Quasi-periodic Microstructure Research Articles

DETECTION OF POLARIZED QUASI-PERIODIC MICROSTRUCTURE EMISSION IN MILLISECOND PULSARS

ABSTRACT Microstructure emission, involving short timescale, often quasi-periodic, intensity fluctuations in subpulse emission, is well known in normal period pulsars. In this Letter, we present the first detections of quasi-periodic microstructure emission from millisecond pulsars (MSPs), from Giant Metrewave Radio Telescope observations of two MSPs at 325 and 610 MHz. Similar to the characteristics of microstructure observed in normal period pulsars, we find that these features are often highly polarized and exhibit quasi-periodic behavior on top of broader subpulse emission, with periods of the order of a few μs. By measuring their widths and periodicities from single pulse intensity profiles and their autocorrelation functions, we extend the microstructure timescale–rotation period relationship by more than an order of magnitude down to rotation periods ∼5 ms, and find it to be consistent with the relationship derived earlier for normal pulsars. The similarity of behavior is remarkable, given the significantly different physical properties of MSPs and normal period pulsars, and rules out several previous speculations about the possible different characteristics of microstructure in MSP radio emission. We discuss the possible reasons for the non-detection of these features in previous high time resolution MSP studies along with the physical implications of our results, both in terms of a geometric beam sweeping model and temporal modulation model for micropulse production.

Comparative study of periodicity estimation methods using ultrasonic signals

Introduction Various signal-processing techniques have been proposed to extract quantitative information about internal structures of tissues from the original radio frequency (RF) signals instead of an ultrasound image. The quantifiable parameter called the mean scatterer spacing (MSS) can be useful to detect changes in the quasi-periodic microstructure of tissues such as the liver or the spleen, using ultrasonic signals. Methods We evaluate and compare the performance of three classic methods of spectral estimation to calculate the MSS without operator intervention: Tufts-Kumaresan, SAC (Spectral Autocorrelation) and MUSIC (MUltiple SIgnal Classification). Initially the evaluations were performed with 10,000 signals simulated from a model in which the variables of interest are controlled, and then, real signals from sponge phantoms were used. Results For the simulated signals, the performance of all three methods decreased with increasing Ad or jitter levels. For the sponges, none of the methods accurately estimated the pore size. Conclusion For the simulated signals, Tufts-Kumaresan had the lowest performance, whereas SAC and MUSIC had similar results. For sponges, only Tufts-Kumaresan was able to detect the increase in the size of the pores of the sponge, although most often, it estimated sizes larger than expected.

An investigation into wire electrochemical micro machining of pure tungsten

Tungsten-based microstructures have attracted great interest in many industrial advanced applications. Nevertheless, with a disadvantageous combination of high hardness, toughness and brittleness, the micro machining of pure tungsten poses significant difficulty. In this paper, an investigation into the wire electrochemical micro machining (WEMM) of pure tungsten at low alkaline electrolyte concentration and small pulse duration is presented. Under the optimal machining conditions, tungsten-based microstructures with a side gap of 4μm, slit width of 18μm and aspect ratio of 5.6, as well as with a side gap of 5μm, slit width of 20μm and aspect ratio of 15, were obtained. In order to improve productivity in the machining of multi-slit microstructures, multi-wire electrochemical micro machining of tungsten was introduced. Using a 3-wire electrode, a 9-slit microstructure with a slit width of approximately 24μm was produced and the machining efficiency was improved by a factor of three. The results revealed that it was a promising method for the fabrication of tungsten-based periodic or quasi-periodic microstructures, such as the gratings used in the X-ray absorption contrast system of imaging.

斐波纳契数列准周期微结构光吸收特性

斐波纳契数列准周期微结构光吸收特性

Laser-Induced Formation of Periodic Structures on the Metal Surfaces and Surface Plasmons Excitation

Quasiperiodic microstructures are formed on the surfaces of metals under irradiation with high-power femtosecond laser pulses. Interpretation of microstructures as a result of interference of the incident plane wave and surface waves leads to the logical conclusion about the relationship of dislocations in the interference fringes with optical vortices in surface wave. Other peculiarities observed in these structures contain different periods and nanogranular fine structure. It is demonstrated that such laser-induced structures can find applications for surface plasmon excitation and surface enhanced Raman scattering.

Manifestation of optical vortices on the surface of solids under irradiation with femtosecond laser pulses

Quasi-periodic microstructures containing dislocations are formed on the surfaces of metals and semiconductors under irradiation with high-power femtosecond laser pulses. Interpretation of microstructures as a result of interference of the incident plane wave and surface waves leads to the logical conclusion about the relationship of dislocations in the interference fringes with optical vortices in surface wave.

