I Chu, Phys. Rev. A 50, 4099 (1994)] for one-color detachment, and the many-mode Floquet theory [T. S. Ho, S.-I Chu, and J. V. Tietz, Chem. Phys. Lett. 96, 464 (1983)]. The generalization of this procedure is performed for both cases of commensurable and incommensurable frequencies of the two-color fields. The procedure consists of the following elements: (i) Determination of the resonance wave function and complex quasienergy by means of the non-Hermitian Floquet Hamiltonian formalism. The Floquet Hamiltonian is discretized by the complex-scaling generalized pseudospectral technique recently developed [J. Wang, S.-I Chu, and C. Laughlin, Phys. Rev. A 50, 3208 (1994)]. (ii) Calculation of the angular distribution and partial widths based on an exact differential formula and a procedure for the rotation of the resonance wave function back to the real axis. The method is applied to a nonperturbative study of multiphoton above-threshold detachment of H- by 10.6-μm radiation and its third harmonic (the commensurable case). The results show strong dependence on the relative phase δ between the fundamental frequency field and its harmonic. For the intensities used in calculations (1010 W/cm2 for the fundamental frequency, 108 and 109 W/cm2 for the harmonic), the total rate has its maximum at δ=0 and minimum at δ=π. However, this tendency, though valid for the first several above-threshold peaks in the energy spectrum, is reversed for the higher-energy peaks. The energy spectrum for δ=π is broader, and the peak heights decrease more slowly compared to the case of δ=0. The strong phase dependence is also manifested in the angular distributions of the ejected electrons

Two-color phase control of high-order harmonic generation in intense laser fields

This is the published version, also available here: http://dx.doi.org/10.1103/PhysRevA.52.3988.We present a time-independent generalized Floquet approach for nonperturbative treatment of high-order harmonic generation (HG) in intense onea (i) determination of the complex quasienergy eigenvalue and eigenfunction by means of the non-Hermitian Floquet formalism, wherein the Floquet Hamiltonian is discretized by the complex-scaling generalized pseudospectral technique [Wang, Chu, and Laughlin, Phys. Rev. A 50, 3208 (1994)], and (ii) calculation of the HG rates based on the approach that implies the classical treatment of the electromagnetic field and quantal treatment of the atom. The method is applied to the nonperturbative study of HG by the hydrogen atom in strong laser fields with the fundamental frequencies 532 and 775 nm and their third harmonics. The results show a strong dependence on the relative phase δ between the fundamental frequency field and its harmonic. For the intensities used in calculations (1×1013 and 5×1013 W/cm2 for the fundamental frequency 532 nm and 1×1013 and 3×1013 W/cm2 for the fundamental frequency 775 nm, the harmonic intensity being 10 and 100 times weaker), the total photon emission rate has its maximum at δ=0 and minimum at δ=π. However, this tendency, while valid for the first several HG peaks, is reversed for the higher HG peaks. The HG spectrum for δ=π is broader and the peak heights decrease more slowly compared to the case of δ=0. These results have their analog in the multiphoton above-threshold detachment study performed recently for H- ions [Telnov, Wang, and Chu, Phys. Rev. A 51, 4797 (1995)]

Spin-Hall interface resistance in terms of Landauer type spin dipoles

We considered the nonequlibrium spin dipoles induced around spin independent elastic scatterers by the intrinsic spin-Hall effect associated with the Rashba spin-orbit coupling. The normal to 2DEG spin polarization has been calculated in the diffusion range around the scatterer. We found that although around each impurity this polarization is finite, the corresponding macroscopic spin density, obtained via averaging of individual spin dipole distributions over impurity positions is zero in the bulk. At the same time, the spin density is finite near the boundary of 2DEG, except for a special case of a hard wall boundary. The boundary value of the spin polarization can be associated with the interface spin-Hall resistance determining the additional energy dissipation due to spin accumulation.Comment: 7 page

Spin-Hall effect on edge magnetization and electric conductance of a 2D semiconductor strip

The intrinsic spin-Hall effect on spin accumulation and electric conductance in a diffusive regime of a 2D electron gas has been studied for a 2D strip of a finite width. It is shown that the spin polarization near the flanks of the strip, as well as the electric current in the longitudinal direction exhibit damped oscillations as a function of the width and strength of the Dresselhaus spin-orbit interaction. Cubic terms of this interaction are crucial for spin accumulation near the edges. As expected, no effect on the spin accumulation and electric conductance have been found in case of Rashba spin-orbit interaction.Comment: 4 pages, 1 figure, some changes in the tex

Anisotropic in-plane optical conductivity in detwinned Ba(Fe1-xCox)2As2

We study the anisotropic in-plane optical conductivity of detwinned Ba(Fe1-xCox)2As2 single crystals for x=0, 2.5% and 4.5% in a broad energy range (3 meV-5 eV) across their structural and magnetic transitions. For temperatures below the Neel transition, the topology of the reconstructed Fermi surface, combined with the distinct behavior of the scattering rates, determines the anisotropy of the low frequency optical response. For the itinerant charge carriers, we are able to disentangle the evolution of the Drude weights and scattering rates and to observe their enhancement along the orthorhombic antiferromagnetic a-axis with respect to the ferromagnetic b-axis. For temperatures above Ts, uniaxial stress leads to a finite in-plane anisotropy. The anisotropy of the optical conductivity, leading to a significant dichroism, extends to high frequencies in the mid- and near-infrared regions. The temperature dependence of the dichroism at all dopings scales with the anisotropy ratio of the dc conductivity, suggesting the electronic nature of the structural transition. Our findings bear testimony to a large nematic susceptibility that couples very effectively to the uniaxial lattice strain. In order to clarify the subtle interplay of magnetism and Fermi surface topology we compare our results with theoretical calculations obtained from density functional theory within the full-potential linear augmented plane-wave method.Comment: 17 pages, 9 figure