The autocorrelation ${\mathit{g}}^{\mathrm{}\left(2\right)\mathrm{}}$(τ) of the photon-counting signal from laser-induced light scattering off a single trapped and laser-cooled ${\mathrm{Ba}}^{+}$ ion shows dramatic nonclassical effects: ${\mathit{g}}^{\mathrm{}\left(2\right)\mathrm{}}$(τ) increasing upon delay τ close to zero (‘‘antibunching’’) is accompanied (i) by its extreme peak values, (ii) by long transients and long appearance of subunity values (‘‘sub-Poissonian light’’), both extending much beyond the lifetime of the ion’s resonance level, and (iii) by modulation from magnetic precession, as is common with quantum beats. These phenomena highlight the internal multilevel structure of the ion, hint at optical pumping, and prove the coherent evolution of the ion’s internal state. The preparation of the ion in a coherent superposition by spontaneous decay is demonstrated. The counting variance of the scattered light, measured in terms of Mandel’s Q parameter, deviates from that of classical light by up to ΔQ=-7×${10}^{\mathrm{-}4}$.

We present systematic experimental studies of high-order harmonic generation in He and Ne that have been performed with an intense, femtosecond Ti:sapphire laser at intensities in the tunneling regime, where special attention is concentrated on the strong-field effect. A broad plateau is formed in the harmonic distribution that includes harmonic peaks at low and high orders and the observed highest orders (shortest wavelengths) are the 103rd (7.6 nm) for He and 95th (8.2 nm) for Ne. The laser-ellipticity-dependent harmonics observed suggest different generation processes for the low orders below and around the atomic ionization limit and for the high orders in and above the plateau region. The medium ionization is found to induce strong self-defocusing of the incident laser beam, and its effects on the harmonic generation are studied. The results for the defocusing and the blueshifts of the harmonic spectra observed show that the generation of a higher-order harmonic requires a higher intensity in time and space. The theoretically predicted harmonic cutoff is demonstrated to agree well with experiment when we take into account the interaction intensity limited in the ionizing medium. The characteristic harmonic behaviors observed are discussed based on the interaction intensity and the propagation effect in the medium.

The dynamics of a system composed of two strongly coupled ions trapped in a time-dependent potential well and cooled by a laser beam is studied. In the present classical treatment the photon-ion interaction is simulated with an effective noise acting on the ions, which produces a stochastic transfer of momentum. The existence of noise-induced fluctuations in the regime where previous analysis indicated that the two-ion system dynamics was stable is discussed.

Two mechanisms for production of an ensemble of atoms in a highly excited state with very narrow distribution of the atomic center-of-mass momentum (cold Rydberg atoms) are proposed. The mechanisms are based on velocity-selective $\pi$ pulses and coherent population trapping in three-level cascade systems with the uppermost state being the Rydberg one. Both methods permit one to reach subrecoil temperatures of the Rydberg atoms.

We study interactions of light with a sample of two-level atoms, with full inclusion of angular momentum degeneracy, at temperatures and densities such that the quantum statistics of the atoms may have an effect. Coupled propagation equations are given for light and matter fields, and plausible general simplifications are enumerated. In particular, the motion of the atoms during the excited-state lifetime may often be ignored. The propagation equations of light and matter fields are decoupled within the assumption that the detuning of the driving light from atomic resonance is large, and the spectrum of scattered light is studied for ideal Bose and Fermi gases. In addition to the expected image of the velocity distribution, the spectra contain qualitatively distinct features that depend on the statistics of the atoms. This is because for bosons (fermions), those scattering events in which an atom recoils to an already occupied state are enhanced (inhibited).

We study theoretically the behavior of two atomic Rydberg series, coupled by a light field that strongly drives the optical transition between the core states to which these series converge. It is shown that this strong-field version of isolated-core excitation leads to a strong modification of the photoionization spectrum and to transitions between adjacent Rydberg states when the Rabi frequency associated with the core transition becomes larger than the Rydberg spacing. The intensities and pulse durations (≈ 2 GW/cm2 and 1 ps, respectively, for $n\approx 20$ Rydberg states) needed for the observation of these effects are within reach of present-day lasers.

