We have calculated cross sections of the radiative electron capture into a projectile K-shell orbit (K-REC) by using the relativistic impulse approximation, fully incorporating the internal conversion process, that is, $e^{+}$$e^{\mathrm{-}}$ pair creation and successive $e^{+}$$e^{\mathrm{-}}$ pair annihilation. The internal conversion process (ICP) is found to play a vital role with an increase of the projectile nuclear charge, while contributions to the ICP from an increase of the relative velocity are less pronounced for, especially, light-ion impacts. Furthermore, the K-REC photon is distributed according to a ${\cos }^2$(${\theta }_L$/2) rule with respect to a photon emission angle ${\theta }_L$ in the laboratory frame in the ultrarelativistic energy domain. This feature is quite different from the conventional ${\sin }^2$ ${\mathrm{\theta }}_{\mathrm{L}}$ dependence, which is valid at most up to an impact velocity below 0.9 of the velocity of light. Calculations are also carried out for the linear polarization correlation of emitted photons using the same approximation. A crossover feature, i.e., a sign inversion of the polarization correlation, comes out in the forward direction relative to 90° with an increase in the impact velocity. This effect is also due to the relativistic effect of the REC.

The linear Stark effect of hydrogen was investigated for the metastable H(2s) state measuring the degree of linear polarization of field-induced Lyman-α radiation. For sufficiently large electric fields (few kV/cm) the measured degree of linear polarization P is almost complete (P≊-100%). This result is in excellent agreement with theoretical predictions, if the fine structure of hydrogen is taken into account.

An ‘‘all-optical’’ technique is presented for the study of multiphoton-ionization processes, based on fluorescence detection of the ions which are produced. A vapor is utilized in a moderate-density regime (∼${10}^{11}$–${10}^{15}$ ${\mathrm{cm}}^{\mathrm{-}3}$) such that collisional effects are negligible. The technique is applied to multiple-photon ionization of Sr in the wavelength range of 558–590 nm at laser intensities ∼1–50 GW/${\mathrm{cm}}^2$. Fine-structure-resolved emission of the ${\mathrm{Sr}}^{+}$ (5$p^2$$P_{3/2/,1/d}^0$) ions which are formed is observed at λ=407.8 and 421.5 nm, following either four- or five-photon absorption. No emission is observed from higher ionic states, with a detection sensitivity ∼${1:10}^3$ with respect to the ionic-resonance emission.

The 1s electrons of Ne have been ionized and excited to np (n=3,4,. . .) Rydberg levels using monochromatized synchrotron radiation of mean energy 865 eV. The Auger spectrum resulting from the excited 1$s^1$2$s^2$2$p^6$3p state has been studied both experimentally and theoretically by carrying out a detailed comparison with the spectrum resulting from the ionized 1$s^1$2$s^2$2$p^6$ state. General agreement between the theoretical and experimental results is very good.

When two optical beams copropagate inside a single-mode fiber, the intensity-dependent refractive index couples the two beams through a nonlinear phenomenon known as cross-phase modulation (XPM). Such an XPM-induced interaction between the two waves can destabilize the steady state and lead to temporal modulations (self-pulsing) in the presence of group-velocity dispersion. This phenomenon is analogous to the modulation instability of a single beam occurring in the anomalous-dispersion regime of the fiber. The XPM-induced modulation instability can occur even in the normal-dispersion regime. The features of XPM-induced modulation instability are discussed in detail. This phenomenon may be useful for generating ultrashort pulses (<100 fs) in the visible region of the optical spectrum.

It has been demonstrated recently that it is possible to generate a pure number state, or Fock state, of the electromagnetic field in a resonant cavity when a ‘‘micromaser’’ is operated under the appropriate conditions. This prospect is examined here by a direct analysis of the equation for the density-matrix-governing operation of the lossless micromaser, without having to solve the equation or perform numerical analyses. This model micromaser affords a unique example of an open quantum system whose von Neumann entropy may increase at first, but must subsequently vanish.

