The photoelectron spectra in photodetachment of a negative ion and in atomic photoionization by an exponentially decaying laser pulse are analyzed. The possibility of a multipeak structure for these spectra in a highly nonperturbative regime is predicted. Various properties of the spectra are also discussed.

Velocity distribution and spatial distribution of ion clouds in Paul traps have been investigated both theoretically and experimentally. The kinetic energy and the width of the spatial distribution have been measured as function of the trap parameters. This is compared with theoretical results obtained from model calculations based on Brownian motion of trapped particles.

The broad white-light continuum generated on the passage of ultrashort pulses of laser light through suitable media has found numerous applications in the physical, chemical, and biological sciences. Many of these are based on use of the continuum to probe the absorption characteristics of transient species, in order to obtain information on their decay kinetics. The processing of data from such experiments is generally based on the implicit assumption that the absorption of white light is subject to the normal Beer-Lambert law, in that the absorption at any particular frequency is linearly proportional to the intensity of the probe light at that frequency. In this paper it is shown that the validity of this underlying assumption should be seriously questioned. At high intensities, the uncertainty principle allows for photon pairs with differing frequencies to provide the energy for the cooperative excitation of molecules in close proximity. This nonlinear absorption, which depends on the frequency autocorrelation function and intensity of the continuum light, results in a rate of absorption which may depart significantly from the normal Beer-Lambert behavior. Hence the interpretation of continuum-probe experiments may need to be reexamined.

The propagation of optical pulses in single-mode nonlinear dispersive fibers in the vicinity of the zero-dispersion wavelength has to take into account the third-order dispersion term. We show that, for short dispersion, the broadening of the pulses, described by the rms and full width at half maximum pulse width evolution, is reduced by a red shift from the zero-dispersion wavelength. Numerical resolution of the associated propagation equation, for initially 1.2-ps Gaussian pulses, shows that this spreading reduction remains valid to a few tenths of kilometers of propagation. Wavelength shift evaluations are obtained in a very simple way, by use of a moment expansion near the origin of propagation. A simple (nonexhaustive) explanation of the time-domain and Fourier-domain pulse evolution, based on the comparison of the phase velocities with the group and pulse mass-center velocities is proposed.

An envelope-function approach is used to give a theoretical description of the electromagnetic properties of nonlinear periodic structures. This method, in which the electric field is separated into slow and fast spatial components, shows that the slow field component satisfies the nonlinear Schrödinger equation. The well-known soliton solutions of this equation provide a theoretical description of the gap solitons found by Chen and Mills [Phys. Rev. Lett. 58, 160 (1987)] in their numerical studies of these structures. The more general solutions of the nonlinear Schrödinger equation provide a framework for understanding the properties of finite nonlinear periodic stacks. Our method allows us to find these solutions analytically.

By inserting an optical diode consisting of a variable Faraday rotator and a quartz rotator, whose loss is different for waves traveling in opposite directions, into the cavity of a two-mode dye ring laser, we have investigated the statistical fluctuations of the laser output as a function of controlled asymmetry. We relate the results of the measurements to analytic solutions of the third-order coupled equations of motion without pumping fluctuations, and also to computer solutions with pump fluctuations. The latter are generally in reasonable agreement with experiment. In addition, we find that the two solutions agree with each other very approximately if the difference of the pump parameters in the expressions for the mean intensities in the analytic theory is scaled up by a factor of 20. Possible reasons for this are pointed out.

We extend our previous comparison of classical and quantum theories for the excitation and dissociation of a sinusoidally driven Morse oscillator [Phys. Rev. A 37, 796 (1988)] to the case of two-frequency driving. In both the classical and quantum theories the total threshold intensity for dissociation can be considerably smaller in the two-frequency case. This is consistent with results obtained with more artificial models of laser-driven molecular systems, and we provide an approximate resonance-overlap analysis to explain this trend. We also compare results obtained with adiabatic and sudden turn-on of the applied field, and comment on the use of absorbing boundaries for the computational identification of dissociation or ionization.

A two-level atom illuminated by a laser may be driven into a population-inverted steady state if it is coupled to a cavity. This is a result of the quantum nature of the electromagnetic field and is forbidden by the semiclassical theory. We numerically and analytically analyze this inversion. We find the maximum possible inversion and determine the rate of approach to the steady state. The quantized cavity modifies the electromagnetic vacuum seen by the atom; this connects our work to that on cavity-enhanced spontaneous emission and on dynamical line narrowing. Experimental signatures of the inversion and potential experimental difficulties are considered. For example, neither the presence of many atoms in the mode nor many modes in the cavity destroys the inversion.

