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Linear Polarization (linear + polarization)
Selected AbstractsLinear polarization scans for resonant X-ray diffraction with a double-phase-plate configurationJOURNAL OF SYNCHROTRON RADIATION, Issue 6 2009Valerio Scagnoli An in-vacuum double-phase-plate diffractometer for performing polarization scans combined with resonant X-ray diffraction experiments is presented. The use of two phase plates enables the correction of some of the aberration effects owing to the divergence of the beam and its energy spread. A higher rate of rotated polarization is thus obtained in comparison with a system with only a single retarder. Consequently, thinner phase plates can be used to obtain the required rotated polarization rate. These results are particularly interesting for applications at low energy (e.g. 4,keV) where the absorption owing to the phase plate(s) plays a key role in the feasibility of these experiments. Measurements by means of polarization scans at the uranium M4 edge on UO2 enable the contributions of the magnetic and quadrupole ordering in the material to be disentangled. [source] Linear polarization in small radio sourcesASTRONOMISCHE NACHRICHTEN, Issue 5-6 2006A. Rossetti Abstract Polarimetric observations at 5 and 8.4 GHz have been performed for 3 CSS quasars. These observations are part of a programme aimed at studying the polarization characteristics of the core regions, jets, and lobes in such objects and at possibly finding of jet-cloud interactions in CSSs. The behaviour of the fractional polarization and of the observed polarization angle as a function of ,2 provides information on the density distribution of the ISM embedding the radio source, on its clumpiness and on the ordered and random components of the interstellar magnetic field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] An ellipticity criterion in magnetotelluric tensor analysisGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2004M. Becken SUMMARY We examine the magnetotelluric (MT) impedance tensor from the viewpoint of polarization states of the electric and magnetic field. In the presence of a regional 2-D conductivity anomaly, a linearly polarized homogeneous external magnetic field will generally produce secondary electromagnetic fields, which are elliptically polarized. If and only if the primary magnetic field vector oscillates parallel or perpendicular to the 2-D structure, will the horizontal components of the secondary fields at any point of the surface also be linearly polarized. When small-scale inhomogeneities galvanically distort the electric field at the surface, only field rotations and amplifications are observed, while the ellipticity remains unchanged. Thus, the regional strike direction can be identified from vanishing ellipticities of electric and magnetic fields even in presence of distortion. In practice, the MT impedance tensor is analysed rather than the fields themselves. It turns out, that a pair of linearly polarized magnetic and electric fields produces linearly polarized columns of the impedance tensor. As the linearly polarized electric field components generally do not constitute an orthogonal basis, the telluric vectors, i.e. the columns of the impedance tensor, will be non-orthogonal. Their linear polarization, however, is manifested in a common phase for the elements of each column of the tensor and is a well-known indication of galvanic distortion. In order to solve the distortion problem, the telluric vectors are fully parametrized in terms of ellipses and subsequently rotated to the coordinate system in which their ellipticities are minimized. If the minimal ellipticities are close to zero, the existence of a (locally distorted) regional 2-D conductivity anomaly may be assumed. Otherwise, the tensor suggests the presence of a strong 3-D conductivity distribution. In the latter case, a coordinate system is often found, in which three elements have a strong amplitude, while the amplitude of the forth, which is one of the main-diagonal elements, is small. In terms of our ellipse parametrization, this means, that one of the ellipticities of the two telluric vectors approximately vanishes, while the other one may not be neglected as a result of the 3-D response. The reason for this particular characteristic is found in an approximate relation between the polarization state of the telluric vector with vanishing ellipticity and the corresponding horizontal electric field vector in the presence of a shallow conductive structure, across which the perpendicular and tangential components of the electric field obey different boundary conditions. [source] Instrument-independent specification of the diffraction geometry and polarization state of the incident X-ray beamJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2009Marc Schiltz This work augments the proposal of Schwarzenbach & Flack [J. Appl. Cryst. (1989), 22, 601,605], who have advocated the use of a diffractometer-independent definition of the azimuthal angle , to specify the diffraction geometry of a Bragg reflection. It is here proposed that one additional angle ,, which is also based on a diffractometer-independent definition, is needed to encode the direction of linear polarization