We study all possible resonant three-level configurations where a low-frequency coherent pump can be converted into a high-frequency field in the absence of population inversion at both atomic high-frequency transitions. We identify two different mechanisms that provide such inversionless amplification.

The gain process of modes of a nonlinear symmetric planar waveguide is investigated. It is shown using numerical simulations that the gain process is adiabatic at low values of gain factor and that the wave envelope changes slowly, remaining close to the shape of the stable nonlinear mode. The representative point of the wave on the plane (energy integral, propagation constant) moves continuously along the stable branches of dispersion curves. When the representative point achieves the region of instability, the spatial solitons are split off from the guided mode. This process is repeated periodically due to the gain. We considered a number of typical cases of guided-wave propagation with symmetric as well as asymmetric initial conditions.

Numerical results are presented showing the generation of gap solitons by external pulses incident on a nonlinear periodic medium. It is shown that the required energy strongly depends on whether the effective index of the periodic structure is well matched to the refractive indices of the surrounding media.

A variational principle for the time-dependent Schrödinger equation with an anisotropic Gaussian wave packet as a trial wave function is used to calculate the nonperturbative response of a hydrogen atom to a strong linearly polarized laser field. This method leads to equations of motion for the position expectation value and allows one to study a classical (ħ=0) limit as well as the first quantum corrections. These corrections are shown to be responsible for a strong enhancement in the generation of the third and fifth harmonic by the bound-state part of the electronic wave function for field intensities of the order of ${10}^{14}$ W/${\mathrm{cm}}^2$ and a laser frequency corresponding to three-photon ionization. For a low-frequency field a strong nonperturbative enhancement of harmonics of orders 9 to 15 is observed. It is concluded that the plateau at even higher harmonics observed recently in experiment and in numerical calculations is caused by the ionizing parts of the wave function that are not represented in the present calculation.

We present some exact analytic results for the steady state of a driven three-level atom in cascade or in a Raman configuration interacting with broadband squeezed reservoirs. The two transitions in the three-level atom are assumed to decay into two independent broadband reservoirs. The effects of squeezing on the population trapping in the Raman configuration are also discussed.

We present a measurement of the beam-pointing fluctuations of a transient Raman-amplifier output using an interferometric method and a theoretical treatment of the three-dimensional, spatial propagation of transient stimulated Raman scattering (SRS) including the fluctuations of the Stokes field giving rise to the beam-pointing fluctuations. The theory is based on an effective steady-state model for the transient SRS process. By using nonorthogonal transverse modes to expand the field, we incorporate the effects of excess spontaneous-emission noise associated with gain-guided amplifiers. The agreement of the measurements with the theoretical treatment demonstrates that the beam-pointing jitter is caused by quantum noise. The beam-pointing fluctuations increase as the Fresnel number of the interaction volume increases because more spatial modes are excited.

By use of the squeezed-number-state basis, we study several features of the squeezed thermal states. Higher-order squeezing properties are found to be entirely determined by the second-order ones. The photon-number distribution is expressed in closed form in terms of a Legendre polynomial. We point out that at a critical value of the squeeze parameter, pairwise oscillations of the photon-number distribution set in. Finally, a compact analytical expression of the exact lth-order correlation function is derived and examined.

We present an experimental demonstration of a fourth-order interference effect first suggested by Franson [Phys. Rev. Lett. 62, 2205 (1989)], where pair photons are directed to two separated Mach-Zehnder interferometers. Apparent nonlocal control of the output ports in which coincident photons are detected can be demonstrated. We show that this effect is present when one of the photons has passed through 170 m of multimode optical fiber. Such a system could form the basis of an interception-proof communication system.

We present a theory of dressed-state lasers, i.e., lasers that operate on an inverted transition between dressed states of a coupled atom-field system, in the bad-cavity limit. We discuss the validity of the effective Hamiltonian approach in such a case and generalize the theory beyond secular and effective Hamiltonian approximations. We demonstrate that higher-order instabilities that lead to self-pulsing and optical chaos may occur in experiments on dressed-state lasers. With the help of the generalized theory we study the competition between the resonant one-photon and two-photon processes and try to understand it physically by looking at the spectrum of the emitted radiation.

A simple expression for the energies of a class of doubly excited states derived from first-order perturbation theory is compared with recent results based on large numerical calculations for H- and He. It concerns (n′K,T)nA2S+1Lπ states with A=+1, L=0, or 1, K=n-1-T, T=12[1-π(-1)S]. (Lin's standard notation is used.) This expression involves both intrashell and intershell cases.

We construct a finite-difference scheme for numerical integration of the Schrödinger equation. Asymptotically (r→∞), the method gives the exact solution correct to terms of order ${\mathit{r}}^{\mathrm{-}2}$.

Using a hydrodynamic formulation of Kohn-Sham orbital density-functional theory, static dipole polarizabilities ${\mathrm{\alpha }}_{\mathit{d}}$ are calculated for ${\mathrm{Cr}}^{+}$, ${\mathrm{Mn}}^{2+}$, ${\mathrm{Fe}}^{3+}$, ${\mathrm{Co}}^{4+}$, ${\mathrm{Ni}}^{5+}$ (3${\mathit{d}}^5$ series); ${\mathrm{Cu}}^{+}$, ${\mathrm{Zn}}^{2+}$, and ${\mathrm{Ga}}^{3+}$ (3${\mathit{d}}^{10}$ series); ${\mathrm{Ru}}^{3+}$(4${\mathit{d}}^5$); ${\mathrm{Sn}}^{2+}$(5${\mathit{s}}^2$); and ${\mathrm{Eu}}^{2+}$(4${\mathit{f}}^7$). The calculated ${\mathrm{\alpha }}_{\mathit{d}}$ values are found to be in good agreement with other available theoretical results of comparable accuracy.

