Monochromatic Laser Field Research Articles

Dynamics of intersubband transitions in triangular quantum wells due to static magnetic and laser fields

We study dynamics of intersubband and bound–continuum transitions of the electron bound in the conduction band of triangular quantum wells interacting with perpendicular static magnetic field in the presence of strong monochromatic laser field. We use non-perturbative Floquet theory and finite differential method to study the influence of static magnetic field on quantum well (QW) system in the presence of strong laser field. We show that the system dynamics gets modified due to the perpendicular magnetic field. The sensitivity to the ratio of strengths of magnetic and laser field intensity (Bs/I) of the state populations is studied.

Laser Asisted Scattering of an Optical Polaron in Presence of an Impurity Center

Momentum transfer cross-sections are studied for polarons inside a polar medium in presence of a coulomb impurity center as well as an external linerly polarised,homogeneous annd monochromatic laser field. The strongly bound polaron is found to be reluctant to contribute to the conductivity which is justified physically. The present study may find application in development of electronic devices using polar materials like organic semiconductors.

Quantum-orbit analysis of high-order-harmonic generation by resonant plasmon field enhancement

We perform a detailed analysis of high-order harmonic generation (HHG) in atoms within the strong field approximation (SFA) by considering spatially inhomogeneous monochromatic laser fields. We investigate how the individual pairs of quantum orbits contribute to the harmonic spectra. We show that in the case of inhomogeneous fields, the electron tunnels with two different canonical momenta. One of them leads to a higher cutoff and the other one develops a lower cutoff. Furthermore, we demonstrate that the quantum orbits have a very different behavior in comparison to the homogeneous field. We also conclude that in the case of the inhomogeneous fields, both odd and even harmonics are present in the HHG spectra. Within our model, we show that the HHG cutoff extends far beyond the semiclassical cutoff as a function of inhomogeneity strength. Our findings are in good agreement both with quantum mechanical and classical models.

Nonlinear interaction between radiation and an electron beam in a free-electron laser oscillator based on a two-frequency wiggler

A two-frequency wiggler was used to enhance the efficiency and the radiation power in a far-infrared free-electron laser oscillator. The interaction between the electron beam and the radiation was numerically studied using multi-particle and multi-pass simulation coding to understand the effect of the two-frequency wiggler, and the wiggler’s frequency ratio parameter was optimized. The influence of the electron beam energy spread was also investigated in the two-frequency wiggler for a monochromatic laser field.

Photon emission statistics and photon tracking in single-molecule spectroscopy of molecular aggregates: Dimers and trimers

Based on the generating function formalism, we investigate broadband photon statistics of emission for single dimers and trimers driven by a continuous monochromatic laser field. In particular, we study the first and second moments of the emission statistics, which are the fluorescence excitation line shape and Mandel's Q parameter. Numerical results for this line shape and the Q parameter versus laser frequency in the limit of long measurement times are obtained. We show that in the limit of small Rabi frequencies and laser frequencies close to resonance with one of the one-exciton states, the results for the line shape and Q parameter reduce to those of a two-level monomer. For laser frequencies halfway the transition frequency of a two-exciton state, the photon bunching effect associated with two-photon absorption processes is observed. This super-Poissonian peak is characterized in terms of the ratio between the two-photon absorption line shape and the underlying two-level monomer line shapes. Upon increasing the Rabi frequency, the Q parameter shows a transition from super- to sub- to super-Poissonian statistics. Results of broadband photon statistics are also discussed in the context of a transition (frequency) resolved photon detection scheme, photon tracking, which provides a greater insight in the different physical processes that occur in the multi-level systems.

Laser-assisted spin-polarized transport in graphene tunnel junctions

The Keldysh nonequilibrium Green’s function method is utilized to theoretically study spin-polarized transport through a graphene spin valve irradiated by a monochromatic laser field. It is found that the bias dependence of the differential conductance exhibits successive peaks corresponding to the resonant tunneling through the photon-assisted sidebands. The multi-photon processes originate from the combined effects of the radiation field and the graphene tunneling properties, and are shown to be substantially suppressed in a graphene spin valve which results in a decrease of the differential conductance for a high bias voltage. We also discuss the appearance of a dynamical gap around zero bias due to the radiation field. The gap width can be tuned by changing the radiation electric field strength and the frequency. This leads to a shift of the resonant peaks in the differential conductance. We also demonstrate numerically the dependences of the radiation and spin valve effects on the parameters of the external fields and those of the electrodes. We find that the combined effects of the radiation field, the graphene and the spin valve properties bring about an oscillatory behavior in the tunnel magnetoresistance, and this oscillatory amplitude can be changed by scanning the radiation field strength and/or the frequency.