Strong infrared absorber: surface-microstructured Au film replicated from black silicon

With quasi-periodic microstructures, great enhancement of infrared light absorption of Au film over a broad wavelength band (2.7~15.1 μm) was realized experimentally for the first time. The microstructured Au film was prepared by replica molding of the surface of femtosecond (fs) laser microstructured silicon (black silicon). This unique absorption characteristic is mainly ascribed to good impedance match from free space to Au film. The surface of the sample was examined by X-ray photoelectron spectroscopy (XPS) and the four peaks of absorptance were ascribed to residual polydimethylsiloxane (PDMS), H2SO4, adsorbed water and CO2 in the air, respectively.

Tweed and Nanoscale Cubic / Tetragonal Phase Mixtures in Decomposing Alloys: the Pseudospinodal Concept

In this paper the concept of pseudospinodal decomposition introduced by Ni and Khachaturyan [1] as a symmetry-lifting continuous phase separation, which can produce coherent nanoscale morphologies ranging from nanowires to nanolaminates, is reviewed. The term spinodal arises from the continuous change in the compositions of emerging cubic and tetragonal phases resulting in quasi-periodic microstructures stemming from the attendant transformation strain and surface energy anisotropies. It is argued here that important features of the pseudospinodal mechanism can be understood in terms of conventional classical and non-classical nucleation and that the behaviour is more general than the cubic → tetragonal transformation context articulated by its authors. Also, the possible relevance of the pseudospinodal mechanism to studies of decomposition of hypostoichiometric Fe-Pd alloys will be presented.

Fine-scale density wave structure of Saturn's rings: A hydrodynamic theory

Aims. We examine the linear stability of the Saturnian ring disk of mutually gravitating and physically colliding particles with special emphasis on its fine-scale ∼100 m density wave structure, that is, almost regularly spaced, aligned cylindric density enhancements and optically-thin zones with the width and the spacing between them of roughly several tens particle diameters. Methods. We analyze the Jeans’ instabilities of gravity perturbations (e.g., those produced by a spontaneous disturbance) analytically by using the Navier-Stokes dynamical equations of a compressible fluid. The theory is not restricted by any assumptions about the thickness of the system. We consider a simple model of the system consisting of a three-dimensional ring disk that is weakly inhomogeneous and whose structure is analyzed by making a horizontally local short-wave approximation. Results. We demonstrate that the disk is probably unstable and that gravity perturbations grow effectively within a few orbital periods. We find that self-gravitation plays a key role in the formation of the fine structure. The predictions of the theory are compared with observations of Saturn’s rings by the Cassinispacecraft and are found to be in good agreement. In particular, it appears very likely that some of the quasi-periodic microstructures observed in Saturn’s A and B rings ‐ both axisymmetric and nonaxisymmetric ones ‐ are manifestations of these effects. We argue that the quasi-periodic density enhancements revealed in Cassini data are flattened structures, with a height to width ratio of about 0.3. One should analyze high-resolution of the order of 10 m data acquired for the A and B rings (and probably C ring as well) to confirm this prediction. We also show that the gravitational instability is a potential cluster-forming mechanism leading to the formation of porous 100-m-diameter moonlets of preferred mass ∼10 7 g each embedded in the outer A ring, although this has yet to be directly measured.

Realization of periodic and quasiperiodic microstructures with sub-diffraction-limit feature sizes by far-field holographic lithography

The authors experimentally demonstrate a far-field holography for the realization of Ag nanoparticles-embedded periodic and quasiperiodic microstructures with feature sizes beyond the diffraction limit. Periodic cylindrical nanoshell arrays with about 240nm hole diameter and 12-fold symmetry quasiperiodic structures with 220nm feature sizes are achieved, respectively, by using a 632.8nm laser beam. Our results imply that conventional far-field optical technology is capable of fabricating nanostructures in modern micromanufacture.

Singular Spectrum Analysis Applied to Backscattered Ultrasound Signals from In Vitro Human Cancellous Bone Specimens

Mean scatterer spacing (MSS) holds particular promise for the detection of changes in quasiperiodic tissue microstructures such as may occur during development of disease in the liver, spleen, or bones. Many techniques that may be applied for MSS estimation (temporal and spectral autocorrelation, power spectrum and cepstrum, higher order statistics, and quadratic transformation) characterize signals that contain a mixture of periodic and nonperiodic contributions. In contrast, singular spectrum analysis (SSA), a method usually applied in nonlinear dynamics, first identifies components of signals corresponding to periodic structures and, second, identifies dominant periodicity. Thus, SSA may better separate periodic structures from nonperiodic structures and noise. Using an ultrasound echo simulation model, we previously demonstrated SSA's potential to identify MSS of structures in quasiperiodic scattering media. The current work aims to observe the behavior of MSS estimation by SSA using ultrasound measurements in phantom materials (two parallel, nylon-line phantoms and four foam phantoms of different densities). The SSA was able to estimate not only the nylon-line distances but also nylon-line thickness. The method also was sensitive to the average pore-size differences of the four sponges. The algorithms then were applied to characterize human cancellous bone microarchitectures. Using 1-MHz center-frequency, radio-frequency ultrasound signals, MSS was measured in 24 in vitro bone samples and ranged front 1.0 to 1.7 mm. The SSA MSS estimates correlate significantly to MSS measured independently front synchrotron microtomography, r/sup 2/=0.68. Thus, application of SSA to backscattered ultrasound signals seems to be useful for providing information linked to tissue microarchitecture that is riot evident from clinical images.

