We calculate both linear and nonlinear optical response in the static limit for AlP, AlAs, GaP, and GaAs in the local-density approximation with and without a scissors operator, including local-field effects. The calculation agrees with the measurement of Levine and Bethea to within 15%. The derivatives d ln${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$/d lna, for lattice constant a, are predicted to be from 1.7 to 6.6 times the values obtainable from existing models. The calculated static dielectric constants ${\mathrm{\epsilon }}_{\mathrm{\infty }}$ are within 2% of measured values; calculated results for d ln${\mathrm{\epsilon }}_{\mathrm{\infty }}$/d lna are also presented. The scissors operator alters ${\mathrm{\epsilon }}_{\mathrm{\infty }}$ up to 25% but alters ${\mathrm{\chi }}^{\mathrm{}\left(2\right)\mathrm{}}$ by a factor of 2.

The interaction of transverse eigenmodes with a relativistic electron beam is analyzed in an overmoded cyclotron autoresonance maser amplifier, using a nonlinear self-consistent model and kinetic theory. It is shown that all of the coupled modes grow with the dominant unstable mode at the same growth rate, but suffer different launching losses. The phases of coupled modes are locked in the linear and nonlinear regimes. Simulations indicate that the rf power distribution among the interacting modes at saturation is insensitive to input power distribution but sensitive to detuning.

The photoelectron counts of spontaneous light from the wiggler in the Brookhaven electron storage ring obey the negative-binomial distribution, in accord with the predictions of a multielectron, multimode theory. The bending-magnet light emerging from the Pyrex exit port of the storage ring obeys the Neyman type-A distribution.

High-wave-number modes are eliminated in a band characterized by its width parameter λ in k space. The requisite conditional average is evaluated as an approximation in which coupling effects are neglected to order ${\lambda }^2$, for small λ. A fixed point was found under renormalization-group transformation, which corresponded to the Kolmogorov ‘‘-5/3’’ spectrum with spectral constant α=1.60.

We develop the principle of ‘‘speckle-pattern tomography’’ in the multiple-scattering regime, i.e., the technique of using correlations between complex interference patterns to determine the position of a special scatterer for either electromagnetic or acoustical wave propagation through a diffusive scattering medium. We focus on two concrete examples which are relevant to experiments and possible applications: (i) detecting the position of a stationary special scatterer in a medium of moving scatterers; (ii) detecting the appearance and position of a new added scatterer.

We show that a qualitative theory of bifurcations can provide an understanding of the spontaneous symmetry breaking and the appearance of traveling waves leading to spatiotemporal complexity in the transverse patterns of the intensity of a ${\mathrm{CO}}_2$ laser.

A new regime of density-channel guiding of a relativistic electron beam in air has been found using a three-dimensional charged-particle simulation code, and confirmed in a double-pulse electron-beam experiment. The guiding results from the temperature dependence of the electron-neutral momentum-transfer frequency ${\nu }_{\mathit{m}}$. The mechanism does not require a deep channel to obtain a significant guiding force. For the 13-kA MEDEA II (and beams of similar parameters), guiding persists 10 ns into the beam pulse with the force per channel displacement as high as 4 G/cm.

Solutions are reported for the problem of the motion of a two-dimensional interface between a viscous and a nonviscous fluid. The solution has the interface moving uniformly while the viscous fluid has a nontrivial potential flow. In an alternative interpretation, there are gravitational forces in the plane which allow the interface to be at rest while the fluid is in motion.

Particle-in-cell simulations of a high-power X-band dielectric Cherenkov maser experiment indicate strong particle bunching for drive currents greater than a few kA. Good agreement between the calculations and experimental measurements have been obtained. Depending on the parameters and configuration of the device, the simulations demonstrate power levels of up to 900 MW with rf conversion efficiencies of 37%. One-third of the total beam power can be contained in well-defined bunches which may be used to drive compact high-gradient linear acclerators.

A nonlinear simulation of the Cherenkov maser amplifier is presented for a configuration in which an electron beam propagates through a dielectric-lined cylindrical waveguide. The parameters used correspond to an experiment at General Dynamics which measured a total efficiency of 11.5% at 8.6 GHz. The simulation is in agreement with this but indicates that the system was too short to reach saturation and that an efficiency of 30% would have been possible for a longer system, and the performance is not significantly degraded by thermal spreads up to 20%.

