A dielectric omnidirectional reflector   总被引：3，自引：0，他引：3  

Y Fink  JN Winn  S Fan  C Chen  J Michel  JD Joannopoulos  EL Thomas 《Science (New York, N.Y.)》1998,282(5394):1679-1682

A design criterion that permits truly omnidirectional reflectivity for all polarizations of incident light over a wide selectable range of frequencies was used in fabricating an all-dielectric omnidirectional reflector consisting of multilayer films. The reflector was simply constructed as a stack of nine alternating micrometer-thick layers of polystyrene and tellurium and demonstrates omnidirectional reflection over the wavelength range from 10 to 15 micrometers. Because the omnidirectionality criterion is general, it can be used to design omnidirectional reflectors in many frequency ranges of interest. Potential uses depend on the geometry of the system. For example, coating of an enclosure will result in an optical cavity. A hollow tube will produce a low-loss, broadband waveguide, whereas a planar film could be used as an efficient radiative heat barrier or collector in thermoelectric devices.  相似文献   

Femtosecond dynamics of electron localization at interfaces     

N Ge  CM Wong  RL Lingle  JD McNeill  KJ Gaffney  CB Harris 《Science (New York, N.Y.)》1998,279(5348):202-205

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.  相似文献   

A long-snouted predatory dinosaur from africa and the evolution of spinosaurids     

PC Sereno  AL Beck  DB Dutheil  B Gado  HCE Larsson  GH Lyon  JD Marcot  OWM Rauhut  RW Sadleir  CA Sidor  DD Varricchio  GP Wilson  JA Wilson 《Science (New York, N.Y.)》1998,282(5392):1298-1302

Fossils discovered in Lower Cretaceous (Aptian) rocks in the Tenere Desert of central Niger provide new information about spinosaurids, a peculiar group of piscivorous theropod dinosaurs. The remains, which represent a new genus and species, reveal the extreme elongation and transverse compression of the spinosaurid snout. The postcranial bones include blade-shaped vertebral spines that form a low sail over the hips. Phylogenetic analysis suggests that the enlarged thumb claw and robust forelimb evolved during the Jurassic, before the elongated snout and other fish-eating adaptations in the skull. The close phylogenetic relationship between the new African spinosaurid and Baryonyx from Europe provides evidence of dispersal across the Tethys seaway during the Early Cretaceous.  相似文献   

Tracking the long-term decline and recovery of an isolated population     

RL Westemeier  JD Brawn  SA Simpson  TL Esker  RW Jansen  JW Walk  EL Kershner  JL Bouzat  KN Paige 《Science (New York, N.Y.)》1998,282(5394):1695-1698

Effects of small population size and reduced genetic variation on the viability of wild animal populations remain controversial. During a 35-year study of a remnant population of greater prairie chickens, population size decreased from 2000 individuals in 1962 to fewer than 50 by 1994. Concurrently, both fitness, as measured by fertility and hatching rates of eggs, and genetic diversity declined significantly. Conservation measures initiated in 1992 with translocations of birds from large, genetically diverse populations restored egg viability. Thus, sufficient genetic resources appear to be critical for maintaining populations of greater prairie chickens.  相似文献   

Transduction and Transplantation of Spermatogonia into the Testis of Ram Lambs through the Extra-testicular Rete     

JR Rodriguez-Sosa  JD Silvertown  RA Foster  JA Medin  A Hahnel 《Reproduction in domestic animals》2009,44(4):612-620

Spermatogonial transplantation will provide a new way to study spermatogenesis in domestic animals, disseminate male genetics and produce transgenic animals, if efficiency can be improved. We evaluated a 'surgical' method for transplanting donor cells into testes of ram lambs, where the head of the epididymis is reflected, and a catheter introduced into the extra-testicular rete testis. We also tested transduction of ram spermatogonia with a lentiviral (LV) vector as a means to identify permanent colonization, and introduce genes into donor cells. Eight ram lambs, 11- to 13-week olds, were the recipients: in five, spermatogonia were injected into one testis, and the contralateral testis was an un-manipulated control: in two, spermatogonia were injected into one testis and the contralateral was sham-injected: in one, both testes were injected. Six lambs received spermatogonia labelled with a cell-tracking dye and these were collected 1 or 2 weeks after transplantation; three lambs received spermatogonia transduced with a LV vector driving the expression of enhanced Green Fluorescence Protein and these were collected after 2 months. Donor cells were detected by immunohistochemistry in tubules of seven of nine recipient testes. Approximately 22% of tubule cross-sections contained donor cells immediately after transplantation, and 0.2% contained virally transduced cells 2 months after transplantation. The onset of spermatogenesis was delayed, and there were lesions in both injected and sham-injected testes. Despite the effects of the surgery, elongated spermatids were present in one recipient testis 2 months after surgery. The results suggest that, after modifying the surgical and transduction techniques, this approach will be a means to produce good colonization by donor spermatogonia in sheep testes.  相似文献   

Self-certification for medicines compliance?     

Wray JG 《The Veterinary record》2011,168(1):28; discussion 28

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Computed tomographic findings,surgical management and postoperative outcomes of large intermuscular lipomas in the hindlimb of 11 dogs     

JD Crowley  G Hosgood  NV Crawford  JL Richardson 《Australian veterinary journal》2020,98(4):135-139

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Comparison of Prosthetic Materials for Abdominal Wall Reconstruction in the Presence of Contamination and Infection     

GL Brown  JD Richardson  MA Malangoni  GR Tobin  D. Ackerman  HC Polk 《Veterinary surgery : VS》1985,14(4):306-306

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Efficacy of laidlomycin propionate for increasing rate and efficiency of gain by feedlot cattle     

H R Spires  A Olmsted  L L Berger  J P Fontenot  D R Gill  J G Riley  M I Wray  R A Zinn 《Journal of animal science》1990,68(10):3382-3391

One thousand twenty steers and heifers were used in six feeding trials to examine the influence of laidlomycin propionate on feedlot performance and to determine the most efficacious dietary concentrations of that ionophore. Cattle were fed diets ranging in energy content from 1.08 to 1.49 Mcal NEg/kg of DM. Laidlomycin propionate improved rate of gain and feed conversion in both steers and heifers. Improvements in performance were not evident when laidlomycin propionate was fed at only 3 mg/kg. However, both average daily gain and feed conversion were improved by laidlomycin propionate within the range of 6 to 12 mg/kg of DM (P less than .001). Feed consumption was not substantially affected by inclusion of laidlomycin propionate in the diet. Improvements in ADG and feed conversion were greater on lower-energy diets than on higher-energy diets, but both these performance characteristics were improved regardless of the type of diet fed. Average daily gain was maximized with laidlomycin propionate at 6 mg/kg, whereas improvements in feed conversion were sustained through 12 mg/kg. Carcasses of cattle fed diets containing 6 to 12 mg/kg of laidlomycin propionate weighed 7.3 kg more (P less than .001) than carcasses of cattle fed the control diets. Yield grade and quality grade were not affected by laidlomycin propionate (P greater than .05).  相似文献   

A survey of Western Australian dogs for Sarcocystis spp and other intestinal parasites     

G SA VINI  JD DUNSMORE  ID ROBERTSON 《Australian veterinary journal》1993,70(7):275-276

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