The effect of saturable absorption on second-harmonic generation in a quasi-phase-matched structure is studied, including both plane-wave and diffractive coupling of the fields and detuning of the pump field from resonance. In the regime of large Fresnel numbers, the main effect of saturable absorption is to suppress the parametric instability, which limits the efficiency of the second-harmonic generation and places a bound on the device length in order to fit sufficient harmonic power. The optical limiting nature of resonance absorption places an upper bound on the gain that quasi-phase-matching can achieve. In the diffractive regime, a variety of output transverse structures may be initiated by modulation of the relative phase and mutual displacement between the input beams of the pump and seeding harmonic signal. In particular, second-harmonic dragging, i.e., the asymmetric interaction between initially overlapping pump and second-harmonic beams propagating at different angles, is observed such that a pump beam can drag a second-harmonic one out a spatial aperture, thereby implementing a colored logical switch with gain.

We present a universal algorithm for an efficient deterministic preparation of an arbitrary two-mode bosonic state. In particular, we discuss in detail preparation of entangled states of a two-dimensional vibrational motion of a trapped ion via a sequence of laser-stimulated Raman transitions. Our formalism can be generalized for multimode bosonic fields. We examine stability of our algorithm with respect to a technical noise.

We consider the quantum field theory of two bosonic fields interacting via both parametric (cubic) and quartic couplings. In the case of photonic fields in a nonlinear optical medium, this corresponds to the process of second-harmonic generation (via ${\chi }^{\mathrm{}\left(2\right)\mathrm{}}$ nonlinearity) modified by the ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ nonlinearity. The quantum solitons or energy eigenstates (bound-state solutions) are obtained exactly in the simplest case of two-particle binding, in one, two, and three space dimensions. We also investigate three-particle binding in one space dimension. The results indicate that the exact quantum solitons of this field theory have a singular, pointlike structure in two and three dimensions—even though the corresponding classical theory is nonsingular. To estimate the physically accessible radii and binding energies of the bound states, we impose a momentum cutoff on the nonlinear couplings. In the case of nonlinear optical interactions, the resulting radii and binding energies of these photonic particlelike excitations in highly nonlinear parametric media appear to be close to physically observable values.

We study the response of an inhomogeneously broadened atomic three-level ladder system to a strong probe beam under conditions in which electromagnetically induced transparency would be induced on a weak probe beam. Theoretical analysis and experimental data show that saturation effects associated with the strong probe can substantially alter the behavior of the system and can result in enhanced probe absorption under exactly the same circumstances in which a weak probe would experience enhanced transparency. Strong probe effects are also shown to lead to a breakdown of the correspondence between population inversion and gain. A power-series solution of the density-matrix equations describing the ladder system provides a physical picture for strong probe effects in terms of interfering multiphoton transition pathways.

Resonant interaction of a collection of two-level atoms with a single-mode coherent cavity field is considered in the framework of the Dicke model. We focus on the role of collective atomic effects in the phenomenon of collapses and revivals of the Rabi oscillations. It is shown that the behavior of the system strongly depends on the initial atomic state. In the case of the initial half-excited Dicke state we account for a number of interesting phenomena. The correlations between the atoms result in a suppression of the revival amplitude, and the revival time is halved, compared to the uncorrelated fully excited and ground states. The phenomenon of squeezing of the radiation field in the atom-field interaction is also discussed. For the initial fully excited and ground atomic states, the field is squeezed on the short-time scale, and squeezing can be enhanced by increasing the number of atoms. Some empirical formulas are found which describe the behavior of the system in excellent agreement with numerical results. For the half-excited Dicke state, the field can be strongly squeezed on the long-time scale in the case of two atoms. This kind of squeezing is enhanced by increasing the intensity of the initial coherent field and is of the same nature as revival-time squeezing in the Jaynes-Cummings model. The appearance of this long-time squeezing can be explained using the factorization approximation for semiclassical atomic states.

A theoretical model of a microscopic laser system incorporating two identical atoms coupled to a single resonator mode is investigated in detail. It is established that a dramatic improvement in laser quality results from the addition of a second atom into the cavity. The enhanced laser action, arising from a cavity-induced mutual atomic coherence, is inferred from comprehensive studies of the steady-state and dynamical properties of the cavity output field. Evidence of laser action is also found in the fluorescence field where position-dependent spectral profiles occur. Counterintuitively, we establish that antibunching phenomena in the fluorescence can be enhanced by incoherent pumping.

The practical feasibility of a conditional experimental scheme for generating bimodal number states, taking into account from the beginning some important technological limits of the current experimental setup, is analyzed. The influence of the unavoidable occurrence of a nonideal performance in a realistic apparatus on the interaction mechanism chosen for guiding the cavity field towards the desired bimodal Fock state is pointed out.

One-photon coupling of the upper level of an atomic transition to a metastable state can generate a dark line in the atom’s emission spectrum. We show in this paper that if such a coupling is replaced by a dominant two-photon coupling, the spectrum is dynamically narrowed instead. The dominant two-photon coupling is achieved here by coupling these two levels to a common broad intermediate state using two laser fields. We also show that if one exchanges the metastable state with the broad one in this system, the one-photon coupling of all coupled states becomes dominant, causing the generation of a dynamically damped dark line effect. We explain these effects in terms of constructive and destructive quantum interferences between a direct and a three-photon Ramam path in the former case, and between a direct and a two-photon Raman path in the latter case. Additionally, we show how these interfering paths provide elements of coherent control of the atomic emission spectra, in general.