The Feshbach-type projection method is applied to the 2${\mathit{s}}^2$2p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}^{\mathit{o}}$ and 2s2${\mathit{p}}^2$ ${}_{}{}^4{}_{}{}^{}$${\mathit{P}}^{\mathit{e}}$ resonances in ${\mathrm{He}}^{\mathrm{-}}$. The results are compared with the saddle-point technique results and some other calculations by methods closely related to the Feshbach-type projection method.

We apply the continuum-distorted-wave approximation within the framework of the independent-event model to calculate the ratio (R) of double-to-single ionization cross sections of He by ${\mathrm{N}}^{7+}$ projectiles. Following Deb and Crothers [J. Phys. B 23, L7999 (1990)], we calculate the double-ionization probability as ${\mathit{P}}_1$${\mathit{P}}_2$, whereas the single-ionization probability is taken to be 2${\mathit{P}}_1$(1-${\mathit{P}}_1$). Here, ${\mathit{P}}_{\mathit{i}}$ (i=1,2) is the probability of ionizing the ith electron from He. The values of R found in the 10–40-MeV/u impact-energy range are similar to those obtained in the somewhat different forced-impulse theory of Reading and Ford [Phys. Rev. Lett. 58, 543 (1987)] and by the semiempirical fit of Knudsen et al. [J. Phys. B 17, 3545 (1984)], but are much smaller in absolute value than the corresponding measurements of Heber et al. [Phys. Rev. Lett. 64, 851 (1990); Nucl. Instrum. Methods B 56/57, 15 (1991)].

The cross sections of resonant charge transfer between atoms and positive ions for all non-transition-elements in the range of low collision velocities (<${10}^8$ cm/sec) have been calculated by the impact-parameter and close-coupling methods. The calculated results are in fairly good agreement with the experimental results for the elements for which data are available. In these results the relation between the cross sections and the ionization potentials of elements was found, and based on this relation a useful and simple formula for the cross section was successfully obtained.

We have observed laser-excited continuum emission spectra in the far wings of the 6${\mathit{s}}^2$ ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}_0$–6s6p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ resonance line of Yb broadened by He at seven different temperatures (405–560 °C). We find that the YbHe molecular excited state 6s6p ${}_{}{}^1{}_{}{}^{}\mathrm{\Sigma }$ is repulsive.

The individual decay channels of samarium L-subshell vacancy states were experimentally investigated by the synchrotron photoionization method. For the Coster-Kronig yield ${\mathit{f}}_{13}$, i.e., the relative contribution of the nonradiative ${\mathit{L}}_{1\mathrm{-}}$${\mathit{L}}_3$ channel to the total decay of ${\mathit{L}}_1$-vacancy states, the value ${\mathit{f}}_{13}$=0.19±0.03 was obtained. This is a surprisingly small value regarding both prediction from theory (e.g., ${\mathit{f}}_{13}$=0.32 according to Chen, Craseman, and Mark [Phys. Rev. A 24, 177 (1981)]) and extrapolation from neighboring elements (${\mathit{f}}_{13}$≊0.32 for the elements with Z=72–74 according to our previous work [U. Werner and W. Jitschin, Phys. Rev. A 38, 4009 (1988)]).

We focus on the question of accuracy of prediction in the application of nonrelativistic wave functions for deriving incoherent scattering functions for photons. The present investigation involves the measurement of incoherent scattering intensity using an annular-target arrangement for momentum-transfer values in the range 1.5 up to 7.5 A${\mathrm{̊}}^{\mathrm{-}1}$. Extrapolation of measurements performed using a minimum of three target weights for each angle of scattering provides both a means for correction of target attenuation and a system for monitoring departures from systematic experimental procedures. Results of measurements, good to within a conservative experimental uncertainty of 10%, reveal no systematic trend in departure from agreement with tabulated incoherent-scattering-function predictions based upon nonrelativistic wave functions. We also compare measured total-atom, total-scatter (coherent plus incoherent) differential cross sections with the best available predictions.

Total-attenuation cross sections of the oxides of rare-earth elements such as La, Ce, Pr, Nd, Sm, Gd, Dy, Ho, and Er, and also ${\mathrm{NaNO}}_3$ and ${\mathrm{NaNO}}_2$ have been measured in a narrow-beam geometry setup at 323 keV. The total-attenuation cross section for oxygen was obtained as the difference in ${\mathrm{NaNO}}_3$ and ${\mathrm{NaNO}}_2$ cross sections. Using this, the total-attenuation cross sections of the individual lanthanides have been obtained with the aid of the mixture rule. From these, the photoeffect cross sections were derived by subtracting the scattering contribution. These values are found to agree well with Scofield’s theoretical data [University of California Report No. UCRL 51326, 1973 (unpublished)].

We measured the ionization and the fluctuations in ionization produced in a Si(Li) detector by silicon atoms at five particular kinetic energies ranging from about 4 to 109 keV. The method is simple and precise, yet untried until now, and provides excellent calibration points for silicon. Our results are in good agreement with the predictions of Lindhard et al. [Mat. Fys. Medd. 33, 10 (1963)].