We develop the theory of an atomic beam splitter in which a monoenergetic beam of two-level atoms is incident normally to a classical standing-wave light field. The incident atomic wave function can be split into two coherent components with transverse momenta ±(2n+1)ħk using velocity-tuned resonances, where n is the order of the resonance. We discuss the cases of zero- and first-order resonances in detail, and show that the velocity-tuned resonances are renormalized due to a high-frequency Stark shift. Numerical results that display the effects of a finite momentum spread in the incident atomic beam are presented.

Conditions for enhanced backward-stimulated emission are formulated for a thin layer of gain medium. A comparison is made between the enhanced backward-stimulated emission and the reflection from a dielectric film. A type of nonreciprocal reflective structure using a gain layer and a dielectric film is described. Such a device can be used to construct ring lasers with controllable unidirectional operation.

We study the properties of gap solitary waves in periodic nonlinear media in the presence of gain and loss. To do so we numerically integrate the appropriate set of nonlinear coupled-mode equations, and interpret the results using an effective-particle picture.

Ionization suppression and electron localization have been observed in numerical modeling of two-photon ionization by very intense short-wavelength laser fields. In this paper we present similar results for one-photon ionization. We find that a one-dimensional atom follows one of three distinct routes to ionization depending whether the field is weak, strong, or superstrong. The first route can be described by perturbation theory. The second route is characterized by very rapid depletion of the bound-state population, leading to 100% ionization. The third route features incomplete ionization and electron localization. The behavior of the atom in both strong and superstrong fields is strongly correlated with the deformation of the atomic binding potential associated with the Kramers-Henneberger frame transformation.

The quantum theory of the spontaneous emission (SpE) from an active microscopic cavity (microcavity) is given with emphasis on mirror separations of the order of the optical wavelength. The theory is based on a complete set of orthonormal-mode functions that include both transverse polarizations and span the infinite three dimensional space that pervades and surrounds the microcavity. SpE rates for different active-dipole orientations and cavity configurations are calculated. The SpE pulse shape detected outside the cavity is shown to be generally nonexponential. A detailed computer simulation of the process is presented on the basis of the given theory in the perspective of our experiment, for a cavity terminated by mirrors bearing either metal- or semiconductor-multilayered coatings. We then report an extensive experimental verification of the theory by adopting an Eu-dibenzoylmethane complex as active medium with SpE from the ${}_{}{}^5{}_{}{}^{}$${\mathit{D}}_{0\mathrm{-}}^7$${\mathit{F}}_2$ line at λ=6111 Å, under coherent uv excitation at ${\lambda }_{\mathit{p}}$=3547 Å. The results show evidence of ‘‘SpE inhibition’’ and ‘‘enhancement,’’ of nonexponential decay of SpE signals, and of competition with superradiance and stimulated emission. Finally we report the results of an experimental test of the algorithm adopted in all computer calculations of the optical parameters of the multilayered structures used for cavity confinement.

A quantum version of the classical paraxial approximation is established. After expanding the classical free-field wave equation in powers of θ (the characteristic opening angle of the paraxial rays), the familiar time-dependent paraxial equation appears in zeroth order. The arguments employed in deriving this approximation scheme are used in turn to identify the subspace of the photon Fock space consisting of paraxial states of the field. The quantum version of the paraxial approximation is then obtained by restricting the Maxwell field operators to this domain. Exact equal-time commutation relations are recovered as expansions in θ. In zeroth order, the theory yields a quantized analog of the classical paraxial wave equation, and formally resembles a nonrelativistic many-body theory. This formalism is applied to show that Mandel’s local-photon-number operator and Glauber’s photon-counting operator reduce, in zeroth order, to the same true-number operator. In addition, it is shown that the O(${\mathrm{\theta }}^2$) difference between them vanishes for experiments described by stationary coherent states.

A full linear stability analysis of the sideband instability in the free-electron laser is presented. All processes affecting the instability are identified and taken into account; these include, in addition to the usual nonparametric process leading to direct excitation of the Stokes and anti-Stokes processes, a parametric four-wave-mixing process in which the sideband modes are coupled through the pump mode, as well as further refractive terms, over and above that due to simple linear refraction, brought about by the presence of the potential wells created by the strong pump mode. The net result is an equation for the growth rate Γ that depends on all parameters describing these processes, and which is therefore more complicated than that usually quoted; moreover, the unstable mode is no longer ‘‘pure’’ Stokes, but is a mixed mode containing both sidebands.

Within the framework of the recently advanced formulation of QED based on self-energy, we calculate the relativistic rates of the 2${\mathit{S}}_{1/2}$→1${\mathit{S}}_{1/2}$+1γ transition in the hydrogen isoelectronic sequence for values of Z ranging between 1 and 92. We compare our results with those of Johnson [Phys. Rev. Lett. 29, 1123 (1972)] and Parpia and Johnson [Phys. Rev. A 26, 1142 (1982)], analytically and numerically. Although the two approaches are quite different, the formulas for decay rates are shown to be equivalent.

Using the canonical density matrix as the main tool, an expression is derived for the current density for an independent-particle model of an atom in an arbitrarily strong magnetic field. This contains the weak-field result of Harris and Cina [J. Chem. Phys. 79, 1381 (1983)] as one limit. The extreme high-field limit is also analytically derived. For intermediate fields, numerical procedures should be straightforward to apply. Finally, some consideration is also given to the single-particle kinetic energy.

