We generalize a recently formulated theory of high-order harmonic generation by low-frequency laser fields [Anne L’Huillier et al., Phys. Rev. A 48, R3433 (1993)] to the case of an elliptically polarized light. Our theoretical description includes both the single-atom response and propagation. Phase matching significantly modifies the results obtained in the single-atom response. The results of our calculations, including propagation for both the intensity and polarization properties of harmonics as a function of laser ellipticity, compare very well with recent experimental observations. © 1996 The American Physical Society.

We compute the spectrum of emitted radiation for a particle in a triangular potential well and driven by an electromagnetic field. For intense fields, the spectrum exhibits a plateau structure, a series of harmonic peaks of comparable strength up to some high-harmonic order, similar to that found in experiments and similar theoretical models. By comparing the quantum and classical dynamics, we see that the onset of high-harmonic generation in the quantum system appears to be correlated to the onset of chaos in the classical system. We also consider the effect of a slow turn on of the field and show why the acceleration should be used instead of the induced dipole moment to compute the spectrum. © 1996 The American Physical Society.

Highly selective photoionization of the ytterbium atom by coherent two-photon excitation was demonstrated experimentally. The Yb atom was excited by two single-mode lasers and ionized by a time-delayed broadband laser. The isotope ratio of the ions was analyzed by a time-of-flight mass spectrometer. The ion yield and the selectivity were recorded as a function of detuning of the first exciting laser. The maximum of ion yield was observed when the laser was detuned from resonance. The selectivity of ${}_{}{}^{168}{}_{}{}^{}\mathrm{Yb}$ was increased higher than 20 000 when the laser was blue-detuned to the most efficient position. Also, numerical analysis was performed to explain the experimental results. © 1996 The American Physical Society.

Multiphoton above-threshold ionization (ATI) is considered for hydrogen in the framework of the Keldysh-Faisal-Reiss (KFR) models but treating the Coulomb effects of the residual ions on the ionized electrons more completely than in previous work. The spectrum of the ATI peaks is evaluated for linearly polarized laser light, and it is shown that with the appropriate inclusion of the Coulomb interaction more hot electrons are predicted than by the original KFR theories. Moreover, the angular distributions of the ATI electrons are evaluated and are shown to have sidelobes. © 1996 The American Physical Society.

We describe the results of our solutions of the time-dependent Schrödinger equation for polarized states of atomic hydrogen exposed to a short pulse of intense, long-wavelength, continuum radiation. The duration of the π-polarized unipolar pulse is of the order of the Kepler period of the state being irradiated, and the pulse is sufficiently intense to ionize a significant fraction of the initial state. Markedly different behavior is observed for the maximally ‘‘redshifted’’ versus the maximally ‘‘blueshifted’’ initial state, depending upon whether the pulse duration is less than or greater than the underlying Kepler period. © 1996 The American Physical Society.

We report the observation of steady-state amplification of a weak probe laser without population inversion in the bare states in a Doppler-broadened Λ-type Rb atomic system. The probe amplification was observed with the Rabi frequency of a coupling laser much smaller than the Doppler width and can be viewed as due to the dressed-state inversion. A 10% single-pass amplification of a weak probe laser was obtained, and the measured linewidth of the gain spectral feature approaches the Rb natural linewidth. A theoretical analysis based on a three-level Doppler-broadened Λ system is presented, and qualitative agreement with the experiment results is achieved. © 1996 The American Physical Society.

We report observations of self-focusing and filamentation, i.e., the formation of one or more self-trapped beams, of ∼3 ns laser pulses in strontium (Sr) vapor. The experimental arrangement consists of a Sr vapor cell and a pulsed neodymium-doped yttrium aluminum garnet pumped dye laser tuned to the blue of the 461 nm transition in Sr. The spatial intensity distribution of individual self-focused beams has been measured at the exit window of a vapor cell by using a magnifying lens and video camera. Measurements of power dependence, spatial distributions, and scaling behavior for filament formation are in good agreement with theory over a wide range of vapor densities, laser detunings, and laser powers. © 1996 The American Physical Society.

