Time-resolved four-wave-mixing experiments are usually interpreted in terms of noninteracting two-level systems in order to obtain information on the polarization dephasing time ${\mathit{T}}_2$. Recent experiments involving excitonic resonances in semiconductor quantum wells (including results presented in this paper) show striking qualitative deviations from this simple picture. In particular, an exponential tail is observed at low excitation for negative time delays. At high excitation, the four-wave-mixing signal is found to evolve into two distinct temporal maxima. We demonstrate that the microscopic origin of this time dependence can be understood in terms of coherent exciton-exciton interactions. We show in fact that this behavior is more general and should be seen in numerous dense media where strong nonlinear interactions of polarizations occur. In addition to presenting rigorous numerical results, we analyze two simple situations in which such interactions exist: dielectric media with local-field effects and the anharmonic oscillator. We derive analytical expressions for their time-dependent four-wave-mixing response and discuss the physical origin of these new nonlinear-optical effects.

The nonlinear equation of motion for the laser field is obtained by using a perturbation method up to third order. The linear gain and intensity of the laser field are obtained. A perfect lasing without inversion can be achieved when ${\mathrm{\delta }}_3$${\mathrm{\mu }}_2$ √${\mathrm{\Gamma }}_2$ =-${\mathrm{\delta }}_2$${\mathrm{\mu }}_3$ √${\mathrm{\Gamma }}_3$ .

Using a singular perturbation expansion, we study the evolution of a Raman loss compensated soliton in an optical fiber. Our analytical results agree quite well with the numerical results of Mollenauer, Gordon, and Islam [IEEE J. Quantum Electron. QE-22, 157 (1986)]. However, there are some differences in that our theory predicts an additional structure that was only partially seen in the numerical calculations. Our analytical results do give a quite good qualitative and quantitative check of the numerical results.

Enhanced and inhibited spontaneous emission of two close atoms in an overdamped cavity is studied taking into account the dipole-dipole interaction between them. The system is described in the minimal-coupling scheme and the advantages of this choice for our problem are discussed. The dynamics is investigated using both quantum Langevin equations and Master equation techniques. This last approach allows one to describe the behavior of the atoms plus field system as a one-step process. The case in which both atoms are initially in their excited state and the mode is in its vacuum state is discussed in some detail. It is shown that, different from what occurs to two close atoms in free space, in an overdamped cavity dipolar interaction may modify the decay rate. In particular, we find that the two transitions the atomic system performs to reach its equilibrium configuration can have different rates, and that for a suitable choice of the detuning between the cavity mode and the unperturbed atomic frequency, one of the transitions occurs at an enhanced rate while the second is inhibited.

We have observed the phase dependence of microwave above-threshold ionization (ATI) of Rydberg potassium atoms at a microwave intensity of 3×${10}^4$ W ${\mathrm{cm}}^{\mathrm{-}2}$. By using 5-ps pulses derived from a mode-locked tunable dye laser to create the initial Rydberg state, and phase locking the microwave field to a high harmonic of the mode-locking frequency, we are able to excite the atom at a specific phase of the microwave field. Measurement of the ejected electrons’ energy as a function of the initial Rydberg state and the phase of the microwave field show that it is the phase of the field at which the electron becomes free of the atom that determines its kinetic energy. The energy dependence is described by a simple, classical theory that reveals the relationship between the so-called ‘‘quiver’’ and ‘‘drift’’ terms in the electron energy previously observed in ATI experiments at higher frequencies.

A theoretical study of the resonance fluorescence emitted in a radiative collision is presented. A treatment of the spontaneous emission based on the short-pulse approximation is shown to be suitable for an analysis of the spectral distribution of the scattered radiation. Following an adiabatic dressed-state picture, a seven-peak spectral structure is predicted, in agreement with the three-level nature of the problem. Some of the spectral lines are found to be due to emission processes taking place only during the collision, allowing a characterization of the process as resonance fluorescence of a transient molecule.

There is a theorem proved by Svendsen which implies that a zero-range (δ-function) potential cannot be defined for the Dirac equation in two and three space dimensions. We give an elementary proof of the theorem by examining the solution of the Dirac equation with a square-well potential of finite range and taking the zero-range limit.

If the interaction generating a quantum measurement is too weak to completely ‘‘collapse’’ the wave function, a partial quantum Zeno effect may result. An experimental test is proposed.

The scalar optical transmittance function is given by the Fourier transform of a path integral. Here we try to express it directly by a path integral. Applications both to the calculation of complex amplitude distributions and ‘‘instantaneous’’ optical transformations in the paraxial region are developed.

In this work, we compare the saddle-point method with the Feshbach-type projection-operator method using experimental and theoretical data in the literature. It appears that the saddle-point energy gives a better approximation to the resonance energy. This result is explained in terms of the variational principle of quantum mechanics. It is also pointed out that the existing Feshbach-type projection method is not suitable for general triply excited states such as (2snln’l’) where n,n’≥3.

