We show that a photoconductive crystal, placed in a rapidly alternating ac field and exposed to nonuniform light, exhibits singularities of the induced space charge and discontinuities of the corresponding space-charge field. The singularities appear at the local intensity maxima when the curvature of the intensity profile exceeds a certain (often very low) threshold value. We analyze the characteristic features of the singular ac response and consider its possible optical manifestations.

We report on the generation of stable dark-vortex solitons in large-phase-mismatched second-harmonic generation of self-defocusing type, sustained by a combined effect of transverse walk-off and finite beam size.

Self-compression of high-intensity femtosecond pulses has been observed in a number of atomic and molecular gases and solid bulk material. The evolution of the femtosecond pulse parameters during the self-compression has been studied under a variety of experimental conditions. Generation of spatiotemporal solitons has been achieved by the combined action of self-compression and self-focusing.

We introduce a mesoscopic model for the formation and evolution of shear bands in loose granular media. Numerical simulations reveal that the system undergoes a nontrivial self-organization process which is governed by the motion of the shear band and the consequent restructuring of the material along it. High density regions are built up, progressively confining the shear bands in localized regions. This results in an inhomogeneous aging of the material with a very slow increase in the mean density, displaying an unusual glassylike system-size dependence.

The quantum kicked rotor map is embedded into a continuous unitary transformation generated by a time-independent quasi Hamiltonian. In some vicinity of a quantum resonance of order $q$, we relate the problem to the regular motion along a circle in a $(q^2-1\mathrm{})$ component inhomogeneous “magnetic” field of a quantum particle with $q$ intrinsic degrees of freedom described by the $\mathrm{SU}\left(q\right)$ group. This motion is in parallel with the classical phase oscillations near a nonlinear resonance.

Exact breather solutions are constructed in piecewise linear (PWL) versions of the discrete nonlinear Schrödinger and Klein-Gordon equations. These solutions correspond to intersections of stable and unstable manifolds of relevant fixed points in associated 2D mappings, an exact construction of which is possible due to the PWL nature of the models. Such exact solutions give us insight into several aspects of breather properties. The problem of dynamical stability of the breathers is mentioned as an instance, detailed results on which will be presented in a future paper.

A new type of high-selective Bragg resonator having a step of corrugation inside the interaction region was used as a microwave system for a free-electron maser (FEM). Using a LINAC LIU-3000 (1 MeV/200 A/200 ns) to drive the FEM oscillator, a single-mode single-frequency operation was achieved at a frequency of 30.74 GHz with an output power of about 50 MW, which corresponded to a record efficiency of $35\%$ for a millimeter wavelength FEM.

A detailed analysis of the statistical properties of the transmitted field in corrugated waveguides in the transition from ballistic to diffusion transport is presented. The field distributions are found to be highly dependent on the incident mode excited. When the speckle contrast is one, our results agree with recent experiments where the intensity distribution follows the predictions of the random-phasor-sum model, even though the field phase is not uniformly distributed. Clear deviations from the Gaussian statistics are found which can be interpreted in terms of “ $K$” distributions arising either from a small number of scatterers or from perturbations of the speckle pattern on propagation.

We investigate theoretically the self-focusing dynamics of an ultrashort laser pulse both near and above the threshold at which the pulse effectively undergoes catastrophic collapse. We find that, as a result of space-time focusing and self-steepening, an “optical shock” wave forms inside the medium that gives rise to a broad blueshifted pedestal in the transmitted pulse spectrum. Our results are in good agreement with the primary features observed in experiments and thus provide a theoretical understanding for the underlying process that gives rise to “white-light” generation.

We show that an intense pump pulse, detuned far from the Bragg resonance of a nonlinear periodic structure, can excite a gap soliton at a wavelength within the band gap that corresponds to the Raman shift of the medium. This Raman gap soliton is a stable, long-lived, quasistationary excitation that exists within the grating even after the pump pulse has passed. We find both stationary solitons as well as slow Raman gap solitons with velocities as low as $1\%$ of the speed of light. The predicted phenomena should be observable in fiber Bragg gratings and other nonlinear photonic band gap structures.

