We examine the interaction of a one-dimensional model molecule with two electrons and two nuclei in a very short pulse of light fields of different strength. Due to their high masses and the shortness of the pulse, the dynamics of the nuclei can be regarded as frozen during the pulse and only the electronic part of the wave function has to be taken into account. After the pulse the molecule will dissociate on a larger time scale if the electrons are not in a binding state, especially if both electrons are removed from the molecule. One finds that the ionization and dissociation probability decrease with increasing field strength for very high laser intensities, a phenomenon which is already known for the ionization of atoms. By examination of the shape of the wave function during the pulse one sees that the electrons are trapped in bound states of the time-averaged Kramers-Henneberger potential.

We have investigated the multiphoton ionization of helium at wavelengths between 310 and 330 nm at intensities between 8×${10}^{13}$ and 5×${10}^{14}$ W/${\mathrm{cm}}^2$ and at 630 nm at intensities of 1×${10}^{15}$ W/${\mathrm{cm}}^2$. We characterize the ionization processes from photoelectron energy and angular distributions observed concurrently with photoion spectra. At the shorter wavelengths we find that resonant enhancement via the ac Stark shifted six-photon resonant states (1s3d and 1s3s) is a dominant ionization path as described previously by Perry, Szöke, and Kulander [Phys. Rev. Lett. 63, 1058 (1989)] and by Rudolph et al. [Phys. Rev. Lett. 66, 3241 (1991)]. At intensities above those required for resonant enhancement, and at wavelengths longer than those required for six-photon resonance, we observe that nonresonant seven-photon ionization dominates. This process gives rise to continuous distributions of low-energy electrons with characteristic angular distributions that peak near 0° and 60° relative to the laser polarization. At yet higher intensities, above the threshold where the nonresonant seven-photon channel closes, the dominant ionization path occurs via seven-photon resonant states with odd parity. This path gives rise to angular distributions characteristic of intermediate states with f character.

Using well-known concepts from semiclassical dynamics, we simulate spectra of a hydrogen Rydberg electron under the influence of parallel electric and magnetic fields. The spectra show peaks characteristic of recurring orbits as well as periodic orbits, which we have identified. The results agree qualitatively with experiments.

Theory relevant to the treatment of resonance collisions between two-level dressed atoms has been extended to the three-level case in a self-contained calculation of field-dependent collision rates. With the laser field tuned close to the ‖${\mathit{e}}_1$〉⇆‖${\mathit{e}}_2$〉 transition, dressing of the upper two-level system is achieved. We solve coupled equations of motion for two-atom states that describe one atom in the ground state and the other in one of two dressed states. Dressed-state collision rates are calculated within the impact approximation using S-matrix theory. The coupled equations of motion are simplified in the limit of intense field. We have applied the theory to describe approximately collisions between Ne atoms in fields intense enough to dress the collision complex.

It is well known that in the presence of a magnetic-field gradient an increase in the transverse relaxation rate will occur in a spin-polarized gas sample. Recent theories related to diffusion through magnetic-field gradients have been applied to spherical and cylindrical gas cells in helium and alkali-metal-atom systems. A simple first-order theory based on the Redfield model is extended to include oscillating gradients in spherical and cylindrical gas sample cells. Experimental gradient measurements are made using a ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ free-precession magnetometer. The experimental results are in agreement with the theory.

A generalization of the standard rotating-wave Jaynes-Cummings models is proposed to study the dynamics of resonantly driven doublet-to-doublet transitions. Under restrictions similar to those involved in the standard two-level models, our four-level model can be solved analytically. The model is then used in a tunneling context to analyze the effects of the resonant driving field in the tunneling dynamics of the system. In particular, two effects, drastic increase in the tunneling rates and suppression of tunneling, are illustrated. The doublet nature of the spectrum also induces interesting effects on the field such as the appearance of displaced Fock states.

We have studied the frequency shift in the Rb electron-paramagnetic-resonance (EPR) line due to the presence of nuclear polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. By measuring the resonant frequency in a cylindrical cell oriented both parallel and perpendicular to an applied magnetic field, we determined the classical EPR shift due to long-range macroscopic fields. We thus calibrated the larger, geometry-independent shift, due to the Fermi-contact interaction between the Rb electron and the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nucleus, to 2.5%. The EPR frequency shift can be used to determine the ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclear polarization.

Laser cooling of three-level ions of the cascade, vee, and lambda configuration is considered with simultaneous excitation on a weak and strong transition. Choosing appropriate parameters for the Rabi frequencies and the respective detunings allows one to eliminate adiabatically one of the levels. The thus designed two-level ion can acquire decay rates Γ’ smaller than the trap frequency ν such that sideband cooling becomes possible and accordingly allows cooling of the ion to its lowest oscillator state 〈n〉=0 in the trap. Explicit expressions for the detunings and the Rabi frequencies for sideband cooling of the effective two-level system are derived.

We have demonstrated the slowing of a rubidium atomic beam by isotropic monochromatic light. The results agree with a model calculation, thus allowing its use for designing a practical isotropic light slower. The large hyperfine splittings of rubidium lead to natural multifrequency slowing, which is also included in our model.

Nonclassical states of light may be generated by processes involving the creation or annihilation of photons in pairs. A quadratic coupling, characteristic of a parametric amplifier, generates a squeezed vacuum from a normal vacuum, and a two-photon absorber can also generate a squeezed state (though not a minimum-uncertainty state) even though it is a purely dissipative process. We consider here the simultaneous action of a quadratic pump on a two-photon absorber and demonstrate how superpositions of distinct coherent states may be generated by their combined effects. We use standard master equations to describe the time development, employing split operators and direct numerical integrations to determine the field density-matrix elements and quasiprobabilities. The purities of the nonclassical states are determined by evaluating the field entropy. Provided one-photon dissipative processes may be ignored, a pure superposition state is formed in the steady state. This superposition is destroyed if one-photon loss processes are important.

