Spatially uniform illumination of Turing structures in the chlorine dioxide-iodine-malonic acid reaction-diffusion system affects the pattern characteristics and, at larger intensities, eliminates them. Periodic illumination is more effective than constant illumination with the same average light intensity. We observe the fastest suppression of pattern at a frequency of illumination equal to the frequency of autonomous oscillations in the corresponding well-stirred system. Numerical simulations demonstrate a similar resonant behavior of periodically illuminated Turing structures.

By using a nonlinear waveguide array we experimentally demonstrate dynamic features of solitons in discrete systems. Spatial solitons do not exhibit these properties in continuous systems. We experimentally recorded nonlinearly induced locking of an initially moving soliton at a single waveguide. We also show that discrete solitons can acquire transverse momentum and propagate at an angle with respect to the waveguide direction, when the initial excitation is not centered on a waveguide. This is to our knowledge the first time that the effect of the Peierls-Nabarro potential has been observed in a macroscopic system.

We have performed optical reflectivity experiments and band structure diffraction calculations on fcc photonic crystals, consisting of air spheres in titania ( $\mathrm{TiO}{}_2$). The spectra reveal Bragg peaks whose large widths agree well with the calculations. The peaks overlap in many directions, causing the propagation of light to be limited by Bragg reflection to $45\%$ of the $4\pi$ solid angle. This illustrates that the air-sphere crystals interact so strongly with light that an unprecedented limitation in the directions of propagation of light in photonic crystals is achieved.

Acoustic wave propagation in liquid media containing many parallel air-filled cylinders is considered. A self-consistent method is used to compute rigorously the propagation, incorporating all orders of multiple scattering. It is shown that, under proper conditions, multiple scattering leads to a peculiar phase transition in the acoustic propagation. When the phase transition occurs, a collective behavior of the cylinders appears and the acoustic waves are confined in a region of space in the neighborhood of the transmission source. A novel phase diagram is used to illustrate the distinct phase states in the acoustic propagation.

We report on the observation of a highly efficient transformation of laser radiation into higher-order Stokes and anti-Stokes components in impulsively vibrationally excited gaseous S${\mathrm{F}}_6$. The specific (Bessel-like) behavior of their intensity with the gas pressure reveals the mutual coherence of the generated frequencies. We propose this new regime of Raman frequency conversion for generation of sub-fs pulses.

The fifth harmonic pulses of an intense femtosecond Ti:sapphire laser are experimentally shown to be negatively chirped by using a LiF plate as a positive dispersive medium. The chirp of the harmonic pulse is originated from the intensity-dependent atomic dipole phase, which is estimated to be proportional to ${25U}_p$, where $U_p$ is the ponderomotive energy. Consequently, we have succeeded in compressing the chirped pulses to 13 fs by compensating the intrinsic negative chirp. Chirp effects of the fundamental laser on the pulse width of the fifth harmonic were also investigated.

Since solitons of the parametrically driven damped nonlinear Schrödinger equation do not have oscillatory tails, it was suggested that they cannot form bound states. We show that this equation does support solitonic complexes, with the mechanism of their formation being different from the standard tail-overlap mechanism. One of the arising stationary complexes is found to be stable in a wide range of parameters, others unstable.

A systematic analysis is carried out for the randomly forced Burgers equation in the infinite Reynolds number (inviscid) limit. No closure approximations are made. Instead the probability density functions of velocity and velocity gradient are related to the statistics of quantities defined along the shocks. This method allows one to compute the dissipative anomalies, as well as asymptotics for the structure functions and the probability density functions. It is shown that the left tail for the probability density function of the velocity gradient has to decay faster than $|\xi {|}^{-3\mathrm{}}$. A further argument confirms the prediction of E et al. [Phys. Rev. Lett. 78, 1904 (1997)] that it should decay as $|\xi {|}^{-7/2\mathrm{}}$.

A Comment on the Letter by V. M. Canuto and M. S. Dubovikov, Phys. Rev. Lett. 80, 281 (1998). The authors of the Letter offer a Reply.

A Reply to the Comment by R. E. Ecke.