We consider a semiconductor microcavity driven by a coherent and stationary holding beam, in two distinct configurations. In the first, no carriers are injected in the multiple-quantum-well structure and the optical nonlinearity is governed by an excitonic resonance. The second corresponds to that of a vertical-cavity surface-emitting laser kept slightly below threshold. We describe both configurations using a unified model that includes both field diffraction and carrier diffusion. We calculate numerically both the time evolution and the stationary profile of the solitonic solutions, using a generalization of the radial integration technique introduced by Firth and Scroggie [Phys. Rev. Lett. 76, 1623 (1996)]. We analyze the instability that forms spatial patterns and especially cavity spatial solitons. We predict the existence of these solitons in various parametric domains for both configurations. We demonstrate that these results are independent of the periodic boundary conditions used in the simulations. We show that, introducing a simple phase modulation in the holding beam, one can eliminate the motions of solitons that arise from noise and from amplitude gradients. The solitons are robust with respect to parametric variations, to carrier diffusion, and even to some amount of self-defocusing. This picture points to the possibility of realizing arrays of solitonic pixels using semiconductor microresonators.

Inversionless gain is predicted in a Doppler-broadened $V$ scheme for which the probe laser frequency is approximately twice that of the coupling laser. A comparison is made of inversionless gain achieved via electromagnetically induced transparency in Doppler-broadened matched and mismatched wavelength systems. We show that the presence of Doppler broadening does not preclude inversionless lasing for mismatched wavelengths; importantly, this is shown to hold even for Rabi splittings significantly less than the Doppler width. Our analysis demonstrates that inversionless gain is realizable in Doppler-broadened media, at modest continuous wave laser powers, for a probe frequency well in excess of the coupling field frequency. The presented theory is related to a practical mismatched system in atomic rubidium vapor. Approximately 8% gain is predicted in a 422 nm probe field coherently driven by a 780 nm coupling field, in the presence of a modest level of incoherent excitation.

The influence of carrier correlation processes on the interaction and propagation of femtosecond optical pulses in an inverted semiconductor is theoretically investigated. Microscopically treated Coulomb scattering described from a first-principles theory, including both diagonal and nondiagonal dephasing, is seen to cause considerably different behavior on the dynamics than that modeled by employing a rate-equation approximation. Semiconductor intrinsic optical nonlinearities such as gain saturation and adiabatic following, controlled by microscopic dephasing and relaxation mechanisms, manifest dominantly in ultrafast transient effects that, experimentally, can appear as a typical Kerr-type (instantaneous) nonlinearity. It is shown that one cannot invoke an adiabatic elimination for the polarization equation on femtosecond time scales. The present self-consistent theory successfully explains recent experimental measurements without recourse to parametrized equations. The possible role of non-Markovian relaxation is also discussed.

A recently proposed method for the stabilization of unstable patterns uses an instantaneous feedback derived from the Fourier transform of the output [R. Martin et al., Phys. Rev. Lett. 66, 4007 (1996)]. We successfully extend this method to regimes of spatiotemporal disorder. By focusing on two different nonlinear optical systems, a laser and a Kerr slice with feedback, we demonstrate the usefulness, effectiveness, and generality of the technique. Our method allows for high power outputs without loss of spatial and temporal coherence. It also provides the possibility of pattern selection and, in some instances, tracking within a disordered regime.

An optical magnetometer based on electromagnetically induced transparency is analyzed. Utilizing the different orientation characteristics for different field polarizations, the magnitude and orientation of an applied magnetic field can be measured. As a model system we consider the sodium $D_1$ transition taking into account hyperfine structure and optical pumping effects.

By generalizing to a mixed-state environment the treatment recently given to a model advanced by Cooper et al. [Phys. Rev. Lett. 72, 1337 (1994)], we show that some characteristics of the so-called semiquantum chaos can be described by recourse to a special motion invariant of the problem, that thus becomes a chaos indicator.

Based on Maslov’s multidimensional semiclassical theory, a semiclassical Green function in a mixed position-momentum space is derived, which can be used to locally fix the semiclassical divergency in singular regions in configuration or momentum space. It is shown that when a semiclassical Green function is transformed from one representation to another, its form is invariant. The mixed representation includes both coordinate and momentum representations of semiclassical Green functions as two special cases.

The accuracy of the time-dependent local-density approximation (TDLDA) for group-IB elements is preserved under the additional approximation of using pseudopotentials to treat the effects of core electrons. This extends the work of Zangwill and Soven, who showed the utility of the all-electron TDLDA in the atomic response problem, and justifies using pseudopotentials in calculating the far-ultraviolet response of multiatom systems.

The cross section for isolated-core excitation of the Ca ${4p}_{1/2,3/2}\mathrm{nd}$ $J=1,3\mathrm{}$ series from the Ca $4snd$ ${}^1{D}_2$ states has been measured for $14<n<\mathrm{}28.\mathrm{}$ The series exhibit strong configuration interaction and rapid decay via autoionization. The energies, quantum defects, and autoionization rates for the observed resonances are presented.

We present a theoretical analysis of the spectrum of swift electrons resulting from collisions of 5.9 MeV/u ${\mathrm{U}}^{29+}$ with Xe atoms. Our calculations include, within an independent model, all sources of electrons (i.e., target L-O shells and projectile O-P shells). We show that there exists clear evidence for a ternary ridge of swift electrons originating from head-on collisions between target electrons and the impinging projectile followed by elastic scattering at the target core. These findings provide a theoretical confirmation of an experimental observation of a ternary ridge in isolated ion-atom collisions.

Antihydrogen formation in the laser-assisted positron-antiproton (nonrelativistic) radiative recombination is investigated. The state of incident positron is given by the Coulomb-Volkov wave function. The perturbative dressed wave function of the atom is obtained in the soft-photon approximation. Our calculation shows that for a geometry of laser polarization parallel to the incident direction, the formation cross section of antihydrogen is greatly reduced. Especially at high impact energy, the reduction is remarkable.