We study the spectrum of a one-photon micromaser through the approach of the two-time correlation function. First, we derive two general formulas for the two-time correlation function and for the micromaser spectrum. Then, we focus our attention on trapped states of the micromaser. We examine the line shapes and linewidth of the micromaser spectrum. We find that the half width at half maximum of the spectrum can be less than the smallest decay rate of the off-diagonal density matrix elements ${\mathrm{\rho }}_{\mathit{n},}$${\mathit{n}}_{+1\mathrm{}}$(t), that a hole can appear at the center of the spectrum, and that the spectrum can be split by shifted spectral components. Using a simple example in which the micromaser spectrum is exactly Lorentzian, we also show that two very close photon-number distributions can correspond to two very different natural linewidths.

The problem of modulation instability of a self-focused beam in a homogeneous nonlinear medium with saturation and anomalous group-velocity dispersion is solved numerically. It is shown that the results of this instability is beam breakup into a periodic train of three-dimensional (3D) spatial solitary waves. It is also shown that other types of periodic initial conditions can produce a periodic train of 3D spatial solitary waves. Our numerical simulations show that 3D solitary waves are attractors (foci or limit cycles) in the Hilbert space of solutions of the 3D nonlinear Schrödinger equation. A field of another configuration can converge to them upon propagation and after the emission of a certain amount of radiation.

An experimental and theoretical study of magnetic-field-induced sum-frequency mixing in sodium vapor has been carried out using two single-frequency, continuous-wave dye lasers as the sources of the fundamental radiation. In addition to the usual two-photon resonant enhancement of the three-wave-mixing nonlinear-optical process, the use of near- or on-resonant single-photon transitions for further resonant enhancement was investigated. The macroscopic phase-matching behavior of the sum-frequency mixing was examined with the intermediate states off resonance. Experimental and theoretical results are presented to illustrate the differences that occur between the coherent quadrupole emission and the incoherent cascade fluorescence when the intermediate states in the two-photon absorption are resonant. Control over the single-photon resonant enhancement and the sum-frequency-mixing phase matching was achieved to increase the power conversion efficiency to 1.2×${10}^{\mathrm{-}5}$.

We study theoretically the spectrum of resonance fluoresence from one-dimensional molasses consisting of two-level atoms with Zeeman substructure. The center-of-mass motion of the atom is treated fully quantum mechanically. The spectrum shows sidebands due to transitions between vibrational levels in optical potentials generated by the laser light. Detailed results are presented for the ${\mathit{J}}_{\mathit{g}}$=1/2→${\mathit{J}}_{\mathit{e}}$=3/2 atomic transition in a laser configuration with two counterpropagating waves with orthogonal polarizations. We have solved the corresponding quantum master equation and calculated the relevant autocorrelation function of the atomic dipole using (i) a direct numerical solution of the master equation, (ii) a wave-function simulation of the master equation and correlation function employing periodic time-dependent Bloch wave functions, and (iii) a semiclassical bipotential calculation for the spectrum including a simple quantum correction.

This paper describes the atomic dynamics when a Rydberg atom is in a laser field which is resonant with a dipole-allowed core transition. The main approximation is to completely ignore the (short-range, direct) interaction of the outer electron with the resonant laser which is the same approximation used with great success in calculating the spectrum due to isolated core excitations (ICE). The atom autoionizes when the core absorbs a photon, because the electron can then inelastically scatter from the excited core state, gaining enough energy to escape the atom. Despite neglecting the direct interaction between the outermost electron and the laser, the laser profoundly affects the autoionization dynamics. This effect is incorporated through a frame transformation between the dressed and undressed core states which only utilizes the field free atomic scattering parameters. A two-color experiment is proposed which might be able to measure nonperturbative effects arising from the dressed core states. The usual ICE transition rate is obtained through a perturbative expansion. Generic effects are examined through a model problem. A calculation of the Mg spectrum when the driving laser is tuned to the 3${\mathit{s}}_{1/2\mathrm{-}}$3${\mathit{p}}_{1/2}$ or the 3${\mathit{s}}_{1/2\mathrm{-}}$3${\mathit{p}}_{3/2}$ transition is presented.

