It is shown that, in an imploding spherical shell, the surface instabilities are of two different types. The first, which occurs at the outer surface, is the well-known Rayleigh-Taylor instability. The second instability occurs at the inner surface. The characteristics of this instability are similar to those of amplified sound waves in an imploding plasma shell. It is suggested that this instability is driven by the amplified sound waves.

The photoabsorption response of the spherical, neutral ${\mathrm{Na}}_8$,${\mathrm{Na}}_{20}$, and ${\mathrm{Na}}_{40}$ clusters is studied in the random-phase approximation built upon a jellium background. Special attention is given to ${\mathrm{Na}}_{40}$, which displays a particularly broad line shape. The theory provides an overall account of the observation in terms of the interplay between collective and individual electronic motion, and thus evidence for the importance of Landau damping in small metal clusters. The physics of this relaxation phenomenon as a quantum-size signature of the Coulomb interaction is analyzed through simple models.

We have measured the angular distribution of electrons produced by microwave ionization of Na in a 9.4-GHz field. Microwave fields of up to 10.5 kV/cm with linear, circular, and elliptical polarizations were generated in a rotatable Fabry-Pérot cavity. The electron angular distributions obtained are consistent with the predictions of a classical theory of above-threshold ionization in the low-frequency limit.

Saturation spectroscopy, velocity-selective atomic orientation, and velocity-selective alignment have been applied to a high-resolution investigation of the 3${\mathit{s}}^5$ ${\mathit{S}}_{2–}$3${\mathit{p}}^5$ ${\mathrm{P}}_{1,2,3}$ transitions at 777 nm of atomic oxygen. Pure Lorentzian lineshapes were obtained, which allowed an accurate measurement of the radiative linewidth. Using an isotopically enriched sample, isotope shifts and hyperfine structure were resolved and measured.

Methods are presented that allow numerical calculations of the friction and momentum diffusion coefficients for atomic motion in laser fields with periodicity in one, two, and three dimensions. Polarization gradients in the fields lead to increased friction on atoms with a Zeeman sublevel structure, and new features arising in the cases of motion in two and three dimensions are discussed. There is a marked position dependence of the calculated quantities, and the consequence for the forces on atoms following specified trajectories is studied. Temperatures determined from the position-averaged friction and diffusion coefficients scale with the laser power divided by the frequency detuning, except in some special configurations. Of relevance for experiments on cooling, the relative merits of four different laser configurations in three dimensions are studied.

We describe the spectral properties of the resonance fluorescence emitted in radiative collisions. The line shapes of the anelastic components of the spectrum are found to be strongly asymmetric, with extended wings related to the van der Waals shift of atomic levels. The specific case of europium and strontium atoms is considered for a combined collisional-radiative interaction following adiabatic dynamics.

Field-induced resonances in ${\mathrm{H}}_2^{+}$ dressed by an intense laser field are calculated in the electric-field gauge (length gauge) in a fully coupled adiabatic electronic-field representation. Distinctions between diabatic and adiabatic resonance states are made. Adiabatic resonances are characterized in terms of shape and Feshbach resonances.

We derive a Fokker-Planck equation (FPE) for the center-of-mass Wigner function of an atom with an arbitrary level structure, moving in a given three-dimensional light field. This FPE is obtained by eliminating adiabatically the internal degrees of freedom from the expansion of the equation of motion of the atomic density operator into a power series of photon momentum. The force and the diffusion tensor in the FPE may be computed by solving a set of differential equations closely related to the time-evolution equations of the density matrix for the internal state of the atom, as encountered in the conventional recoilless spectroscopy. By studying spontaneous, induced, and free-evolution terms in the equations of motion of the density operator for various level configurations, we devise algorithms that enable a computer to build all the required equations automatically for an arbitrary atomic level scheme.