We discuss the discrepancy between the Cung et al. [Phys. Lett. B 68, 474 (1977)] calculation of the three-photon-annihilation contribution to the positronium ground-state energy, performed using the binding energy to regulate the infrared divergences, and the recent calculation of Adkins, Bui, and Zhu [Phys. Rev. A 37, 4071 (1988)], which used a photon mass to regulate these divergences. By using a simpler version of the binding-energy approach advocated by these authors, we confirm the value of the discrepancy they obtained. Furthermore, it is shown that the additional term needed in the binding-energy approach to produce agreement between the two methods is precisely the one required to make the binding-energy regularized amplitudes gauge invariant to all orders in the binding.

Based on a photoabsorption experiment on Be i, Caldwell et al. [Phys. Rev. A 41, 542 (1990)] have shown that the 1s2${\mathit{s}}^2$2p ${}_{}{}^1{}_{}{}^{}$P° state decays mainly through the (1${\mathit{s}}^2$2p,e) Auger channel. It is also remarked that this can be regarded as a common feature for the Be-like series. We present accurate energies of ${}_{}{}^1{}_{}{}^{}$P° on Be i, B ii, C iii, and O v that lead to identifications in high-resolution Auger experimental measurements from ion-atom and ion-molecule collisions. We show that, while the 1s2${\mathit{s}}^2$2p ${}_{}{}^1{}_{}{}^{}$P°→(1${\mathit{s}}^2$2p,e) decay channels are clearly seen in the reported spectra, the 1s2${\mathit{s}}^2$2p ${}_{}{}^1{}_{}{}^{}$P°→(1${\mathit{s}}^2$2s,e) channels are either extremely weak or not reported at all. This gives unambiguous support to the conclusion of Caldwell et al. The calculated 1s2${\mathit{s}}^2$2p ${}_{}{}^1{}_{}{}^{}$P° energy in this work is 115.52 eV above the 1${\mathit{s}}^2$2${\mathit{s}}^2$ level of Be i. This agrees well with the 115.5 eV found in the experiment.

First-order calculations of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pair production in relativistic heavy-ion collisions exceed unitarity for heavy nuclei and γ values of the order of γ>1000, at impact parameters of the order of the Compton wavelength of the electron. It is shown how unitarity can be restored by using the sudden approximation, which is very appropriate for this kind of collision. It is found that the probability of creating N ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{-}}$ pairs in a single collision is given by a Poisson distribution. Such strong-field effects will occur and can in principle be studied with relativistic heavy ions in the planned colliders like the relativistic heavy-ion collider at Brookhaven National Laboratory or the large hadron collider at CERN.

Electron-capture processes of a proton from a hydrogen atom are studied at a collision energy of 5 MeV by the classical-trajectory Monte Carlo method. Thomas double-scattering processes and knock-on captures, which are characteristic of high-energy ion-atom collisions, are detected in a two-dimensional treatment. Another type of capture that takes place through a momentum-matching mechanism is also investigated. Three-body effects modify these processes considerably in some cases from the idealized pictures given in a simplified treatment of successive two-body collisions published before [see, e.g., R. Shakeshaft and L. Spruch, Rev. Mod. Phys. 51, 369 (1979)]. Three-dimensional calculations designed for knock-on captures are also performed, and the cross section, 1.9×${10}^{\mathrm{-}29}$ ${\mathrm{cm}}^2$, is in good agreement with quantum-mechanical calculations.

Differential and total cross sections have been studied for ground-state positronium (Ps) formation using the Glauber-eikonal approximation in positron-lithium atom collisions. The valence electron of the alkali-metal target and the effect of the core have been treated in the framework of a pseudopotential formalism. Results are reported for intermediate and high energies along with the first-order Born-approximation results. Even at very high energies the eikonal cross sections are found to be different from the corresponding first-order Born approximation showing the importance of higher-order effects.

All recent determinations of the atomic transition probabilities for the prominent ArI 4s-4p lines in the red part of the visible spectrum have been carried out separately from determinations of the equally prominent 4s-5p lines in the blue part of the spectrum. A cross connection of these data is thus desirable to establish their mutual consistency. We have measured several transition probability ratios for 4s-4p to 4s-5p transitions and find very close consistency.

We discuss some of the consequences of scaling freedom in a one-dimensional atom model. We show that results already obtained with this model for a specified ionization energy and laser intensity can easily be reinterpreted for other ionization potentials and intensities. One example relevant to the experimental realizability of atom stabilization and wave function localization in strong fields is worked out explicitly.

The probability distributions for the electric-field strength, containing the full information on higher-order squeezing, are studied for special squeezed fields occurring in the k-photon Jaynes-Cummings model. Small modifications of the probability distributions at the wings may substantially alter the higher-order squeezing effects.

The statistical fluctuations in dye-laser transients are investigated when the laser is operated far above threshold. Exact analytic expressions for the mean, variance, and skewness of the first-passage-time distributions are presented when both additive and multiplicative white noise are included in two different single-mode laser models. These results are in good agreement with the numerical simulations of Zhu, Yu, and Roy [Phys. Rev. A 34, 4333 (1986)].