A measurement is made of the 3d-4f transition energies in the Ni-like ions ${\mathrm{Re}}^{47+}$, ${\mathrm{Ir}}^{49+}$, ${\mathrm{Pt}}^{50+}$, ${\mathrm{Au}}^{51+}$, and ${\mathrm{Bi}}^{55+}$ and the 2p-4d transition energies in the Ne-like ions ${\mathrm{Br}}^{25+}$, ${\mathrm{Kr}}^{26+}$, ${\mathrm{Rb}}^{27+}$, and ${\mathrm{Y}}^{29+}$ using the Livermore electron-beam ion trap. The ions studied are candidates for an x-ray laser scheme based on resonant photopumping which predicts lasing among the 3p-3s transitions in a Ne-like ion. The results of the measurements are compared to multiconfiguration Dirac-Fock calculations and systematic differences are found. The best resonance is found for the Pt-Rb pair at 2512 eV, whose energies differ by 0.4±0.1 eV, that is, by only 160 ppm.

The effects of phase-interrupting and velocity-changing collisions between active and perturber atoms on the spectral profile of a gas laser are studied. Exact analytical results for the density matrix are presented for the case of strong collisions in a unidirectional ring laser, and the laser intensity is then evaluated in the Doppler limit under the condition of self-consistency. The washout of the hole burning in the steady-state population difference and the broadening of the laser line shape are discussed. The results of the third-order perturbation theory are compared with the exact calculations valid for an arbitrarily intense pump.

The interaction of a model atom with very intense radiation fields is well approximated by a map that describes a free particle being kicked periodically by a double-well potential. Both the classical and quantum versions of the map are studied and are shown to provide a compact description of high-field stabilization in this strongly perturbed quantum system. In particular, we find that the ‘‘stabilized’’ wave function results from the excitation of a superposition of eigenstates of the periodic, time-evolution operator that are localized near classically unstable and weakly stable orbits. These ‘‘scarred’’ wave functions correspond to localized wave packets that oscillate back and forth in the strong external field while in the neighborhood of the attractive Coulomb field. The periodic bremsstrahlung, emitted as this wave packet passes the nucleus, provides a distinct experimental signature of the stabilized state.

We solve the problem of a laser with variable pumping statistics for any relative magnitude of the atomic- and cavity-decay constants, and obtain a different regime of sub-Poissonian light generation. We show that, even for Poissonian pumping, the noise in the amplitude quadrature outside the cavity can be reduced up to 50% below the shot-noise level when the polarization but not the populations can be adiabatically eliminated. Maximum noise reduction in this case is obtained when the lower level decays much faster than the upper one and occurs at a frequency given by the geometrical mean of the decay rates of the field and the lower-level population. Furthermore, the full consideration of atomic memory effects leads to a generalization of previous results on regularly pumped lasers. We find that, for regular pumping, maximum amplitude-noise reduction (up to complete quieting) still occurs at zero frequency in all cases. However, a long-living polarizaton leads to an increase in amplitude noise and may even eliminate the dip at zero frequency, at the same time leading to a significant quenching of the laser linewidth in the bad-cavity limit.

We present experimental results of harmonic generation at 1064 nm using a 36-ps neodymium-doped yttrium aluminum garnet laser tightly focused in a gas jet of xenon. The harmonic yields are studied as a function of the position of the focus in the jet at several laser intensities and as a function of intensity at several focus positions. These results bring information about how the high harmonics are phase matched in the medium and also about the single-atom dynamics of the process. The harmonic yields as a function of the position of the focus in the jet exhibit regular oscillations that are due to the interference effects inherent to the generation of harmonics in the nonlinear medium. The intensity dependences present structures that we attribute to single-atom resonance effects.

We use approximate Sommerfeld-Maue plus Volkov states to investigate the relativistic and final-state Coulomb-potential effects in strong-field multiphoton-ionization processes. Nonperturbative calculations are carried out for Dirac hydrogenlike atoms in the length gauge. Both ionization rates and angular distributions of photoelectrons are obtained with elliptically polarized light for various laser intensities. Qualitative agreement of ‘‘twofold’’ symmetry of azimuthal angular distribution of photoelectrons with experimental measurements is presented, and a tendency toward ‘‘fourfold’’ symmetry at the laser fields of intensity higher than 1 a.u. is pointed out. Stabilization of atoms against ionization by strong fields is also discussed.

The quantum Zeno effect is the inhibition of the free evolution of a system when that system is subject to a measurement process. The authors of one recent experiment study, W. M. Itano et al., [Phys. Rev. A 41, 2295 (1990)] claimed a demonstration of the quantum Zeno effect. This was done by performing discontinuous atomic-level measurements on a coherently evolving two-level atomic stystem. The measurement was implemented using a pulsed laser strong enough to saturate the two-level atomic system. In this paper, we examine the simpler case where the measurement laser is continuously on and does not saturate the two-level atomic system. In contrast to previous treatments, we show certain parameter regimes wherein an effective state reduction does occur. We ascertain the experimental parameter regimes under which a measurement can be said to have been performed. We show that, in some regimes, fully efficient photon detection does not give good information about the atomic-level populations and hence no measurement has been performed. Throughout, we emphasize that the design of an atomic level-measurement device requires more than the analysis of a mechanism for photon detection or of coherence loss. We gain insight into the operation of a particular measurement process by following the selective evolution of various systems using the quantum-trajectories approach.

