Picosecond laser pulses are used to study the forbidden resonant second harmonic generation (SHG) in atomic vapor. The results can be understood quantitatively by an ionization-initiated dc-field-induced SHG model taking into account the temporal buildup of the dc field and transient behavior of the two-photon coherent excitation of the vapor.

We report intersubband-scattering times $\left(T_1\right)$ in a semiconductor heterostructure with intersubband spacing below the LO phonon energy. $T_1$ is determined by simultaneous measurements of the intersubband absorption and the photovoltage induced by far-infrared radiation (FIR) near the intersubband transition frequency. At the lowest temperature $(T=10\mathrm{}\mathrm{K})$ and FIR intensity $(I=10\mathrm{}\mathrm{mW}/{\mathrm{cm}}^2)$, $T_1=1.2\mathrm{}\pm 0.4\mathrm{ns}$, several times longer than predicted theoretically. $T_1$ decreases strongly with increasing temperature and FIR intensity, to 20 ps at $T=50\mathrm{}\mathrm{K}$ in the linear regime, and to 15 ps at $T=10\mathrm{}\mathrm{K}$ and $I=2\mathrm{}\mathrm{kW}/{\mathrm{cm}}^2$.

Atomistic simulations of the vertical propagation of perturbations in deep gravitationally compacted granular columns characterized by the intergrain potential $V\left(\delta \right)\propto {\delta }^n$, with $n\ge 2$ and $\delta$ the grain overlap, are shown to recover the results of elasticity theory for weak perturbations. For strong perturbations, the sound velocity $c_{\mathrm{strong}}\to c_{\mathrm{weak}}$ as $z\to \infty$ with the deviations from $c_{\mathrm{weak}}$ best expressed via a certain recursion relation in $z$. We predict that voids in real granular columns lead to $c\propto 1-\epsilon$, when the void fraction $\epsilon$ is small, and show that the velocity power spectrum of a grain resembles that of a harmonic oscillator chain as $z\to \infty$.

A physical interpretation of a recent Navier-Stokes based theory for scaling in developed hydrodynamic turbulence is presented. It is proposed that corrections to the normal Kolmogorov scaling behavior of the $n$th order velocity structure functions are finite Reynolds number effects which disappear when the inertial interval exceeds 5–6 decades. These corrections originate from the correlation between the velocity differences and energy dissipation which are characterized by an anomalous (subcritical) exponent. The values of the experimentally observed scaling indices for the $n$th order structure functions for $n$ between 4 and 14 are in agreement with our findings.

Ruelle's principle for turbulence leading to what is usually called the Sinai-Ruelle-Bowen (SRB) distribution is applied to the statistical mechanics of many particle systems in nonequilibrium stationary states. A specific prediction, obtained without the need to construct explicitly the SRB itself, is shown to be in agreement with a recent computer experiment on a strongly sheared fluid. This presents the first test of the principle on a many particle system far from equilibrium. A possible application to fluid mechanics is also discussed.

A novel nonequilibrium molecular dynamics simulation technique has been developed whereby the flux of particles, flowing between explicitly defined regions of different constant chemical potential, is measured directly by counting particles. The method uses both stochastic and dynamic steps. A linear relationship between flux and concentration gradient (Fick's law) is found for methane in a carbonaceous slit micropore with a diffuse wall condition. Diffusion coefficients thereby calculated are larger than transport diffusivities from equilibrium simulations using Darken's rule.

We examine scaling laws for dynamical hysteresis in an optically bistable semiconductor laser. An analytic derivation of these laws from multidimensional laser equations is outlined and they are expected to be universal for systems that exhibit a cusp catastrophe. The scaling laws for the hysteresis loop area or width are numerically verified and experimentally measured for operation of the bistable laser above and below threshold. Excellent agreement with theory is obtained in the limit of low switching frequencies.

We show how to extract messages that are masked by a chaotic signal in a system of two Lorenz oscillators. This mask removal is done for two different modes of transmission: a digital one, where a parameter of the sender is switched between two values, and an analog mode, where a small amplitude message is added to the carrier signal. We achieve this without using a second Lorenz oscillator as a receiver, and without doing a full reconstruction of the dynamics. This method is robust with respect to transformations that impede the unmasking using a Lorenz receiver, and is not affected by the broadband noise that is inherent to the synchronization process. We also discuss the limitations of this way of extraction for messages in high frequency bands.

We have controlled chaos in a pendulum with vertically driven pivot over a range of drive frequencies and damping levels. Using two variations on the OGY method [Ott, Grebogi, and Yorke, Phys. Rev. Lett. 64, 1196 (1990)], and with eddy-current damping as a control parameter, we have stabilized the motion of the pendulum about several of its unstable period-1, -2, and -4 limit cycles. Sampling the angular position at a correct choice of phase within the drive cycle simplified the OGY control rule. A time-proportioning rule was also found to provide robust control.

