We report the first ab initio quantum treatment of microwave driven alkali Rydberg states. We find that nonhydrogenic atomic initial states exhibit fingerprints of classically chaotic excitation, and identify the cause of their experimentally observed enhanced ionization, as compared to Rydberg states of atomic hydrogen.

We propose a single-sided locally averaged adaptive coupling scheme for the synchronization of spatially extended systems. Coupling and synchronization are analyzed from the viewpoint of image filter construction and numerical dissipation. Single-sided locally averaged coupling is introduced based on the resolution argument of control process. Control sensors are adaptively selected and automatically adjusted according to the magnitude of local oscillations. We demonstrate that the present scheme can effectively suppress and control spatiotemporal oscillations and, thus, provide a powerful approach for shock capturing. Both the Navier-Stokes equation and Burgers' equation are used to illustrate the idea.

We have examined the propagation of femtosecond laser pulses in an absorbing dye solution through a short to a long range of propagation distance. The transmitted pulses show strong spectral shift and a superluminal to subluminal transition in the propagation velocity keeping its initial shape almost intact. It is verified that the peak velocity is well described by a modified group velocity $v_S$ defined within the framework of the saddle-point method as well as by a recent prediction of the net group delay of surviving frequency.

We investigate the photoacoustic effect generated by heat deposition at a point in space in an inviscid fluid. Delta-function and long Gaussian optical pulses are used as sources in the wave equation for the displacement potential to determine the fluid motion. The linear sound-generation mechanism gives bipolar photoacoustic waves, whereas the nonlinear mechanism produces asymmetric tripolar waves. The salient features of the photoacoustic point source are that rapid heat deposition and nonlinear thermal expansion dominate the production of ultrasound.

We study the nonequilibrium state of heat conduction in a one-dimensional system of hard point particles of unequal masses interacting through elastic collisions. A BBGKY-type formulation is presented and some exact results are obtained from it. Extensive numerical simulations for the two-mass problem indicate that, even for arbitrarily small mass differences, a nontrivial steady state is obtained. This state exhibits local thermal equilibrium and has a temperature profile as predicted by kinetic theory. The temperature jumps typically seen in such studies are shown to be finite-size effects. The thermal conductivity appears to have a very slow divergence with system size, different from that seen in most other systems.

When a drop of fluid detaches from a capillary, singular behavior ensues. We show that the addition of very small amounts of polymer inhibits this singularity in an abrupt way and gives rise, after a period of self-similar dynamics as for simple liquids, to long-lived cylindrical necks or filaments which thin exponentially in time. This abrupt change occurs when the elongation rate $\dot{\epsilon }$ becomes comparable to the inverse of the polymer relaxation time leading to a large elongational viscosity ${\eta }_E$ of the dilute polymer solution.

In general, trace formulas relate the density of states for a given quantum mechanical system to the properties of the periodic orbits of its classical counterpart. Here we report for the first time on a semiclassical description of microwave spectra taken from superconducting billiards of the Limaçon family showing mixed dynamics in terms of a generalized trace formula derived by Ullmo et al. [Phys. Rev. E 54, 136 (1996)]. This expression not only describes mixed-typed behavior but also the limiting cases of fully regular and fully chaotic systems and thus presents a continuous interpolation between the Berry-Tabor and Gutzwiller formulas.

We have actively stabilized the comb of frequencies from a mode-locked femtosecond laser using a Fabry-Perot reference cavity. This technique offers the ability to synthesize a comb of highly stable radio frequencies directly from optical transitions. The measured fractional frequency instability of the components of the frequency comb relative to the reference cavity was $<5\mathrm{}\times {10}^{-13\mathrm{}}$ in 0.1 sec. The variation of the optical mode spacing versus frequency of the reference cavity was also directly measured using this technique.

We have observed extreme nonlinear optical phenomena produced by intense midinfrared (MIR) pulses in semiconductors. These phenomena include multiple off-resonance optical sidebands (up to $\pm 3\mathrm{MIR}$ photons interacting with a near-infrared photon), multiple MIR harmonics (up to the seventh harmonic), and significant broadening and modification of MIR harmonic spectra. The generation of these extreme MIR nonlinear optical phenomena is primarily aided by cross-phase modulation.

We show that two regimes of diffraction exist in arrays of waveguides, depending upon the input conditions. At higher powers, normal diffraction leads to self-focusing and to the formation of bright solitons through the nonlinear Kerr effect. By slightly changing the input conditions, light experiences anomalous diffraction and is nonlinearly defocused. For the first time, self-focusing and self-defocusing have been achieved for the same medium, structure, and wavelength.

Direct imaging of settling, non-Brownian, hard sphere, particles allows measurement of particle occupancy statistics as a function of time and sampling volume dimension. Initially random relative particle number fluctuations, $〈N^2-〈N{〉}^2〉/〈N〉=1\mathrm{}$, become suppressed, anisotropic, and $〈N〉$ dependent. Fitting to a simple Gaussian pair correlation model suggests a minute long ranged correlation leads to strong if not complete suppression of number fluctuations. Calflisch and Luke predict a divergence in velocity fluctuations with increasing sample volume size based on random (Poisson) statistics. Our results suggest this is not a valid assumption for settling particles.