We show how the squeezed vacuum fields generated in collective fluorescence within a high-finesse cavity can be transformed into squeezed coherent states with high brightness but with the same degree of noise suppression. The transformation is achieved by an injected signal that creates a mean polarization and squeezing in the high rather than the low branch of the optical bistable collective radiation. Analytical expressions are obtained for the limit of weak excitation of the atomic media. Comparisons with results obtained using rigged-reservoir theories and also from a multi- stage attenuator are made. The limitations of the model are discussed.

A time-dependent slow-time-scale theory is developed for gyrotron oscillators driven by an external signal. The signal is introduced either directly into the cavity output or via a beam prebunching cavity. The theory is applied to a high-voltage gyrotron configuration and the numerical, nonlinear calculations are compared with simple analytical estimates of the frequency bandwidth for phase-locked operation. For the case of direct injection, the nonlinear slow-time-scale calculations are in good agreement with Adler’s relation. Two approaches are investigated for the case of phase locking with a prebunching cavity. In the first approach the induced ac current density due to the prebunching cavity is treated as a small perturbation on the ac current density in the oscillator. In this approach the equations for the time-dependent wave amplitude and phase are similar in structure to the equations for gyrotrons driven by direct injection and lead to a simple analytical estimate of the locking bandwidth. The accuracy of the perturbation approach is investigated by comparing it with the results of an alternate approach used in the analysis of multicavity gyrotrons (gyroklystrons). The maximum phase-locking bandwidth obtainable with the prebunching cavity approach is discussed.

The mirrorless optical bistability problem is approached by analyzing the local relationship between the population inversion and the intensity of the laser excitation source. The active mechanism leading to bistability is the nonlinear Lorentz shift that changes sign along with the population inversion. Homogeneous broadening works against bistability. For a collision-broadened gas these factors are related and we show that the relationship is such that optical bistability is forbidden.

We undertake a theoretical investigation of the coherent optical propagation of Stokes light in a collisionally broadened three-level Raman amplifier. The Stokes light consists of two distinct spectral components: off-resonance stimulated Raman scattering and on-resonance stimulated fluorescence, induced by collisional transfer of population to a near-resonant intermediate state of the atomic system. Our model is based on the semiclassical Maxwell-Bloch formalism where the separation of the Stokes light into two components results in an expansion from the usual 9 to 13 atomic variables. We investigate the model both analytically and numerically. We point out the physical contributions to the simultaneous evolution of pump, fluorescence, and Raman light. We discuss the development during propagation of these separate pulses, emphasizing the nonlinear effects that lead to saturation.

Time-dependent features of the fluorescence radiation from a strongly driven three-level atom in a Raman configuration are studied applying the Eberly-Wódkiewicz counting-rate definition of the spectrum. Using our earlier formalism, where we assume that one or both of the driving fields are intense, we obtain analytical expressions for the time-dependent fluorescence spectra. We have also studied the influence on the transient spectrum of the initial preparation of the atom in three different dressed states.

Experiments on two of the recent manifestations of Berry’s phases in optics and their mutual influence are reported. The first is the phase that arises from a cycling in the directions of a beam of light, so that the tip of the spin vector of a photon in this beam traces out a closed curve on the sphere of spin directions. The second is Pancharatnam’s phase that arises from a cycling in the polarization states of the light while keeping the direction of the beam of light fixed, so that the tip of the Stokes vector traces out a closed curve on the Poincaré sphere. What happens when the two phases are combined in the same experiment is examined. We find that the two phases are additive if we use the notion of a generalized Poincaré sphere. The classical theory of the effect based on the Jones calculus is presented, as well as a general analysis of Berry’s phases for Maxwell’s equations. The signs of these phases have been determined experimentally, and they agree with theory.

Spectral correlations from theoretical and experimental studies of the fluctuations in the single-shot stimulated Raman scattering spectrum are presented. The coherence-mode representation of the Stokes field is demonstrated as a convenient and somewhat intuitive method of generating ensembles of the fields and spectra. The theory is shown to reproduce the observed spectral fluctuations. In addition, other theoretical approaches are shown to be in agreement with coherence-mode theory.