Periodic-intensity self-oscillation caused by modulational instability is observed for the first time in a nonlinear dispersive fiber ring cavity. The oscillation stops immediately when the phase coherence is disturbed by cavity detuning. It is found that a noninstantaneous response of the nonlinear index competes with the modulational instability gain and produces asymmetrical Stokes sidebands.

We calculate the reflection coefficient for a light beam incident on the interface between a dielectric and a resonant atomic vapor to first order in the dipole polarization in the vapor. The angle of incidence and the polarization direction are chosen arbitrarily, and saturation is fully accounted for. The atoms are supposed to get deexcited at collisions with the surface. The resulting transient behavior of atoms leaving the surface is responsible for a nonlocal response. This spatial dispersion near the surface is known to produce a natural-linewidth-limited resonance in the vapor reflection coefficient at normal incidence. Our analysis shows that this sub-Doppler structure is broadened by the residual Doppler effect for non-normal incidence. This structure disappears at the critical angle for total internal reflection, where one predicts a conventional Voigt-type dispersion response, based on the complex-refractive-index approach. We also calculate the saturation broadening of the atomic response for large intensities. Beyond the critical angle, the spectral response suddenly shifts from dispersion to absorption line shapes. In the case of total internal reflection, the spatial dispersion leads to an additional Lorentzian broadening, which results from an effective imaginary Doppler shift at passage through the evanescent wave (transit-time broadening).

We report the observation and investigation of synchronous energy exchange between nonrelativistic electrons and the ponderomotive (beat) force of two counterpropagating intense pulsed ${\mathrm{CO}}_2$ laser beams, operating at different frequencies in a stimulated Compton-scattering scheme. The interaction occurred in the nonlinear (trapping) regime, the physics of which is the same as that which occurs in laser accelerators and efficiency-enhanced free-electron lasers (FEL’s) with long wigglers. The experiment is a first demonstration of the principle of inverse FEL acceleration and electromagnetic pump FEL operation in the nonlinear (trapping) regime. It can also be described as a demonstration of a ‘‘traveling beat-wave’’ Kapitza-Dirac effect in the nonlinear regime. Two different mechanisms of enhanced energy transfer were observed—electron trapping and phase-area displacement. Experimental results and computer simulations of both mechanisms are presented.

We investigate instabilities which develop when a laser interacts with a nonlinear dielectric slab. Feedback between the scattered field and inhomogeneities in the material can lead to growing fluctuations on a length scale of the wavelength of light. In the thin-film limit an instability criterion is developed that complements present work on laser-induced periodic surface structure. Evolution and saturation of an unstable mode of the system are examined.

A nonlinear theory of correlated emission lasers is presented. It is shown that if three-level atoms in the V configuration are injected in a coherent superposition of the upper two states in a doubly resonant cavity, the diffusion coefficient for the relative phase vanishes under certain conditions.

We examine the buildup of atomic dipoles by a single broadband light pulse and their subsequent rephasing to produce a coherent transient signal. Within the assumption of short-time correlation for the light field, the theoretical model predicts the inhibition of inhomogeneous dephasing by the strong stochastic field. A geometrical picture of the effect is presented that uses a diffusion model of the Bloch vector headings on the Bloch sphere. The model is applied to the specific case of a stimulated photon echo in an angled-beam configuration. The corresponding experiment is performed in calcium vapor, and the expected inhibition effect is observed. The decay of the echo intensity as a function of the delay between the two first weak pulses is found to agree with the theoretical model.

We derive exact equilibrium fluctuation expressions for the nonlinear Burnett coefficients for the electrical conductivity of a fluid. The Burnett coefficients define a Taylor-series expansion of the conductivity in powers of the electrical current density. The fluctuation expressions may be calculated from an analysis of the time-dependent fluctuations of the equilibrium system constrained so that the mean electrical current is zero.