Two autoionizing levels which are separated by a few decay widths may exhibit a sharp interference or window in their absorption profile and also make canceling contributions to the refractive index at the absorption minimum. A correct choice of intermediate mixing levels prevents a similar cancellation in the nonlinear susceptibility. Using uv lasers with energies of about a mJ and pulse lengths of 5 ns, we generate 0.23 μJ per pulse at 104.8 nm.

We have studied turbulent drag reduction using the technique of photon-correlation homodyne spectroscopy to measure turbulent velocity gradients in a dilute polymer solution. A large anisotropic suppression of turbulent velocity differences is found in the bulk region of the turbulent fluid and along the direction of the mean flow. The suppression effect is associated with an enhancement of the turbulent drag reduction. This anisotropy caused by polymer molecules in turbulent flow may be important in understanding turbulent drag reduction.

The photonic band structure of a face-centered-cubic lattice of spheres is calculated using the plane-wave method for Maxwell’s equations. Comparisons with the available experimental results show rather good agreement, except that we do not find a true gap for this configuration. This we believe is due to symmetry reasons at the W point.

We examine the propagation of electromagnetic waves in periodic dielectric structures by solving the vector Maxwell equations with the plane-wave method. Contrary to experimental reports, as well as results of scalar-wave calculations, we do not find a true gap extending throughout the Brillouin zone in the fcc structure. However, there is a depletion in the photon density of states, seemingly a remnant of the Mie resonance, giving rise to a pseudogap in the spectrum that is quite strong for dielectric-sphere packing fraction β∼0.3–0.4. An effect analogous to the Borrmann effect in x-ray diffraction is predicted, where certain photon modes will propagate an anomalously long distance before getting absorbed.

The defects of a system where hexagons and rolls are both stable solutions are considered. On the basis of topological arguments we show that the unstable phase is present in the core of the defects. This means that a roll is present in the penta-hepta defect of hexagons and that a hexagon is found in the core of a grain boundary connecting rolls with different orientations. These results are verified in an experiment of thermal convection under non-Boussinesq conditions.

A transition analogous to the strongest 4d-4p nickel-like x-ray laser transition has been observed in cobaltlike ions within the same plasma. Amplified spontaneous emission from Co-like ions has been identified in laser-produced exploding-foil plasmas of Yb and Ta. The Co-like line exhibits similar time history to the Ni-like laser line and has a gain coefficient consistent with a significant Co-like population. The observation of this transition is of interest as fluorinelike analogs to neonlike 3p-3s x-ray lasers have never been seen although they are predicted to have gain.

We have observed convective flow in a thin freely suspended film of smectic-A liquid crystal when a sufficiently large electric field is applied in the plane of the film. The convective flow neither reorients the director nor deforms the free surfaces of the film. Thus the film behaves as a two-dimensional isotropic fluid. We have studied the onset of this flow in two film holders, one which allows injection of charges from the electrodes into the film and one which does not. We present measurements of the threshold voltage and of the wavelength of the vortex pattern as a function of the frequency of the applied field, and briefly discuss the nature of the mechanisms driving the flow.

Mode competition in a far-infrared, free-electron laser operating in the quasi-cw regime has been investigated by means of direct spectral measurement with a heterodyne receiver system. About 30% of the spectra had a single longitudinal mode. The observed data compare reasonably well with theory. It is found that even in a regime of multimode operation the laser signals are only weakly modulated in time, as observed with a video detector. Such an observation is consistent with a phase locking of multiple modes, as predicted by theory.

Infrared fluorescence has been observed from a monolayer of ${}_{}{}^{13}{}_{}{}^{}\mathrm{C}_{}^{16}{}_{}{}^{}$O on NaCl(100) following vibrational excitation of its fundamental absorption band with a pulsed CO gas laser. Direct time-domain measurements of the fluorescence reveal a lifetime of 4.3 ms. Since fluorescence occurs in the overtone region from high vibrational states, these results demonstrate that energy can be stored in the adsorbate long enough to allow vibrational up-pumping between neighboring molecules. It is suggested that the primary channel for CO vibrational relaxation is Δv=-1 transfer to phonons of the NaCl substrate.

Measurements of the spatial and spectral intensity correlation of microwave radiation allow us to identify the leading terms in a perturbation expansion of the correlation function. This function is shown to depend upon two average properties of damped eigenstates of the random medium. These are the average level width δν and the number of eigenstates within the level width δ, whose inverse is the expansion parameter for the correlation function.