Monochromatic Laser Field Research Articles

Narrow spectral feature in resonance fluorescence with a single monochromatic laser field

We describe the resonance fluorescence spectrum of an atomic three-level system where two of the states are coupled by a single monochromatic laser field. The influence of the third energy level, which interacts with the two laser-coupled states only via radiative decays, is studied in detail. For a suitable choice of parameters, this system gives rise to a very narrow structure at the laser frequency in the fluorescence spectrum which is not present in the spectrum of a two-level atom. We find those parameter ranges by a numerical analysis and use the results to derive analytical expressions for the additional narrow peak. We also derive an exact expression for the peak intensity under the assumption that a random telegraph model is applicable to the system. This model and a simple spring model are then used to describe the physical origins of the additional peak. Using these results, we explain the connection between our system, a three-level system in V-configuration where both transitions are laser driven, and a related experiment which was recently reported.

Approximate model of intense field stabilization for hydrogen atom

An approximate model is proposed to study the stabilization problem of hydrogen atoms under monochromatic intense laser field. The stabilization regime for system parameters such as laser field strength, laser field frequency and atomic magnetic quantum number are obtained by stability analysis of fixed points of the model. The results are consistent with those obtained by other methods.

Stabilization of bound state decay in an intense monochromatic high-frequency field

The formalism of complex quasienergies is used for exact calculation of the field-dependent decay rate for a weakly bound particle (in the model of a three-dimensional zero-range potential) in a strong monochromatic laser field. It is shown that the adiabatic (quasistationary) stabilization regime in this model occurs at frequencies ω exceeding the binding energy and only in a limited intensity range. A simple estimate is obtained for the critical field of stabilization breakdown. The effect may be observed for the decay of H− ions in the field of a neodymium laser of femtosecond duration.

Influence of quantum transitions in the continuum on ionization of atoms in strong fields

The tight-binding method is used to analyze the ionization of a hydrogenlike atom by an intense monochromatic laser field. The orthogonal and normalized basis in which the solution of the time-dependent Schrodinger equation is expanded contains unperturbed wave functions of the discrete spectrum and generalized Coulomb wave functions of the continuum. In the solution of the coupled equations we make use of the fact that the bound-free and free-free transitions are efficient in different regions of complex time. Simplified equations are constructed and investigated. Results of calculations for ionization of a hydrogen atom from its ground state and of the energy distribution of the electrons in strong and superstrong linearly polarized fields are presented. It is shown that in this case the ground state decays completely, and free-free transitions play a defining role in the dynamics of the process. It is established that the total probability of population of the upper Rydberg states abutting the continuum does not exceed 0.05. The range of applicability of the approach is discussed. A comparison with numerical results obtained by other authors is given.

Phase control of x-ray-atom scattering in the presenceof a bichromatic laser field

We consider the scattering of 50 eV x-rays by hydrogen atoms in the presence of a bichromatic, linearly polarized laser field with frequencies and r, where r = 2,3, = 1.17 eV, and with relative phase between the bichromatic laser field components. Numerical results for the differential cross-section (DCS) as a function of the number n, where n is the energy exchanged with the laser field, are presented. For either a monochromatic laser field or a bichromatic laser field with the frequencies and 3, the integer n can only be even, while for a bichromatic laser field with the frequencies and 2, the integer n can have any value. For small values of |n| we find pronounced maxima in the DCS. For larger |n| we find plateaus which are different for negative and positive values of n. For lower values of the laser field intensity the plateau for negative values of n is much more extended and greater in magnitude than that for positive values of n. The height of either plateau is also higher for the bichromatic laser field than for either monochromatic field. In the (, 2) case we find the symmetry DCS(+) = DCS(). We show that the relative phase influences the DCS so that phase control of the x-ray-atom scattering process is possible. For some values of the energy of the outgoing x-ray photons can be increased. Finally, a quasi-classical explanation of the results is presented.

Phase-Dependent Resonance Fluorescence and Time Ordering in Photodetection

We measure phase-dependent fluorescence spectra of two-level atoms driven by a monochromatic laser field, for off-resonance excitation. Pairs of spectra with phases of opposite sign are found to display striking differences that arise entirely from time ordering.