We consider the motion of a symmetric finite bubble in a two-dimensional Hele-Shaw cell. In the absence of surface tension, the Saffman-Taylor solution contains two free parameters, U and λ, for a given bubble area J, where U is the dimensionless speed and λ is the dimensionless width of the bubble. It is shown that, in the presence of surface tension, a solution does not exist for U>2 for any bubble area. We also derive the following scaling relations: (a) 2-U≊${\epsilon }^{2/3}$ for large J; and (b) 2-U≊${\epsilon }^2$ for small J, where ε is a small parameter which is proportional to the surface tension. We show that by creating a cusp at the tip of the bubble, one can increase the speed of the bubble U>2. We present predictions for the shape and speed of the symmetric bubble as a function of the external parameters in the presence of a cusp at the tip. This picture may explain the recent experimental results of Maxworthy, where substantially enhanced velocities were measured for the anomalous bubbles with a tiny bubble at the tip.

Neutrons can be detected with a suspension of superheated drops in an immiscible and compliant holding medium. The mechanism of this radiation-induced bubble nucleation has been studied. A model to predict the minimum neutron energy required to nucleate a vapor bubble in superheated liquids has previously been reported [R.E. Apfel, S. C. Roy, and Y.-C. Lo, Phys. Rev. A 31, 3194 (1985)]. Now, the energy-dependent response function for the detector has been calculated based on the model and a detailed consideration of the possible interactions of neutrons with superheated materials. The response of the detector has been measured with monoenergetic neutrons (from 0.025 eV to 14 MeV) at facilities of the National Bureau of Standards and the Radiological Research Accelerator Facility at the Nevis Laboratories of Columbia University. The experimental results agree reasonably well with that of the calculations.

We explore the anomalous scattering of light observed in a number of different crystals growing from their melt within the framework of irreversible thermodynamics. Based on recent experimental work, we treat the scattering as resulting from small ‘‘objects’’ that are produced at the interface by the growth process and that are confined, thereafter, to one phase or the other. The thermodynamic analysis is based on the form of the entropy production at the Gibbs surface separating the solid from the melt and is independent of the nature of the ‘‘objects.’’ Two scalar processes at the interface that, in principle, can be coupled dynamically, are found, one involving the growth rate of the crystal, v, and the other involving the rate of creation of the ‘‘objects.’’ If the Onsager coupling coefficient is a sigmoidal function of the surface density of ‘‘objects,’’ then multiple steady states are possible at the interface, leading to critical behavior and hysteresis in light scattering as a function of v. As a consequence of the thermophoretic effect, the critical growth velocity is a decreasing function of the temperature gradient when the objects reside in the liquid and an increasing function when they reside in the solid. In the absence of multiple steady states, which is the case when the coupling coefficients are essentially constant, neither critical behavior nor hysteresis will be observed. In either case, the resulting light scattering will be localized in a layer adjacent to the interface and have a Lorentzian spectrum with a half-width equal to the product of the diffusion constant of the ‘‘objects’’ and the square of the scattering wave vector. Comparing to recent experiments, we conclude for all systems in which anomalous scattering has been found save one, namely, pure water, that the ‘‘objects’’ are microbubbles in the melt arising from dissolved gaseous impurities. In water of sufficient purity the only plausible mechanism for scattering appears to be the formation of defects residing in the solid.

We present modeling, measurements and analysis that extend our understanding of laser-plasma coupling and x-ray conversion processes to shorter laser wavelengths at kilojoule energies. We have studied the x-ray emission from gold-disk targets irradiated at 0.26-μm wavelength, using 0.5–1.5-kJ, 1-ns full width at half maximum Gaussian pulses at intensities of 1×${10}^{14}$–2×${10}^{15}$ W/${\mathrm{cm}}^2$. We interpret measurements of the absolute broadband x-ray spectrum, the time history of sub-keV x-ray emission flux, and the broadband hard-x-ray spectrum. We find negligible hot-electron levels (${<10\mathrm{}}^{\mathrm{-}3}$$E_{\mathrm{inc}}$). We infer increased conversion of incident light to soft x rays (72%±15% at 1×${10}^{14}$ and 38%±12% at 2×${10}^{15}$), compared with longer wavelengths, as predicted. We find good overall agreement between the modeling and the data, but identify some areas that need to be better understood, including certain aspects of the x-ray emission spectrum and the decay rate of the emission pulse.

The behavior of laser-driven shocks in aluminum-on-gold targets was examined using a purely hydrodynamic code. The results revealed the importance of the detailed structure of the shock as it propagated through the low- and high-impedance layers. Optimal design and proper interpretation of experiments would require considerations of the thicknesses of both layers. The validity of the purely hydrodynamic calculations was checked using a hydrocode incorporating the laser-driven ablation process. The predicted shock paths in the targets were also verified in experiments.