Resonance-enhanced four-wave-mixing processes were studied in molecular iodine by detection of a wave generated at frequency ${\mathrm{\omega }}_3$=2${\mathrm{\omega }}_1$-${\mathrm{\omega }}_2$ for a wide range of Raman frequencies (${\mathrm{\omega }}_1$-${\mathrm{\omega }}_2$). On the basis of known spectroscopic constants of the two electronic states involved, the electronic ground state ${\mathit{X}}^1$${\mathrm{\Sigma }}_{\mathit{g}}^{+}$ and the electronic excited state ${\mathit{B}}^3$${\mathrm{\Pi }}_{\mathit{u}0}^{+}$, an interpretation of the spectral features in terms of three essentially different four-wave-mixing processes is given. Resonance enhancement through bound states with discrete energy levels and the enhancement effect by dissociative continuum states are considered. In addition to resonance-enhanced coherent anti-Stokes Raman-scattering (CARS) processes, in which the Raman resonances are in the electronic ground state, two resonant schemes are identified in ${\mathrm{I}}_2$, in which Raman resonances in an excited state are probed: excited-state parametric and nonparametric CARS. The latter process is an example of electronic population transfer in a four-wave-mixing cycle.

We show that a ${\mathrm{CO}}_2$ laser driven from below to above threshold by pump sweeping displays a linear dependence of the peak intensity on the switch-on time. This feature is a consequence of the small spread of the switch-on times. Experimental results for the slope of the linear relation are well reproduced by a calculation based on the rate equations.

A combination of dielectric reflectors and thin (thickness much less than the wavelength) gain layers can be tailored to provide a suitable amplifier medium for laser gyro operation. The potential advantage is an all-solid-state device, in which spontaneous-emission noise can be reduced and the coupling between counterpropagating waves minimized.

The double detector, which is used in experiments for the fast measurement of photon correlations, is analyzed in detail. The scheme is shown to measure photon correlations properly only in the limit that the stochastic intensity fluctuations are either very small or possess very short correlation times. Perturbation theory is employed to study the scheme but fails to converge for some realistic fields. Simulations of fields with complex Gaussian fluctuations and random telegraph signals allow for a numerical study of the double-detector model, and we find that, despite the limits of the perturbation theory, the theoretical curves match the simulated curves quite well if the perturbation curves are simply renormalized.

An asymptotic result is derived for the Jaynes-Cummings model of a two-level atom interacting with a quantized single-mode field, which is valid when the field is initially in a coherent state with a large average photon number. It is shown that for certain initial atomic states the joint atom-field wave function factors into an atomic and a field part throughout the interaction, so that each system remains separately in a pure state. The atomic part of the wave function displays a crossing of trajectories in the atom Hilbert space that leads to a unique state for the atom, independent of its initial state, at a specific time ${\mathit{t}}_0$ (equal to half the revival time). The field part of the wave function resembles a crescent squeezed state. The well-known collapses and revivals are investigated from this perspective. The collapse appears to be associated with a ‘‘measurement’’ of the initial state of the atom with the field as the measuring apparatus. The measurement is not complete for finite average photon number: the system is instead left in a coherent superposition of macroscopically distinct states. At the half-revival time ${\mathit{t}}_0$ this superposition of states is entirely in the field part of the state vector, so that the (pure) state of the field at that time is of the form sometimes referred to as a ‘‘Schrödinger cat.’’ The revivals of the population inversion are seen to be entirely due to the fact that the linear superposition of the two macroscopically distinct field states is coherent (i.e., a pure state), as opposed to an incoherent mixture.

We have made a quantitative study of quantum electrodynamic corrections to the atomic energy levels between parallel conducting mirrors. Within the framework of simple two-level atoms, we evaluate the level shifts numerically and discuss the van der Waals, Casimir, and cavity-resonant radiative shifts as their asymptotic forms. In particular, we find that the level shifts of an excited state are substantially different not only from those of a classical dipole radiator but also from those of an excited atom near a single mirror. For the ground state, however, the shifts are qualitatively similar to those near a single mirror. We also apply the results to some simple atomic systems that may be used to test the cavity QED theory.

We present a high-intensity theory of the angular distribution of electrons from multiphoton ionization by an elliptically polarized laser field in which the initial state is described by a dressed atomic state and the final state by a Coulomb scattering state. The essential approximation is the neglect of the Coulomb interaction compared to the electron-laser interaction in the intermediate state. We find the destruction of ‘‘twofold’’ symmetry, as in a recent experiment, and qualitative agreement with that experiment. Extensive results for various photon numbers, polarization angles, and intensities are presented. We also compare distributions from initial S states with those from initial P states with some surprising results.

