Bichromatic Laser Field Research Articles

Photoelectric effect in the presence of a bichromatic laser field

Photoelectric effect in atomic hydrogen in the presence of bichromatic laser fields composed of a fundamental frequency and its second or third harmonics is investigated. The expression of the S-matrix for the laser-assisted photoelectric effect is derived in the length gauge, which can simplify the elements of the matrix. A general dressed wavefunction of the atom is derived in perturbation theory. The continuum state of the ejected electron is described by a Coulomb–Volkov wavefunction. Numerical results of photoionization cross section for the distinct second component are obtained and compared with the monochromatic field dressing case. It is shown that the photoionization cross section is modified notably by introducing the second component of field, especially when the colliding system absorbs or emits a considerable number of laser photons.

Positron-Impact Ionization of Atomic Hydrogen in a Bichromatic Laser Field in the Second Born Approximation

The positron impact-ionization of atomic hydrogen in the presence of a linearly polarized bichromatic field is investigated in the second Born approximation. The field is composed of a fundamental frequency and its second harmonic. The state of positron in the field is described by the Volkov wavefunction, and the continuum state of the ejected electron is described by the Coulomb—Volkov wavefunction. The dressed ground state of target is a first order time-dependent perturbative wavefunction. The triple differential cross sections and their dependencies on laser field parameters are discussed. Numerical results demonstrate that the second-order effect plays a crucial role in understanding the laser-assisted positron scattering process.

Coherent phase control in electron–argon scattering in a bichromatic laser field

This paper theoretically investigates the coherent phase control in electron-argon scattering assisted by a bichromatic laser field. The laser field is composed of a fundamental component and its second harmonic. The incoming and out going states of electron are described by the Volkov wave functions, and the electron-target interaction is treated as a screening potential. Numerical results for differential cross section of multiphoton processes vs the phase difference between the two components of laser field are discussed for several scattering angles and impact energies.

10.1007/s11490-008-1012-3

The application of resonant light pressure in the presence of nonuniform bichromatic laser fields for the localization and control of ultracold electron-ion plasma in which the quantum transitions of ions are resonant with respect to the laser radiation is analyzed. It is demonstrated that the so-called rectified radiation forces that selectively act upon plasma ions and give rise to a dissipative optical superlattice (for ultracold plasma) whose period is significantly greater than the radiation wavelength can be used for this purpose.

The effect of coherent phase control in (e, 2e) process of a hydrogenic ion in presence of a two colour laser field

We study the coherent phase control (CPC) of ionization (TDCS) of a hydrogenic ion under the action of a bichromatic laser field consisting of a fundamental frequency and one of its harmonics by varying the phase difference between the two components. A significant reduction is noted particularly in the recoil peak (dominant for the ionic target) with the two color field as compared to the monochromatic one, while the modification of the binary peak is comparatively less. The CPC is found to be more effective for the odd harmonics than the even one.

Electromagnetically induced absorption in a bichromatic laser field

In this paper we describe experimentally and theoretically the increase in the electromagnetically induced absorption (EIA) resonance under the influence of an additional optical field. EIA was observed on the cycling transition of the rubidium ${\text{D}}_{2}$ line in the Hanle configuration in the presence of the second-laser field acting on an open transition coupled with a cycling one via a common ground state. We present results obtained for different polarization orientations and propagation directions of the two laser beams. A systematic theoretical analysis based on the optical Bloch equations explains most of the experimental data. A lot of attention is also paid to a qualitative understanding of the EIA---its resonance shape, amplitude, and the response to the additional optical 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.

Dressed states analysis of lasing without inversion in a three-level ladder system: the steady-state regime

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bi-chromatic laser field in the ladder configuration is presented. In our model we allow the atom to be dressed by both of the coupling and probe fields. The system under consideration is explored both analytically and numerically within the steady-state regime. A parameter study is performed in which we explore the influence of the change of control parameters (Rabi frequencies and incoherent pump rates, etc) on the system behaviour. The parameter study also applies to situations where both coherent fields have comparable intensities. Expressions for steady-state populations and coherences are derived, from which the constraints on the system control parameters allowing gain or absorption of various types, are calculated. The system is demonstrated to possess gain without inversion (GWI), as well as regular gain with inversion, for the appropriate choice of system control parameters. Inversion without lasing (IWL) is also found. The dressed-state approach offers a different insight into the system dynamics in which Rabi oscillations and spectral features appear natural. We believe that the dressed-state approach provides a more adequate description of the system dynamics than that provided by the bare state basis.