We calculate the modes in a stable gravitational cavity where the atomic mirror at the bottom of the bounce is caused by an internally reflected intense Gaussian-mode laser beam. In contrast to Wallis, Dalibard, and Cohen-Tannoudji [Appl. Phys. B 54, 407 (1992)], who treated the bottom bouncing mechanism as due to an infinite potential, we include the effect of the exponentially decaying potential caused by the evanescent wave. Nevertheless, we find that above this evanescent wave potential the mode structure corresponds exactly to that obtained using an infinite potential. The allowed energies differ slightly due to the phase change through the evanescent wave. The broadening of the modes due to the damping effects of spontaneous emission was calculated and because of the intrinsic transverse width of the modes was found to depend strongly on energy and transverse excitation, which differs from a simple one-dimensional classical calculation. The finesse and $Q$ of the cavity due to the damping effects of spontaneous emission were also calculated and in contrast to an optical Fabry-Pérot cavity were found to depend strongly on energy.

We consider the transition from regular motion to dynamical chaos in a classical model of a diatomic molecule which is driven by a circularly polarized resonant ir field. Under the conditions of a nearly two-dimensional case, the Hamiltonian reduces to that for the nonintegrable motion of a charged particle in an electromagnetic wave [A. J. Lichtenberg and M. A. Lieberman, Regular and Stochastic Motion (Springer-Verlag, City, 1983)]. In the general case, the transition to chaos is connected with the overlapping of vibrational-rotational nonlinear resonances and appears even at rather low radiation field intensity, S≳1 GW/${\mathrm{cm}}^2$. We also discuss the possibility of experimentally observing this transition.

An iterative predictor-corrector finite-difference time-domain method is used to solve the semiclassical Maxwell-Bloch system numerically without invoking any of the standard approximations such as the rotating-wave approximation. This approach permits a more exact study of self-induced transparency effects in a two-level atom. In addition to recovering the standard results, for instance, for π, 2π, and 4π pulses, several features in the results appear at the zeros of the driving pulse, where its time derivatives are maximum. Several ultrafast-pulse examples demonstrate that time-derivative-driven nonlinearities have a significant impact on the time evolution of a two-level atom system. Moreover, typical small-signal gain results are also obtained with our Maxwell-Bloch simulator. We illustrate that these time-derivative effects can be used to design an ultrafast, single-cycle pump pulse that completely inverts the two-level atom population. A pump-probe signal set is then used to illustrate gain in the probe signal.

We report on the observation of planar dark spatial solitons due to the bulk photovoltaic effect in lithium niobate, with intensities of the order of 10 W/cm2 and widths of approximately 20 μm. Photovoltaic solitons display a characteristic tensorial dependence on their direction of propagation, on their polarization, and on the orientation of the amplitude profile, with respect to the principal axes of the crystalline medium. The index perturbation associated with a dark soliton persists in the dark, and it can trap and guide a second beam.

We present an experimental study of nonlinear selective reflection at a dielectric–Cs-vapor interface, in a three-level cascade-up scheme. The nonlinear reflection at the Cs ${\mathit{D}}_2$ resonance line is monitored in the presence of resonant irradiation at 621 nm or 658 nm (6${\mathit{P}}_{3/2}$→8${\mathit{D}}_{5/2}$,9${\mathit{S}}_{1/2}$). Experimental reflection spectra with either counterpropagating or copropagating light beams are analyzed and compared with the theoretical predictions of F. Schuller et al. [Phys. Rev. A 47, 519 (1993)]. For a blue-detuned pump and copropagating geometry an alternative type of three-level coherent reflection resonances, specifically associated with desorbing atoms, is observed. Also observed is the nonlinear reflection associated with the off-resonant, two-photon dispersion of atoms moving parallel to the window surface. Probe beam saturation is responsible for the appearance of additional resonant features in reflection spectra. Finally we analyze crossover spectral resonances which arise from nondegenerate, four-wave-mixing processes induced by the combined influence of counterpropagating pump and probe beams, and the pump beam being partially reflected by the window.

The interaction between radiation and matter in an x-ray laser occurring in a cylindrical laser-produced plasma is investigated by means of the Maxwell-Bloch equations with level degeneracies explicitly taken into account. Radiation is initiated by spontaneous emission and amplified while propagating along the cylinder axis. This theory generalizes the current simple theory of the small-signal gain coefficient and is suited to correctly describe the gradual transition to the saturation regime. A detailed numerical study of the germanium x-ray laser is performed to illustrate the theory for the case of the Ne-like ion collisional laser. The effect of electron-ion collisions leading to transitions between degenerate states on linewidths, as well as on saturation behavior is outlined.

