Search Results (12 total)

First-order multiple-scale perturbation theory is used to derive a set of coupled-mode equations valid for electromagnetic-wave propagation in a weakly periodic, nonlinear medium with periodicity on the order of a wavelength. We apply this to a problem where the medium has a χ⁽²⁾ response and find that the second-harmonic signal generated is enhanced when the fundamental is tuned near the band edge. Results are given for a possible experiment with optical fibers.

We present a numerical study of second-harmonic (SH) generation in a one-dimensional, generic, photonic band-gap material that is doped with a nonlinear χ⁽²⁾ medium. We show that a 20-period, 12-μm structure can generate short SH pulses (similar in duration to pump pulses) whose energy and power levels may be 2–3 orders of magnitude larger than the energy and power levels produced by an equivalent length of a phase-matched, bulk medium. This phenomenon comes about as a result of the combination of high electromagnetic mode density of states, low group velocity, and spatial phase locking of the fields near the photonic band edge. The structure is designed so that the pump pulse is tuned near the first-order photonic band edge, and the SH signal is generated near the band edge of the second-order gap. This maximizes the density of available field modes for both the pump and SH field. Our results show that the χ⁽²⁾ response is effectively enhanced by several orders of magnitude. Therefore, mm- or cm-long, quasi-phase-matched devices could be replaced by these simple layered structures of only a few micrometers in length. This has important applications to high-energy lasers, Raman-type sources, and frequency up- and down-conversion schemes.

We have directly imaged the percolation network at the surface of a three-dimensional carbon-black-polymer composite using an electric force microscope. At intermediate length scales the conductive area exposed at the surface increases with surface area according to a power law corresponding to a three-dimensional infinite cluster of fractal dimension D=2.6±0.1. This value is in good agreement with the scaling theory prediction, D=2.53. At large length scales the behavior is homogenous with the classical exponent D=3.

Molecular-resolution atomic force microscope images of Ba arachidate Langmuir-Blodgett multilayers show two previously unknown structures, a tilted (26°) triclinic packing with a 3×1 sawtooth height modulation, and a tilted (19°) rectangular herringbone packing with an alternating vertical offset, along with untilted disordered regions, all within the same molecular layer. The submicron extent of each region, the extent of twinning and defects, and the complexity of the molecular packing make the determination of the lattice structure of these materials impossible by any other technique.

We have quantitatively determined the nature of the surface order of the outermost layer of monolayer and multilayer Langmuir-Blodgett films of cadmium arachidate, in air and under water, using atomic-force microscopy. Molecular-resolution images of 1200-nm² areas show that the alkyl chains of 2–5-layer films have a noncentered rectangular lattice with lattice constants of 0.482±0.004 and 0.748±0.006 nm, in good agreement with diffraction measurements of bulk aliphatic systems. We show unambiguously that the unit cell consists of two molecules. In contrast with the highly ordered multilayers, monolayer films have a disordered surface. Images taken under water of the head-group surface show that the bilayer surface is disordered, while thicker films show order of the alkyl chains. The presence of an underlying head-group–head-group interface with its associated cadmium ion bridging is shown to be the critical feature needed to obtain a stable and ordered surface layer.

The normal-state resistivity of the A15 superconductors V₃Si, Nb₃Pt, and Nb₃Al has been studied as a function of neutron damage. Resistivity data have been taken from the superconducting transition temperature T_c to room temperature on unirradiated samples and irradiated samples with degraded T_c's ranging down to 2-3 K. The V₃Si data are the most extensive and both a single-crystal and polycrystalline samples have been studied. The Nb₃Pt and Nb₃Al data were taken for comparison. The data are fitted to several theoretical expressions put forth to explain the normal-state resistivity in A15 superconductors at low temperatures, high temperatures, and the full range of temperatures. The results are discussed in light of these theories. The most striking feature of the V₃Si data is that there is no observable change in the shape of the temperature-dependent contribution to the resistivity down to a fractional degradation of T_c of ∼0.5. This does not appear to be the case for the Nb-based A15 superconductors. It is suggested that this difference in behavior may be related to the different sensitivity of T_c to disorder in V₃Si as opposed to the Nb-based A15 superconductors.

The superconducting transition temperature T_c and its width ΔT_c were determined for neutron-irradiated V₃Si for various fluences ϕt, using heat-capacity and resistivity measurements. It was found that, although the two determinations do not give identical results, in both cases ΔT_c increases with fluence indicating the inhomogeneous nature of the damage.

We have measured the heat capacity and resistivity of a V₃Si single crystal as a function of neutron irradiation up to a fluence of 22.2 × 10¹⁸ n/cm². The structural transformation was found to be extremely sensitive to very-low-level irradiations (∼ 0.25 × 10¹⁸ n/cm²) whereas the superconducting transition temperature T_c was not affected up to ∼ 3 × 10¹⁸ n/cm². The residual resistivity showed minor variations up to ∼ 3 × 10¹⁸ n/cm² and started to change appreciably beyond that fluence. The electronic-heat-capacity coefficient and hence the density of states at the Fermi level was found to be considerably affected for all fluences. Further there is some evidence for the inhomogeneous nature of the irradiated state.

Reported here is a direct comparison of the thermodynamic properties of the tetragonal and cubic phases of a single crystal V₃Si based on our ac heat-capacity data obtained before and after arresting the structural transformation by copper cladding. Whereas the bulk superconducting temperatures are the same to within 0.1 K, the entropy of the tetragonal phase at the transformation temperature (∼ 20 K) is greater than that of the cubic phase by ∼ 240 ± 50 mJ/mole K.

Low-temperature calorimetric measurements have been made on some fcc Ni-Co alloys. The observed variation of electronic-specific-heat coefficient with the average number of valence electrons per atom, n_e, for this system is in a remarkably good agreement with that for the Ni-Fe system in Ni-rich alloys over a significantly large n_e range. Based on these measurements and the validity of the rigid-band model, an approximate shape of the 3d band is proposed for fcc alloys.