Scaling behavior of the first arrival time of a random-walking magnetic domain

We report a universal scaling behavior of the first arrival time of a traveling magnetic domain wall into a finite space-time observation window of a magneto-optical microscope enabling direct visualization of a Barkhausen avalanche in real time. The first arrival time of the traveling magnetic domain wall exhibits a nontrivial fluctuation and its statistical distribution is described by universal power-law scaling with scaling exponents of 1.34+/-0.07 for CoCr and CoCrPt films, despite their quite different domain evolution patterns. Numerical simulation of the first arrival time with an assumption that the magnetic domain wall traveled as a random walker well matches our experimentally observed scaling behavior, providing an experimental support for the random-walking model of traveling magnetic domain walls.     

Properties of antennas focused in the Fresnel zone

The paper deals with the calculation of the electric field strength distribution of a circular antenna focused in the Fresnel zone. To express the electric field strength distribution ⁿ W _v–1 (w, x) functions defined by Lommel functions of two variables are used. On the basis of the calculated electric field strength distribution the fundamental properties of an antenna focused in the Fresnel zone are determined. The width and depth of the focused beam are calculated and the results of the numerical computations are plotted. Formulae for determining the wavelengths in the focused beam are expressed as a function of the circular aperture illumination.The author would like to express his gratitude to J. atlov, CSc, and F. áek for their kind discussion.     

On the Cherenkov radiation in the Fresnel zone for finite charge track

First- and second-order corrections to the Tamm formula are obtained at small ratios of the point charge track length in the case of Cherenkov radiation to the distance to the reception point for the finite track in the Fresnel zone. The result is discussed in the context of actual conditions of the LORD lunar experiment on measurements of radio emission from cascades produced by neutrinos and cosmic rays near the lunar surface.     

微聚焦菲涅耳波带板聚焦特性研究

通过激光轰击Ti平面靶,用微聚焦菲涅尔波带板做成像器,测到了在放大倍数为66倍时X射线焦斑图像.利用Fresnel-Kirchhoff衍射积分公式数值模拟了微聚焦菲涅尔波带板的点扩展函数,模拟结果表明该微聚焦菲涅尔波带板在两倍焦距处强聚焦.改变物距和像距但保持透镜的物像距公式,也可得到类似的结果.模拟和实验表明微聚焦波带板可以应用于X射线点对点成像,实现激光等离子体X射线高空间分辨成像. 关键词： 菲涅尔波带板 Fresnel-Kirchhoff衍射 数值模拟 点扩展函数     

Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10(-2) . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.     

A study of the image contrast in Fresnel zone plate imaging of extended sources

The image contrast is experimentally demonstrated to be a particular function of various parameters including the area of the source divided by the area of the central Fresnel zone. Over 150 images were analyzed and the best fit parameters are reported. It is also shown that the best contrast is obtained by a new technique in which the projection transparency is photographically reversed before image reconstruction.     

Time of arrival of the first relativistic solar protons to the Earth

The anticoincidence system of the spectrometer onboard the INTEGRAL spacecraft (ACS SPI) is sensitive to primary and secondary γ-rays with energies higher than 150 keV and effectively responds to the arrival of solar protons and electrons. When the flux of primary γ-rays is small, ACS SPI registers the arrival of solar protons much earlier than the network of neutron monitors (NMs). For example, on December 13, 2006, ACS SPI registered the arrival of solar protons approximately 10 min earlier than the CR intensity started increasing on the Earth according to the NM data. However, when the flux of solar γ-rays is sufficiently high, ACS SPI observes the arrival of solar protons simultaneously with NMs (the event of January 20, 2005) or later (October 28, 2003). Found times of arrival of relativistic solar protons to the Earth do not contradict proton acceleration during γ-ray flares.     

Fourier treatment to the diffractive optical element composed of Fresnel zone and pinholes

Currently, the minimum feature size fabricated is about 15-40 nm. The resolution of Fresnel zone plates is limited by the width of the outermost zone. Although the photon sieves make it possible to focus soft X-rays to spot sizes smaller than the diameter of the smallest pinhole they have low diffraction efficiency. In order to foster strengths and circumvent weaknesses of them, we propose a new model which is composed of Fresnel zone and pinholes and give its fast Fourier algorithm. The simulation results demonstrate that the resolution of photon sieves is not better than that of Fresnel zone plates in low order of local rings. In this case, we can substitute pinholes for Fresnel zone in photon sieves. Resolution is nearly a constant, and also the diffraction efficiency improves.     

Probabilistic time and the quantum gravity interpretation

We analyze the perturbed minisuperspace models of quantum gravity through the analogy with the time-independent Schrödinger equation. We show that a time variable defined in a previous work, the probabilistic time, is the variable which yields the backreaction Einstein equations.     

