Microstructured optical fiber for single-polarization air guidance

An air-core microstructured fiber design that supports a single-polarization, circularly symmetric nondegenerate mode is presented. The fiber design is modeled directly, and the microstructured cladding is analyzed by use of band diagrams to elucidate the mechanism through which polarization nondegeneracy is achieved.     

Analytic Results for Gaussian Wave Packets in Four Model Systems: II. Autocorrelation Functions

The autocorrelation function, A(t), measures the overlap (in Hilbert space) of a time-dependent quantum mechanical wave function, (x, t), with its initial value, (x, 0). It finds extensive use in the theoretical analysis and experimental measurement of such phenomena as quantum wave packet revivals. We evaluate explicit expressions for the autocorrelation function for time-dependent Gaussian solutions of the Schrödinger equation corresponding to the cases of a free particle, a particle undergoing uniform acceleration, a particle in a harmonic oscillator potential, and a system corresponding to an unstable equilibrium (the so-called inverted oscillator.) We emphasize the importance of momentum-space methods where such calculations are often more straightforwardly realized, as well as stressing their role in providing complementary information to results obtained using position-space wavefunctions.     

Quasi-atomic fine structure in the Auger spectra of solid silver and indium

High resolution M₄,₅N_4,5N_4,5 Auger spectra of Ag and In reveal fine structure, which may be interpreted in terms of spin-orbit splitting of the initial state and multiplet structure in the final state. Interaction between d holes in Ag is an important factor in determining the shape of the Auger spectrum, and so the Auger profile is not related in any simple way to the one-particle density of states.     

Optical study of H2O ice to 120 GPa: dielectric function, molecular polarizability, and equation of state

The refractive index of H2O ice has been measured to 120 GPa at room temperature using reflectivity methods. The refractive index increases significantly with pressure on initial compression and exhibits small changes with pressure at previously identified phase transitions. Pressure dependencies of the molecular polarizability show changing slopes in different pressure regions. A general molar refractivity analysis of this change in slope reveals features at 60 GPa due to the onset of the ice VII-X transition. Band gap closure in H2O ice is constrained by the dispersion data using a single oscillator dielectric model. Sample thickness measurements obtained from interference patterns yield pressure-volume relations in excellent agreement with those measured by x-ray diffraction.     

Deviatoric stress driven formation of large single-crystal PbS nanosheet from nanoparticles and in situ monitoring of oriented attachment

Two-dimensional single-crystal PbS nanosheets were synthesized by deviatoric stress-driven orientation and attachment of nanoparticles (NPs). In situ small- and wide-angle synchrotron X-ray scattering measurements on the same spot of the sample under pressure coupled with transmission electron microscopy enable reconstruction of the nucleation route showing how enhanced deviatoric stress causes ordering NPs into single-crystal nanosheets with a lamellar mesostructure. At the same time that deviatoric stress drives SC(110) orientation in a face-centered-cubic supercrystal (SC), rocksalt (RS) NPs rotate and align their RS(200) and RS(220) planes within the SC(110) plane. When NPs approach each other along the compression axis, enhanced deviatoric stress drives soft ligands passivated at RS(200) and RS(220) surfaces to reorient from a group of SC(110) in-planes to the interspace of SC[110]-normal planes. While the internal NP structure starts a rocksalt-to-orthorhombic transition at 7.1 GPa, NPs become aligned on RS(220) and RS(200) and thus become attached at those faces. The transition-catalyzed surface atoms accelerate the inter-NP coalescing process and the formation of low-energy structure nanosheet. Above 11.6 GPa, the nucleated single-crystal nanosheets stack into a lamellar mesostructure that has a domain size comparable to the starting supercrystal.     

Deviatoric stress: a nuisance or a gold mine?

Both synchrotron radiation and deviatoric stress were once considered to be nuisances. Now synchrotron radiation is one of the most important tools available to scientists of all disciplines and deviatoric stress is one of the most useful aspects of x-ray diffraction at extreme conditions. Samples in high-pressure devices are under true hydrostatic pressure only when surrounded by a fluid, thus limiting true hydrostatic pressure studies at ambient temperatures to pressures below about 11?GPa. Elevated temperature is able to extend this limit but has rarely been used for this purpose. Instead, noble gases have been used as pressure media as their solids are especially soft. Deviatoric stress and resultant anisotropic elastic strain in solid samples and solid media have led to many subtle errors in determinations of elastic properties and crystal structures, especially in the days before it was realized that they could be measured and were potentially a valuable source of information. In recent years, measuring anisotropic elastic strain by x-ray diffraction has provided new insights into materials strength, elastic properties, crystal structures, mechanisms of phase transitions, slip systems, lattice preferred orientation, and, of course, ways to make corrections when deviatoric stress is indeed a nuisance.     

Polymer Morphology: A Guide to Macromolecular Self-Organization

The study of polymer morphology continues to be the principal means of acquiring knowledge and understanding of macromolecular self-organization. Longstanding problems of the nature of melt-crystallized lamellae and spherulitic growth have been resolved, bringing understanding of how characteristic properties such as a broad melting range and spatially-varying mechanical response are inherent in spherulitic morphologies. This reflects the distinctive features of the crystallization of long molecules, i.e. that they impede each other and, for faster growth, form rough basal surfaces. Knowledge of morphology is an essential accompaniment to the informed development of advanced polymeric materials and a full understanding of their structure/property relations.