Driven coupled Morse oscillators: visualizing the phase space and characterizing the transport

Recent experimental and theoretical studies indicate that intramolecular energy redistribution (IVR) is nonstatistical on intermediate timescales even in fairly large molecules. Therefore, it is interesting to revisit the old topic of IVR versus quantum control and one expects that a classical-quantum perspective is appropriate to gain valuable insights into the issue. However, understanding classical phase space transport in driven systems is a prerequisite for such a correspondence based approach and is a challenging task for systems with more than two degrees of freedom. In this work we undertake a detailed study of the classical dynamics of a minimal model system – two kinetically coupled Morse oscillators in the presence of a monochromatic laser field. Using the technique of wavelet transforms a representation of the high dimensional phase space, the resonance network or Arnold web, is constructed and analysed. The key structures in phase space which regulate the dissociation dynamics are identified. Furthermore, we show that the web is non-uniform with the classical dynamics exhibiting extensive stickiness, resulting in anomalous transport. Our work also shows that pairwise irrational barriers might be crucial even in higher dimensional systems.

Steady-state correlations of two atoms interacting with a reservoir

We consider two two-level atoms fixed at different positions, driven by a resonant monochromatic laser field, and interacting collectively with the quantum electromagnetic field. A Born-Markov-secular master equation is used to describe the dynamics of the two atoms and the steady-state is obtained analytically for a configuration of the atoms. The steady-state populations of the energy levels of the free atoms, entanglement, quantum and geometric discords and degree of mixedness are calculated analytically as a function of the laser field intensity and the distance between the two atoms. It is found that there is a possibility of considerable steady-state entanglement and left/right quantum discord and that these can be controlled either by increasing/decreasing the intensity of the laser field or by increasing/decreasing the distance between atoms. It is shown that the system of two atoms can be prepared in a separable mixed state with non-zero quantum discord that turns into an $X$-state for high laser field intensities. The behavior and relationships between the different correlations are studied and several limiting cases are investigated.

Light-induced anisotropy of atomic response: prospects for emission spectrum control

We present the quantum mechanical theory of the nonlinear atomic response in a strong laser field. The proposed approach provides the self consistent description of the atomic response both in the long (THz) and the short (XUV) wavelength regions. At the subrelativistic intensity of a laser pulse, our approach is valid for the arbitrary ratio between the laser and the intra-atomic field strengths. Instead of the traditional basis of the “free atom” eiegenfunctions, the basis of eiegenfunctions of the boundary value problem for “the atom in an external field” is used to calculate the matrix elements of the quantum mechanical operators. In this case the matrix elements of the generalized momentum operator and the Hamiltonian of the time-dependent Schrodinger equation are directly related with the experimentally measured dipole matrix elements and the energy spectrum of a free atom. The response of the atomic argon in the two-color laser field of arbitrary polarized components has been calculated numerically. The results of computer simulations show that the efficiency of the atomic response depends non-monotonically on the angle between the polarization vectors of the laser pulses. We also show that the most effective mechanism of THz emission in the two-color laser field appears mainly due to the atomic non-linearity, and not to the plasma non-linearities playing the dominant role in the case of the monochromatic laser field. The predictions of our theory coincide with the numerical and experimental results. This fact gives us a reliable base to provide the straightforward explanation of the observed dependencies, and to propose the methods of enhancement of efficiency of the nonlinear transformations both in the long and short wavelength spectral regions.

Ionization of a lithium ion by electron impact in a strong laser field

The modification in the dynamics of the electron-impact ionization process of a ${\mathrm{Li}}^{+}$ ion due to an intense linearly polarized monochromatic laser field ($n\ensuremath{\gamma}e,2e$) is studied theoretically using coplanar geometry. Significant laser modifications are noted due to multiphoton effects both in the shape and magnitude of the triple-differential cross sections (TDCSs) with respect to the field-free (FF) situation. The net effect of the laser field is to suppress the FF cross sections in the zeroth-order approximation [Coulomb-Volkov (CV)] of the ejected electron wave function, while in the first order [modified Coulomb-Volkov (MCV)], the TDCSs are found to be enhanced or suppressed depending on the kinematics of the process. The strong FF recoil dominance for the ($e$,2$e$) process of an ionic target at low incident energy is destroyed in the presence of the laser field. The FF binary-to-recoil ratio changes remarkably in the presence of the laser field, particularly at low incident energies. The difference between the multiphoton CV and the FF results indicates that for the ionic target, the Kroll-Watson sum rule does not hold well at the present energy range in contrast to the neutral atom (He) case. The TDCSs are found to be quite sensitive with respect to the initial phase of the laser field, particularly at higher incident energies. A significant qualitative difference is noted in the multiphoton ejected energy distribution (double-differential cross sections) between the CV and the MCV models. Variation of the TDCSs with respect to the laser phase is also studied.