The main goal of this paper is to analyze a two-photon degenerate four-wave-mixing process that occurs in an isotropic media modeled by homogeneously broadened two-level systems having unequal permanent dipole moments and subject to rotational diffusion. Our theoretical treatment is based upon a third-order perturbative solution of the density-matrix equations and does not require the rotating-wave approximation. The medium being excited by three stationary fields, we give an analytical expression of the macroscopic polarization up to the third order, including permanent dipole moments, orientational diffusion, and field polarizations. We apply these general results to the description of a degenerate four-wave-mixing process by selecting, in the expressions, the contributions in the direction ${\mathbf{k}}_{\mathrm{\alpha }}$-${\mathbf{k}}_{\mathrm{\beta }}$+${\mathbf{k}}_{\gamma }$, ${\mathbf{k}}_{\mathrm{\alpha }}$, ${\mathbf{k}}_{\mathrm{\beta }}$, and ${\mathbf{k}}_{\gamma }$ being, respectively, the wave vectors of the exciting fields. We show that the two-photon degenerate four-wave mixing is dependent on the permanent dipole moments and is not present if these dipole moments are equal or zero. We analyze the influence of rotational diffusion. Different cases of field polarizations are also considered, and we discuss a comparison of the well-known one-photon degenerate four-wave-mixing process and this two-photon process. This process is also discussed in the particular case of phase conjugation.

The physics of Raman-type self-oscillations in highly dispersive saturable two-level-medium devices is revisited by using the gain and reflectivity matrices. The probe gain when two identical pumps counterpropagate through a two-level system is demonstrated to be very different from the gain for a probe in a one-way device, and typically peaked near a frequency twice the one-way Rabi frequency, while multiwave mixing displays a generalized mirror behavior of the atomic cell. The multimode instability in a Fabry-Pérot interferometer is also discussed and shown to be mainly a result of counterpropagating probe coupling while the coupling between copropagating probe and idler is negligible.

We study the operational properties of the lasing field generated from strongly driven two-level atoms in a ring cavity by including the field saturation to all orders. Lasing without inversion between bare atomic states can occur for both off-resonant– and resonant-pump fields. In the case of an off-resonant–pump field, lasing occurs at one of the Rabi sidebands, for which there exists population inversion between the relevant dressed-atom–field states. In the case of a resonant-pump field, lasing can occur near either of two Rabi sidebands, for which there is no population inversion between the dressed states; here the gain is due to the atomic coherence between the dressed states. While the operation of the off-resonantly–pumped dressed-state laser is similar to that of an ordinary laser, the operation of the resonantly pumped dressed-state laser exhibits some interesting features, such as (1) mode pushing and (2) larger laser intensity coming from smaller linear gain as the detuning is changed. In order to show such features, the calculations have to go beyond the secular approximation.

The possibility of observing a self-induced transparency (SIT) in an erbium-doped optical fiber is investigated. A general equation that includes the SIT, the group-velocity dispersion (GVD) of the fiber, and the nonlinear refractive index is discussed. It is shown that for one particular value of waveguide parameters the nonlinear index can balance the GVD, that is, the nonlinear Schrödinger equation (NLS), while maintaining a SIT soliton. This is called a SIT-NLS soliton. The phase change of the SIT-NLS soliton is governed solely by the NLS component, and the pulse delay due to resonance is determined solely by the SIT component when the detuning is zero. Practical parameters for silica-based fibers do not allow the coexistence of a mixed soliton state. A simple derivation of a condition for a SIT-NLS soliton is also presented. The SIT-NLS soliton is computer-run, and it is shown that a stable 2π/N=1 SIT-NLS soliton exists. However, high-order SIT-NLS solitons always split into multiple 2π/N=1 solitons. The NLS property can be preserved when ${\mathit{Z}}_{\mathrm{sp}}$≪${\mathit{L}}_{\mathrm{abs}.\mathrm{}}$ Finally, the NLS soliton that interacts coherently with erbium ions is studied, and the coherent pulsation that produces multiple narrow pulses is described.

It is shown that a linear combination of ‘‘even’’ and ‘‘odd’’ Sp(2,R) coherent states can be more highly squeezed than a coherent state belonging to a single Sp(2,R) representation. Physical mechanisms for generating such superpositions are introduced and studied.