Although the signal and idler light beams produced in the process of spontaneous parametric down-conversion are mutually incoherent and exhibit no one-photon interference, interference effects should be observable after either the signal or the idler is reflected by a phase-conjugate mirror. The degree of coherence can, in principle, be as large as unity. We refer to the relationship between the signal and the idler produced by down-conversion as anticoherent and introduce a natural measure for the degree of anticoherence. © 1996 The American Physical Society.

The coherent states of a Hamiltonian linear in SU(1,1) operators are constructed by defining them, in analogy with the harmonic-oscillator coherent states, as the minimum-uncertainty states with equal variance in two observables. The proposed approach is thus based on a physical characteristic of the harmonic-oscillator coherent states which is in contrast with the existing ones which rely on the generalization of the mathematical methods used for constructing the harmonic-oscillator coherent states. The set of states obtained by following the proposed method contains not only the known SU(1,1) coherent states but also a different class of states. © 1996 The American Physical Society.

Standard damping theory is generalized to incorporate the effects of finite mirror transmittivity. The correction to the standard Langevin equation for the quasimode annihilation operator is determined in first order in the transmittivity of the mirrors. From the Langevin equation an effective master equation is derived. As an example, we study the decay of a single two-level atom at a fixed position in a nonideal cavity. For this case we find a modification of the damped Rabi oscillations, which depends on the atomic position. © 1996 The American Physical Society.

The classical behavior and the quantum fluctuations are analyzed for the system of nondegenerate second-harmonic generation inside a cavity, where two frequency degenerate but orthogonally polarized harmonic modes are coupled to a second-harmonic mode via a ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ medium. We consider the consequences of asymmetrically pumping the two harmonic modes. Asymmetric pumping causes large changes in the classical dynamical behavior of the system; the pitchfork bifurcation, predicted by Ou [Phys. Rev. A 49, 4902 (1994)] and Eschmann and Reid [Phys. Rev. A 49, 2881 (1994)] for symmetric pumping, is transformed into a saddle node for any antisymmetric pumping. This new dynamics brings about the possibility of the system reaching a Hopf and a saddle-node–Hopf bifurcation point. Squeezed-state production is investigated at the critical points of the system. Large squeezing is predicted at the saddle-node and Hopf bifurcation points. The wealth of squeezing predicted by this model at the pitchfork bifurcation point is interpreted in terms of the dynamics of the system. For symmetric pumping, this system is shown to be able to produce two coherent amplitude and phase correlated beams both above and below the pitchfork bifurcation point and so is a candidate for an Einstein-Podolsky-Rosen experiment. © 1996 The American Physical Society.

We demonstrate the application of electromagnetic-field-induced transparency and quantum interference effects in the cooperative phenomenon, such as optical bistability. The control field used in tandem with the usual electromagnetic field of the two-level scheme results in a considerable lowering of the threshold intensity. We discuss the transient response of the system in the mean-field limit and describe the regression to the steady state when perturbed away from it; the regression exponent is itself dependent on the control field. We also demonstrate the possibility of control-field-induced multistability in two-level systems. © 1996 The American Physical Society.

A quantization scheme for the radiation field in dispersive and absorptive linear dielectrics is developed, which applies to both bulk material and multilayer dielectric structures. Starting from the phenomenological Maxwell equations, where the properties of the dielectric are described by a permittivity consistent with the Kramers-Kronig relations, an expansion of the field operators is performed that is based on the Green function of the classical Maxwell equations and preserves the equal-time canonical field commutation relations. In particular, in frequency intervals with approximately vanishing absorption the concept of quantization through mode expansion for dispersive dielectrics is recognized. The theory further reveals that weak absorption gives rise to space-dependent mode operators that spatially evolve according to quantum Langevin equations in the space domain. To illustrate the applicability of the theory to inhomogeneous structures, the quantization of the radiation field in a dispersive and absorptive one-interface dielectric is performed. © 1996 The American Physical Society.

