The photonic band structure in a two-dimensional dielectric array is investigated using the coherent microwave transient spectroscopy (COMITS) technique. The array consists of alumina-ceramic rods arranged in a regular square lattice. The dispersion relation for electromagnetic waves in this photonic crystal is determined directly using the phase sensitivity of COMITS. The experimental results are compared to theoretical predictions obtained using the plane-wave expansion technique. Configurations with the electric field parallel and perpendicular to the axis of the rods are investigated.

We present Monte Carlo results for the deposition of particles on a line, in a two-dimensional semi-infinite flow simulated by a lattice gas automation. Instead of straight inclined needles, as observed for diffusion-limited deposition with a drift, we obtain a distribution of clusters which are inclined with an angle increasing linearly with the distance to the baseline. The kinetics is different than in diffusion-limited deposition, whereas the static properties of the clusters are similar.

The dynamics of a metastable beam of particles ‘‘injected’’ into artificially excited resonance islands in the Fermilab Tevatron is studied. Evolution of the captured beam distribution is measured by an externally detectable coherent signal. Dynamical partitions observed in the control space of amplitude and tune of a focusing modulation are consistent with an equivalent mode of an ensemble of driven gravity pendula. The model’s single parameter, the island tune ${\mathit{Q}}_{\mathit{I}}$ of small oscillations, is inferred as the central frequency of resonant detrapping under this weak drive.

We present microscopic semiclassical simulations of the femtosecond nonlinear-optical polarization (${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$) of a solute in water. ${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$ is dominated at short times by intermolecular vibrations that cannot be accounted for by the Bloch equations. The applicability of the Bloch, stochastic, and Brownian oscillator models to the interpretation of photon-echo and pump-probe measurements is discussed. We find that only microscopic semiclassical simulation and the Brownian oscillator model can account for the essential features of ${\mathit{P}}^{\mathrm{}\left(3\right)\mathrm{}}$.

Dislocations are important for wave-number adjustment in the almost periodic patterns found in convecting fluids and a wide variety of other physical contexts. Using a model for infinite-Prandtl-number fluids, we show that, far from onset, climbing dislocations can undergo local, finite-amplitude instabilities that lead to the formation of disclinations, bridgelike structures, sudden roll disappearance, and gliding.

A general description is given to the unlimited acceleration of charged particles by a wave in the presence of a uniform magnetic field. The wave can have an arbitrary mixture of electrostatic and electromagnetic components and the propagation direction in the magnetic field is also arbitrary. Exact limits on parameters for trapping and acceleration are derived. The probability of trapping in a gyration period is also presented, and it is shown that a particle once trapped remains so and undergoes unlimited acceleration.

For pointlike singularities localized well away from the symmetry axis, axisymmetric flows with swirl are arbitrarily well approximated by two-dimensional Boussinesq convection. An adaptive mesh simulation of the latter equations was continued until the maximum three-dimensional vorticity showed a factor of ${10}^7$ increase, allowing a reasonable determination of exponents, and elucidation of the mechanism of blowup.

Turbulent Taylor vortex flow is studied in experiments for Reynolds numbers ${10}^3$<R<${10}^6$. Simple scaling of the torque with Reynolds number is m/Inot observed for any range of R, although the characteristic time scales and the transport of passive scalars are found to scale with the global torque measurements. Above a nonhysteretic transition observed at R=1.3×${10}^4$, the torque has a Reynolds number dependence similar to the drag observed in wall-bounded shear flows such as pipe flow and flow over a flat plate.

We present experimental and theoretical results on the shear thinning of electrorheological fluids. Rheological measurements on a model fluid consisting of monodisperse silica spheres immersed in a dielectric liquid show a power-law dependence μ∝γ${\mathrm{̇}}^{\mathrm{-}\mathrm{\Delta }}$ of the apparent viscosity μ on the strain rate γ̇, with Δ in the range Δ=0.68–0.93. We present a theoretical treatment of steady-state cluster formation in applied electric and shear fields, which correctly predicts the observed power-law shear thinning albeit with an exponent Δ=2/3. We observe no true yield stress in this material.

The acceleration of electrons using a laser and a static electric field perpendicular to the former is considered. The coupled particle-field equations are reduced to a second-order nonlinear inhomogeneous equation which determines the trajectory of the particle. The particle energy equation is considered and found to exhibit fold catastrophes. At these catastrophes, which may occur whenever the wave phase is nπ if the direction of the applied weak field is reversed at (2n-1)π/2, further net acceleration occurs. The trajectory equation is found amenable to analytical treatment.

The coherent effect in spontaneous emission has been observed in a low-energy, low-gain, waveguide-mode free-electron laser. The intensity of the radiation was more than ${10}^3$ times stronger than that of the incoherent radiation. This enhancement is due to coherent radiation emitted by the electron beam, which is randomly modulated by micropulses whose typical pulse width (∼35 ps) is comparable to the radiation wavelength. The measured power tends towards a quadratic dependence on the electron-beam current.

