The mutual interaction of counterpropagating waves in a ring resonator containing a medium with second-order dispersive nonlinearity is studied. The theoretical treatment of nonreciprocal propagation is based upon the novel coupling of counterpropagating and mode-locked first- and second-harmonic waves. The experimentally observed transfer characteristics of a high-frequency model system reveal a directionally asymmetric bistability. This nonreciprocity can also be attributed in the time domain to the occurrence of dissipative structures, i.e., to solitons traveling within the resonator.

Working with a beam of two-level–atom–like ${}_{}{}^{174}{}_{}{}^{}\mathrm{Yb}$, we have measured the temporal evolution of strong-field resonance fluorescence spectra during square-pulse excitation. In the case of resonant excitation, it is found that driving-field phase shifts transform the normal three-peaked Mollow spectrum into a transient doublet spectrum asymmetric with respect to the atomic transition frequency. This qualitatively new type of spectrum provides fundamental insight into the connection between atomic dynamics and spectra, and results from a nearly perfect polarization of the dressed atom-field states.

Enhancement of the sensitivity of a polarization interferometer beyond the limit set by vacuum fluctuations of the electromagnetic field has been obtained with use of squeezed light generated by a ${\mathrm{KTiOPO}}_4$ optical parametric amplifier. The increase in signal-to-noise ratio relative to the shot-noise limit is 2 dB. The corresponding improvement in the response time of the interferometer is measured to be a factor of 1.6.

We report the first observation of amplification of spontaneous emission at soft x-ray wavelengths by ions in the nickel-like sequence. The ions are created by high-intensity laser irradiation of a thin foil. Gains of order 1 ${\mathrm{cm}}^{\mathrm{-}1}$ are observed on J=0-1, 4d-4p transitions in ${\mathrm{Eu}}^{35+}$ at 65.83 and 71.00 Å. There is some evidence for amplification on the analogous lines at 50.26 and 56.09 Å in ${\mathrm{Yb}}^{42+}$. The pumping mechanism is believed to be electron collisional excitation from the ground state. The scheme should be readily scalable to produce amplification in ${\mathrm{W}}^{46+}$ at 43.15 Å.

Extreme ultraviolet laser amplification has been observed for the C vi Balmer-α transition at 18.2 nm, with use of a novel optical system to irradiate up to 1 cm length of carbon fiber target. The measurements were time resolved and indicated peak single-transit amplification of about 30 times.

A tokamak discharge has been formed and maintained through helicity injection, by use of only an external dc low-energy electron beam. The discharge, in a 5-kG toroidal field, evolved to a steady-state circular cross section with q(a)=10, q(0)=4, which was maintained for more than 400 L/R periods, the time (60 msec) limited by the cathode bias supply. The density profile reached a line-averaged n${¯}_e$=2×${10}^{13}$ ${\mathrm{cm}}^{\mathrm{-}3}$, while $T_e$=25 eV and $T_i$=15 eV. Strong central peaking occurs and inward pinching is indicated for both density and current.

Tunneling images of vicinal Si(001) tilted 4° about the [11¯0] direction have been obtained. Straight double steps running along the [11¯0] direction, separated by terraces of relatively uniform width, dominate the topography of the surface. The structure of the bi- atomic steps is best described by a model that involves a rebonded geometry with buckling.

The structure of steps on cleaved silicon (111) surfaces is studied by scanning tunneling microscopy. Predominantly [21¯1¯] oriented steps are observed. Ordered regions of individual steps have unit periodicity along the step edge. A π-bonded reconstruction of the step edge is deduced from the images.

We report direct observations of surface reconstructions on CdTe by high-resolution electron microscopy in the profile-imaging geometry. The predominant surface was (111) which was often found to be twinned. Image simulations confirmed that the (110) reconstruction was similar to that derived from low-energy electron diffraction studies. A metastable 3×n reconstruction was observed on the (100) surface.

The position of Ga and As atoms at monolayer coverages of heteroepitaxial GaAs on clean Si(111) have been measured by x-ray standing waves in UHV. Though both As and Ga are incident on the surface As atoms choose to occupy the upper half of the (111) double plane about 5% higher on the average, relative to the Si(111) d spacing, than the bulk silicon position. Ga atoms are exclusively located on the lower half of the (111) plane about 3% higher than the corresponding bulk positions.