Nonlinear Compton scattering in ultrashort laser pulses

A detailed analysis of the photon emission spectra of an electron scattered by a laser pulse containing only very few cycles of the carrying electromagnetic field is presented. The analysis is performed in the framework of strong-field quantum electrodynamics, with the laser field taken into account exactly in the calculations. We consider different emission regimes depending on the laser intensity, placing special emphasis on the regime of one-cycle beams and of high laser intensities, where the emission spectra depend nonperturbatively on the laser intensity. In this regime we in particular present an accurate stationary phase analysis of the integrals that are shown to determine the computed emission spectra. The emission spectra show significant differences with respect to those in a long pulsed or monochromatic laser field: the emission lines obtained here are much broader and, more important, no dressing of the electron mass is observed.

Space and surface-state effects in multiphoton emission

We investigate multiphoton emission of electrons from solid surfaces in the presence of nonperturbative monochromatic laser fields. We study the dependence of transmission probabilities on surface states, the penetration depth of laser fields in solids and the size of the laser focus in vacuum. To this end we present a one-space-dimensional model which permits to study these problems and develop a numerically stable algorithm for solving the corresponding time-dependent Schrödinger equation.

Multiphoton ionization and multiphoton resonances in the tunneling regime

The rate of ionization of an atom of helium, argon, or hydrogen exposed to an intense monochromatic laser field and the quasienergy spectrum of their dressed states are studied for values of the Keldysh parameter between 1 and 0.6 and wavelengths between 390 and 1300 nm. The calculations are carried out within the non-Hermitian Floquet theory. Resonances with intermediate excited states significantly affect ionization from the dressed ground state at all the intensities and all the wavelengths considered. The dressed excited states responsible for these structures are large-${\ensuremath{\alpha}}_{0}$ states akin to the Kramers-Henneberger states of the high-frequency Floquet theory. Within the single-active-electron approximation, these large-${\ensuremath{\alpha}}_{0}$ states become species independent at sufficiently high intensity or sufficiently long wavelength. Apart for the resonance structures arising from multiphoton coupling with excited states, the ab initio Floquet ionization rate is in excellent agreement with the predictions of two different calculations in the strong field approximation, one based on a length-gauge formulation of this approximation and one based on a velocity-gauge formulation. The calculations also confirm the validity of the ${\ensuremath{\omega}}^{2}$ expansion as an alternative to the strong field approximation for taking into account the nonadiabaticity of the ionization process in intense low-frequency laser fields.

Photoelectron spectra in strong-field ionization by a high-frequency field

We analyze atomic photoelectron momentum distributions induced by bichromatic and monochromatic laser fields within the strong-field approximation (SFA), separable Coulomb-Volkov approximation (SCVA), and ab initio treatment. We focus on the high frequency regime---the smallest frequency used is larger than the ionization potential of the atom. We observe a remarkable agreement between the ab initio and velocity gauge SFA results while the velocity gauge SCVA fails to agree. Reasons of such a failure are discussed.

Multiphoton ionization of H+2 at critical internuclear separations: non-Hermitian Floquet analysis

We present ab initio time-dependent non-Hermitian Floquet calculations of multiphoton ionization (MPI) rates of hydrogen molecular ions subject to an intense linearly polarized monochromatic laser field with a wavelength of 800 nm. The orientation of the molecular axis is parallel to the polarization vector of the laser field. The MPI rates are computed for a wide range of internuclear separations R with high resolution in R and reproduce resonance and near-threshold structures. We show that enhancement of ionization at critical internuclear separations is related to resonance series with higher electronic states. The effect of two-centre interference on the MPI signal is discussed.

Quantum-electrodynamic effects for atoms in a laser field

Quantum-electrodynamic radiation corrections to atomic energy levels in the presence of monochromatic laser field are considered as radiation shifts of quasienergies. General expressions are obtained for self-energy part of radiation shifts for an arbitrary multilevel atomic system. As an appendix, the radiation shifts of spectral lines of atomic resonance transitions are investigated.