Singular spectrum analysis applied to backscattered ultrasound signals from in vitro human cancellous bone specimens

Mean scatterer spacing (MSS) holds particular promise for the detection of changes in quasiperiodic tissue microstructures such as may occur during development of disease in the liver, spleen, or bones. Many techniques that may be applied for MSS estimation (temporal and spectral autocorrelation, power spectrum and cepstrum, higher order statistics, and quadratic transformation) characterize signals that contain a mixture of periodic and nonperiodic contributions. In contrast, singular spectrum analysis (SSA), a method usually applied in nonlinear dynamics, first identifies components of signals corresponding to periodic structures and, second, identifies dominant periodicity. Thus, SSA may better separate periodic structures from nonperiodic structures and noise. Using an ultrasound echo simulation model, we previously demonstrated SSA's potential to identify MSS of structures in quasiperiodic scattering media. The current work aims to observe the behavior of MSS estimation by SSA using ultrasound measurements in phantom materials (two parallel, nylon-line phantoms and four foam phantoms of different densities). The SSA was able to estimate not only the nylon-line distances but also nylon-line thickness. The method also was sensitive to the average pore-size differences of the four sponges. The algorithms then were applied to characterize human cancellous bone microarchitectures. Using 1-MHz center-frequency, radio-frequency ultrasound signals, MSS was measured in 24 in vitro bone samples and ranged from 1.0 to 1.7 mm. The SSA MSS estimates correlate significantly to MSS measured independently from synchrotron microtomography, r2 = 0.68. Thus, application of SSA to backscattered ultrasound signals seems to be useful for providing information linked to tissue microarchitecture that is not evident from clinical images.

Quasi-periodic structure optimization of the torsional rigidity

The composite materials with quasi-periodic microstructure play an important role in optimal structural design. In this paper we study the torsional rigidity of an ε-perforated material, subjected to a volume constraint. It is proved that for any ε>0 there exist a distribution of the holes which maximizes the torsional rigidity functional. Also, we prove the existence of an optimal control of the material governed by the system obtained by homogenization.The main result is that when ε→0 the torsional rigidity of the optimal ε-perforated material tends to the torsional rigidity of the optimal distribution of the macroscopic material.

On elastic relaxation and long wavelength microstructures in spinodally decomposed InxGa1−x.AsyP1−yepitaxial layers

Abstract The origin of the long-wavelength (100–300 nm) quasi-periodic microstructure observed in transmission electron microscopy studies of spinodally decomposed In Ga1−x As y P1−y alloys has been investigated. The contrast is well explained by diffraction effects arising from lattice plane bending near the surfaces of the thinned specimens, such as would be induced by elastic relaxation of shear stresses accompanying a quasi-periodic lattice modulation. Excellent qualitative agreement between calculated and experimental contrasts lends weight to the claim that these contrasts, observed over part of the composition range of liquid phase epitaxy grown InxGa1−xAs y P1−y alloys, are associated with spinodal decomposition. The accompanying strain modulation amplitude is of the order of 10−3. It is suggested that speckle contrast on a smaller scale (5–15 nm) in this material may also be related to composition variations. More generally, the nature of near-surface elastic relaxation in compositionally modulat...

Correlation analyses of microstructure and noiselike intensity fluctuations from pulsar 2016 + 28

Correlation analyses of pulses obtained at 430 MHz with 12 ..mu..s resolution show that the pulsar signal is accurately described as amplitude-modulated Gaussian noise. In the average autocorrelation function of many pulses, microstructure decorrelates at a 0.29 ms lag and is quasiperiodic with a 0.9 ms period. The microstructure in individual subpulses is usually quasi-periodic with a period that is usually in the range 0.6 to 1.1 ms. Microstructure that occasionally has periods between 0.3 and 0.5 ms tends to be weaker and is of shorter duration than the longer period microstructure. The hypothesis of D.C. Backer that there is a pulse to pulse correlation of the microstructure along a band of drifting subpulses was tested by computing cross-correlation functions of successive subpulses. Although average cross-correlations show no evidence of any such correlation, it does appear that the period of the quasi-periodic microstructure exhibits some pulse to pulse correlation in a few bands of subpulses. In general, the period changes markedly from pulse to pulse and between the subpulses in the same pulse. It is therefore implausible that stellar vibration, which has a 1 ms period and a 1 s damping time for a neutron star, is the cause ofmore » microstructure periodicities.« less