We develop a theory of electromagnetically induced transparency in a three-level, ladder-type Doppler-broadened medium, paying special attention to the case where the coupling and probe beams are counterpropagating and have similar frequencies, so as to reduce the total Doppler width of the two-photon process. The theory is easily generalized to deal with the Λ configuration, where the ideal arrangement involves two copropagating beams. We discuss different possible regimes, depending on the relative importance of the various broadening mechanisms, and identify ways to optimize the absorption-reduction effect. The theory is compared to the results of a recent experiment (on a ladder-type system), using the Rb D2 line, with generally very good agreement. The maximum absorption reduction observed (64.4%) appears to be mostly limited by the relatively large (∼5 MHz) linewidth of the diode lasers used in our experiment.

We describe experimental studies of high-order harmonic generation in rare-gas-like ${\mathrm{Na}}^{+}$ and ${\mathrm{K}}^{+}$ ions. The ions were created in a laser-produced plasma and the harmonics were generated with a short-pulse, high-power, titanium-sapphire laser operating at 794 nm. The highest harmonic order observed was the 27th (29.4 nm), while much higher harmonic orders were observed in a jet of neutral Ne gas under similar conditions. Pronounced ring structures were observed in the harmonic far-field distribution and these structures varied with focusing condition. The cutoff in the harmonic spectra at high orders may be due to ionization-induced defocusing of the fundamental laser beam. Calculations illustrating how this effect reduces the peak intensity obtained in the plasma compared with the intensity obtained in the Ne gas are presented.

The role of collision-induced transfer of electronic state coherence on the spectral response of an atomic vapor is investigated. It is shown that such collisional effects can lead to the appearance of narrow resonances in the nonlinear spectroscopy of three-level systems. Collisional transfer between different multipoles of electronic state coherence is shown to lead to a breakdown of several percent in Becquerel’s relation between the Faraday rotation and dispersion of a vapor. All calculations are carried out in the impact limit of pressure broadening theory. Particular emphasis is given to the order in which the various averages over the velocity distributions of both the active atoms and perturbers must be performed. In particular, for the effects described herein, the various collisional transfer rates vanish when averaged over the active atom velocity distribution. It is essential, therefore, to obtain the final line-shape formulas, including velocity-dependent collisional transfer rates, before carrying out the average over the active atom velocity distribution. The experimental observation of these spectral features would provide direct evidence for the existence of these collisional transfer processes.

The model problem of photodetachment of an electron initially bound by a zero-range potential in a homogeneous static magnetic field of arbitrary strength is solved exactly. Expressions for the cross sections are derived within the nonrelativistic theory for various photon polarizations. In particular, photodetachment from states that exist only in the presence of a magnetic field is studied. In the limit of small fields the results obtained agree with findings of other authors.

The dynamics of short-lived quasibound Rydberg-electron wave packets above the classical field-ionization limit are investigated in the presence of parallel electric and magnetic fields. The observed variations in relative peak intensities in the recurrence spectra are understood both in terms of a quantum-mechanical picture and in terms of the classical electron trajectories. The linear Zeeman term causes the Rydberg wave packet to rotate around the magnetic field axis. This rotation is observed experimentally as a sinusoidal intensity modulation superimposed on the periodic structure which is attributed to motion in the angular momentum coordinate. The diamagnetic term stabilizes the Rydberg wave packet. This stabilization is observed experimentally as an increase in the recurrence peak intensities and the observation of recurrences after longer delays (up to twice as long as in the absence of the magnetic field). Classical trajectory calculations identify the dominant closed orbits which contribute to the experimental recurrence spectra.

Based on analytical calculations and numerical simulations it is shown that cooling can result in the melting of Coulomb crystals stored in a Paul trap. The effect is dynamical in origin and relies on the modification of the stability region of the Paul trap in the presence of damping. A diagnostic tool for beam crystallization in storage rings is suggested.