for those experiments where this quantity is of importance. This definition is then extended to the cases of partially and/or elliptically polarized X-ray beams, and the use of three normalized Stokes parameters, P1, P2 and P3, together with ,, is advocated in order to characterize exhaustively the polarization state of the incident beam. The conventions proposed here present a general, unambiguous and economical means of encoding the information about the diffraction geometry, without the need to record any further information about the instrument, crystal orientation matrix and goniometer angles. Data-processing software using these definitions to analyse polarization-dependent phenomena becomes instrument-independent and completely general. These methods have been implemented in the macromolecular phasing program SHARP for exploiting the polarization anisotropy of anomalous scattering in protein crystals. [source] Single linearly polarized, widely tunable Yb3+ -doped fiber laser with alternative polarization and wavelengthLASER PHYSICS LETTERS, Issue 2 2010D.-F. Liu Abstract A novel single linearly polarized, widely tunable Yb3+ -doped fiber laser with alternative two orthogonal polarization (wavelength) laser beams was proposed and demonstrated experimentally. Using a polarization beam splitting (PBS) as the separating of the two tunable laser beams in a linear Yb3+ -doped fiber cavity, the laser generates two alternatively tunable wavelength laser. Beam 1 is from 1001.9 nm to 1083.4 nm with single linear polarization and 368 mW (at 1030.1 nm) output power. Beam 2 is from 1000.0 nm to 1082.5 nm with 395 mW (at 1018.4 nm) output power and orthogonally polarized with beam 1. In addition, the laser can also work in widely tunable two wavelengths oscillating simultaneously. (© 2010 by Astro Ltd., Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] GeV-acceleration of electron by a superintense ultrashort laser pulseLASER PHYSICS LETTERS, Issue 8 2004A. Bahari Abstract New mechanism of laser acceleration of a charged particle is discovered and explained. Particle acceleration with focused beam of superintense ultrashort laser pulse is determined by a combination of ponderomotive forces at rising and falling edges of laser pulse and a longitudinal component of laser electric field. We found that acceleration of electron, which moves along the laser wavevector, is crucially depends on whether or not the electron reaches the region z , zR behind the laser focus (here zR is the Rayleigh length). Interpretation of this effect consists in that the laser longitudinal electric field at the electron trajectory in this region is a unidirectional one (oscillatory in the case of laser linear polarization and slowly varying in the case of laser circular polarization). Due to this effect it is possible to overcome the negative influence of a phase slippage in the particle- wave interaction, which substantially suppresses electron acceleration. We revealed also that the physical reason of a unidirectional influence of laser longitudinal electric field on accelerating electron in the region z , zR consists in the difference in phase velocities of transverse and longitudinal components of a focused laser field. Owing to this mechanism, lasers of ultimate present-day parameters enable electron acceleration up to the energy , , 1 GeV. Moreover, electron acceleration along the laser wavevector (in contrast to techniques currently considered) is not sensitive to field initial phase (there is no bunch effect), it is possible to accelerate slow electrons (electrons need not to be preaccelerated to relativistic velocities), and there are no problems with a removal of accelerated electron from the laser field. (© 2004 by ASTRO, Ltd. Published exclusively by WILEY-VCH Verlag GmbH & Co. KGaA) [source] Testing procedure to obtain reliable potentiodynamic polarization curves on type 310S stainless steel in alkali carbonate meltsMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 4 2006S. Frangini Abstract Potentiodynamic polarization measurements have been employed to evaluate the anodic behavior of a type 310S stainless steel in the eutectic Li/K molten carbonate. In general, the electrochemical tests yield useful information to predict the stability of the oxide films formed on the surface at the initial period of corrosion, although some precaution is required in the testing procedure as the reproducibility of results is seen to be adversely affected by the passage of large currents. Especially when the steel is in a passive state, erratic results are easily observed if the corrosion layer is being damaged by uncontrolled large currents. This is because the acid-base properties of the melt are susceptible to deep changes by applied currents in the milli-ampere range resulting in hysteresis phenomena in the polarization plot. Hysteresis is caused, on one hand, by acidic dissolution of the passive layer at high anodic currents and, on the other hand, by increased melt basicity due to oxide ion build-up at high cathodic currents. An optimized testing procedure is therefore suggested that minimizes these effects by imposing a 2 mA/cm2 threshold current during polarization measurements. Moreover, the conditions for the applicability