A number of parametric effects, such as suppression of period doubling, shift of the bifurcation point, scaling law relating the shift and the perturbation amplitude, influence of the detuning on the suppression, reaching of the maximum gain between the original and shifted bifurcation points, and scaling law for idler power are experimentally observed near period doubling bifurcation in a loss-driven ${\mathrm{CO}}_2$ laser that is subjected to periodic loss perturbations at a frequency that is close to a parametric resonance of the unperturbed system. © 1996 The American Physical Society.

We examine the various two-time fluorescence intensity correlations emitted by a driven three-level atom in a cascade configuration. For a strongly driven atom, the two intensity autocorrelation functions, one from the upper transition, the other from the lower transition, exhibit different behaviors. In addition to the an oscillation similar to the two-level intensity correlation we find an additional oscillation for the upper transition only. This is shown to be due to the spontaneous decay between the lower transition levels. © 1996 The American Physical Society.

We describe theoretical and experimental investigations of steady-state gain without population inversion that can occur on the ${\mathit{D}}_1$ transition of potassium, upon strong cw-laser excitation of the ${\mathit{D}}_2$ transition. A Raman-driven four-level model, in which a noninverted population in the 4${\mathrm{}}^2$${\mathit{P}}_{1/2}$ level is established via collisional transfer from the 4${\mathrm{}}^2$${\mathit{P}}_{3/2}$ level, is in good qualitative agreement with a variety of experimental results. These results include, among other studies, an examination of the impact of coherence dephasing on the realizable gain, as well as measurements of atomic energy-level populations for a case of line-center pumping that clearly displays gain. The measured relative populations for this case are n(4${\mathit{P}}_{1/2}$)/${\mathit{n}}_{\mathrm{total}}$=0.098±0.034 and n(4${\mathit{P}}_{3/2}$)/${\mathit{n}}_{\mathrm{total}}$=0.19±0.08, where ${\mathit{n}}_{\mathrm{total}}$ is the total population. These measurements indicate that the probed level 4${\mathit{P}}_{1/2}$ population is not inverted with respect to the ground state. © 1996 The American Physical Society.

The amplification of a visible laser-diode beam in a gain-guided, multipass Raman amplifier is studied as a function of laser-diode and pump-laser powers. At a relatively high laser-diode power (178 nW) the amplified output grows as a simple exponential from very low gains to amplifier saturation. At a relatively low laser-diode power (2.3 nW) the behavior is more complicated, since the seeded growth and the growth from spontaneous scattering are comparable in power. The experiments, which also include an experiment with no injected laser-diode light, are modeled by a nonorthogonal-mode theory that accounts for the three-dimensional nature of the amplification process. The theory and data are found to be in close agreement in all experimental regimes studied, except at low gains with low input seed power. © 1996 The American Physical Society.

We show how the internal dynamics of a multilevel atom are modified in the vicinity of the interface between a vacuum and a simple or multilayered lossless dielectric medium. Optical Bloch equations are derived, which take into account the modifications of spontaneous emission rates and energy levels experienced by the atom. van der Waals level shifts are evaluated using the method of images for dielectrics. Spontaneous emission rates and radiation patterns are calculated in a simple way using the Lorentz reciprocity theorem. © 1996 The American Physical Society.

We apply an optimal control technique (OCT) to design a secondary coupling pulse that can control the amplification of a given primary optical pulse in a dense media of coherently prepared three-level λ-type atoms without population inversion. Effects of pulse absorption, amplification, and reshaping by the medium are incorporated in the OCT. We demonstrate numerically that shape-invariant as well as shape-distorted amplification without population inversion can be achieved by properly designing the shape of the coupling pulse. The maximum amplification obtainable is shown to be dependent on the initial atomic coherence as well as the upper bound of the coupling pulse intensity. The group velocity of the primary pulse can also be controlled by the coupling pulse. © 1996 The American Physical Society.

The states generated from the vacuum in two parametric down-conversion crystals with aligned idler beams are studied. It is shown that after a measurement of the photon number in some of the modes, the emerging states can be related to SU(2) and SU(1,1) coherent and minimum-uncertainty states. © 1996 The American Physical Society.