We report on the first experimental observations of dark, planar, spatial photorefractive solitons, and photorefractive vortex solitons that are trapped in a bulk (three-dimensional) photorefractive media. Both the dark and vortex solitons possess the “signatures” of the photorefractive solitons: they are independent of absolute intensity, can afford significant absorption, and are inherently asymmetric with respect to the transverse dimensions of trapping.

We propose a new continuum description of the dynamics of sandpile surfaces, which involves two populations of grains: immobile and rolling. The latter move down the slope with a constant mean velocity and a certain dispersion constant. We introduce a simple bilinear form for the interconversion processes of “sticking” (below the angle of repose) and “dislodgment” (for greater slopes). We find a “spinodal” angle, larger than the angle of repose, at which the surface of a tilted immobile sandpile first becomes unstable to an infinitesimal perturbation. The effect of noise on our dynamical equations is briefly outlined.

Experimental evidence is given that the hierarchy of the energy dissipation moments, recently proposed by She and Leveque [Phys. Rev. Lett. 72, 336 (1994)] for fully developed turbulence, is in agreement with experimental data in different experimental configurations.

Berry's phase may be viewed as arising from the parallel transport of a quantal state around a loop in parameter space. In this Letter, the classical limit of this transport is obtained for a particular class of chaotic systems. It is shown that this “classical parallel transport” is anholonomic—transport around a closed curve in parameter space does not bring a point in phase space back to itself—and is intimately related to the Robbins-Berry classical two-form.

An additional weak harmonic forcing is added to the Duffing equation, and its ability of suppressing chaos is analyzed in the global parameter space. It is found that the phase difference between the two sinusoidal forces plays a very important role in suppressing chaos. A new type of intermittency characterized by periodic appearance of regular and chaotic motions, called the breathing effect in this Letter, is observed when the two harmonic forces deviate slightly from resonance.

Chaotic synchronization involving a yttrium iron garnet film in ferromagnetic resonance at 1.2 GHz has been achieved through small perturbations. The perturbations were derived from the difference between the current system's chaotic signal and a prerecorded chaotic signal from the same attractor, and were applied continuously. A model of the system's dynamics predicts the observed synchronization.

We study self-oscillation in a photorefractive double phase-conjugate mirror and show that it undergoes an order-disorder phase transition at a critical gain-threshold point. We present theoretical and experimental results on two key properties of this phase transition: critical slowing down and the characteristic order parameter. We show that the correlation distance of the refractive index variation responsible for the double phase conjugation changes abruptly from very small values (disorder, below threshold) to a large unique value (order, above threshold) determined primarily by the boundary conditions. Finally, we point out similarities and important differences between this combined light and matter phase transition and the mean field and Ginzburg-Landau theories.

We (analytically) calculate the energy spectrum corresponding to various experimental and numerical turbulence data analyzed by Benzi et al. We find two bottleneck phenomena: While the local scaling exponent ${\zeta }_r\left(r\right)$ of the structure function decreases monotonically, the local scaling exponent ${\zeta }_p\left(p\right)$ of the corresponding spectrum has a minimum of ${\zeta }_p\left(p_{\min }\right)\approx 0.45$ at $p_{\min }\approx (10\eta {)}^{-1\mathrm{}}$ and a maximum of ${\zeta }_p\left(p_{\max }\right)\approx 0.77$ at $p_{\max }\approx {8L}^{-1\mathrm{}}$. A physical argument starting from the constant energy flux in $p$ space reveals the general mechanism underlying the energy pileups at both ends of the $p$-space scaling range. In the case studied here, they are induced by viscous dissipation and the reduced spectral strength on the scale of the system size, respectively.

For the complex Ginzburg-Landau equation on a large periodic interval, we show that the transition from defect to phase turbulence is more accurately described as a smooth crossover rather than as a sharp continuous transition. We obtain this conclusion by using a parallel computer to calculate various order parameters, especially the density of space-time defects, the Lyapunov dimension density, and correlation lengths. Remarkably, the correlation length of the field amplitude fluctuations is, within a constant factor, equal to the length scale defined by the dimension density.

High-resolution Navier-Stokes simulations are used to test the Kolmogorov refined similarity hypothesis (RSH). It is found that the statistical dependence between velocity increments and dissipation rate invoked by RSH is mostly due to kinematics at moderate Reynolds numbers. However, a dynamically induced contribution can be demonstrated. The longitudinal velocity increment is correlated more strongly with the purely longitudinal component of dissipation rate than with other components. An alternative formulation of the RSH is offered.

A Comment on the Letter by Mario Liu, Phys. Rev. Lett. 70, 3580 (1993).