Effective-range theory for an electron in a short-range potential and a laser field

The time-dependent effective-range (TDER) theory introduced by Frolov et al. [Phys. Rev. Lett. 91, 053003 (2003)] and used in numerous applications for processes involving a linearly polarized intense laser field is presented in detail for the general problem of an electron, initially bound in a short-range potential, interacting with a laser field of arbitrary polarization. The TDER theory combines the well-known effective-range theory for a weakly bound electron in a short-range potential with the quasistationary, quasienergy state (QQES) or Floquet formulation for an electron interacting with a harmonic, time-dependent field, such as a monochromatic laser field. We present the basic underlying assumptions, the relevant boundary conditions, and the key equations for both the quasienergy $ϵ$ of the system and its associated, three-dimensional, time-dependent wave function ${\ensuremath{\Phi}}_{ϵ}(\mathbf{r},t)$. The specific equations appropriate for two initial state symmetries (i.e., $s$- or $p$-valence electrons) and for three cases of laser field polarization (linear, circular, and elliptical) are derived and discussed. Results of the formulation are presented for the specific case of the Stark shift (and level splitting, in the case of an initially degenerate $p$ state) and detachment rates for ${\mathrm{H}}^{\ensuremath{-}}$, ${\mathrm{O}}^{\ensuremath{-}}$, the negative alkali-metal ions, and the negative halogen ions.

Low-frequency atomic stabilization and dichotomy in superintense laser fields from the high-intensity high-frequency Floquet theory

The Floquet problem for the interaction of an atom with a monochromatic laser field of frequency $\ensuremath{\omega}$ was studied long ago for the case of high $\ensuremath{\omega}$ and arbitrary intensity using the ``high-frequency Floquet theory'' (HFFT). The two parameters of the theory are the frequency $\ensuremath{\omega}$ and the classical excursion parameter ${\ensuremath{\alpha}}_{0}\ensuremath{\equiv}{E}_{0}{\ensuremath{\omega}}^{\ensuremath{-}2}$, where ${E}_{0}$ is the electric field strength. HFFT solves the Floquet system by successive iterations. Convergence of the iteration procedure was shown to be ensured by the condition that $\ensuremath{\omega}$ be suffiently large with respect to some typical atomic excitation energy. We now establish that the same iteration procedure is capable of handling the case of low frequency at sufficiently high intensity. This leads to the conclusion that in this case the ionization rates display quasistationary stabilization also at low $\ensuremath{\omega}$. The concept is thus not exclusively related to high frequencies, as widely assumed. In addition, it suggests that a more appropriate designation for the theory should be ``high-intensity, high-frequency Floquet theory'' (HIHFFT). Our general results are applied to a frequently used one-dimensional (1D) soft-core potential model, for which explicit analytic results can be obtained for the quasienergies and wave functions from the general HIHFFT formulas. The relevance of these quasistationary results for the case of laser pulses is pointed out.

Electromagnetically Induced Absorption Resonance Sign Reversal

The coherent population trapping (CPT) [1] and related electromagnetically induced transparency (EIT) phenomena have found a large number of applications in science and technology. The degenerate two-level systems provide many possibilities for investigation of CPT. Zeeman optical pumping destroyed by magnetic field (MF) allows the observation of EIT [2], as well as electromagnetically induced absorption (EIA) [3] in the atomic medium. CPT resonances prepared in the Hanle configuration have been investigated for atoms irradiated by monochromatic laser field in a way that different polarization components couple the Zeeman sublevels of single hyperfine (hf) ground level to a common excited state and introduce coherence between ground magnetic sublevels at zero value of a scanned around it magnetic field B, orthogonal to the orientation/alignment of atoms. As shown in [3], in absence of depolarizing collisions of the excited state, and depending on the ratio of the degeneracies of the two hf levels involved in the optical hf transition, EIT and EIA sub-natural width resonances can be observed for degenerate two-level systems. EIT is realized when the conditions Fg → Fe = Fg − 1, Fg are met, while EIA is observed for Fg → Fe = Fg+1. Here, Fg and Fe are the hf quan∗corresponding author; e-mail: stefka-c@ie.bas.bg

Landau-Dykhne approximation for multiphoton dipole-forbidden transitions

A two-level system in a monochromatic laser field is considered in the Landau-Dykhne approximation under the violation of dipole selection rules. An analytic expression is obtained for the rate of transitions. The multiphoton and tunneling limits are found.