A notion of quantized photons of any polarization has been introduced recently in descriptions of free-electron lasers in a small-signal regime, where photons essentially are assumed to be interacting with a classical electron current. Here we show that these photons (of any polarization), which we call scalarized photons, can be described with a scalar field and that this description can be used whenever photons interact with an external current (no matter what specific form the current has). The interaction Lagrangian now is the scalar-scalar type with a real quantized field of scalarized photons coupled to the real electron scalar source related to the scalar product of the Fourier-transformed vector current densities. Now the polarization of the radiation is directly related to the vector nature of the original current density and, in fact, can be even quantitatively described through functional differentiations. Scalarized photons are readily applicable to the spontaneous emission and, in a special situation, also to the stimulated emission; as such they can be used in all known free-electron lasers in the small-signal regime. Here they are used to address the following two problems appropriate for the spontaneous emission: (1) finding the general expression for the average four-momentum that an electron loses to the radiation over an infinite time period and (2) giving the specific expression for the spontaneously radiated energy by an electron in a uniform magnetic field free-electron laser over a finite interaction length.

Frequency locking and transition to chaos from quasiperiodicity has been observed in an optical bistable device with two delayed feedbacks. f(α) spectra have been obtained along the winding number W=[4,1,1, . . .]. At the transition point the f(α) curve is in good agreement with the universal result.

We present here an extensive study of the technique of numerical simulations, applied to the solution of a system of equations obtained by the positive P representation. The model we introduce to aid in these investigations has the distinct advantage of an immediate physical interpretation. In these simulations we experience the divergent trajectory or ‘‘spike,’’ noted by other authors; however, due to the simplicity of our equations, we are able to categorize and explain these spikes. Finally, we report a disturbing anomaly in some of the simulation results which indicates a possible problem with the use of the positive P representation.

We investigate theoretically the selection in Saffman-Taylor fingers in the presence of a small bubble. Experimentally anomalous fingers have been found in this case, with relative width much smaller than what is found for the standard fingers. We study the case of a small bubble far ahead of the finger and solve the problem for zero surface tension. We then show that selection due to surface tension leads indeed to narrow fingers. Finally we consider the more complicated case (but more relevant experimentally) of a bubble sticking to the finger and find again that narrow fingers are allowed.

We report measurements of the noise level of light emissions in the frequency band of the stimulated Raman scattering (SRS) instability. Long-scale-length planar plasmas were produced from thin gold targets using 0.35-μm laser light. At laser intensities up to 5×${10}^{14}$ W/${\mathrm{cm}}^2$, the SRS was completely quenched by collisional damping, and the emissions were thermal, as evidenced by their time dependence and by their spectrum. The onset of SRS was consistent with theory, and the SRS spectral power was five orders of magnitude larger than thermal at a laser intensity of 4×${10}^{15}$ W/${\mathrm{cm}}^2$.

The problem of anisotropy of the electron-velocity distribution in a weakly ionized gas in an electric field is discussed. In contrast to the conventional multiple-term truncated spherical-harmonics expansion method for weakly anisotropic distributions, the present treatment is based on a new two-term perturbative solution of the kinetic equation. The expansion parameter δ in the theory is the ratio between the electron-energy gain on a mean free path along the electric field and the electron energy itself. The method allows us evaluation of the angular velocity distribution for an arbitrary degree of anisotropy, provided δ is small. For instance, both the conventional two-term spherical-harmonics expansion result for a weakly anisotropic case and the beamlike distribution characteristic of gases with large inelastic cross sections are obtained as two limiting cases of the theory. The new two-term expansion allows evaluation of coefficients $f_n$ of the conventional spherical-harmonics expansion of the distribution function to arbitrary order via a recurrent relation. The asymptotic form of these coefficients for n≫1 is obtained from this relation, allowing estimation of the convergence rate of the conventional expansion.

Dielectronic recombination (DR) rate coefficients are calculated for selected heliumlike ions with atomic number Z=10, 14, 18, 22, 24, 26, and 28 at plasma temperatures 0.1–10 keV. The total DR rate coefficient has been obtained by summing over partial DR rates of recombination processes that proceed via intermediate lithiumlike resonance states ‖1s2lnl’LSJ〉. The effects of configuration interaction and spin-orbit coupling have been included for all states with n≤8; for higher-lying autoionizing states, the 1/$n^3$ scaling was employed. The results from the present calculation are compared with available theoretical and experimental values of DR rate coefficients.