The dipole coupling term between a system of N particles with total charge zero and the electromagnetic field is derived in the presence of a weak gravitational field. It is shown that the form of the coupling remains the same as in flat space time if it is written with respect to the proper time of the observer and to the measurable field components. Some remarks concerning the connection between the minimal and the dipole coupling are given.

Atom photoionization by the superposition of a fundamental field and its second harmonic is considered. The finite analytical expressions for the photoionization probability are obtained using the adiabatic approximation. They demonstrate that the photoelectron angular distribution has a polar symmetry when the electrical field strength has a maximal polar asymmetry and the distribution is asymmetrical when the field is symmetrical. A strict proof of the polar symmetry of the photoionization probability in the case of the electrical field with maximal asymmetry is deduced using the Keldysh-Faisal-Reiss theories. The obtained results are in agreement with the experimental data available.

We consider the dynamical behavior of light in a cavity that contains birefringent elements with time-dependent settings. The evolution of the two-dimensional polarization vector can be described by an equation of the Schrödinger type if the intracavity elements change little on the time scale of the round-trip time. A coupled-mode analysis is given, which models the multimode dynamics of the optical system. We show that a description in terms of isolated two-level systems is justified if the optical band gap is small.

We report the observation of ‘‘tunneling’’ effects in a classical optical two-level system. In particular, we present experimental results for the optical implementations of the Landau-Zener model and of a hidden level-crossing model. The experimental results are in good agreement with theory for both the transition probability and the detailed time evolution of the wave function.

Nonresonant gratings have been generated in gases using as excitation beams the second-harmonic output of a multimode Nd:YAG (yttrium aluminum garnet) laser (pulse duration τ=8 ns, coherence time ${\mathrm{\tau }}_{\mathit{c}}$=20 ps). The grating reflectivity is measured by scattering a probe beam off the grating. It shows the damped oscillation of a standing acoustic wave (period ${\mathit{T}}_{\mathit{p}}$). By varying the time delay ${\mathrm{\tau }}_{\mathit{d}}$ between the excitation beams, the influence of the temporal laser coherence is analyzed. The signal for incoherent excitation (${\mathrm{\tau }}_{\mathit{c}}$≪‖${\mathrm{\tau }}_{\mathit{d}}$‖≪τ) is nonvanishing and depends only weakly on ${\mathrm{\tau }}_{\mathit{d}}$. It is reduced by about a factor ${\mathrm{\tau }}_{\mathit{c}}$/τ compared to coherent excitation, if ${\mathit{T}}_{\mathit{p}}$≫τ. The measurements are interpreted within a calculation of electrostrictively generated gratings from beams having Gaussian temporal pulse shape, Gaussian statistics for the intensity, and a Gaussian frequency spectrum. Furthermore, the measurement of acoustic quantities (sound velocity and attenuation) with the described setup is discussed.

We have observed, in experiments with a loss-driven ${\mathrm{CO}}_2$ laser operating near the onset of a period-doubling bifurcation, strong amplification of a perturbation signal at half the driving frequency. However, deamplification has been observed above the bifurcation point. Using a two-level laser model we have obtained reasonably good agreement between numerical and experimental results.

The role of spontaneous scattering and nonlinear refraction on the nonlinear dynamics of stimulated Brillouin scattering with feedback is investigated. We find that feedback suppresses stochasticity in the amplification process of the Stokes emission, giving rise to deterministic behavior, the forms of which are found to be critically dependent on the strength of the nonlinear refraction, which is shown to depend on the polarization-preserving properties of the medium. Our results provide an explanation of the chaotic dynamics observed in polarization scrambled fibers and also account for the simpler forms of dynamical behavior reported using polarization-preserving fibers.