We propose a simple density-functional theory for the isotropic-nematic transition of a system of hard ellipsoids which yields results in fair agreement with the computer simulation studies and which reduces exactly to Onsager’s theory in the limit of infinitely dilute systems of infinitely long ellipsoids. When the free energy is expanded with respect to the Maier-Saupe quadrupole order parameter an explicit Landau theory is produced which yields good results for small nonsphericities. The theory also predicts a Lindemann rule for orientational freezing.

We study a model for the protonated polyanilines in which the polarizability of the extended π electrons on the polymer promotes the ordering of the protonated sites in a series of commensurate composition waves on the polymer. The model predicts a succession of first-order transitions between locked commensurate structures for the protonated system, with the period of the structure determined by the electronic band filling. Several implications for the physical properties of the protonated polyanilines are suggested.

We report a synchrotron x-ray diffraction study of the tungsten (100) surface reconstruction induced by the chemisorption of hydrogen at room temperature. Our experiments verify previously deduced phase boundaries but they suggest a new interpretation of the phases, the most interesting being that the weakly incommensurate ‘‘solid’’ is actually a surface fluid. These experiments also illustrate the utility of x-rays in obtaining quantitative intensity and line-shape information in chemisorbed surface systems.

Some applications of the n-phonon density of states and related computational methods are discussed. A new algorithm, with computation time linear in n, is presented, to replace the traditional exponential time algorithm.

With the use of the high-resolution spectral-hole-burning technique, we have measured the distribution of barrier heights in doped glasses and polymers. We show that this distribution follows closely the tunneling model.

A newly developed method of backside time-resolved reflectivity measurement is useful for probing the interface between solid and transient liquid Si. Measurements indicate that explosive crystallization starts very near the Si surface from a highly undercooled liquid Si layer thinner than 3 nm for laser irradiation with long pulses ranging from 65 to 200 ns. During the laser irradiation, surface melt-in continues into fine-grained polycrystalline Si produced by explosive crystallization, followed by solidification of the surface-liquid layer.

We have developed a lattice theory capable of providing, within certain mean-field restrictions, a realistic description of the surface-initiated melting taking place at a low-index crystal-vapor interface near the bulk triple-point temperature ${\mathrm{T}}_{\mathrm{M}}$. Direct application is presented to the (100) and (110) surfaces of an fcc Lennard-Jones crystal. A quasiliquid layer of thickness growing like (${\mathrm{T}}_{\mathrm{M}}$-T${)}^{\mathrm{-}1/3}$ is found, in good qualitative agreement with recent experiments on Ar films.

For a localized vibration of linearly bonded adsorbate with displacement coordinate u interacting with a substrate atom of coordinate v, we propose a new dephasing contribution to the vibrational linewidth arising from the cubic term $u^2$v by going to fourth-order perturbation theory. Such a contribution is shown through a specific example to be equally as important as the leading-order dephasing contribution of the quartic term $u^2$$v^2$, which has been considered by others. It is shown that this cubic term produces much stronger sidebands than the quartic term; it is suggested that this fact may be used in some cases to verify or eliminate phonon dephasing as the linewidth mechanism.

Using density-functional theory we investigate the mechanism of ion and neutral desorption via electronic transitions. F on Al is used as a prototype. We find that fluorine adsorbs as ${\mathrm{F}}^{\mathrm{-}}$ and is bound by an imagelike interaction. The bonding of ${\mathrm{F}}^{+}$ and ${\mathrm{F}}^{++}$ is, however, very different. The 3s, 3p levels which were empty in ${\mathrm{F}}^{\mathrm{-}}$ are now partially occupied by metal electrons. The resulting states are locally nearly neutral but Franck-Condon transitions to them can lead to ion desorption.

Optical transmission and reflection spectra (mid IR through uv) and Raman spectra of superior-quality 90-, 180-, 400-, and 1000-nm-thick superconducting Y-Ba-Cu-O films are reported. Characteristic excitonic bands, and in particular the absorption band at ≃0.37 eV reported earlier, are not observed. It therefore seems unlikely that the high-$T_c$ superconductivity in cuprates could arise from exciton-mediated electron pairing.