of the linear polarization technique to estimate kinetic parameters have been discussed in relationship with the corrosion mechanisms analysed by impedance spectra. It is concluded that the presence of diffusional impedance terms and formation of surface resistive films in molten carbonates may result in not reliable polarization resistance values obtained with the linear polarization. [source] Electrochemical impedance spectroscopy study of surface films formed on copper in aqueous environmentsMATERIALS AND CORROSION/WERKSTOFFE UND KORROSION, Issue 9 2005A. Srivastava Abstract The electrochemical behavior of pure copper has been studied in aqueous environments using linear polarization and electrochemical impedance spectroscopy (EIS) techniques as a function of immersion time. The effect of pollutants (like chloride, sulfide and ammonium ions) on the nature of films formed on the copper surface has been studied. All the surfaces revealed the presence of a porous oxide layer. The corrosion resistance decreased with increasing amount of chloride ions. The addition of Na2S in the environments in the absence of any chloride species was beneficial for corrosion resistance. EIS data suggested that the capacitance of the films formed in chloride environments was higher. The surfaces obtained in presence of chloride ions were relatively rough and deeply attacked. The total impedance decreased after 432 h of immersion in solutions possessing chloride ions. The damaging role of chloride ions and the relatively less severe effect of sulfide ions were noted. [source] Asteroid photometric and polarimetric phase curves: Joint linear-exponential modelingMETEORITICS & PLANETARY SCIENCE, Issue 12 2009K. Muinonen Here we model the two phase curves jointly at phase angles ,25° using a linear-exponential model, accounting for the opposition effect in disk-integrated brightness and the negative branch in the degree of linear polarization. We apply the MCMC methods to V-band phase curves of asteroids 419 Aurelia (taxonomic class F), 24 Themis (C), 1 Ceres (G), 20 Massalia (S), 55 Pandora (M), and 64 Angelina (E). We show that the photometric and polarimetric phase curves can be described using a common nonlinear parameter for the angular widths of the opposition effect and negative-polarization branch, thus supporting the hypothesis of common physical mechanisms being responsible for the phenomena. Furthermore, incorporating polarimetric observations removes the indeterminacy of the opposition effect for 1 Ceres. We unveil a trend in the interrelation between the enhancement factor of the opposition effect and the angular width: the enhancement factor decreases with decreasing angular width. The minimum polarization and the polarimetric slope at the inversion angle show systematic trends when plotted against the angular width and the normalized photometric slope parameter. Our new approach allows improved analyses of possible similarities and differences among asteroidal surfaces. [source] A shorted square-ring slot antenna with a branched slot for the 1575 MHz and 2.4 GHz dual-band operationsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 2 2009Chin-Long Tsai Abstract A shorted square-ring slot antenna with a branched slot is presented for the 1575 MHz and 2.4 GHz dual-band operations. This proposed antenna can be designed by two steps. First, the shorted square-ring slot antenna is designed for circular polarization in the GPS band. Then, a branched slot is added to the original structure to result linear polarization in the 2.4 GHz band. Because the added branched slot almost has no influence in the GPS band's characteristics, the design rule is very simple. The impedance and axial ratio bandwidths for the GPS band is 23.3% and 4.2%, respectively. Besides, the AR and antenna gain is 1.23 dB and 2.3 dBic at the frequency of 1575 MHz. The impedance bandwidth for the 2.4 GHz band is 4.5% and antenna gain at the center frequency of 2.445 GHz is 2.8 dBi. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 402,405, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24073 [source] Analysis of circular waveguides with soft and hard surfaces realized by strip-loaded walls using asymptotic boundary conditionsMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2001Ahmed A. Kishk Abstract Soft and hard walls are realized for a circular waveguide with walls coated by a uniform dielectric layer of appropriate thickness and loaded with conducting strips. The strips are oriented along the waveguide axis (z -axis) for hard surfaces or ,-directed for soft surfaces in a periodic form. The simple approximate asymptotic strip boundary condition (ASBC) is used to analyze the structure. For the dominant mode, when the waveguide walls are hard or soft, a linear field distribution across the waveguide cross section can be obtained. Then the waveguide can be used as an antenna with good linear polarization. The characteristics of such waveguides are presented with sample radiation patterns as compared with hollow waveguide radiation patterns. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 433,436, 2001. [source] Probing the circumstellar structure of Herbig Ae/Be starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002Jorick S. Vink Abstract We present H, spectropolarimetry observations of a sample of 23 Herbig Ae/Be stars. A change in the linear polarization across H, is detected in a large fraction of the objects, which indicates that the regions around Herbig stars are flattened (disc-like) on small scales. A second outcome of our study is that the spectropolarimetric signatures for the Ae stars differ from those of the Herbig Be stars, with characteristics changing from depolarization across H, in the Herbig Be stars, to line polarizations in the Ae group. The frequency of depolarizations detected in the Herbig Be stars (seven out of 12) is particularly interesting as, by analogy with classical Be stars, it may be the best evidence to date that the higher-mass Herbig stars are surrounded by flattened structures. For the Herbig Ae stars, nine out of 11 show a line polarization effect that can be understood in terms of a compact H, emission that is itself polarized by a rotating disc-like circumstellar medium. The spectropolarimetric difference between the Herbig Be and Ae stars may be the first indication that there is a transition in the Hertzsprung,Russell diagram from magnetic accretion at spectral type A to disc accretion at spectral type B. Alternatively, the interior polarized line emission apparent in the Ae stars may be masked in the Herbig Be stars owing to their higher levels of H, emission. [source] Linear polarized photoluminescence from GaN quantum dots imbedded in AlN matrixPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010K. S. Zhuravlev Abstract We report microphotoluminescence studies of GaN/AlN quantum dots grown along the (0001) crystal axis by molecular-beam epitaxy on sapphire substrates. To obtain quantum dots with different density and size a nominal GaN coverage was varied from 1 to 4 monolayers. The highest density of quantum dots was about 1011 cm -2, so about 103 quantum dots was excited in experiments. We found that the photoluminescence intensity of a sample with the smallest amount of deposited GaN decreases in more than two orders of magnitude under continuous-wave laser exposure during about 30 minutes and then it remains stable. The photoluminescence intensity of the rest samples was time-independent quantity. The emission band of the former sample exhibits a prominent linear polarization along the growth plane. We assume that the quite high degree of polarization can be due anisotropy of strain and/or shape of the quantum dots formed near dislocations which act also as recombination centers causing photoluminescence quenching. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Structural and optical properties of ZnSe-based diluted magnetic semiconductor quantum-well wire arrays by wet chemical etchingPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2010Tsutomu Muranaka Abstract The ZnSe-based mesa structures with well-defined and smooth facets were successfully demonstrated by using molecular beam epitaxy (MBE) and wet chemical etching. The ZnSe structures were found to be bound by (111) and (-1-11) facets along the [-110] direction, and bound by (1-11) and (-111) facets along the [110] direction, respectively. The intensities of the non-polarized PL peaks from the [-110]- and [110]-oriented QWW structures were almost proportional to the unetched region and additional decreases were not observed. The PL peaks from the QWW structures were found to be highly polarized when the polarization angle was aligned parallel to the wire direction. The degrees of the linear polarization were 18% for the [-110]-oriented QWW structure and 26% for the [-110]-oriented QWW structure, respectively. The results of the PL and magneto-PL measurements show no process-induced damage to degrade magneto-optical performance of the DMS structures by using this method. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Angular switching of the linear polarization of the emission in InGaAs microcavitiesPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 11 2005A. Amo Abstract The angular dependence of the degree of linear polarization of the emission is presented for an InGaAs microcavity at several detunings. For emission angles (, ) close to the growth direction, polarizations as high as +80% for lower branch polaritons at negative detuning are found. This polarization degree abruptly switches to negative values (up to ,90%) for emission angles outside a narrow cone of about ±2º. A similar behaviour, with smaller values of the polarization, is found at 0 and positive detunings. The relation between these effects and cavity birefringence is discussed. