In the region between two lines of a doublet of an inverted medium, there exists a point of zero group-velocity dispersion, where highly superluminal effects may be observable without significant gain, loss, distortion, or broadening. The results of this group-velocity analysis hold for sufficiently narrow-band, analytic pulses, and do not constitute a violation of causality, although the group, ‘‘signal,’’ and energy velocities as defined by Sommerfeld and Brillouin may all exceed c or even become negative. No sharp disturbance in the pulse (a real signal) could propagate faster than light, but the scheme offers an unusual noiseless amplification scheme for the leading edge of a pulse, both at the classical and at the single-photon level.

Spatial pattern pulsations are observed at frequencies much less than the intermode beat frequencies in a He-Xe laser operating at 3.51 μm. Numerical studies indicate that such low-frequency pulsations can arise from secondary beat notes between the frequencies of unequally spaced modes. The decay rate of the active medium is an important parameter which sets the scale for mode pulling without locking, which leads to these pulsations.

Measurements of light-intensity fluctuations have been carried out during cw nondegenerate two-wave mixing at visible wavelengths in an artificial Kerr medium. Taking advantage of the distinctive properties of artificial Kerr media, such as the high nonlinear efficiency and slow response, the zero-average component of optical noise has been revealed and completely characterized with regard to statistical and spectral features. Experimental results are discussed in the frame of a stochastic noise model including thermal fluctuations in the linear dielectric constant of the medium.

This paper considers the chaotic dynamics of semiconductor lasers in the presence of phase-conjugate feedback (PCF). Bifurcation diagrams are used to explore the chaotic dynamics and show the difference between the conventional feedback and the PCF. In general, semiconductor lasers display richer chaotic dynamics in the case of PCF. Period-doubling, quasiperiodic, and intermittency routes to chaos are observed in numerical simulations performed by using realistic parameter values. For weak values of PCF, the laser can be phase locked to the phase provided by the phase-conjugate mirror, resulting in dramatic narrowing of the laser linewidth. Higher values of feedback result in periodic output, as the laser relaxation oscillations become destabilized. At a critical value of feedback, the laser enters the chaotic regime, resulting in increased low-frequency intensity noise and substantial broadening of the laser line. Finally, the effect of detuning between the solitary-laser frequency and the frequency of the phase-conjugate-mirror pump laser is explored.

By solving linearized wave equations of nonlinear polarization for a medium, we derive the input-output relationship for the propagation of the quantum fluctuations in single-pass second-harmonic generation with arbitrary interaction length. Spectra of quadrature-phase squeezing are calculated from the input-output relations. It is found that the intensity fluctuations of both harmonic and fundamental fields are squeezed below the vacuum fluctuation level. For large interaction length, an arbitrary amount of squeezing can be achieved for the fundamental field whereas only 50% squeezing is possible for the harmonic field. The bandwidth of squeezing is determined by the phase-matching condition and can be very broad in this case. The possibility of a quantum-nondemolition measurement in the process is discussed. Some interesting features in quantum fluctuations of the fields are presented.

We present a simple, analytic, and fully quantum theory of high-harmonic generation by low-frequency laser fields. The theory recovers the classical interpretation of Kulander et al. in Proceedings of the SILAP III Works hop, edited by B. Piraux (Plenum, New York, 1993) and Corkum [Phys. Rev. Lett. 71, 1994 (1993)] and clearly explains why the single-atom harmonic-generation spectra fall off at an energy approximately equal to the ionization energy plus about three times the oscillation energy of a free electron in the field. The theory is valid for arbitrary atomic potentials and can be generalized to describe laser fields of arbitrary ellipticity and spectrum. We discuss the role of atomic dipole matrix elements, electron rescattering processes, and of depletion of the ground state. We present the exact quantum-mechanical formula for the harmonic cutoff that differs from the phenomenological law ${\mathit{I}}_{\mathit{p}}$+3.17${\mathit{U}}_{\mathit{p}}$, where ${\mathit{I}}_{\mathit{p}}$ is the atomic ionization potential and ${\mathit{U}}_{\mathit{p}}$ is the ponderomotive energy, due to the account for quantum tunneling and diffusion effects.

A general theory of quantum-limited feedback for continuously monitored systems is presented. Two approaches are used, one based on quantum measurement theory and one on Hamiltonian system-bath interactions. The former gives rise to a stochastic non-Markovian evolution equation for the density operator, and the latter a non-Markovian quantum Langevin equation. In the limit that the time delay in the feedback loop is negligible, a simple deterministic Markovian master equation can be derived from either approach. Two special cases of interest are treated: feedback mediated by optical homodyne detection and self-excited quantum point processes.