The Schrödinger equation for the S state of the three-body system with arbitrary charge and mass has been solved directly using finite-element analysis. In this analysis, the wave function is approximated piecewise using polynomial interpolation functions. The energy and wave function converge to their exact values as the number of elements is increased. In contrast to standard variational calculations, the error in the expectation value of physical observables is comparable to the error in the energy. Results are reported here for the helium atom and the muonic molecular ion dd${\mathrm{\mu }}^{+}$.

It is shown that in the semiclassical (WKB) Coulomb approximation the radial integrals for multipole transitions in nonhydrogenic atoms obey simple recurrence relations. As a result, any such integral can be analytically expressed in terms of Anger functions plus an algebraic part proportional to sinπs, where s=ν’-ν is the difference between the effective principal quantum numbers of the states involved. We retrieve, in particular, various formulas established previously for dipole, quadrupole, and octupole transitions.

We employ a parameter-free positron-correlation-polarization (PCOP) potential to calculate total (rotationally summed) cross sections for the positron-${\mathrm{H}}_2$ (${\mathrm{N}}_2$) collisions below the positronium formation threshold energy. A laboratory frame close-coupling technique, retaining six rotational states (j=0,2,4,6,8,10), has been used. It is found that the PCOP model compares reasonably well with measured values. Results on rotationally elastic and inelastic cross sections are also reported. We also make a comparison between PCOP and electron-correlation-polarization models for all the cross sections reported here.

Autoionizing levels of beryllium near its first ionization threshold are investigated using the multiconfiguration relativistic random-phase approximation. The 2pns ${}_{}{}^1{}_{}{}^{}$P${\mathrm{°}}_1$ and 2pnd ${}_{}{}^1{}_{}{}^{}$P${\mathrm{°}}_1$ Rydberg series of doubly excited states manifest as autoionization resonances in the 2s photoionization cross section. Theoretical predictions and available experimental data are in excellent agreement within 0.05 eV.

Two previously unclassified helium optical emission continua, seen in arc and other discharges, are found to result from a transition between excited states of ${\mathrm{He}}_2^{+}$. Ab initio potential curves are calculated for the states involved, and semiempirical spontaneous emission spectra are obtained that fit experimental data. The corresponding cross section for stimulated emission suggests the possibility of a laser with a tuning range of 2500 to 10000 Å peaking at ∼6000 Å. A broadband emission at ∼6050 Å may be evidence of laser gain.

A nonunitary operator scaling mechanism is presented, which produces number-phase squeezing, useful at decreased signal power. It accounts for the performance of nonunitary schemes—better than unitary—in attaining squeezing in the number.

In this Brief Report we propose a simple phenomenological model of gain saturation for free-electron lasers operating with a bunched electron beam. The model is based on an analogy with that of conventional lasers, and in spite of its simplicity, the obtained results are in close agreement with the numerical simulations performed within the context of a more complete theory.

We calculate the phase-difference and phase-sum fluctuations of the quantum-beat laser, using the nonlinear model of Bergou et al. [Phys. Rev. A 38, 754 (1988)], and also making use of the phase operators defined by Pegg and Barnett [Phys. Rev. A 39, 1665 (1989)]. We found a vanishing phase-difference diffusion constant, in agreement with Bergou et al. However, the present formalism is more general, and includes shot noise as well as saturation effects for the relative and sum phase fluctuations.

Generation of squeezing via a two-photon transition in a system of three-level atoms, strongly driven by two resonant fields is investigated. Spontaneous emission, cavity damping, and collective effects are accounted for. It is shown that in the steady-state regime, strong squeezing can be generated.

We present relative spectral line intensities for 16 3s-3p and 3p-3d transitions in the neonlike ion ${\mathrm{Fe}}^{16+}$, obtained from solar-flare spectra. The spectra were obtained by a Naval Research Laboratory slitless spectrograph flown on the Skylab manned space station. The results are of relevance to x-ray-laser experiments involving collisional pumping of excited states of neonlike ions, and to the analysis of solar-flare spectra. The relative line intensities are measured to an accuracy of about 30%. The measured line intensities are compared with theoretical line intensities calculated assuming electron collisional excitation within the ${\mathrm{Fe}}^{16+}$ ion followed by deexcitation and radiative decay. The measured and theoretical intensities agree to within less than a factor of 2 for all but one line. The important J=0 lines (3 ${}_{}{}^1{}_{}{}^{}\mathrm{S}_0^{}{}_{}{}^{}$–3s ${}_{}{}^1{}_{}{}^{}\mathrm{P}_1^{}{}_{}{}^{}$,3s ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}_1$) at 254.87 and 204.65 Å are about a factor of 1.4 weaker than predicted, which is almost within experimental error. The 3${\mathit{s}}^3$${\mathit{P}}_2$–3p ${}_{}{}^3{}_{}{}^{}$${\mathit{S}}_1$ line at 409.69 Å is 2.3 times weaker than predicted, and this is unexplained.

The nonresonant, multiphoton ionization of many-electron atoms is explained theoretically within the Thomas-Fermi model with the inclusion of an electromagnetic field. Numerical results are presented for the charge state and induced dipole moment versus the strength of the applied electric field, which, because of the scaling properties of the model, can be expressed as universal functions of the field. Our results compare favorably with those from recent ionization experiments performed on noble gases using laser fields.