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Spectropolarimetric analysis of 3D MHD sunspot simulationsASTRONOMISCHE NACHRICHTEN, Issue 6 2010J.M. Borrero Abstract We have employed 3D non-grey MHD simulations of sunspots to compute theoretical Stokes profiles and compare the levels of circular and linear polarization in the simulations with those observed in a real sunspot. We find that the spatial distribution and average values of these quantities agree very well with the observations, although the polarization levels in the simulations are slightly larger. This can be explained by a slightly larger magnetic field strength or a larger temperature gradient in the simulated penumbra as compared to the observations (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] A Relativistic Disk in Sagittarius A*ASTRONOMISCHE NACHRICHTEN, Issue S1 2003Siming Liu The detection of a mm/Sub-mm "bump" in Sgr A*'s radio spectrum suggests that at least a portion of its overall emission is produced within a compact accretion disk. This inference is strengthened by observations of strong linear polarization (at the 10 percent level) within this bump. No linear polarization has been detected yet at other wavelengths. Given that radiation from this source is produced on progressively smaller spatial scales with increasing frequency, the mm/Sub-mm bump apparently arises within a mere handful of Schwarzschild radii of the black hole. We have found that a small (10-Schwarzschild-radii) magnetized accretion disk can not only account for the spectral bump via thermal synchrotron processes, but that it can also reproduce the corresponding polarimetric results. In addition, the quiescent X-ray emission appears to be associated with synchrotron self-Comptonization, while X-ray flares detected from Sgr A* may be induced by a sudden enhancement of accretion through this disk. The hardening of the flare-state X-ray spectrum appears to favor thermal bremsstrahlung as the dominant X-ray emission mechanism during the transient event. This picture predicts correlations among the mm, IR, and X-ray flux densities, that appear to be consistent with recent multi-wavelength observations. Further evidence for such a disk in Sgr A* is provided by its radio variability. Recent monitoring of Sgr A* at cm and mm wavelengths suggests that a spectral break is manifested at 3 mm during cm/Sub-mm flares. The flat cm spectrum, combined with a weak X-ray flux in the quiescent state, rules out models in which the radio emission is produced by thermal synchrotron process in a bounded plasma. One possibility is that nonthermal particles may be produced when the large scale quasi-spherical inflow circularizes and settles down into the small accretion disk. Dissipation of kinetic energy associated with radial motion may lead to particle acceleration in shocks or via magnetic reconnection. On the other hand, the identification of a 106-day cycle in Sgr A*'s radio variability may signal a precession of the disk around a spinning black hole. The disk's characteristics imply rigid-body rotation, so the long precession period is indicative of a small black-hole spin with a spin parameter a/M around 0.1. It is interesting to note that such a small value of a/M would be favored if the nonthermal portion of Sgr A*'s spectrum is powered by a Blandford- Znajek type of process; in this situation, the observed luminosity would correspond to an outer disk radius of about 30 Schwarzschild radii. This disk structure is consistent with earlier hydrodynamical and recent MHD simulations and is implied by Sgr A*'s mm/Sub-mm spectral and polarimetric characteristics. For the disk to precess with such a long (106-day) period, the angular momentum flux flowing through it must be sufficiently small that any modulation of the total angular momentum is mostly due to its coupling with the black-hole spin. This requires that the torque exerted on the inner boundary of the disk via magnetic stresses is close to the angular momentum accretion rate associated with the infalling gas. Significant heating at the inner edge of the disk then leaves the gas marginally bounded near the black hole. A strong wind from the central region may ensue and produce a scaled down version of relativistic (possibly magnetized) jets in AGNs. [source] Measurement of linear polarization in the H, line in solar flaresASTRONOMISCHE NACHRICHTEN, Issue 4 2003Article first published online: 28 MAY 200, P. Kotr Abstract Large solar telescopes built at places with a quite excellent seeing, equipped with a sophisticated optics and control system are too expensive and unique to be used currently in hunting of sudden and short-lasting activity events, e.g. flares and eruptive prominences. For a systematic observation of selected kinds of active phenomena it is still necessary to use smaller or medium-sized telescopes equipped with a special setup of devices. Detection of linear polarization in the H, line emitted in a flare seems to be just a right task and delicate matter for such a systematic observation. This kind of polarization is supposed to be generated by particle beams accelerated in thke corona and directed towards denser chromospheric layers where the particle beams deposit their kinetic energy. As the accelerated particle beams possess a preferred direction of velocity they can produce a linearly polarized light. However, the occurrence of the accelerated particle beams and the related linear polarization in the H, line may have a tendency to appear: 1) at the early beginning of a flare 2) in pulses lasting just a few seconds or even less. To measure the linear polarization in flares regularly we have built an additional branch in the Ond,ejov multichannel flare spectrograph. In this paper we describe the optical system, the detectors, the method used for data recording and reduction and we also briefly discuss the first results. [source] | |||||||||||||