Free, buoyancy-driven convection has been observed experimentally in three-dimensional highly fluidized granular flows for the first time. Positron emission particle tracking was used to determine the position of a tracer grain in a vibrofluidized bed, from which packing fraction distributions as well as the velocity fields could be determined. The convection rolls, although small compared to the magnitude of velocity fluctuations $(<5\mathrm{}\%)$, were consistently observed for a range of grain numbers and shaker amplitudes. Density variations are a signature of free convection and, with negative temperature gradients also present, were interpreted as the mechanism by which the convection rolls were initiated.

We present an experiment which aims to investigate the mechanical properties of a static granular assembly. The piling is a horizontal 3D granular layer confined in a box. We apply a localized extra force at the surface and the spatial distribution of stresses at the bottom is obtained (the mechanical Green's function). For different types of granular media, we observe a linear pressure response whose profile shows one peak centered at the vertical of the point of application. The peak's width increases linearly with increasing depth. This Green's function seems to be in at least qualitative agreement with predictions of elastic theory.

Several workers have established that the Larkin domains for two three-dimensional nonmetallic elastic solids in contact with each other at a disordered but atomically flat interface are enormously large, implying that there should be negligible static friction per unit area in the macroscopic solid limit. In contrast, the present Letter argues that when the Larkin domains are calculated for disorder on the multiasperity scale, they are much smaller than the interface size. This can account for the virtual universal occurrence of static friction.

The Rayleigh-Bénard theory by Grossmann and Lohse [J. Fluid Mech. 407, 27 (2000)] is extended towards very large Prandtl numbers Pr. The Nusselt number Nu is found here to be independent of Pr. However, for fixed Rayleigh numbers Ra a maximum in the Nu(Pr) dependence is predicted. We moreover offer the full functional dependences of Nu(Ra,Pr) and Re(Ra,Pr) within this extended theory, rather than only give the limiting power laws as done in J. Fluid. Mech. 407, 27 (2000). This enables us to more realistically describe the transitions between the various scaling regimes.

We present measurements of the Nusselt number $N$ as a function of the Rayleigh number $R$ and the Prandtl number $\sigma$ in cylindrical cells with aspect ratios $\Gamma =0.5\mathrm{}$ and 1.0. We used acetone, methanol, ethanol, and 2-propanol with Prandtl numbers $\sigma =4.0\mathrm{}$, 6.5, 14.2, and 34.1, respectively, in the range $3\times {10}^7\lesssim R\lesssim {10}^{11}$. At constant $R$, $N(R,\sigma )$ varies with $\sigma$ by only about $2\%$. This result disagrees with the extrapolation of the Grossmann and Lohse theory beyond its range of validity, which implies a decrease by $20\%$ over our $\sigma$ range, but agrees with their recent extension of the theory to small Reynolds numbers.

We report the first saturated amplification of an optical-field-ionization soft x-ray laser. The amplifying medium is generated by focusing a circularly polarized 330-mJ, 35-fs, 10-Hz Ti:sapphire laser system in a few-mm cell filled with xenon. A gain of $67\mathrm{cm}{}^{-1\mathrm{}}$ on the ${4d}^{9}5p-{4d}^{9}5d$ transition at 41.8 nm in Pd-like xenon and a gain-length product of 15 have been inferred at saturation. This source delivers about $5\times {10}^9$ photons per pulse. The influence of the pumping energy and the laser polarization on the lasing output are also presented.

Using near-field optical microscopy, we investigate propagation and scattering of surface plasmon polaritons (SPP's) excited in the wavelength range of 780–820 nm at nanostructured gold-film surfaces with areas of 200-nm-wide scatterers arranged in a 400-nm-period triangular lattice containing line defects. We observe the SPP reflection by such an area and SPP guiding along line defects at 782 nm, as well as significant deterioration of these effects is 815 nm, thereby directly demonstrating the SPP band gap effect and showing first examples of SPP channel waveguides in surface band gap structures.

Experimental measurements of acoustic transmission through a solid-solid two-dimensional binary-composite medium constituted of a triangular array of parallel circular steel cylinders in an epoxy matrix are reported. Attention is restricted to propagation of elastic waves perpendicular to the cylinders. Measured transmitted spectra demonstrate the existence of absolute stop bands, i.e., band gaps independent of the direction of propagation in the plane perpendicular to the cylinders. Theoretical calculations of the band structure and transmission spectra using the plane wave expansion and the finite difference time domain methods support unambiguously the absolute nature of the observed band gaps.

We address the situation in which vibration is applied to thin layers of granular material with water filling the pore space, but with air above. Beyond a critical drive parameter we observe heap-shaped convecting domains of nontrivial topology, which exhibit cracks as the forcing amplitude is further increased. We summarize these results in a phase diagram, understand the onset of the convecting heaps as a manifestation of the Rayleigh-Taylor instability for fluids, and measure the response of isolated convecting structures.