We show that filamentation initiated by quantum fluctuations is a process that limits the intensity of a laser beam that can propagate stably through a nonlinear optical medium. We also describe the experimental signatures of this process, which allow it to be distinguished from classical processes such as filamentation induced by wave-front irregularities.

A method is described for determining the quantum correlations at different times in optical pulses by using balanced homodyne detection. The signal pulse and sequences of ultrashort test pulses (local oscillators) are superimposed, where for chosen distances between the test pulses their relative phases and intensities are varied from measurement to measurement. The correlation statistics of the signal pulse is obtained from the time-integrated difference photocurrents measured.

Observation of the optical analog of the delayed nutation echo [V. S. Kuz'min et al., Zh. Éksp. Teor. Fiz. 99, 215 (1991) [Sov. Phys. JETP 72, 121 (1991)] recently shown in nuclear magnetic resonance is reported. We call this echo a stimulatednnutationnecho (SNE) in analogy to the three-pulse-stimulated echo that is produced from a hole pattern burnt into an inhomogeneous line. In a SNE two-pulse sequence, the first pulse burns a train of holes into the inhomogeneous absorption spectrum and its duration ${\mathrm{t}}_{\mathrm{p}}$ is stored. After a long delay time the second pulse burns its own train of holes. When coincidence of these holes takes place, the SNE occurs at a time t=${\mathrm{t}}_{\mathrm{p}}$ following the start of the second pulse. Thus the second pulse reads exactly the duration of the first pulse. It is shown that for a Gaussian-shaped beam, the echo becomes sharper and well recognizable. This echo may be used for Rabi frequency measurement and spectral diffusion studies.

We consider the case of two cavity modes of the electromagnetic field, which are coupled via the action of a parametric amplifier. The fields are allowed to leak from the cavity and homodyne measurement is performed on one of the modes. Because of the correlations between the modes, this leads to a reduction of the variance in a quadrature of the other mode, although no measurement is performed on it directly. We discuss how this relates to the Einstein-Podolsky-Rosen Gedankenexperiment.

We report the observation of discharge-pumped extreme ultraviolet lasing in collisionally excited ions of a material ablated from a solid target. Excitation of sulfur plasmas by a capillary discharge resulted in amplification of the J=0–1 line of Ne-like sulfur at 60.8 nm, with a gain coefficient of 0.45 ${\mathrm{cm}}^{\mathrm{-}1}$ and a gain-length product of 7.5. Overheating of the electron temperature and transient population effects are computed to make a significant contribution to the measured gain.

We develop a model to compute the line shape of an injected gas laser. The model describes the competition that occurs between the injected field and the usual resonant field of the laser, which are both considered as separate dynamic variables. A small signal stability analysis of the stationary solutions is used to study this competition: the laser field may be destroyed due to gain quenching by the injected field, which explains the frequency locking phenomenon. We give the connection with the standard Adler model. Theoretical and experimental results that describe the line shape inside or outside the locking range are given for the typical He-Ne laser operating at 3.39 μm.

Photonic structures are considered with the emphasis on dispersive materials and on one-dimensionally periodic structures. The electromagnetic frequency bands and the corresponding field distributions are determined for a GaAs/AlAs one-dimensional photonic structure and the field quantization is established. The interaction is considered between the electromagnetic fields and an oscillating electric dipole located deep within the layers. This involves coupling to the polariton modes of this inhomogeneous medium, with each type of mode providing a dipole relaxation channel. Relaxation processes due to emission into the polariton channels are analyzed. The results are displayed in terms of the emission rate Γ versus the dipole position ${\mathrm{Z}}_0$ for a fixed dipole frequency ${\mathrm{\omega }}_0$ and against ${\mathrm{\omega }}_0$ for a fixed ${\mathrm{Z}}_0$ . These results strongly indicate the existence of band-edge effects associated with the dispersive response of the materials comprising the layered system.

