We analyze the origin of gain in systems that do not exhibit population inversion either in terms of the bare states or dressed states. Such systems involve light amplification by coherence. We show that gain arises from coherence (a) among Fano states in Arkhipkin-Heller-Harris-like systems (b) between the two dressed states in the Λ system of Imamoglu, Field, and Harris [Phys. Rev. Lett. 66, 1154 (1991)].

We investigate the theory of cancellation by interference between the absorption of three fundamental laser photons and one third-harmonic photon. The theory is formulated in terms of the density matrix so as to take detunings, dephasing, and and laser bandwidth into account. The result is a theory of cancellation for finite detuning that explains how four-photon resonances can be canceled by a three-photon mechanism, if there is an atomic level at near three-photon resonance. We obtain explicit conditions for this to happen, and perform calculations pertaining to a recent experiment [D. Charalambdis et al., Phys. Rev. A (to be published)] where cancellation of 4+1 resonantly enhanced multiphoton ionization has been observed.

We observe transitions to chaos by type-III intermittency of a laser. A family of transitions is found in which the onset of intermittency is preceded by subharmonic period-N states. The laser used is an optically pumped far-infrared ring laser.

Using a spatially filtered pulsed laser at 486 nm we have photoexcited positronium to the 2S state and detected it via a delayed and spatially separated 532-nm photoionization pulse. We have extended our observations down to low 486-nm power to explore the possibility of a cw measurement of the positronium 1S-2S interval. A model calculation taking into account the velocity distribution of the 235-K positronium from an oxygenated cold Al(111) target, the parameters of the laser beams, and the nonlinear excitation probability explains our data. The same model indicates that a cw measurement is presently feasible.

Velocity-fluctuation spectra are proposed as a probe of the dynamic scaling of moving interfaces. Using the theory of Kardar, Parisi, and Zhang [Phys. Rev. Lett. 56, 889 (1986)] it is shown that the spectrum of the spatially averaged displacement velocity diverges as 1/f at low frequencies, where α=1/3 (α≃0.7) for a one- (two-) dimensional interface. This implies superdiffusive motion of the average interface position, which is verified numerically. The fluctuation spectrum of the stationary interface width diverges as 1/${\mathit{f}}^{2+\mathrm{\alpha }}$ Simulations of interfaces driven by non-Gaussian noise are also presented.

A mechanism for nucleating pairs of topological defects in non- equilibrium pattern-forming systems in two space dimensions plus time is studied. Its relation to the creation of phase slips in space-time plane is revealed, stressing its deterministric origin from smooth dynamics. The role of this mechanism in the context of spatiotemporal chaos is discussed.

We analyze the quantum level statistics of the zero-obstacle-size limit of the pseudointegrable billiard While the nearest-neighbor-spacing distribution approaches to the Poissonian shape in the semiclassical limit, it is shown to become the Wigner distribution in low-energy region. This fact is interpreted as a signature of the chaotic motion generated by the quantum uncertainty.

The effect of strong space disorder in a one-dimensional medium on the macroscopic segregation of A and B in the A+B→0 reaction is studied via an effective-medium approximation. The strongest disorder (percolative medium) shows an increase of the segregation due to the existence of isolated diffusive regions. Poisson strong disorder shows a slowing down both in the extinction and segregation. In the presence of sources a disorder-induced transition from bounded to unbounded growth is observed.

We propose a computationally efficient method to simulate ordering processes in binary fluids with a long-range hydrodynamic interaction, and the results of a computer simulation of spinodal decomposition in binary fluids in three dimensions using this method are presented. A linear growth law of characteristic length scale l(t), namely l(t)∼t, is observed in the very late scaling stage. The scaling function obtained is in good quantitative agreement with the experiments.

A mean-field theory is presented of the transition at which a crystalline array of rigid chain molecules transforms to a nematic phase in which relative longitudinal motion of the molecules occurs. The transition temperature in commonly encountered materials is found to be proportional to the geometric mean of a very high temperature that is characteristic of the intrachain forces and a lower one characteristic of the interchain forces.

The interactions of a lyotropic liquid crystal with boundary amorphous glass surfaces are investigated using optical techniques. An unusual phenomenon is observed: the gliding of the director at the surface. A qualitative model is proposed to explain the experimental results.

We present an exact expression for the rate of screening with time of an arbitrary point on a growing diffusion-limited aggregate, and use it to study the multifractal singularities α that correspond to strongly screened sites. We find that the time evolution of these singularities is controlled by the field at the lips (the outer corners) of the fjord. We show quantitatively that if the time evolution of the strongest singularity ${\mathrm{\alpha }}_{\max }$ is self-consistent with the growth process, the key issue is the process by which long fjords are generated. We analyze this process and find an asymptotic linear slope for the decreasing part of f(α). This form agrees with recent measurements [Barabasi and Vicsek, J. Phys. A 23, L729 (1990)] and excludes an infinite value of ${\mathrm{\alpha }}_{\max }$.

