An analysis of near-resonant collisions between dissimilar atoms in the presence of a radiation field reveals their effect on the far wings of the absorption profile. Extending a model due to Cavalieri, Arimondo, and Matera, we take into account the collisional shifts of atomic levels, as well as the collisional coupling between atoms and the coupling with the radiation field. A general formula for the absorption profile is derived and expressed in terms of the universal line-shape function of the unified Franck-Condon treatment of pressure broadening. We show that in some cases collision-induced coherence can give rise to a strongly depressed red wing that exhibits transparency for some values of the laser detuning. © 1996 The American Physical Society.

The excitation of nuclear states by electric and magnetic multipole transitions of bound electrons in the presence of a strong laser field is studied. The excitation probability is calculated for general time-dependent electronic states in first-order perturbation theory. An adiabatic description of the dressed electron states in the laser field allows an easy calculation of the excitation function. Ionization effects caused by the laser are considered in a simple manner. The probability for the excitation of the nuclei ${}_{66}{}^{161}{}_{}{}^{}\mathrm{Dy}$, ${}_{76}{}^{189}{}_{}{}^{}\mathrm{Os}$, ${}_{77}{}^{193}{}_{}{}^{}\mathrm{Ir}$, ${}_{79}{}^{197}{}_{}{}^{}\mathrm{Au}$, ${}_{92}{}^{235}{}_{}{}^{}\mathrm{U}$, and ${}_{93}{}^{237}{}_{}{}^{}\mathrm{Np}$ is investigated as a function of the laser intensity and the photon energy. The increase of the excitation probability is limited by the ionization of the atom and the lifetime of the states. The de-excitation of the 3/$2^{+}$ (3.5±1.0 eV) state in ${}_{90}{}^{229}{}_{}{}^{}\mathrm{Th}$ is an example of the laser-assisted discrete internal conversion. © 1996 The American Physical Society.

The dynamics of strong-field (∼${10}^{14}$ W/${\mathrm{cm}}^2$) multiphoton processes in molecules can be investigated using a simple, collinear model for ${\mathrm{H}}_2^{+}$. We discuss properties of this model and report preliminary calculations related to recent experimental results. We find that the excitation and ionization dynamics of the molecular ion depend strongly on the laser wavelength, pulse shape, intensity, and particularly on the initial vibrational state of the molecule. © 1996 The American Physical Society.

The effects of permanent dipole moments and those due to the randomness of molecular orientation in the phase control of molecular excitation are discussed for the simultaneous one- and three-photon excitation of a two-level model molecule. In this transition scheme both transitions can occur with or without the presence of permanent dipoles and the results are contrasted to those corresponding to the one- and two-photon excitation of a two-level molecule, which requires the presence of permanent dipoles. The dependence of the temporal evolution of the excited state and the associated resonance profiles on the relative phase of the lasers is used to monitor the control of the excitation process. Analytical perturbation theory, the rotating-wave approximation, and exact Floquet results for those observables are used for this purpose. Both fixed molecule-laser configurations and situations where the absorbing molecules assume random orientations with respect to the laser beams are considered. © 1996 The American Physical Society.

We present the experimental and theoretical investigation of the effects of the strong electromagnetic coupling of two autoionizing states (AIS’s) on the photoionization properties of an atomic system. These effects are observed by probing the ladder system from the ground state of the atom through a weak two-photon transition to the region of the lower AIS. The experimental observation of the modification of the total ionization and partial photoelectron spectra at different wavelengths of the coupling and probe lasers are accurately reproduced by a theoretical analysis which uses a density-matrix approach. Both theory and experiment indicate a strong inhibition of the ionization of the system in the case when both lasers are resonant with the two atomic transitions, while displaying a rapidly varying asymmetry induced as the detuning between the two states and the fields increases. In the extreme case of a large detuning, two separate structures appear as distinct total ionization and photoelectron peaks. The above findings combined with the full theoretical treatment indicate that the phenomena observed are an experimental demonstration of ac Stark splitting between two AIS’s. © 1996 The American Physical Society.

A general Floquet approach is used to investigate the nonlinear response of a multilevel quantum system driven by a strong, in general off-resonance, pump field. The response to a weak probe field is studied near the quasienergy interval resonances. A general condition is derived for a phenomenon in which the transition matrix elements for a given pair of Floquet states simultaneously vanish for an infinite number of equally spaced resonances beyond a unique frequency edge. This phenomenon is illustrated with several models involving modulating interactions and possibilities for its observation are analyzed. © 1996 The American Physical Society.