We discuss a model for interferometric gravitional-wave antennas without dissipative or active elements. It is predicted that the even and odd coherent states may play an alternative role to squeezed vacuum states in reducing the optimal power of the input laser.

We analyze squeezing of both the fundamental and harmonic fields undergoing traveling-wave, second-harmonic generation (SHG) in second-order (${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$) nonlinear media. We take into account depletion of the fundamental field as well as the phase mismatch between the fundamental and harmonic fields. The behavior of the quantum noises on the propagating fields is studied by linearizing the nonlinear operator equations around the mean-field values. We first consider the degenerate case that is applicable to type-I phase-matching geometries, obtaining expressions for squeezing in both the fundamental and harmonic fields under the conditions of perfect phase matching and large phase mismatch. We show that in the case of a large phase mismatch, the intensity-dependent self-phase shift of the fundamental field, arising due to cascading of the ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ nonlinearity, is responsible for the squeezing generation. We also numerically solve the linearized quadrature-operator equations together with the nonlinear mean-field equations. We find that in the case of a finite phase mismatch the harmonic field can be highly squeezed. This is in contrast to the perfectly phase-matched case where the maximum squeezing is limited to 50%. Finally, we analyze the nondegenerate case that applies to type-II phase-matching geometries. Here we show that the commonly used type-II phase-matched SHG process with the input harmonic field in the vacuum state is equivalent to a type-I SHG process in parallel with a degenerate optical-parametric process. The latter causes squeezing in the mode that is polarized orthogonal to the fundamental beam. In the perfect phase-matching case, the squeezing in the orthogonally polarized mode follows the simple expression S=1-γ, where γ is the harmonic conversion frequency.

A displacement-angle interferometer capable of ${10}^{\mathrm{-}6}$ resolution in fringe division was developed for the precise measurement of the silicon (220) lattice spacing by x-ray and optical interferometry. With a view to achieving ${10}^{\mathrm{-}8}$ measurement uncertainty, the interference pattern was studied by the Fresnel (Gaussian) scalar approximation of the free-space propagation of interfering beams. Imperfect alignment and diffraction phenomena having been identified, and subsequently experimentally proved, as important error sources, remedial steps were identified and taken with consequent improvement of the experiment accuracy. The investigation brought into light theoretical and experimental evidences of corrections to the interference phase which were overlooked in previous analyses.

We report on the polarization dependence of the ionization of helium and neon by 120-fs pulses at 614 nm. For linearly polarized pulses the data for ${\mathrm{He}}^{2+}$ and ${\mathrm{Ne}}^{2+}$ show enhancements over sequential tunneling. For circularly polarized pulses the data show no distinct enhancements. Dynamic resonances, which produce similar enhancements in the multiphoton regime, do not explain the data in the tunneling regime. Two direct-ionization models are considered: a shake-off model, and a semiclassical model for rescattering of an electron by the ion core. The exhibited polarization dependence of the ion yield enhancement is consistent with the semiclassical rescattering mechanism. Definitive isolation of a direct, nonsequential ionization mechanism in the optical tunneling regime requires continued scaling experiments on the laser wavelength, pulse width and ellipticity, and the target species.

The usual uncertainty relation Δ${\mathit{A}}^2$Δ${\mathit{B}}^2$≥〈C${\mathrm{〉}}^2$/4 between two Hermitian operators A and B satisfying the noncanonical commutation relation [A,B]=iC, where C is not a constant multiple of the unit operator, fails to give a nontrivial lower bound on the product of the variances Δ${\mathit{A}}^2$ and Δ${\mathit{B}}^2$ when 〈C〉=0. For those operators, therefore, the general uncertainty relation Δ${\mathit{A}}^2$Δ${\mathit{B}}^2$≥[〈C${\mathrm{〉}}^2$+〈F${\mathrm{〉}}^2$]/4 where 〈F〉=〈AB+BA〉-2〈A〉〈B〉 is better suited to determine the lower bound on Δ${\mathit{A}}^2$Δ${\mathit{B}}^2$. The implications of the general uncertainty relation and the properties of the minimum-uncertainty states, i.e., the states for which the general uncertainty relation is satisfied with equality, are discussed. The minimum-uncertainty states are found to fall in two different classes. One is the usually studied class for which 〈F〉=0, that is, the case when the usual uncertainty relation holds. The other is the hitherto unnoticed class for 〈C〉=0, that is, when the usual uncertainty relation is redundant. The squeezing properties of the present class of minimum-uncertainty states is discussed by defining squeezing in the light of the general uncertainty relation.