It is shown that a particular combination of so-called even coherent state dubbed the orthogonal-even coherent state, while exhibiting no ‘‘ordinary’’ or second-order squeezing, can furnish near-optimal simultaneous-quadrature fourth-order squeezing [where the latter term is defined in the sense of Hong and Mandel, Phys. Rev. Lett. 54, 323 (1985)]. The Wigner function of the particular quantum-mechanical superposition of states is calculated and various plots compared to the Wigner function of a simple statistical mixture of the same states. Rather uninteresting behavior is found in the latter case, which illustrates that the higher-order squeezing effect is due to nonclassical, quantum-mechanical ‘‘interference in phase space.’’

It is found that dissipative types of stable soliton structures can exist in nonlinear optical media with broadband gain and group-velocity dispersion (GVD). These structures resemble ionization or combustion waves and are essentially self-accelerating pulses with a stationary-envelope form and a permanently shifting wave spectrum. Contrary to the conservative solitons, the dissipative ones exist for any sign of GVD. Being an attractor in the development of arbitrary initial distributions, the dissipative structures cause the fundamental Schrödinger solitons to disappear in the course of evolution in weakly nonconservative systems.

Squeezing of the fluorescence light from a single trapped and laser-cooled ion is studied for atomic localization conditions far from the Lamb-Dicke limit. The quantum center-of-mass motion significantly influences the quantum fluctuations of the fluorescence light field. Both for traveling- and standing-wave excitation squeezing is found to depend on the observation direction.

A systematic procedure for deriving effective Hamiltonians to any order is presented, which is applicable to any time-independent Hamiltonian. The method is based on a continued-fraction approach and avoids the singularities which may occur with perturbation theory. It is illustrated by deriving the effective Hamiltonian for the one-photon, dressed-state laser to second order.

We demonstrate that a single-ended optical cavity can be used to generate squeezed light from a pair of intensity-correlated light beams. One beam serves as an input to the cavity, while the measured intensity in the other beam serves to control various cavity parameters such as input coupling, losses, or detuning. Depending on the mode of operation, the resulting output beam from the cavity can be squeezed in amplitude or in phase. We derive the noise spectrum for the case of optimal amplitude squeezing, provided that there are no initial correlations between amplitude and phase in the input beams. Practical examples are given in which an optical parametric oscillator serves as the twin-beam source. As a general consequence of twin-beam control, we also derive a Heisenberg-like inequality governing the amplitude and phase spectra of intensity-correlated beams.

We construct a solution to the linear complex absorption coefficient for the closed Raman-driven four-level symmetrical model. We derive, from this solution, asymptotic expressions for the spectra around and away from the two-photon resonance in the absence as well as in the presence of the Doppler broadening. They are used for the derivations of analytical conditions and the discussion of the physical origins of various phenomena that do not appear in the traditional two-level system.

The photon dynamics proposed by Cook [Phys. Rev. A 25, 2164 (1982); 26, 2754 (1982)] is reformulated in terms of conventional quantum mechanics. Here, the subject is confined to a photon in vacuum. The photon density D(x,t) and photon current density C(x,t) in the Cook theory are here regarded as the probability density and probability current density of a single photon, respectively. A wave function Ψ for a photon is defined by a 6×1 column matrix ${(}_{\mathrm{\phi }}^{\mathrm{\psi }}$) where ψ and φ correspond to the photon vector fields of the Cook theory. The Schrödinger equation for Ψ is easily obtained from equations of photon fields ψ and φ and the Hamiltonian form is determined as the time-evolution operator. By this reformulation, negative-frequency solutions were shown not to be physical and need not be considered for a physical photon. This explains why, in the Cook theory, they are dismissed from plane-wave expansions of photon fields ψ and φ.

Similarity and other group-invariant solutions of stimulated Raman scattering (SRS) in the presence of dissipation are studied. The group-theoretical analysis reduces the SRS equations to ordinary differential equations which in the most interesting cases, i.e., in the case of the self-similar and soliton solutions, are studied perturbatively to derive the effect of dissipation.

The response of a hydrogen atom initially prepared in the 4s state and subjected to a short, intense laser pulse is studied by calculating field-dependent time-dependent wave functions numerically as a function of r and θ. We calculate the probabilities of bound states that are dipole connected to the 4s state both directly and by two-photon Raman coupling through the continuum. We find that for intensities in the neighborhood of ${10}^{15}$ W/${\mathrm{cm}}^2$ the bound states exchange population relatively slowly in time while ionizing. Space-time plots of the electronic charge cloud show a complex but very stable extended microstructure like that of a stabilized polychotomous wave packet. A calculation of the expectation value of the charge cloud’s position along the axis of laser polarization shows behavior similar to free-electron behavior although the extent of the charge cloud is much greater than the free-electron oscillation amplitude.

The amplitude-phase coupling of a stochastically varying field has been predicted to lead to a shift of the effective resonance frequency when this field interacts with a two-level system. Experimentally, we have generated such a field by passing a semiconductor laser beam through a Michelson interferometer, thereby converting part of the intrinsic phase flucutations into amplitude fluctuations. We find indeed a shifted resonance frequency when this beam interacts with the Rb ${\mathit{D}}_2$ transition. Depending on the path-length difference of the interferometer, shifts of more than 10 MHz have been measured. The experiments have been performed in the weak-field limit. This allows a convolution-type interpretation of the stochastic realization shift.