Grazing angle scattering of electromagnetic waves in gratings with varying mean parameters

Abstract

Highly unusual sensitivity of wave scattering in thick periodic gratings to small step-like variations of mean structural parameters at the grating boundaries is predicted and described for the case when the scattered wave (the +1 diffracted order) propagates almost parallel to the front grating boundary (the geometry of GAS — grazing-angle scattering). The analysis is carried out for GAS of bulk TE electromagnetic waves in holographic gratings with varying mean permittivity, and GAS of optical modes guided by a slab with a periodic groove array and varying mean thickness. In particular, it is shown that variations of the mean permittivity by ～0.007% may result in several times variations in the scattered wave amplitude in the grating. At the same time, it is also demonstrated that near the main GAS resonance, the scattered wave amplitudes are only weakly dependent on small variations of mean structural parameters. Physical explanations of the predicted effects are presented. Possible ways of enhancing and, vice versa, reducing (compensating for) the unusual sensitivity of GAS are indicated. Feasible applications of the predicted effects for the design of new highly sensitive sensors and measurement techniques are discussed.     

Interbunch extinction measurement at the BNL AGS for the KOPIO experiment

The KOPIO experiment at the BNL AGS required an extracted proton beam in which a debunched, coasting beam is forced between empty longitudinal RF buckets to form microbunches. The goal of the measurement described here was to obtain a low background determination of the fraction of protons coming from the AGS slowly extracted beam at times between microbunches (interbunch extinction). The effect on interbunch extinction of variations in the RF cavity voltage, the momentum dispersion of the beam and the main guide field voltage-ripple was studied. It was found that there exists a broad range of operating parameters that could allow the AGS to produce microbunches with the interbunch extinction better than the KOPIO experiment requirement. Results obtained in this study gave interbunch extinctions better than 10⁻⁵ (1%) of KOPIO's upper limit.     

Gap modes of one-dimensional photonic crystal surface waves

The finite-difference time-domain method is employed for the analysis of coupling of the surface modes of two truncated one-dimensional photonic crystals separated by a gap. The wave vector, field distributions, and existence conditions of the coupled surface modes are investigated. The wave vector of symmetric gap modes increases with decreasing gap width, while that of antisymmetric modes decreases-exactly opposite of the situation for surface plasmons on metallic half-spaces separated by a dielectric gap. Photonic crystal gap modes could easily and effectively be used as nondissipating gap-mode waveguides.     

微通道中高黏度油墨流体的黏性耗散生热

为了分析胶印机钢辊和橡胶辊间的微通道中高黏度油墨流体的黏性耗散效应,首先对黏性耗散效应的影响因素进行分析,然后运用流体仿真软件Fluent对微尺度的高黏度油墨流体的黏性耗散生热进行了研究.建立了考虑黏性耗散生热的油墨流体仿真模型,获得了油墨流体的压力分布、速度分布及温度分布等特性;通过不同通道尺度、不同油墨黏度的仿真对比分析,结合理论分析,得到了通道尺度和油墨黏度对黏性耗散生热的影响.结果表明:随通道尺度的减小,油墨流场的温度升高,并且流场的最高温度与通道尺度间近似为非线性的指数关系.随油墨黏度的增大,油墨流场的温度升高,并且流场的最高温度与油墨黏度间为近似线性关系.     

Extremely asymmetrical scattering of optical waves in nonuniform periodic Bragg arrays

The extremely asymmetrical scattering (EAS) of bulk and guided electromagnetic waves in nonuniform periodic Bragg arrays with steplike variations of the grating amplitude is analyzed theoretically by means of a recently developed approach based on allowance for the diffractional divergence of the scattered wave. Arrays of finite and infinite widths are investigated. It is shown that, for thin nonuniform arrays, EAS has the same pattern as for uniform arrays with mean grating amplitude. On the contrary, for wide nonuniform arrays, the scattered wave amplitudes are well determined by local values of the grating amplitude. In this case, the energy of the scattered wave is shown to concentrate mainly in regions with smaller grating amplitude. The sensitivity of EAS to small imperfections of periodic arrays is investigated theoretically. The physical explanation of the observed effects is based on the diffractional divergence of the scattered wave.     

微尺度下速度滑移对油墨流动特性的影响

为了了解微尺度下速度滑移对油墨流动特性的影响,以便提高印刷质量,首先从理论方面分析了速度滑移对流体流动特性的影响.运用专业流体仿真软件FLUENT对微尺度下油墨在墨辊间的流动进行了流体计算模拟,分析了无滑移和有滑移2种情况下油墨在传递过程中流动速度与压力的特性,同时提取墨层有效厚度,并进行对比.通过分析可以得出:当滑移速度与油墨流速方向相同时,随着速度滑移程度的提高,在油墨通道的各个位置处油墨的速度都有增大的趋势,压力有减小的趋势,墨层厚度有变大的趋势;当滑移速度与油墨流速方向相反时,随着滑移程度的增强,油墨的速度有减小的趋势,压力有增大的趋势,墨层厚度有变小的趋势.