The Goos-Hänchen longitudinal shift at total reflection of an optical Gaussian beam is experimentally investigated for only one reflection. The differential experimental method uses the high sensitivity of the eigenstates of a quasi-isotropic laser to small perturbations to measure an intracavity Goos-Hänchen effect for angles of incidence both below and above the critical angle. The measurements are in good agreement with our calculations of the longitudinal shift for Gaussian laser beams.

The overall propagation rate of growing interfaces subject to weak random stirring is investigated, based on a variant of the Kardar-Parisi-Zhang equation. Scaling analysis, supported by computations, indicates that random stirring increases the overall propagation rate in proportion to 4/3 power of the ratio of stirring rate to growth rate. The generally assumed quadratic dependence on this ratio is valid only for special cases such as periodic flow. On this basis, a modified turbulent-flame speed formula is proposed. Implications concerning interpretation of measurements are noted.

Homodyne photon correlation spectroscopy (HCS) and laser Doppler velocimetry (LDV) were used to study the probability density function of velocity differences δv(l) between points in the fluid separated by a distance l. The two measuring schemes yield different results for the probability density, P(δv(l))dδv(l). HCS probes the spatial fluctuations directly, whereas LDV records temporal fluctuations in velocity and relates them to the spatial fluctuations through the Taylor hypothesis. The measurements therefore imply the failure of this hypothesis in the Reynolds number range where we have applied it.

We consider radiation caused by the collective electromagnetic field of one beam deflecting particles of the other. We find the number of photons for a single collision is ${\mathit{dN}}_{\gamma }$≊${\mathit{N}}_0$ ${\mathit{dE}}_{\gamma }$/${\mathit{E}}_{\gamma }$ in the energy range ${\mathit{E}}_{\gamma }$<${\mathit{E}}_{\mathit{c}}$=4ħc${\gamma }^2$<l, where l is the length of the bunch. At the Superconducting Super Collider, for example, ${\mathit{N}}_0$=50 and ${\mathit{E}}_{\mathit{c}}$=6 keV. Specific features of this radiation can be used for a fast control over collisions and for measuring beam parameters. A background due to synchrotron radiation in the magnetic field of a collider is estimated.

A formulation to characterize stimulated emission of synchrotron radiation is presented, based on simplification of a theory developed by Sokolov and Ternov. It is shown that optical gain may be obtained up to gyration harmonics of the order of the critical harmonic number for spontaneous radiation 3${\gamma }^3$, where γ is the relativistic energy factor. Synchrotron-radiation lasers (SRL’s) in the infrared and visible portions of the spectrum are shown to be feasible, provided beams with high energy definition are used. SRL gain can exceed free-electron-laser gain for similar beams.

The broadband radiation emitted from a NaZ pinch is used to photoionize Ne to the He-like ground state and radiation from the Na 1${\mathit{s}}^2$-1s2p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ transition is used to resonantly photoexcite the Ne 1${\mathit{s}}^{2\mathrm{-}}$1s4p ${}_{}{}^1{}_{}{}^{}$${\mathit{P}}_1$ transition. Time-resolved and time-integrated spectral measurements of the Ne K-shell emission demonstrate the first population inversion driven by a Z pinch. This is the first experiment in any medium to demonstrate a soft-x-ray inversion pumped solely by resonant photoexcitation.

The spatial coherence of a neonlike selenium x-ray laser operating at 206 and 210 Å has been measured using a technique based on partially coherent x-ray diffraction. The time-integrated spatial coherence of the selenium x-ray laser was determined to be equivalent to that of a quasimonochromatic spatially incoherent disk source whose diameter is comparable to the line focus of the visible-light laser pumping the x-ray laser. The spatial coherence was improved by narrowing the line focus width.

A new optical near-field microscope with a lateral resolution of 3 nm (λ/200) is presented. Resonantly excited extended surface plasmons generate an optical near field which is locally probed by a sharp tip. The interaction of the surface plasmons with the tip can be understood in terms of elastic plasmon scattering and radiationless energy transfer from the tip to the sample. The new technique may permit the detection and spectrscopic identification of single adsorbed molecules.

Localized traveling-wave trains (LTW) have been observed in various experiments on binary mixture convection. I show that the commonly used complex Ginzburg-Landau equations, which fail to describe a characteristic feature of LTW—their extremely slow drift—break down in these systems and I derive a new set of coupled equations which takes into account the slow dynamics of the concentration field. It possesses slow LTW over a wide range of parameters. In addition, it supports LTW even if it has only real coefficients and is therefore far from the nonlinear Schrödinger limit.