Recently a method for measuring quasidistributions of single-mode optical fields in unbalanced homo- dyne detection has been proposed [S. Wallentowitz and W. Vogel, Phys. Rev. A 53, 4528 (1996); K. Bana- szek and K. Wódkiewicz, Phys. Rev. Lett. 76, 4344 (1996)]. We show that the scheme can be used for direct sampling of the signal-mode density matrix in the Fock basis. In this case it is sufficient to vary only the phase of the local oscillator, keeping its amplitude constant.

We demonstrate the occurrence of the well-known Wigner distribution for density of states, which arises commonly in nuclear physics, in a class of problems in quantum optics. We show how such a distribution can be probed, and analyze the effects of dissipation on this distribution, one of which is to make the underlying random matrix non-Hermitian.

For symmetry reasons, vertical-cavity semiconductor lasers might be expected to show polarization isotropy. Practical devices, however, operate in a well-defined polarization state, which indicates the presence of residual anisotropies. We have performed a systematic experimental study of these residual anisotropies by applying additional anisotropies in a controlled and continuous way. The results are compared with a coupled-mode model that describes the effect of linear anisotropies on the polarization eigenmodes. We conclude that the polarization state of a practical device is dominantly determined by these linear anisotropies; nonlinear anisotropies are found to play at most a minor role.

We extend recent work on two closely spaced atoms interacting through the narrow band of strongly coupled modes at the edge of a photonic band gap. The resonant dipole-dipole interaction (RDDI) is strongly modified for atomic transition frequencies in the vicinity of the band-gap edge, but we show that an analytical approximation to the RDDI agrees very well with the exact RDDI obtained by numerical integration using the exact dispersion relation. Having established the value of the RDDI, we can derive the amplitudes for the two atoms without resorting to the pole approximation which is necessary due to the strongly modified mode structure in the dielectric host. For a wide range of parameters we find beating and population trapping in the long time limit. The distribution of population in the photonic continuum is investigated in the long time limit in the case of one and two atoms. It is found to be strongly asymmetric and to exhibit a strong signature of the unusual mode structure in the material at the band-gap edge.

Spontaneous emission of erbium atoms implanted in various planar multidielectric stacks is investigated. Computational and experimental analyses are performed so as to assess the amount of light leaking into the radiative and the guided modes of the structures. The key point is the demonstration of the dominant emission into the guided modes, even though the atoms are implanted in the spacer of a resonant microcavity structure. The result is of practical interest since it challenges the conventional idea about the electromagnetic confinement, which is likely to be reached within planar microcavities.

A method for density analysis of static polarizabilities (α) and second hyperpolarizabilities (γ) on the basis of the finite-field (FF) many-electron wave packets (MEWP) method is developed and applied to evaluation of the longitudinal α and γ in the dissociation process for a one-dimensional ${\mathrm{H}}_2$ model. Remarkable increases in α and γ are observed in the intermediate dissociation region. The internuclear distance where the γ is maximized is also found to be larger than that where the α is maximized. In order to elucidate the characteristics of α and γ in the dissociation process, we extract their classical pictures describing displacements of two-electron configurations by using (hyper)polarizability densities on the two-electron coordinate plane. It is suggested from these classical pictures that the polarization in the ionic structure contributes primarily to the enhancement of (hyper)polarizability in the intermediate dissociation region, while the polarization in the diradical structure causes the decrease of the (hyper)polarizability at a large internuclear distance. This implies that the experimental search for species with chemical bonds in the intermediate correlation regime is important and interesting in relation to the molecular design of nonlinear optical materials.

The Pancharatnam phase for ordinary and generalized squeezed states of the harmonic oscillator is discussed. In particular, the general expressions for the total and dynamical phases are obtained yielding, from their difference, the geometric phase for a nonadiabatic and noncyclic evolution from an initial generalized squeezed state at time ${\mathrm{t}}^{|\mathrm{I}\mathrm{H}}$ to the final state at a time t″. The special case of a two-photon interaction process, such as parametric amplification, is subsequently discussed. In particular, the conditions leading to cyclic evolution are examined and the corresponding Aharonov-Anandan phase is found from the general expression for the Pancharatnam phase.