Various probability distributions are calculated for photocurrent fluctuations for photoelectric detection and are expressed in terms of quantum averages of field operators. The calculation is based on an extended theory of Kelley and Kleiner for photoelectric counting [Phys. Rev. 136, A316 (1963)], which allows us to obtain the counting distributions for multiple time intervals and multiple detectors. The finite duration of the detector’s response function is also considered in our formalism, as is the possibility of fluctuations in the response function itself. This generalized theory is applied to homodyne and heterodyne detection processes. We find that with ideal conditions (perfectly balanced detectors with unit quantum efficiency), the characteristic function of the photocurrent fluctuations from homodyne detection is connected to the Fourier transformation of the Wigner function of a single-mode field, while that from heterodyne detection is linked to the antinormally ordered characteristic function of the field. More specifically, if the two orthogonal quadratures of the photocurrent are recorded in the heterodyne detection, the joint probability distribution for the two quadratures is simply the Q function of the field without the need for optical tomography. Our formalism is applied to the multimode case and allows one to draw important conclusions about the possibility of measurements of the complete quantum state of a multimode field. In particular, the complete characterization of fields with intermode correlations is a nontrivial undertaking, as we demonstrate with our general formalism as well as by specific examples.

A three-level lambda ($\Lambda$) system driven by two coherent fields at the Raman resonance condition evolves to a nonabsorbing eigenstate of the Hamiltonian called the coherent population trapping (CPT) state. We study the various factors dictating the dynamical evolution of the coherences and the populations to the CPT state. We demonstrate the formation of the CPT state even at low intensities, though it takes much longer to form such a state. For the case of unequal decay rates we demonstrate an interesting sharp dip in the steady-state response of the medium at the Raman resonance condition.

We study the evolution of a single mode of the quantized electromagnetic field interacting with both a collection of resonant two-level atoms and a Kerr medium. Each of these interactions separately leads to a spreading of the initial field coherent states. Acting simultaneously, these two mechanisms of dispersion can cancel each other for special initial atomic states, allowing the propagation of field wave packets without spreading in the quantum phase space.

Using Doppler-free Rb two-photon excitation measurements, we present support for the hypothesis that semiconductor-laser noise is characterized by pure phase-diffusion noise. We measured near-Lorentzian shapes for both the laser and the two-photon excitation spectra and a slope of 3.7±0.3 for the dependence of the two-photon excitation width on the laser width. These represent measurements of the second-order field statistics of a naturally operating laser where the noise is dominated by spontaneous emission. The measured spectral shape and slope are in excellent agreement with the Mollow model, which predicts a Lorentzian spectral shape and a slope of 4 for weak-field, two-photon excitation with a pure phase-diffusion field (Lorentzian spectral density). The dominance of phase-diffusion noise is further corroborated by an analytically solvable microscopic noise model that includes phase and amplitude noise.

A complete description of two families of two-wavelength bright spatial solitons in Kerr media is presented. We calculate field profiles and power dispersion properties, and show that a simple variational approximation can be used to describe the fundamental soliton with great accuracy. Only the family of fundamental solitons is found to be stable on propagation.

The validity of the four-level model is controlled in an experiment with a ${\mathrm{CO}}_2$ laser with electronic feedback. A global application of center manifold theory allows one to reduce the number of varieties from six to four. Qualitative differences are found with the case of loss modulations.

Both stationary and time-dependent regimes of operation, instabilities, and phase squeezing are investigated in the off-resonant two-photon correlated-spontaneous-emission laser by numerical calculation. Initial atomic coherence plays an essential role in lasing without population inversion, phase locking, and phase noise squeezing in the system. Under certain conditions, in the inverted and noninverted regimes alike, the output intensity exhibits bistable behaviors against the initial atomic coherence. Depending on the parameters, the whole or a portion of the upper or lower branch gives stable operations. In the inverted regime, even tristable behavior can be found in a narrow range of parameters. The field evolution and dynamics are studied. Furthermore, phase noise reduction near bistable areas is also investigated. In addition to the lower branch, where from previous studies, it has been known to exist, phase squeezing is also found on the upper branch both with and without population inversion, thus generating a bright source of phase noise squeezed light.

The role of atomic phase coherence in various correlated-emisson laser (CEL) schemes is investigated. A CEL scheme is introduced in which coherence is generated by a Raman-type two-photon process. Ideal CEL action, that is, vanishing phase diffusion in the difference phase of two modes, is predicted even in the presence of nonradiative transverse decay in this scheme and in a previously suggested configuration. The results are applicable to the analysis of dephasing collisions between atoms in a gaseous CEL medium.