Study of multiple hologram recording in lithium niobate

The results of detailed experimental and theoretical considerations relating to multiple hologram recording in lithium niobate are reported. The following problem areas are identified and discussed: (1) the angular selectivity of the stored holograms, (2) interference effects due to the crystal surfaces, (3) beam divergence effects, (4) material recording sensitivity, and (5) scattered light from material inhomogeneities

Functional Morphology of the Sphyrnid Shark Cephalofoil: an Assessment of the Hydrodynamic Lift Hypothesis Using Computational Fluid Dynamics

The hammerhead sharks (Family: Sphyrnidae) are all characterized by a conspicuous lateral expansion and dorsoventral flattening of the head forming a structure known as a cephalofoil, however, there is substantial morphological variation within the clade. Many theories have been proposed regarding the functional aspects of this structure. One of these is that it may produce beneficial dynamic lift as the shark swims (in similar fashion to the cambered wings found on many modern-day aircraft). As sharks do not possess a swim bladder, part of their energy intake is expended on simply maintaining vertical station in the water column. If indeed the cephalofoil constitutes an anterior lift-generating feature as hypothesized, such energy expenditure could theoretically be reallocated. We digitized the head shaped of all eight living species of hammerhead shark, and performed a computational fluid dynamic (CFD) analysis to quantify the lift and drag forces associated with each of the various cephalofoil morphologies. For comparison, three carcharhinid species, the bull shark (C. leucas), the blacktip shark ( C. limbatus), and the leshark (N. brevirostris) were likewise analyzed. It was assumed that addition of a lifting structure to the morphology of the shark should have effected corresponding evolutionary changes in other lift-generating features. To test this hypothesis, morphometric data were gathered from numerous specimens and multiple regression coupled with an information-theoretic approach to model selection were used. The cephalofoil appears only to produce substantial lift forces at positive angles of incidence to the flow. These head morphologies, meanwhile, appear to be characterized by greater drag than their carcharhinid counterparts. Statistical analysis corroborates the current belief that hydrodynamic forces acting on the cephalofoil reduce stability during swimming. The ecophysiological impli

Secondary Structures in Long Compact Polymers

Compact polymers are self-avoiding random walks which visit every site on a lattice. This polymer model is used widely for studying statistical problems inspired by protein folding. One difficulty with using compact polymers to perform numerical calculations is generating a sufficiently large number of randomly sampled configurations. We present a Monte-Carlo algorithm which uniformly samples compact polymer configurations in an efficient manner allowing investigations of chains much longer than previously studied. Chain configurations generated by the algorithm are used to compute statistics of secondary structures in compact polymers. We determine the fraction of monomers participating in secondary structures, and show that it is self averaging in the long chain limit and strictly less than one. Comparison with results for lattice models of open polymer chains shows that compact chains are significantly more likely to form secondary structure.Comment: 14 pages, 14 figure

Holographic data storage crystals for the LDEF

Crystals of lithium niobate were passively exposed to the space environment of the Long Duration Exposure Facility (LDEF). Three of the four crystals contained volume holograms. Although the crystals suffered the surface damage characteristic of that suffered by other components on the Georgia Tech tray, the crystals remained suitable for the formation of volume holograms

A hydrodynamics assessment of the hammerhead shark cephalofoil

© 2020, The Author(s). Hammerhead sharks are characterized by a conspicuous lateral expansion of the head forming a structure known as a cephalofoil. Two theories regarding the function of this structure suggest that it may increase maneuverability as well as produce dynamic lift similar to a cambered airplane wing. Here we report on a family-wide computational fluid dynamics analysis of all eight hammerhead shark species and three sharks with typical head shape. Models cast of the heads of fresh and museum specimens of hammerhead and typical sharks were used to produce pressure surface maps and lift and drag polar diagrams at various angles of attack. These analyses suggested that the cephalofoil (1) provides greater maneuverability that may be important in prey capture efficacy, (2) does not provide significant dynamic lift when held parallel to flow, (3) is characterized by greater drag than typical sharks across all attack angles, and (4) was found to result in a 10-x increase in energetic cost over typical shark head morphologies

Future Orientation as a Protective Factor for African American Adolescents Exposed to Community Violence

For African American youth disproportionately exposed to community violence and the associated risk of externalizing behaviors, developmental assets that reduce the risk for externalizing behaviors and enhance adaptive coping should be explored. In a sample of 572 African American adolescents (Mage = 15.85; SD = 1.42), the current study explored whether future orientation or gender buffered the impact of community violence exposure on externalizing behaviors. The current study also examined the interaction between future orientation, gender, and violence-specific coping strategies to determine their association with externalizing behaviors. Future orientation moderated the relationship between violence exposure and delinquent, but not aggressive, behaviors. Future orientation interacted differently with coping for males and females to predict externalizing behaviors. Research and clinical implications are discussed

Review of rigorous coupled-wave analysis and of homogeneous effective medium approximations for high spatial-frequency surface-relief gratings

A review of the rigorous coupled-wave analysis as applied to the diffraction of electro-magnetic waves by gratings is presented. The analysis is valid for any polarization, angle of incidence, and conical diffraction. Cascaded and/or multiplexed gratings as well as material anisotropy can be incorporated under the same formalism. Small period rectangular groove gratings can also be modeled using approximately equivalent uniaxial homogeneous layers (effective media). The ordinary and extraordinary refractive indices of these layers depend on the gratings filling factor, the refractive indices of the substrate and superstrate, and the ratio of the freespace wavelength to grating period. Comparisons of the homogeneous effective medium approximations with the rigorous coupled-wave analysis are presented. Antireflection designs (single-layer or multilayer) using the effective medium models are presented and compared. These ultra-short period antireflection gratings can also be used to produce soft x-rays. Comparisons of the rigorous coupled-wave analysis with experimental results on soft x-ray generation by gratings are also included