Electron-Helium Scattering in a Bichromatic Laser Field in the Second-Order Born Approximation

The elastic scattering of electrons in atomic helium assisted by a bichromatic laser field is investigated in the second order Born approximation. The target atom is approximated by a simple screening potential. The dependence of the differential cross section on the relative phase between the two laser components is calculated, and compared with the recent results of first order Born approximation [Sun J F, Liang M C and Zhu Z L 2007 Chin. Phys. Lett. 24 2572].

Phase-dependent quantum interference between different pathways in bichromatic harmonic generation

This paper studies the harmonic generation of the hydrogen atom subjected to a collinear bichromatic laser field by numerically solving the time-dependent Schrödinger equation using the split-operator pseudo-spectral method. By adding a frequency variation to the additional field, the contributions of different pathways to particular order harmonic generation can be isolated. The quantum interference pattern between harmonic pathways, which influences the harmonic intensity, is found to be either constructive or destructive with respect to different relative phase of the two field components. Detailed description of up to the 35th-order harmonics and the harmonic pathways for a wide range of field parameters is presented.

Dressed states analysis of lasing without population inversion in a three-level ladder scheme: Approximate analytic time dependent solutions

A dressed-state study of lasing without population inversion from a three level atom interacting with a bi-chromatic laser field, in the ladder configuration, is formulated. We allow the atomic system to be dressed by both laser filed photons (double dressing). The evolution of the system under consideration is being explored both analytically and numerically, within the transient regime. Time dependent approximate analytic solutions for dressed-state populations and coherences are derived, within the so called “secular approximation,” under resonant conditions. We also present time dependent numerical solutions for population and coherences in the off-resonance regime. A spectral analysis is also performed revealing the structure of various dressed states transitions. These are shown to be composed of quintets centered about the frequencies of the coupling and probe laser fields and having sidebands located symmetrically at positions shifted from line center at the Rabi and double Rabi frequencies.

Photoelectron angular distributions of molecules in bichromatic laser fields of circular polarization

Photoelectron angular distributions (PADs) from above-threshold ionization of O 2 and N 2 molecules irradiated by a bichromatic laser field of circular polarization are studied. The bichromatic laser field is specially modulated such that it can be used to mimic a sequence of one-cycle laser pulses. The PADs are greatly affected by the molecular alignment, the symmetry of the initial electronic distribution, and the carrier-envelope phase of the laser pulses. Generally, the PADs do not show any symmetry, and become symmetric about an axis only when the symmetric axis of laser field coincides with the symmetric axis of molecules. This study shows that the few-cycle laser pulses can be used to steer the photoelectrons and perform the selective ionization of molecules.

High-order harmonic generation by carbon nanotubes in bichromatic laser field

The generation of high-order harmonics from a single walled carbon nanotube interacting with a bichromatic laser field is investigated. We study the nonlinear motion of π electrons in metallic carbon nanotubes ((9,0) zigzag and (10,10) armchair) driven by intense laser fields and the induced current spectrum has been analyzed. The effect of variation intensity of the applied laser fields on electron current density and high-order harmonic generation is investigated. Numerical calculations show that with application of the bichromatic laser field both odd and even harmonics are generated.

Electron-Helium Atom Collisions in the Presence of a Bichromatic Laser Field

The differential cross section (DCS) for electron-helium atom collisions in the presence of a bichromatic CO2 laser field is investigated as a function of the scattering angle θ by employing first-Born approximation (FBA) with a simple screening electric potential. We discuss in detail the influence of the scattering geometry, the photon energy and the number of photons exchanged on the DCSs. These illustrate that the three factors have important effects on the elastic scattering and the screening electric potential is effective.

