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排序方式： 共有3695条查询结果，搜索用时 15 毫秒
991.
This paper investigates various types of coded waveforms that could be used for air-coupled ultrasound, using a pulse compression approach to signal processing. These are needed because of the low signal-to-noise ratios that are found in many air-coupled ultrasonic nondestructive evaluation measurements, due to the large acoustic mismatch between air and many solid materials. The various waveforms, including both swept-frequency signals and those with binary modulation, are described, and their performance in the presence of noise is compared. It is shown that the optimum choice of modulation signal depends on the bandwidth available and the type of measurement being made. 相似文献
992.
We present a microscopic theory of parametric polariton amplification in semiconductor microcavities including the influence of exciton–exciton correlations beyond mean‐field. The complexity induced by the Coulomb interaction between electrons determines the noninstantaneous character of exciton–exciton collisions. We find that the observed great enhancement of gain when increasing the polariton splitting origins from the unique interplay of the noninstantaneous nature of exciton–exciton interactions with the strong coupling regime giving rise to polaritons. 相似文献
993.
In this work, we report encouraging results on all‐optical quantitative determinations in binary mixtures of chemical substances using porous silicon technology. Measuring the resonance peak shifts of porous silicon microcavities, after exposure to vapors of several liquid mixtures (iso‐propanol/ethanol, iso‐propanol/methanol, xylene/methanol), we are able to quantify the liquid volume percentage of both component. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
994.
Motivated by the recent attention on superluminal phenomena, we investigate the compatibility between faster-than-c propagation and the fundamental principles of relativity and causality. We first argue that special relativity can easily accommodate—indeed, does not exclude—faster-than-c signaling at the kinematical level. As far as causality is concerned, it is impossible to make statements of general validity, without specifying at least some features of the tachyonic propagation. We thus focus on the Scharnhorst effect (faster-than-c photon propagation in the Casimir vacuum), which is perhaps the most plausible candidate for a physically sound realization of these phenomena. We demonstrate that in this case the faster-than-c aspects are “benign” and constrained in such a manner as to not automatically lead to causality violations. 相似文献
995.
Bertoni-Freddari C Fattoretti P Ricciuti R Vecchioni S Casoli T Solazzi M Ducati A 《Micron (Oxford, England : 1993)》2002,33(5):447-451
We carried out a novel application of the disector sampling and counting method, in a biopsy material from the pathologic human brain, to estimate the synaptic structural dynamics, quantitatively. Parietal cortex biopsies of adult (mean age: 40.0 years) and old (mean age: 66.2 years) patients having undergone surgical intervention were investigated. The tissue samples were excised at the periphery of meningioma masses. Synaptic contact zones were stained en bloc by the ethanol phosphotungstic acid (E-PTA) preferential technique which selectively enhances both the pre- and post-synaptic paramembranous material separated by a sharp cleft against a very faint background, thus facilitating and objectifying synaptic morphometry. The disector method, associated with currently used morphometric formulas, enabled us to measure the number of synapses/m3 of tissue (numeric density: Nv); the total area of the synaptic contact zones/m3 of tissue (surface density: Sv) and the average synaptic size (S). In old vs. adult patients, Nv decreased by 7.5% (Mean (SEM): Adult 2.0040(0.0452); Old 1.6780(0.0623)), while S increased by 17.5% (Adult 0.0203(0.0026); Old 0.0246(0.0035)). Sv did not show any age-related difference. The same negative correlation between Nv and S has also been reported in physiological aging, and this suggests the active presence of age-related synaptic restructuring mechanisms in the nervous tissue surrounding a tumoral mass. 相似文献
996.
Stefano Ossicini 《physica status solidi (a)》1998,170(2):377-390
The optical properties of bulk silicon are deeply modified if the material is manipulated at the nanometre scale. In particular the growth of Si nanostructures constitutes today a promising approach for the development of silicon-based light emitting devices. In this context I discuss theoretical results on the optoelectronic properties of low-dimensional silicon structures, e.g. Si quantum wells, quantum wires, quantum dots. The results are compared with recent experimental data. 相似文献
997.
L. De Stefano D. Alfieri I. Rea L. Rotiroti K. Malecki L. Moretti F. G. Della Corte I. Rendina 《physica status solidi (a)》2007,204(5):1459-1463
In this communication, the design and the fabrication of a silicon-glass microsystem for lab-on-chip sensing applications have been reported. Each component has been realised using wet etch process and the porous silicon (PSi) technology, while the final device has been obtained intimately joining all elements by the Anodic Bonding (AB) process. The flow injection analysis (FIA) principle has been adopted in the microfluidic manipulation of the analyte samples: very small volume of the substances can be quickly and repeatedly monitored. The transducer element of the sensor device is a PSi Fabry–Perot monolayer which allows a very effective interaction with several chemicals and biological molecules. Time resolved measurements revealed very rapid response times in detecting some volatile substances. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
998.
The relative importance of bulk and surface contributions to the second‐harmonic response of isotropic surfaces is addressed by studying the polarization properties of second‐harmonic signals generated by two noncollinear input beams. Measurements on an air‐glass interface reveal that the bulk contributions dominate in transmission but are negligible in reflection. Even when the nonlinear response has a bulk origin, it is strongly suppressed when the interaction volume is localized deep inside the bulk medium. Conversely, bulk contributions are maximized when the interaction volume is localized near the surface. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
999.
We propose a protocol for deterministic communication that does not make use of entanglement. It exploits nonorthogonal states in a two-way quantum channel to attain unconditional security and high efficiency of the transmission. We explicitly show the scheme is secure against a class of individual attacks regardless of the noise on the channel. Its experimental realization is feasible with current technology. 相似文献
1000.
Stefano Marmi 《Meccanica》1988,23(3):139-146
Summary We give a simple proof of Siegel's linearization theorem of germs of complex analytic diffeomorphisms of
N
near a fixed point. The proof leads to realistic bounds when applied to polynomial maps of . Numerical estimates, based on rigorous arguments, are also given. For non-quadratic mappings we find that winding numbers different from the golden mean might determine the stochastic transition.
Sommario Viene data una dimostrazione semplice del teorema di Siegel sulla linearizzazione di germi di diffeomorfismi analitici di N vicino a un punto fisso. La dimostrazione conduce a stime realistiche quando viene applicata a mappe polinomiali di . Vengono date inoltre delle stime numeriche, che si fondano su argomenti rigorosi. Si trova che per mappe non quadratiche numeri di notazione diversi dal numero d'oro potrebbero determinare la transizione al caos.相似文献