We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.
Metamaterials are periodic structures, patterned on a scale much shorter than the operating wavelength. Though they are made of positive index materials at small length scale, these structures exhibit abnormal dispersion characteristics at large scale. The material can indeed exhibit a strong anisotropy, or behave as a medium of negative index of refraction-associated with negative values of the permittivity and permeability. In such periodic media, propagation of light, or generally of electromagnetic waves, can be backward. The wave vector, k, electric field, E, and magnetic field, H, then constitute an indirect trihedron, where k and the Poynting vector, S, are antiparallel. In this case, they are termed left handed materials as proposed by Veselago in 1968 [1].The breakthrough, however, was provided in 1999 by Pendry et al. in [2], when they presented several examples of how one can obtain negative values of and from conductors. The most interesting of them was a split-ring resonator, an array of which provided a negative value of over a portion of its resonance frequency range. The combination of the rods and split-ring resonators yielded an artificial medium possessing both a negative and over a narrow frequency range. Encouraged by the implications of these results, a team at the University of California at San Diego built a composite medium along these lines at microwave frequencies, and made measurements that clearly demonstrated that this medium exhibited a negative index of refraction [3]. These measurements proved that the physical effect is real and leaded to a series of fascinating, novel, and practical microwave components. Among these are forward or backward directional couplers with improved features like arbitrary coupling ratios, shorter coupling lengths, broader bandwidths, etc. Other coupling components with improved features include branch-line couplers and hybrid rings. A different type of novel component that followed directly from the backward-wave property is a leaky wave antenna that can be frequency scanned or electronically scanned from backward endfire through broadside to forward endfire.Beyond this inversion of the phase velocity with respect to the direction of propagation of energy, the main impact of a negative refractive index, n, may be found in the negative refraction [3][4][5]. Both the incident and refracted beam are then on the same side of the normal to the refraction interface, as predicted by Snell's law, with a negative refractive index. Under this condition, any radiating source placed in front of a slab of negative refractive index shows focusing effects within and outside the slab under near field conditions. One can thus build flat lenses (Veselago/Pendry lenses), whose properties are invariant upon translation parallel to their surface. Besides this, as proposed recently by Smith et al. [4], one prospect is also to avoid the diffraction limit with the so-called "superlens effect." In the far field limit using a vanishing index [4] may also contribute to the colli...
Dietary supplementation of yeast or yeast subcomponents (YYS) as commercial preparations of β‐glucan (MacroGard®; Biotec‐Mackzymal, Tromsø, Norway; and Betagard A®; Aqua‐In‐Tech, Inc., Seattle, WA, USA), mannan oligosaccharide (Bio‐Mos™ Aqua Grade; Alltech, Nicholasville, KY, USA), or whole‐cell Saccharomyces cerevisiae (Levucell SB20®; Lallemand Animal Nutrition, Milwaukee, WI, USA) at the manufacturer’s recommended levels was evaluated on the physiological performance of juvenile channel catfish, Ictalurus punctatus. Fish were fed YYS diets for 4 wk, followed by 2 wk of control diet. Fish were sampled at the end of each feeding period (4 and 6 wk) to measure hematological and immune parameters and growth and to determine the effects of dietary β‐glucan on resistance to Edwardsiella ictaluri infection and to low‐water stress (6 wk). Supplementation of YYS in diets did not affect growth performance, hematology, or immune function. Survival from E. ictaluri infection was from 5 to 17.5% higher in fish fed YYS diets than in the control group, but the increases were not significant. Some improvement in stress resistance was observed in YYS‐fed catfish after exposure to low‐water stress. Stress reduction in fish fed diets supplemented with yeast subcomponents has been reported previously, but thus far, no explanation has been proposed for this effect. The present study and the previously published research suggest that dietary YYS supplementation does not appear to improve resistance of channel catfish to E. ictaluri.
Aims: In a search for an antifungal peptide with a high activity against Aspergillus¯avus, Bacillus subtilis AU195 was selected from a collection of isolates with antagonistic activity against A.¯avus. Methods and Results: To identify the antifungal peptides, a protein puri®cation scheme was developed based on the detection of the antifungal activity in puri®ed fractions against A.¯avus. Two lipopeptides were puri®ed with anion exchange and gel ®ltration chromatography. Their masses were determined to be 1045 and 1059 1 m/z with mass spectrometry, and their peptide moiety was identical to bacillomycin D. Conclusions: AU195 synthesized a mixture of two antifungal bacillomycin D analogues with masses of 1045 and 1059, the 14 mass unit difference representing the difference between a C15 and a C16 lipid chain. Signi®cance and Impact of the Study: Both bacillomycin D analogues were active at the same concentration against A.¯avus, but the different lipid chain length apparently affected the activity of the lipopeptide against other fungi.
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