Optical measurements of the condensate phase

We propose a light-scattering scheme to measure the relative phase of atomic Bose-Einstein condensates and its diffusion rate. The proposal relies on the existence of two independent condensates coupled to a common excited state. To this end, we consider a two-well ground-state potential together with excited-state trap wave functions that extend over the whole region. When the first trap is driven by a weak monochromatic laser field, the light scattered from the second trap has a nonzero mean-electric field amplitude with a phase shift proportional to the difference of the condensate phases. When both condensates are driven, the phases of the two laser fields can be adjusted to cancel the scattering completely by a quantum interference. The particular value of the laser phase difference that gives zero scattering determines the relative phase of the two condensates.

Floquet formulation of time-dependent density functional theory

We present a generalized Floquet formulation of time-dependent density functional theory for nonperturbative treatment of both bound-bound and bound-free multiphoton transitions of many-electron systems in intense monochromatic laser fields. It is shown that the time-dependent Kohn-Sham equations can be converted into an equivalent time-independent infinite-dimensional non-Hermitian Floquet Hamiltonian eigenvalue problem. We introduce the notion of complex density and present a procedure for extracting the partial ionization rates from individual electron orbitals. The method is illustrated by a case study of multiphoton ionization of He in both weak and strong fields.

Bremsstrahlung and harmonic generation in laser-assisted electron-nucleus collision

The spectrum of radiation emitted by an electron colliding with a nucleus in the presence of a monochromatic laser field is calculated. The Coulomb potential is treated at all orders and the radiation is seen as spontaneous transition between two free states. The kinetic energy of the electron isT∈100−3000 eV and the laser intensityI∈1010−1013 W/cm2; in this condition we see strong enhancement in thebremsstrahlung cross-section when the emitted frequency is an integral multiple of the laser frequency.

Stimulated emission in a three-level atom

We consider light amplification arising from the competition between absorption and stimulated emission of an excitation, which is induced by a monochromatic laser field at low intensities interacting with a three-level atom. The lifetime of the induced excitation 10 5–10 6 times longer than those of excitations spontaneously emitted. Conditions are established under which the stimulated emission prevails over the induced absorption, and the net negative intensity is 10 2–10 3 times greater than that of excitations spontaneously emitted, indicating that significant amplification occurs near the frequency of the laser field.

A double quantum well in a driving laser field

The dynamic effect of electrons in a double quantum well under the influence of a monochromatic driving laser field is investigated. Closed-form solutions for the quasienergy and Floquet states are obtained with the help of SU(2) symmetry. For the case of weak interlevel coupling, explicit expressions of the quasienergy are presented by the use of perturbation theory, from which it is found that as long as the photon energy is not close to the tunnel splitting, the electron will be confined in an initially occupied eigenstate of the undriven system during the whole evolution process. Otherwise, it will transit between the lowest two levels in an oscillatory behavior.

Excitation of coherent Raman beats in rare earth solids with a bichromatic laser field

A new method is presented for measuring coherent Raman beats in systems with a wide spectral range that works even for weakly allowed optical resonance lines. A bichromatic pump laser field is used to resonantly pump a coherent excitation in the material, and a monochromatic test laser field, whose frequency coincides with that of one component of the pump laser beam.

Born-Floquet theory of laser-assisted electron-atom collisions

We present a non-Hermitian Born-Floquet theory of scattering of fast electrons by atoms in the presence of a strong monochromatic laser field. The interaction of the laser field with both the incident electron and the target atom is treated nonperturbatively, while the interaction of the incident electron with the target atom is treated in first Born approximation. Fluorescence is neglected. Detailed calculations are performed for the ''elastic'' scattering of 500 eV electrons by atomic hydrogen accompanied by the transfer of photons. The contribution of the entire spectrum of unperturbed atomic states to the dressing of the target is exactly taken into account by performing the calculations on a complex Sturmian basis set. In the nonresonant case, and for electric field strengths that are small with respect to the atomic unit, our Born-Floquet results are in agreement with those obtained using the semiperturbative approach of Byron and Joachain (in which target dressing is treated in first-order perturbation theory) even at intensities where multiphoton ionization is nonperturbative. The Born-Floquet approach is particularly useful to study resonant cases, where the laser frequency matches a transition frequency in the atom. Two such situations are analyzed.