Bifurcations as dissociation mechanism in bichromatically driven diatomic molecules

We discuss the influence of periodic orbits on the dissociation of a model diatomic molecule driven by a strong bichromatic laser fields. Through the stability of periodic orbits, we analyze the dissociation probability when parameters, such as the two amplitudes and the phase lag between the laser fields, are varied. We find that qualitative features of dissociation can be reproduced by considering a small set of short periodic orbits. The good agreement with direct simulations demonstrates the importance of bifurcations of short periodic orbits in the dissociation dynamics of diatomic molecules.

Coherent hole-burnings induced by a bichromatic laser field

In a Doppler-broadened three-level Λ-type system driven simultaneously by a coupling laser and two equal-amplitude saturating laser fields with a frequency difference 2δ, the absorption spectrum of a weak probe laser exhibits multiple deep spectral holes through coherent hole-burning CHB with controllable numbers, widths, depths, and positions. More significant, changing δ or lasers directions, CHBs can degenerate into narrower and deeper spectral holes where the slope of the refractive index is very steep. The multiple narrow spectral holes in a single absorption profile are expected to have potential applications in high density storage, optical information processes, and slow-light.

Time-dependent dressed-states analysis of lasing without population inversion in a three-level ladder scheme

A dressed-state study of lasing without population inversion from a three-level atom interacting with a bichromatic laser field in the ladder configuration is being formulated. The system is allowed to be dressed by both laser-field photons (double dressing). The temporal behavior of the system under consideration is being explored numerically. Although the use of dressed-basis density matrix equations introduces major complexity into the equations, this procedure has crucial importance to the understanding of the atomic spectrum and the underlying physics of the processes involved. The system is found to possess a gain without population inversion, as well as a regular gain with inversion for various transitions, both in the transient and steady-state regimes. Enhancement of the refractive index accompanied by vanishingly small absorption was also found. The frequency response of various transitions is explored via numerical Fourier analysis. Some transitions are found to exhibit a dispersion-like line shape, clear evidence of a quantum-coherent interference effect. The spectrum consists of two distinct quintets centered about the coupling and probe-laser frequencies. The spectral features consisting of each quintet are located symmetrically (at resonance conditions only) around the line center at positions shifted from the center by the generalized Rabi and double Rabi frequencies.

Ion trap quantum gates with amplitude-modulated laser beams

In ion traps, entangling gate operations can be realized by a bichromatic pair of laser beams that collectively interact with the ions. In this paper, a new method of modelling the laser–ion interaction is introduced that turns out to be superior to standard techniques for the description of gate operations on optical qubits. The treatment allows for a comparison of the performance of gates based on σz⊗σz and σϕ⊗σϕ interactions on optical transitions where the bichromatic laser field can be realized by an amplitude-modulated laser resonant with the qubit transition. Shaping the amplitude of the bichromatic laser pulse is shown to make the gates more robust against experimental imperfections.

Dissipative optical superlattice for confinement of ultracold plasma with resonant ions

The application of resonant light pressure in the presence of nonuniform bichromatic laser fields for the localization and control of ultracold electron-ion plasma in which the quantum transitions of ions are resonant with respect to the laser radiation is analyzed. It is demonstrated that the so-called rectified radiation forces that selectively act upon plasma ions and give rise to a dissipative optical superlattice (for ultracold plasma) whose period is significantly greater than the radiation wavelength can be used for this purpose.

Polarization properties of a one-atom maser under a step-wise two-photon pump

The theory of a one-atom maser, when the atoms enter the microresonator being excited to the upper maser level from its ground state by the bichromatic laser field resonant with the two adjacent optically allowed transitions with arbitrary angular momenta, is built. This theory is applied to the investigation of the polarization properties of a one-atom maser, i.e., the dependence of the maser field characteristics on the mutual orientations of the polarization vectors of the maser field mode and of the pump laser field. New experiments are proposed to study such polarization properties, which may be performed within the framework of the experimental setup used by Professor H. Walther and collaborators [1–3].