Binding of a positron to a neutral atom is explored within the premise of a three-component density-functional theory. Within this regime, employing model electron-positron correlation potentials (which are crucial for binding), it turns out that the systems ${}_{}{}^2{}_{}{}^{}\mathrm{Li}$;${\mathrm{e}}^{+}$], ${}_{}{}^2{}_{}{}^{}\mathrm{Na}$;${\mathrm{e}}^{+}$], and ${}_{}{}^2{}_{}{}^{}\mathrm{K}$;${\mathrm{e}}^{+}$] exhibit stability with respect to neutral-atom–positron dissociation, but are unstable with respect to positronium-cation dissociation, while the species ${}_{}{}^3{}_{}{}^{}\mathrm{He}$;${\mathrm{e}}^{+}$], ${}_{}{}^1{}_{}{}^{}\mathrm{Be}$;${\mathrm{e}}^{+}$], ${}_{}{}^2{}_{}{}^{}\mathrm{F}$;${\mathrm{e}}^{+}$], ${}_{}{}^1{}_{}{}^{}\mathrm{Mg}$;${\mathrm{e}}^{+}$], ${}_{}{}^2{}_{}{}^{}\mathrm{Cl}$;${\mathrm{e}}^{+}$], and ${}_{}{}^1{}_{}{}^{}\mathrm{Ar}$;${\mathrm{e}}^{+}$] exhibit stability with respect to both these dissociation modes. The indispensability of the inclusion of the electron-positron correlation for atom-positron binding is also emphasized.

Energy-level systems can be seen as sets of a series of discrete points that exhibit fractal properties. To characterize these fractals, we calculated the effective fractal dimension, which behaves as a function of the measuring scale. The computerized box-counting method was employed to obtain the fractal dimension that is irrelevant to the level number. This feature allows a physically meaningful connection that can be established between the fractal and statistics. The procedure was applied to the Poisson- and Wigner-distributed numbers and to the vibrational levels of ${\mathrm{SO}}_2$ .

The ionization potentials of neutral osmium and iridium atoms have been obtained by mass-analyzed optical-optical double resonance spectroscopy. Observation of the photoionization threshold yielded approximate values of 68 058±2 ${\mathrm{cm}}^{\mathrm{-}1}$ for Os I and 72 324±2 ${\mathrm{cm}}^{\mathrm{-}1}$ for Ir I. In addition, high-lying ns and nd Rydberg series were recorded and fit by a Rydberg formula in order to obtain independent values of the ionization potentials. The results were 68 058.9±1.6 ${\mathrm{cm}}^{\mathrm{-}1}$ for Os I and 72 323.9±1.8 ${\mathrm{cm}}^{\mathrm{-}1}$ for Ir I.

The retarded long-range potentials for hydrogen and alkali-metal atoms in their ground states and a perfectly conducting wall are calculated. The potentials are given over a wide range of atom-wall distances and the validity of the approximations used is established.

The total cross sections ${\mathrm{\sigma }}_{\mathrm{t}}$ for positron impact on many diatomic and polyatomic molecules have been calculated using the additivity rule in which the total cross section for a molecule is the sum of the total cross sections for the constituent atoms. The energy range is from 10 eV to 5000 eV. The total cross sections for positron impact on an atom are calculated by employing a complex, spherical potential which is comprised of a static, polarization, and an absorption potential. We have good agreement with the available experimental results for positron energy ⩾30 eV.

Double-differential cross sections for the single ionization of He by protons are measured at collision energies of 2, 2.8, and 6 MeV. At energy losses larger than 0.75 keV, single ionization is shown to be dominated by binary encounters between the proton and a quasifree target electron. The measured scattering angle and energy loss of the protons show a simple two-body kinematic relation. Furthermore, the measured shapes of differential cross sections are in good agreement with calculations of Rutherford scattering from electrons.