We identify the defects of waves by means of topological arguments and study them in the framework of Landau-type analysis. It is shown that they correspond to sinks, sources, or dislocations of traveling waves, and to dislocations of standing waves.

We introduce a simple model for the consumption and fragmentation of a reactive porous medium, and investigate various distinct regimes of such processes. The model simulates the reaction-consumption process in a realistic way, and is capable of simulating the entire range of possible kinetic limitations. We show that a dynamic scaling governs the time evolution of the fragments distribution and calculate the critical exponents. They vary continuously with the reactivity of the porous medium. This dynamic scaling may also hold for the fragmentation of a system as a result of mechanical breakage, such as grinding of minerals, or stress-induced fragmentation of disordered solids, or of rocks and polymers.

Histograms of the normalized values of the helicity and the smallest principal component of the rate of strain are found for one realization of a ${128}^3$ isotropic simulation of forced turbulence. In regions of high dissipation the strain has a structure indicating sheets. Peak-to-valley ratios in the helicity histograms are less than in other numerical flows and this ratio decreases further when the helical large scales are filtered or active regions characterized by the strain are sampled.

A gyrotron oscillator with a single cylindrical cavity has produced output powers up to 645 kW and efficiencies up to 24% at 140.8 GHz, and step-tunable single-mode operation between 126 and 243 GHz. Mode stability and suppression of nearby competing modes are found to persist even for operation in very high-order cavity modes with severe mode competition. These results greatly improve prospects of developing cw megawat gyrotrons relevant to the heating of fusion plasmas.

An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the measurement of phase modulation in an optical interferometer. The experiment makes use of squeezed light to reduce the level of fluctuations below the shot-noise limit. An increase in the signal-to-noise ratio of 3.0 dB relative to the shot-noise limit is demonstrated, with the improvement currently limited by losses in propagation and detection and not by the degree of available squeezing.

In a thermal-convection box the boundaries parallel to the convection rolls are replaced by ramps, i.e., smooth variations in the Rayleigh number from supercritical to subcritical values. Such a geometry selects a small wavelength band. The wavelength selected by the left and right ramps are not necessarily the same—resulting in a wavelength gradient. This gradient leads to forced phase diffusion: The convection pattern drifts from the short-wavelength end to the ramp selecting the longer wavelength. An experimental observation of this phenomenon is presented.

The propagation of light in a disordered medium with purely elastic scattering is considered. The intensity correlation functions for the diffusely reflected and transmitted light from a slab are shown to have long-range power-law behavior. This arises from the interference of the underlying wave fields of the diffusive modes of the light intensity. This interference also gives rise to anomalous fluctuations in the transmission coefficient analogous to conductance fluctuations in metals.

An optical ring cavity containing distributed nonlinear elements is proposed as a promising candidate for investigation of the dynamic stability of spatial disorder in a system far from thermal equilibrium. If the interaction between the elements is unidirectional, the stability of disordered structure can be determined by the spatial Lyapunov exponent. This fact implies that spatial disorder is frozen under quite restricted conditions, and most of the spatially disordered structure is replaced by spatiotemporal chaos. However, in the case of a bidirectional interaction, the spatial disorder is self-induced over a wide range of the control parameter.

A new regime for the generation of squeezed states of the electromagnetic field is described for a collection of atoms within a high-finesse cavity. The process responsible for squeezing is a coupling-induced splitting in the normal-mode structure of the atom-field system. A theoretical analysis is presented that predicts large degrees of squeezing for modest operating conditions. An experimental investigation of this regime has produced noise reductions of 30% relative to the vacuum noise level.

In the free-electron laser the interaction between the radiation and electrons can result in radiation focusing. The radiation tends to follow the electron beam when the centroid of the electron beam is laterally displaced. Spatial modulation of the electron-beam envelope induces a similar modulation in the radiation-beam envelope. These and other phenomena are studied by use of a novel source-dependent modal representation of the fully three-dimensional radiation field. Among the merits of this approach is that few modes are needed to describe the radiation accurately.

We present femtosecond time-resolved transient reflectivity measurements on polydiacetylene performed at the peak of the exciton resonance. The magnitude of the nonlinear refractive index at 1.97 eV was found to be 3.0×${10}^{\mathrm{-}8}$ ${\mathrm{cm}}^2$/W. We discuss a model which accounts quantitatively for the large resonant and nonresonant optical nonlinearity observed in polydiacetylene in terms of phase-space filling by one-dimensional excitons.

We show that in a dynamic multiply scattering system, the intensity-intensity correlation function of universal optical fluctuations does not decay exponentially, but rather drops rapidly to half its initial value and then tails off very slowly to zero. We illustrate this dramatic behavior, and the approach to the ensemble average, with high-speed video images. We also present quantitative results for the correlation function, and provide a theory which is in excellent agreement with the data.

It has recently been shown theoretically that correlations between fluctuations of the source distribution at different source points can produce red shifts or blue shifts of emitted spectral lines. To facilitate experimental demonstration of this effect a simple example is analyzed. It involves only two small appropriately correlated sources and brings out the essential physical features of this new phenomenon.

Wolf has recently shown that the spectrum of radiation from an extended source changes on propagation unless a certain scaling condition is obeyed by the degree of spectral coherence across the source. For a large class of source-coherence functions, the change may be such as to produce red shifts or blue shifts of spectral lines. We have performed an acoustic experiment with two small partially correlated sources and demonstrated Wolf’s prediction of frequency shifts of spectral lines by this mechanism.

Spatial patterns in temporal chaotic regimes of Rayleigh-Bénard convection have been experimentally studied with a measuring technique able to resolve in time the gradient of the temperature field. We find two different time-dependent regimes, one characterized by oscillations localized in space and the other consisting of traveling waves. The transition between these two regimes is also analyzed.

The channel capacity C of a light-wave communication system based on photoevent point process observations cannot be increased by use of photon-number–squeezed light. Under a constraint of maximum photon rate ${\lambda }_{\max }$, the channel capacity C=${\lambda }_{\max /\mathrm{e}}$ is achieved with a Poisson process. On the other hand, the channel capacity of a communication system based on photon counting can be increased by use of photon-number–squeezed light. The improvement vanishes in the limit of very small mean counts.

We report direct quantitative measurements of the light-induced drift velocity in Na–noble-gas mixtures under well-defined conditions. Results are in excellent agreement with predictions from a realistic numerical model incorporating the multilevel structure of the Na atom. The effect of optical hyperfine pumping on light-induced drift is investigated. Limiting values for the drift velocity are predicted.

The fluorescent spectrum for a two-level atom which is damped by a squeezed vacuum shows striking differences from the spectrum for ordinary resonance fluorescence. For strong coherent driving fields the Mollow triplet depends on the relative phase of the driving field and the squeezed vacuum field. The central peak may have either subnatural linewidth or supernatural linewidth depending on this phase. The mean atomic polarization also shows a phase sensitivity.

The stability of modulated Couette flow is studied by modulation of the angular velocity, Ω, of the inner cylinder according to Ω=Ω¯[1+ε cos(ωt)], where Ω¯ is the mean angular velocity of rotation. ω is varied for several radius ratios, and ε is varied for one radius ratio. The results are compared to earlier experimental and theoretical results.

We show that the polarizations of counterpropagating light waves in an isotropic Kerr medium are temporally unstable when their total intensity exceeds a certain threshold value. Periodic and chaotic temporal behavior can occur in the output polarizations and under certain conditions also in the output intensities.