We compute the near-resonant dipole-dipole interaction between atoms in a three-dimensional optical lattice. After a general derivation, we concentrate specifically on the case of a J=1/2→J=3/2 off-resonant transition, in which case the excited electronic states can be adiabatically eliminated. We present in detail the spatial dependence of the resulting effective dipole-dipole potential, and discuss the physical origin of its anisotropy. In addition to impacting the center-of-mass motion of the atoms, the effective dipole-dipole potential can induce electronic transitions within one or both atoms. The symmetries of the matrix elements corresponding to these various mechanisms are also commented upon. © 1996 The American Physical Society.

One-dimensional laser cooling based on velocity-selective coherent population trapping (VSCPT) has been investigated numerically through the solution of the optical Bloch equations and through a Monte Carlo analysis. The 1→1 and 2→2 transitions have been examined as a function of the atomic recoil frequency, the spontaneous-emission decay rate, and the Rabi frequency of the cooling laser. It has been found that for a large set of those parameters, the VSCPT cooling process may be described through scaling-law relations. The scaling laws are not valid at long atom-laser interaction times or large Rabi frequencies, where the atomic Doppler shift plays a significant role in the atomic motion evolution. Similar results for two atomic transitions suggest the validity of the scaling law for any one-dimensional VSCPT process. © 1996 The American Physical Society.

A theory of resonance fluorescence in a monochromatic field and pump-probe spectroscopy is developed using semiclassical dressed states. The semiclassical dressed states are obtained via a rotation from the time-dependent state vectors in a field-interaction representation. Although the semiclassical dressed states are not stationary states as are the dressed states in a quantized-field formulation, they still offer all the conceptual advantages of a dressed-atom approach. Moreover, the line-shape formulas can be obtained directly from physically intuitive equations, without having to invoke the quantum regression theorem and without having to worry about cascades and coherences in the ladder of quantized dressed states. © 1996 The American Physical Society.

There has been much recent discussion of problems or unphysical results occurring from the study of the non-rotating-wave (NRW) master equations. Much of this discussion has centered on the quantum Brownian motion master equation, one particular NRW master equation. These anomalous results can occur because the (NRW) master equation is not of the ‘‘Lindblad form.’’ In this paper we discuss the limitations of the NRW master equation. Two specific examples are considered, the first being a closely spaced two-level system, while the second is the classical damped free particle. © 1996 The American Physical Society.

Two cw Raman lines at 603.0 nm (2${\mathit{p}}_2$→1${\mathit{s}}_4$) and 659.9 nm (2${\mathit{p}}_2$→1${\mathit{s}}_2$), generated when cw dye-laser radiation is mode matched to the 588.2-nm (1${\mathit{s}}_5$→2${\mathit{p}}_2$) transition of Ne I in a commercial He-Ne laser cavity, can both show optically bistable switching simultaneously with the pump beam. A theoretical treatment based on a semiclassical three-state Raman laser model is used to qualitatively reproduce the experimental observations and to identify the mechanisms that produce the experimental signature. Important features of the Raman gain are characterized. These, in combination with the phase-shift properties of the pump and Raman fields, are used to explain the switching behavior. © 1996 The American Physical Society.

The optical dark resonance in a symmetric five-level system with two lasers and two rf (or microwave) fields is studied theoretically. We obtain the analytic steady-state solution for the density matrix of the system. We find that some manifolds of narrower dark resonance due to rf fields appear at the positions that depend on the Rabi frequencies and the detuning frequencies of rf fields. The exact expression of the resonance position, the line shape, and the resonance width are obtained. The resonance width of the dark lines with rf-dressed atoms is proved to be narrower than that of the ordinary three-level Λ system and is desirable for trapping atoms. As the dark lines can simultaneously trap atoms with a nonzero velocity as well as those with a zero velocity, they can be used for the collision experiments. © 1996 The American Physical Society.

The determination of the quantum properties of a single-mode radiation field by heterodyne or double homodyne detection is studied. The realistic case of not fully efficient photodetectors is considered. It is shown that a large amount of quite precise information is available, whereas the completeness of such information is also discussed. Some examples are given and the special case of states expressed as a finite superposition of number states is considered in some detail. © 1996 The American Physical Society.