The present paper establishes the photonic superguiding theory in polar crystals with a high nonlinearity. In quantum theory it is shown that photons can sense an attractive effective interaction by exchange of virtual optical phonons. Such an interaction leads to the superguiding state. In the standing-wave superguiding state, the photons with opposite wave vectors and spins are bound into pairs. In the traveling-wave superguiding state, a propagating photon pair consists of a combination of two photons with opposite transverse wave vectors and spins. We study the particle properties of the photonic superguiding state, the most important property being that the system of photon pairs evolves without scattering attenuations. Quantum fluctuations of the standing-wave superguiding state exceed the vacuum fluctuations while the traveling-wave superguiding state has the squeezing property. We also investigate the wave properties of the photonic superguiding state. It is found that the polar crystals with a high nonlinearity are self-defocusing media. In the standing-wave superguiding state, the system of photon pairs exists in the form of quantized vortices. In the traveling-wave superguiding state, the system of photon pairs exists in the form of quantized temporal solitons.

The singular systems are treated as field systems with constraints. The Euler-Lagrange equations are solved with some constraints. The validity of our proposal has been tested by two examples of singular Lagrangians.

Polarization-coupled spatial solitons in optical planar waveguides are investigated, using Whitham’s average variational principle to cast the problem into a set of ordinary differential equations. The main problem addressed is the stability of the dynamics and the mathematical results derived are compared with linear stability theory. Analytical forms for the stability edges are given together with numerical work that confirms that the true soliton dynamics agree with the mathematical analysis.

In spite of high absorption, nonlinear optical crystals have a high nonlinear susceptibility in the vacuum-ultraviolet and the x-ray regions, providing a basis for second-harmonic generation. In the conventional transmission configuration, a crystal film ∼100 Å thick may generate sufficiently strong second harmonics with a few tenths conversion efficiency under a double-resonance condition. The phase-matching restriction is almost lifted, as a result of the tiny thickness. However, the reflection configuration is shown to be unique and promising under the double-resonance condition. The conversion efficiency may also reach a few tenths, while the crystals need not be thin films, and the problem of heat dissipation is relieved.

We present evidence for the characterization of the intermediate regime of stochastic perturbations acting on a dimer of polar molecules through a steady-state degenerate four-wave-mixing experiment. The influence of the intersite correlation between the local fluctuations is considered as well.

A theoretical study of the dynamics of Rydberg atoms in a microwave cavity driven by a strong bichromatic field is presented. The resonator is assumed to operate in the low-Q regime. As a consequence, photons emitted by the atoms are dissipated in the cavity walls during the interaction time of the atoms inside the resonator. In this situation the cavity field follows the atomic dynamics adiabatically. The transient behavior of the system is analyzed in terms of Floquet states, and cavity-induced transition rates between these states are calculated for a large range of parameters of the bichromatic field. Narrow resonances are found in the transition rates, in agreement with recent experimental investigations of cavity Rydberg atoms subjected to strong bichromatic driving. We explain in detail the structure of the resonances, which is determined by the frequency-dependent cavity-mode density as well as the Rabi frequencies of the applied fields. The intensity-dependent shifts of the resonance frequencies are also calculated and found to be largely insensitive to inhomogeneous broadening. Finally, the numerical results are compared with experimental observations.

By introducing a simple spatially symmetric correlation function, we have modified a hyperspherical harmonic and generalized Laguerre function (HHGLF) into a correlation function HHGLF (CFHHGLF) expansion method and used it to solve directly the Schrödinger equations of ground-state lithium and excited helium (2 ${}_{}{}^1{}_{}{}^{}$S) atoms. In such a scheme, the wave function of the lithium atom is decomposed into the product of a correlation function X (X=exp[-Z(${\mathit{r}}_1$+${\mathit{r}}_2$+${\mathit{r}}_{\mathit{a}}$)]), and a wave function Φ expanded in the same way as in the HHGLF method. The eigenenergy can be calculated explicitly by solving a simple secular equation. We have obtained a ground-state eigenenergy which is much better than the Hartree-Fock in precision and approaches the experimental value. The result indicates that it is possible to generalize the hyperspherical harmonic method to a four-body atomic system more successfully. In addition, the exact eigenenergy of the excited He (2 ${}_{}{}^1{}_{}{}^{}$S) has also been obtained with the CFHHGLF method with X=exp[-2(${\mathit{r}}_1$+${\mathit{r}}_2$)].