We show theoretically that competing optical nonlinearities in a semiconductor étalon, with transverse effects included, result in complex spatiotemporal behavior. A theoretical framework is developed that explains many features of the oscillatory behavior. Numerical simulations exhibit kinks, switching waves, whole-beam, and edge oscillations. Simulations compare favorably with recent experiments.

An alternative derivation of the Pegg-Barnett phase operator is presented. This approach is based on the properties of the representation in quantum mechanics of a nonlinear nonbijective canonical transformation. It does not use as its starting point either a finite-dimensional space or the definition of phase states. The features of this formalism are analyzed in terms of this transformation.

A ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ shape resonance in ${\mathrm{Ps}}^{\mathrm{-}}$ lying above the n=2 Ps threshold is calculated using the method of complex-coordinate rotation with Hylleraas-type wave functions. Comparison is made with a calculation using adiabatic potential curves. We also report new results for ${}_{}{}^3{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ shape resonances lying above the n=3 and 7 thresholds.

It is argued that the results of a recent neutron interference experiment [Kaiser et al., Phys. Rev. A 45, 31 (1992)], in which contrast lost as a result of difference in effective path lengths is restored by wavelength filtering after recombination, cannot be construed as evidence against the notion that a neutron is a physically real wave packet.

It is shown that the Lamb shift for stripped high-Z atoms can be accurately obtained from that for hydrogenic atoms of the same Z by taking into account the correct electron density at the origin of coordinates as given by the Hartree-Fock approximation. This method provides a computationally simple way of evaluating such energy shifts.

The exact expression for the exchange potential of density-functional theory is obtained. This expression involves the inverse of the linear-response function as well as the single-particle Green’s function. The consideration is valid for any interparticle interaction.

The observed trend in the radiative lifetimes of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ excited states of CuF (1.5 μs), CuCl (0.43 μs), CuBr (0.23 μs), and CuI (0.16 μs) is analyzed and explained in the light of very accurate multiconfigurational self-consistent-field multireference configuration-interaction and second-order Möller-Plesset perturbation theory calculations. Results show that this trend is due to a decreasing ionic (increasing molecular) character of the wave functions of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ excited states as the atomic number of the halogen increases. The forbidden ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$:${\mathrm{Cu}}^{+}$(${\mathit{d}}^{10}$)${\mathit{X}}^{\mathrm{-}}$(${\mathit{s}}^2$${\mathit{p}}^6$)${\leftarrow }^1$${\mathrm{\Sigma }}^{+}$:${\mathrm{Cu}}^{+}$(${\mathit{d}}^9$${\mathit{s}}^1$) ${\mathit{X}}^{\mathrm{-}}$(${\mathit{s}}^2$${\mathit{p}}^6$) (almost atomic ${}_{}{}^1{}_{}{}^{}$S${\leftarrow }^1$D) transition becomes allowed due to the increasing importance of the ${\mathrm{Cu}}^{+}$(${\mathit{d}}^{10}$)${\mathit{X}}^{\mathrm{-}}$(${\mathit{s}}^2$${\mathit{p}}^6$) and Cu(${\mathit{d}}^{10}$${\mathit{s}}^1$)X(${\mathit{s}}^2$${\mathit{p}}^5$) configurations in the wave function of the ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ excited state, thus reducing the associated radiative lifetime. Spin-orbit effects are found not to play any role in this process since the only large effects are observed in CuI, where the Ω=$0^{+}$ fine-structure state in question is found to be of pure ΛSΣ ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ symmetry; this excludes a mixture with the ${}_{}{}^3{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ component which would otherwise increase this radiative lifetime because of a spin-forbidden transition towards the X ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }_{}^{+}{}_{}{}^{}$ ground state.

We derive the Schwinger multichannel variational principle [K. Takatsuka and V. McKoy, Phys. Rev. A 23, 2352 (1981); 30, 1734 (1984)] by a method that emphasizes the question of variational stability. The present approach is in some respects more direct than the original method and serves to clarify the role of a certain parameter that occurs in the formulation.