The problem of photon creation from vacuum in an ideal cavity with vibrating walls is studied in the resonance case, when the frequency of vibrations equals twice the frequency of some unperturbed electromagnetic mode. Analytical solutions are obtained in two cases: for the one-dimensional model (scalar electrodynamics) and for the three-dimensional (3D) cavity. In the first example, we have a strong intermode interaction; nonetheless, an explicit solution in terms of the complete elliptic integrals is found. The rate of photon generation in the principal mode rapidly assumes a constant value proportional to the product of the frequency by the dimensionless amplitude of oscillations. The total amount of photons created in all the modes increases in time as ${\mathit{t}}^2$. In the second example, the eigenmode spectrum is nonequidistant and the problem can be reduced to the problem of a single harmonic oscillator with a time-dependent frequency. The number of photons in the resonant mode of a 3D cavity increases exponentially in time and the field appears in a highly squeezed state with a strongly oscillating photon distribution function. The problem of detecting the created photons is analyzed in the framework of a simplified model, when a detector is replaced with a harmonic oscillator. It turns out that the presence of the detector changes the picture drastically: both the detector and the field mode occur in highly mixed (nonthermal) quantum states, with identical nonoscillating photon distribution functions. The detector gains exactly half of the total energy of excitation inside the cavity. The estimations show a possibility of creating up to several hundred or even thousand photons, provided that the cavity’s Q factor exceeds ${10}^{10}$ and the amplitude of the wall’s oscillations is greater than ${10}^{\mathrm{-}10}$ cm at a frequency of the order of 10 GHz. © 1996 The American Physical Society.

The quantized center-of-mass dynamics of two-level atoms in a periodically modulated standing-wave laser field is analyzed, including the decohering effect of spontaneous emission. Under conditions appropriate for the occurrence of dynamical localization as determined in previous work, we predict a random diffusive motion of dynamically localized de Broglie waves due to spontaneous emission and calculate the diffusion constant and the diffusing distribution of wave packets. © 1996 The American Physical Society.

Quantum-state diffusion (QSD) as a tool for solving quantum-optical master equations by stochastic simulation can be made several orders of magnitude more efficient if states in Hilbert space are represented in a moving basis of excited coherent states. The large savings in computer memory and time are due to the localization property of the QSD equation. We show how the method can be used to compute spectra and give an application to second harmonic generation. © 1996 The American Physical Society.

An efficient method of quantum state engineering based on discrete superpositions of coherent states is presented. A systematic method is developed for obtaining optimized superpositions from the one-dimensional representation of the desired state. It is shown that superposition of even a small number of coherent states put along a straight line or on a circle in phase space can approximate nonclassical field states with a high degree of accuracy. An experimental scheme is proposed for generating equidistant coherent-state superpositions on a circle with arbitrary coefficients. © 1996 The American Physical Society.

We study the effect of inhomogeneous broadening of the electronic state on vacuum-field Rabi splitting. The broadening has no effect on the size of the splitting and, in general, does not lead to an inhomogeneous broadening of the split states. From a spectroscopic point of view, these results have interesting consequences, since they allow the extraction of a homogeneous line in an inhomogeneously broadened system. © 1996 The American Physical Society.

The effect on the electromagnetic modes of the interaction with a polarizable medium confined along one direction is studied in the framework of a full quantum-mechanical theory. A general quantum-mechanical method of computing the scattered electromagnetic field in confined systems is presented. A precise link between the semiclassical and the quantum theory of polaritons in confined systems is established. The method is applied to the case of a semiconductor quantum well embedded in infinite barriers. The experimental situation of a classical traveling light wave partly reflected and partly transmitted by the well is found to correspond to a particular coherent photon-exciton mixed state. Simple analytical expressions are obtained for experimental quantities such as transmission and reflectivity. The polariton quanta are identified as the ‘‘physical’’ photons detected in photon counting experiments. © 1996 The American Physical Society.

We give a general definition of unpolarized light that rests on invariance properties rather than specific values of second-order field correlations or, equivalently, the Stokes parameters. It turns out that to require invariance with respect to rotation around the propagation axis and symmetry with respect to left- and right-handed circular polarization is not sufficient to fully characterize completely unpolarized light, such as natural light. Actually, a third requirement demanding invariance with respect to phase retardation is needed. Depending on whether it is satisfied or not, we speak of type I or type II unpolarized light, and we specify the general form of both classical distribution functions and density operators consistent with the respective conditions. © 1996 The American Physical Society.