The resonance lines in Li i and Na i both exhibit a puzzling discrepancy between experiment and accurate ab initio calculations. Only results from a semiempirical method, that in principal neglects core-core correlation, agree with the experiments. The agreement with a multiconfiguration Hartree-Fock calculation, including only core-valence correlation, shows that this might be fortuitous. A method for including some core-core correlation is designed and gives results in excellent agreement with other accurate ab initio methods.

The high-resolution 3d-4p x-ray-emission spectrum of Xe excited by electron impact was measured. In addition to the prominent peak due to 3${\mathit{d}}_{5/2\mathrm{-}}$4${\mathit{p}}_{3/2}$ previously reported, the experimental high-resolution spectrum shows a weak peak, identified as 3${\mathit{d}}_{3/2\mathrm{-}}$4${\mathit{p}}_{3/2}$, and satellites. The satellites are tentatively attributed to 3${\mathit{d}}^{\mathrm{-}1}$${\mathit{X}}^{\mathrm{-}1\mathrm{-}}$4${\mathit{p}}^{\mathrm{-}1}$${\mathit{X}}^{\mathrm{-}1}$ (X=4s, 4p, and 4d), where the initial doubly ionized states are created by the Auger (Coster-Kronig) decay of the 2p and/or 3p level. The experimental 3d-4p x-ray spectrum is compared with a theoretical spectrum calculated by the ab initio Green’s-function method.

We report on a scheme for the detection of sputter-generated metastable atoms that is based on the resonant excitation of an autoionizing state by single-photon absorption from a tunable laser. Using this technique, sputtered silver atoms ejected in the metastable 4${\mathit{d}}^9$5${\mathit{s}}^2$ ${}_{}{}^2{}_{}{}^{}$${\mathit{D}}_{5/2}$ state with an excitation energy of 3.75 eV have been detected. This represents the highest excitation energy of sputtered metastable atoms observed so far.

By analyzing the L-x-ray spectra induced by 2-MeV proton impact, the ${\mathit{L}}_1$- and ${\mathit{L}}_2$-subshell fluorescence yields of La, Nd, Dy, Yb, and Lu have been derived. The present yields are in good agreement with recent calculations employing Dirac-Hartree-Slater wave functions, the semiempirical compilation, and also recent measurements, within the uncertainties.

Predictions of Stone’s tensor-surface-harmonic theory [Mol. Phys. 41, 1339 (1980)] for bonding in small silicon clusters are tested by ab initio calculations. Stable geometries, along with the energies and symmetries of the occupied molecular orbitals, may all be rationalized within the model. For example, local energy minima for ${\mathrm{Si}}_{\mathit{n}}^{2\mathrm{-}}$ clusters are found which are isostructural to the borane and carborane clusters ${\mathrm{B}}_{\mathit{n}}$${\mathrm{H}}_{\mathit{n}}^{2\mathrm{-}}$ and ${\mathrm{C}}_2$${\mathrm{B}}_{\mathit{n}\mathrm{-}2}$${\mathrm{H}}_{\mathit{n}}$. In particular, both ${\mathrm{Si}}_{12}^{2\mathrm{-}}$ and ${\mathrm{Si}}_{13}^{2+}$ exhibit icosahedral geometries which are true minima.

We investigate the influence of noise present in the periodic input signal of a passive optically bistable element. The thermally induced absorption nonlinearity of CdS crystals is used to achieve optically bi- stable behavior. By measuring the signal-to-noise ratio (S/N) of the noisy modulated input signal and of the output signal the phenomenon of stochastic resonance is found: With increasing noise strength the S/N increases up to a maximum value corresponding to the above resonance and then decreases again. The S/N of a noisy input can be enhanced by up to approximately 6 dB with this system. Furthermore, the cases of sinusoidal and rectangular modulation of the bistable element are compared.

We point out a mistake in a recent paper [Tan, Lu, and Harrison, Phys. Rev. A 46, R3613 (1992)] and show that, contrary to the authors’ claim, lasing without inversion is impossible in a closed V system driven by a single coherent field only.