We describe experiments on Bénard-Marangoni convection in horizontal layers of two immiscible liquids. Unlike previous experiments, which used gases as the upper fluid, we find a square planform close to onset which undergoes a secondary bifurcation to rolls at higher temperature differences. The scale of the convection pattern is that of the thinner lower fluid layer for which buoyancy and surface tension forces are comparable. The wave number of the pattern near onset agrees with the linear stability prediction for the full two-layer problem. The square planform is in qualitative agreement with recent two-layer weakly nonlinear theories, which fail however to predict the transition to rolls.

We observe a novel phenomenon which we call “pattern resonance.” It occurs in a theoretical model when the realized wave solution evolves to about the same shape of a saddle steady-wave solution. Here the unstable saddle solution behaves like a “potential.” We find that the resonance triggers the onset of a crisis, leading to a transition from temporal to spatiotemporal chaos. We also show that in our case the pattern resonance is essentially a nonlinear frequency resonance.

Polydomain periodically poled samples of iron-doped lithium niobate allow for phase grating recording with the diffraction efficiency close to that ensured by a single-domain sample with identical thickness. At the same time the optical damage induced by a finite transverse size light beam is considerably reduced in the periodically poled sample as compared with single-domain material.

Experimental evidence of coherence resonance in an optical system is reported. We show that the regularity of the excitable pulses in the intensity of a laser diode with optical feedback increases when adding noise, up to an optimal value of the noise strength. Both phase and amplitude fluctuations of the pulses play a relevant role in the dynamics of the system. We introduce the joint entropy of the two variables to generalize the indicator of coherence, and we put in evidence the mechanism of destruction of the excitable orbit after the resonance.

Four-wave mixing and walk-off between two optical beams are investigated for focusing Kerr media. It is shown that four-wave mixing reinforces the self-focusing of mutually trapped waves by lowering their power threshold for collapse, only when their phase mismatch is small. On the contrary, walk-off inhibits the collapse by detrapping the beams, whose partial centroids experience nonlinear oscillations.

We report the experimental and theoretical study of electrostatically driven granular material. We show that the charged granular medium undergoes a hysteretic phase transition from the immobile condensed state (granular solid) to a fluidized dilated state (granular gas) with a changing applied electric field. In addition we observe a spontaneous precipitation of dense clusters from the gas phase and subsequent coarsening—coagulation of these clusters. Molecular dynamics simulations show qualitative agreement with experimental results.

The flow induced instability in the flow past a soft material is studied in the limit of low Reynolds number where inertial effects are insignificant. A transition from laminar flow to a more complicated flow profile is observed when the strain rate of the base flow increases beyond a critical value; the transition is found to be reproducible. The experimental results are compared with theoretical predictions and quantitative agreement is found with no adjustable parameters.

This Letter reports the measurement of electro-osmotic flows (EOF) in microchannels with surface charge patterned on the $200\mu \mathrm{m}$ scale. We have investigated two classes of patterns: (1) Those in which the surface charge varies along a direction perpendicular to the electric field used to drive the EOF; this type of pattern generates multidirectional flow along the direction of the field. (2) Those in which the surface charge pattern varies parallel to the field; this pattern generates recirculating cellular flow, and thus causes motion both parallel and perpendicular to the external field. Measurements of both of these flows agree well with theory in the limit of thin double layers and low surface potential.

Synchronization of mutually coupled chaotic lasers has been demonstrated in a microchip ${\mathrm{LiNdP}}_4{\mathrm{O}}_{12}$ laser array with self-mixing feedback modulation. An abrupt transition to synchronized chaos by way of “phase squeezing” was observed when coupling between the two lasers was increased. This phenomenon is well reproduced by numerical calculations using model equations. It is also shown that low energy variation as well as high disorder are concurrently established in synchronized chaos.

Chains of parametrically driven, damped pendula are known to support solitonlike clusters of in-phase motion which become unstable and seed spatiotemporal chaos for sufficiently large driving amplitudes. We show that the pinning of the soliton on a “long” impurity (a longer pendulum) expands dramatically its stability region, whereas “short” defects simply repel solitons producing an effective partition of the chain. We also show that defects may spontaneously nucleate solitons.