The discrete multiscale analysis for boundary value problems in nonlinear discrete systems leads to a first order, strictly discrete, modulational instability (disappearing in the continuous envelope limit) above a threshold amplitude for wave numbers beyond the zero of group velocity dispersion. Applied to the electrical lattice [Phys. Rev. E 51, 6127 (1995)], this accurately explains the experimental instability at wave numbers beyond $1.25\mathrm{rad}{\mathrm{cell}}^{-1\mathrm{}}$. The theory is also briefly discussed for the sine-Gordon and Toda lattices.

We analyze experimentally the combined action of convection and a diffusionlike process in a bistable nonlinear optical resonator. We show that the very existence of bistability in this device strongly depends on a competition mechanism between these two processes. This mechanism is described analytically in terms of the switching wave dynamics of a Fisher-Kolmogorov equation.

We demonstrate experimentally collisions between vector (Manakov-like) solitons that involve energy exchange at large collision angles, for which scalar solitons pass through one another practically unaffected.

The profiles of light-induced charge gratings on photorefractive ${\mathrm{Bi}}_{12}{\mathrm{SiO}}_{20}$ crystals are directly observed by electrostatic force imaging with an atomic force microscope. We investigate the temporal evolution of the profiles during grating formation and the dependence of the profiles of saturated gratings on the fringe spacing and the modulation index of the light grating. In the case of saturated gratings, the profiles are well described by a simple theoretical model.

We report the experimental control of chaotic pattern dynamics in Taylor vortex flow with hourglass geometry. During uncontrolled chaos, the one-dimensional vortex pattern undergoes phase diffusion until a phase slip occurs. This process repeats itself aperiodically with an irregular switching of both the vortex drift direction and phase slip location for a range of reduced Reynolds numbers $\varepsilon$. Small perturbations of $\varepsilon$, determined by an Ott-Grebogi-York method, robustly maintain periodic intervals between phase slips while selecting and stabilizing a drift direction and phase slip location.

Random matrix theory predicts parametric correlations for chaotic or disordered systems which are as universal as their spectral counterparts. However, an experimental verification of parametric correlations is, in the vast majority of systems, a difficult task, because the motion of individual levels has to be followed as an external parameter is varied. We present the first statistically highly significant result for a parametric correlator that uses solely experimental information. To this end, we measure the motion of acoustic resonances in quartz blocks as a function of its uniform temperature. We find some deviation between theoretical prediction and experimental results whose possible origin is discussed.

We introduce a new method of controlling chaos that retains the essential features of occasional proportional feedback, but is much simpler to implement. We demonstrate control on a simple piecewise-linear Rossler circuit operating near 1 kHz and a Colpitts oscillator with a fundamental frequency of 19 MHz. As a result of the simplicity of our technique, control of chaos has been accomplished for the fastest chaotic system reported to date.

The dynamics of two coupled Hamiltonian systems is shown to exhibit a transition to coherent evolution as the strength of coupling is increased. Above this transition the two systems are not strictly synchronized, but their orbits cover the same region of the (individual) phase space with identical invariant measures. This measure synchronization is numerically observed in maps and time-continuous systems, for both regular and chaotic evolution, and can be analytically derived for regular systems in the action-angle representation, which suggest that it is a generic form of coherent evolution in Hamiltonian dynamics. Analytical results show that the transition involves logarithmic singularities.

We studied enhanced backscattering of light from anodically and photoanodically etched, macroporous GaP networks. The most strongly scattering material for visible light reported to date, photoanodically etched GaP, features anomalous rounding of the top of the backscatter cone. The phenomenon cannot be attributed to finite sample size or absorption and is most likely the onset of Anderson localization.

We investigate the phase matching of a high-harmonic generation by intense, 20 fs laser pulses in a gas-filled capillary waveguide. We identify several regimes in which the harmonic field can build up coherently: a balance of atomic and waveguide dispersions, noncollinear Cerenkov phase matching, and a balance of atomic and plasma dispersions. The role of atomic dispersion is examined by measuring and calculating the harmonic signal for several gases. We also predict and provide preliminary evidence for a regime where phase matching occurs only at specific fractional ionization levels, where the harmonic signal is sensitive to the absolute phase of the carrier wave.