Dynamic behavior of highly excited molecular states in the strong monochromatic laser fields

The dynamic behavior of highly excited molecular states in an external monochromatic field has been investigated in order to establish the general trends in the Rydberg state manifestations in collisional and radiative processes. The effects of interference between direct (background) and resonant interactions and coupling between the continua on the fine structure of collision cross sections and near-threshold photoabsorption spectra are discussed. Analytical expressions for the widths and intensities of the Rydberg lines induced by mixing the field with other quasistationary states are derived and their dependence on the external field strength and frequency are analyzed. It was found that the appreciable stabilization of isolated Rydberg levels observed previously in superstrong fields is also possible in fields much weaker than atomic fields. The possibility of laser control for the energy averaged cross sections and reaction rate constants are considered. All effects are illustrated for thee− + H2+ system.

Chaos and the taming of free electron laser spectral dynamics

We provide a semi-quantitative analysis of the electron phase space in a uniform wiggler with a broad spectrum laser field, and in an optimized two frequency wiggler with a monochromatic laser field. We show that the two problems are formally equivalent, and for the first one we reach a theoretical interpretation and an estimate for the universal ratio between extracted efficiency and relative spectral width.

Calculation of enhanced slowing and cooling due to the addition of a traveling wave to an intense optical standing wave.

We investigate the force on a two-level atom interacting with intense monochromatic laser fields which are combinations of standing and traveling waves. We present a continued-fraction solution to the optical Bloch equations. Using this solution to calculate the force on an atom, we have examined the slowing and cooling of a thermal Na atomic beam. We find that the addition of a traveling wave to an intense standing wave can significantly improve the slowing rate and simultaneously decrease the final velocity of the cooled beam.

Multiphoton absorption in anharmonic systems

We study the multiphoton absorption of the diatomic HF by time propagating a wave packet on the electronic potential under the action of a monochromatic laser field. We apply the pseudo-spectral splitoperator method for the propagation. We use different approximations for the potential and for the dipole function. We find that the quantitative details of the absorption spectrum are sensitive to the form of the dipole function. © 1992 John Wiley & Sons, Inc.

Dipole force rectification in a monochromatic laser field

We propose a novel scheme which allows to generate a strong rectified dipole force in a simple monochromatic laser field. A basic example demonstrates that this optical force can act on atoms with a Zeeman-split transition in the field of two counterpropagating intense laser beams with different polarization.

Light-pressure-induced nonlinear dispersion of a laser field interacting with an atomic gas.

We report on detailed studies of the effect of resonant light pressure on the optical response of an atomic gas to a single monochromatic laser field. In this very elementary situation of laser spectroscopy, the redistribution of atomic velocities that is induced by spontaneous light pressure leads to a novel contribution to the optical dispersion curve of the medium. This light-pressure-induced dispersion phenomenon displays a pronounced nonlinear dependence on the laser intensity. Moreover, for a given intensity, its strength is closely related to the laser beam diameter. As most important feature, this light-pressure-induced dispersion displays an even symmetry with respect to the optical detuning from line center. As a result, the total Doppler-broadened dispersion curve of the gas can become asymmetric, and a significant shift of the dispersion line center can occur. In addition to a detailed theoretical description of the phenomenon, we report on its experimental investigation on the \ensuremath{\lambda}=555.6 nm $^{1}$${\mathit{S}}_{0}$${\mathrm{\ensuremath{-}}}^{3}$${\mathit{P}}_{1}$ transition in atomic ytterbium vapor with the use of frequency-modulation spectroscopy. The experimental findings are in good quantitative agreement with theoretical predictions.

Coherent Nonlinear Optical Processes in Semiconductors. Physics and Applications

AbstractThe influence of monochromatic nonresonant laser fields is discussed on the elementary excitation spectrum of semiconductors. It is shown that the different renormalizations in inorganic and onedimensional organic materials can be understood in the framework of one and the same theory. Applications, such as generation of ultrashort voltage pulses in biased quantum wells, are discussed.