基于哈达玛积扩展子空间的到达时间和波达方向联合估计

在窄带阵列天线正交频分复用系统的到达时间和波达方向联合估计中, 针对阵元数目较少时波达方向估计精度不高, 特别是多径数目大于阵元数目导致的波达方向无法估计问题, 提出一种基于哈达玛积扩展子空间的到达时间和波达方向联合估计算法. 该算法首先利用各阵元上的频域信道估计构成扩展信道频域响应矢量, 然后计算扩展信道频域响应矢量自相关矩阵, 并进行特征值分解得到哈达玛积扩展噪声子空间, 最后构造伪谱函数并进行二维谱峰搜索, 从而实现到达时间和波达方向的联合估计. 仿真结果表明, 与现有算法相比, 在复杂度没有大幅提高的前提下, 该算法的估计结果均方根误差更加接近克拉美罗界, 且到达时间和波达方向估计能够自动配对, 在多径数目大于阵元数目时依然适用.     

Particle detection and non-detection in a quantum time of arrival measurement

The standard time-of-arrival distribution cannot reproduce both the temporal and the spatial profile of the modulus squared of the time-evolved wave function for an arbitrary initial state. In particular, the time-of-arrival distribution gives a non-vanishing probability even if the wave function is zero at a given point for all values of time. This poses a problem in the standard formulation of quantum mechanics where one quantizes a classical observable and uses its spectral resolution to calculate the corresponding distribution. In this work, we show that the modulus squared of the time-evolved wave function is in fact contained in one of the degenerate eigenfunctions of the quantized time-of-arrival operator. This generalizes our understanding of quantum arrival phenomenon where particle detection is not a necessary requirement, thereby providing a direct link between time-of-arrival quantization and the outcomes of the two-slit experiment.     

On the simultaneous measurement of the energy and arrival time of a photon

The simultaneous measurement of the energy and arrival time of a single-photon packet, as well as the experimental implementation of such a measurement with the use of a parametric transformation upward in the energy (up conversion), is proposed. Original Russian Text ? S.N. Molotkov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 12, pp. 901–906.     

On the simultaneous measurement of the energy and arrival time of a photon

The simultaneous measurement of the energy and arrival time of a single-photon packet, as well as the experimental implementation of such a measurement with the use of a parametric transformation upward in the energy (up conversion), is proposed.     

In-plane attenuation zone and its optimization in a plate with periodic holes

It is difficult to design a plate with wide attenuation zones in low frequency region based on Bragg scattering mechanism. A plate with periodic rhombic holes is optimized and designed. Based on the finite element method under periodic boundary conditions, the in-plane dispersion curves of periodically perforated plate are calculated via COMSOL. The frequency responses of periodically perforated plate axe investigated via ANSYS simulation. The plates with periodic holes are suspended and dynamically tested under sinusoidal excitations. The results show that a periodically perforated plate with rhombus holes has wider attenuation zones than plates with circular and hexagonal holes. Material properties have a great influence on attenuation zones: nitrile-butadiene rubber and silicon rubber can easily obtain low-frequency zones, while increasing porosity creates lower and wider zones. The width of attenuation zone increases as the horizontal angle of the rhombus holes increases. An analysis of the attenuation zones* generation mechanism shows that the plate with periodic holes exhibits characteristics of Bragg scattering and local resonant phononic crystal, indicating an inherent relationship between two types of mechanisms. The optimized plate has a complete attenuation zone ranging from 5281.76-8824.30 Hz. The vibrations are significantly reduced when the number of periods is no less than two. The attenuation range obtained via the numerical method is generally consistent with the experiment. The research sheds light on the noise insulation plate and has the potential to improve the sound environment in various applications due to its simple manufacturing.     

A particle filtering approach for spatial arrival time tracking in ocean acoustics

The focus of this work is on arrival time and amplitude estimation from acoustic signals recorded at spatially separated hydrophones in the ocean. A particle filtering approach is developed that treats arrival times as "targets" and tracks their "location" across receivers, also modeling arrival time gradient. The method is evaluated via Monte Carlo simulations and is compared to a maximum likelihood estimator, which does not relate arrivals at neighboring receivers. The comparison demonstrates a significant advantage in using the particle filter. It is also shown that posterior probability density functions of times and amplitudes become readily available with particle filtering.     

Refracted arrival waves in a zone of silence from a finite thickness mixing layer

Refracted arrival waves which propagate in the zone of silence of a finite thickness mixing layer are analyzed using geometrical acoustics in two dimensions. Here, two simplifying assumptions are made: (i) the mean flow field is transversely sheared, and (ii) the mean velocity and temperature profiles approach the free-stream conditions exponentially. Under these assumptions, ray trajectories are analytically solved, and a formula for acoustic pressure amplitude in the far field is derived in the high-frequency limit. This formula is compared with the existing theory based on a vortex sheet corresponding to the low-frequency limit. The analysis covers the dependence on the Mach number as well as on the temperature ratio. The results show that both limits have some qualitative similarities, but the amplitude in the zone of silence at high frequencies is proportional to omega(-1/2), while that at low frequencies is proportional to omega(-3/2), omega being the angular frequency of the source.