We have used time-resolved 2p-3d absorption spectroscopy to probe the ionization degree of a germanium plasma heated by the soft x rays emitted from the rear side of laser-irradiated gold targets. We have determined the average ionization degree and the distribution of ion stages as a function of time during the expanding, cooling phase of the plasma. They have been compared with collisional-radiative distributions based on hydrodynamic simulations of the plasma parameters. Results show a slow decrease of the ionization degree from 〈Z〉=11.5 to 10.0 during a 3-ns period after gold heating. Comparison with theoretical spectra show matter densities around 0.01 g/${\mathrm{cm}}^3$ and temperatures of about 50 eV.

We analyze in this paper the role of the correlations during double-electron ejection through two-photon absorption from the lowest autoionizing state ${}_{}{}^1{}_{}{}^{}$${\mathit{S}}^{\mathit{e}}$ of helium. Emphasis will be put on the pertinence of the notion of simultaneity for double-electron ejection when this process does not require electron interaction.

The density of states and other closely allied quantum properties of a particle in a spherical cavity are formulated. Germane to this study is the degeneracy of states which, when plotted versus energy, reveals a quasiperiodic array of points. These points define two sets of intersecting curves of constant l and n, respectively, where ${\mathit{z}}_{\mathit{l}\mathit{n}}$ is the nth zero of ${\mathit{j}}_{\mathit{l}}$(z). Graphical analysis further suggests that slopes of curves of constant n asymptotically approach that of the envelope n=1 curve. Differentiation of the cumulative sum of the number of states, with increase in energy, gives an approximate expression for the smooth component of the density of states, D(E)=0.088${\mathit{E}}^{0.63}$ (where E is nondimensional energy). A histogram of the difference of nearest-neighbor eigenenergies roughly indicates a Poissonian form. This finding is consistent with theories concerning the properties of quantum systems with integrable classical counterparts. Application of these results was made in evaluation of the Fermi energy for mesoscopic spherical metallic particles.

A tensorial formalism is presented for the general use of nonorthogonal basis sets in quantum mechanics, both in representation theory and in second quantization. The natural formulation of second quantization for nonorthogonal basis sets defined here is shown to be formally very similar to the usual formulation for orthogonal basis sets. The formalism is used to study orthogonality effects in a Hubbard model defined on a semi-infinite lattice.

Several modifications of the Schrödinger equation that dynamically reduce the off-diagonal elements of the density operator in the position representation (localization) have received recent attention in the literature. Models of quantum open systems provide localization for the system of interest by considering the effects of an environment on the effective master equation that describes the dynamics of the reduced density operator for the system of interest. In contrast, localization is introduced as a fundamental modification to the Schrödinger equation in quantum mechanics with spontaneous localization (QMSL) [G. C. Ghirardi, A. Rhimini, and T. Weber, Phys. Rev. D 34, 470 (1986)]. We compare several environment models and discuss some of the anomalous features of the effective master equations noted by Ballentine [L. E. Ballentine, Phys. Rev. A 43, 9 (1991)] by examining the Ehrenfest relations for position, energy, and momentum. Insight is provided into the relative importance of various terms in the effective master equations for several environment models. We also briefly discuss the implications of similar anomalous features present in the QMSL master equation in light of our analysis of the open-system models.

A different approach to the shifted 1/N expansion technique is developed to deal with the Klein-Gordon particle trapped in a spherically symmetric potential. Properly modifying the definition of the perturbative expansion of the energy eigenvalue, and without making any approximation in the determination of the parameters involved, we obtain sufficiently good results compared with the exact ones for the Coulomb problem. The calculations are carried out to the second-order correction of the energy series.

Radiative lifetimes of the ${\mathit{n}}^1$S (n=3–6) and ${\mathit{n}}^3$P (n=3–5) states in He i have been measured using pulsed excitation by a high-energy electron beam over a broad pressure range of the He target. The observed pressure dependences reveal that these nonresonance transitions are subjected to strong radiative trapping effects for higher pressures that may have distorted earlier precision measurements of He i lifetimes. This effect is particularly large for the 2${\mathrm{}}^3$S–3 ${}_{}{}^3{}_{}{}^{}$P transition for which published experimental values are, on the average, about 10% higher than accurate theoretical values.

We calculate the QED energy-level shifts of an atom between two perfectly conducting mirrors in perturbation theory. We separate the contributions of vacuum fluctuation and self-radiation reaction to the shifts between two mirrors. This provides unambiguous assignment of each effect on the cavity-induced shifts, and allows reinterpretation of previously derived results. In particular, we are able to identify the Casimir interaction as a pure effect of vacuum fluctuation.