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Polarization States (polarization + states)
Selected AbstractsThe graviton propagator with a non-conserved external generating sourceFORTSCHRITTE DER PHYSIK/PROGRESS OF PHYSICS, Issue 11-12 2007E.B. Manoukian Abstract A novel general expression is obtained for the graviton propagator from Lagrangian field theory by taking into account the necessary fact that in the functional differential approach of quantum field theory, in order to generate non-linearities in gravitation and interactions with matter, the external source T,,, coupled to the gravitational field, should a priori not be conserved ,,T,,, 0, so variations with respect to its ten components may be varied independently. The resulting propagator is the one which arises in the functional approach and does not coincide with the corresponding time-ordered product of two fields and it includes so-called Schwinger terms. The quantization is carried out in a gauge corresponding to physical states with two polarization states to ensure positivity in quantum applications. [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] A real-time monitoring photopolarimeter based on a multichannel signal process systemMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 12 2007Ruey-Ching Twu Abstract This study developed and evaluated a photopolarimeter based on a polarization interferometer with four photo detectors. By using the proposed multichannel signal process system, the time varying polarization states of an incident light beam, modulated by an electrically driving liquid crystal cell, can be measured instantly and monitored continually. Research findings contribute to the study of dynamic behaviors of the liquid crystal cell. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 3093,3096, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22947 [source] A dual-layered wideband microstrip reflectarray antenna with variable polarizationMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 7 2006Zhi-Hang Wu Abstract This paper presents a new kind of microstrip reflectarray antenna those polarization could be reconfigurable among all polarization states instead of some fixed states for dual- or multipolarized antenna. The mechanism for polarized variability is so simple that only mechanical rotation is needed. The complete theoretical analysis is given as well as a specific design sample to verify this method. At designed frequency of 10 GHz and in case of circularly polarized (CP) radiation, experimental results in agreement with simulated ones exhibit ,26-dB cross-polar level (CPL), ,13-dB side-lobe level, 13.5° half-power beamwidth, and 17.3-dBi gain corresponding to about 40% aperture efficiency. It also possesses 8% bandwidth from 9.5 to 10.3 GHz for CPL less than ,15 dB. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 1429,1432, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.21640 [source] An empirical model for the polarization of pulsar radio emissionMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Don Melrose ABSTRACT We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincaré sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincaré sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincaré sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible. [source] Coherence control of electron spin currents in semiconductorsPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 10 2006Henry M. van Driel Abstract We provide an overview of some of our recent work on the use of one color and two color optical techniques to generate and control electronic spin currents in semiconductors for which a spin,orbit interaction exists. The generation process relies on the quantum interference between different absorption pathways, such as that between single and two photon absorption or those involving different polarization states of a monochromatic beam. For different crystal orientations and/or beam polarizations it is possible to generate a spin current with or without an electric current, and an electrical current with or without a spin current. In our experiments, which are conducted either at 80 K or 295 K, we typically employ nominally 100 fs pulses centered near 1500 and 750 nm. The currents generated are quasi-ballistic and the carriers typically move distances of ,1,10 nm, determined by the momentum relaxation time, which is of the order of 100 fs. The transient characteristics of spin-polarized electrical currents generated in strained GaAs at room temperature by ,100 fs pulses is detected by the emitted THz radiation. Pure spin currents can be detected by taking advantage of the accumulation of up and down spins on opposite sides of tightly focused pump beams. The spin states are detected through differential transmission measurements of tightly focused right and left circularly polarized, near-band-edge probe pulses, delayed by several picoseconds from the pump pulses to allow carrier thermalization to occur. By spatial scanning across the differential spin profiles and determining the amplitude of the response we are able to translate this into nm spatial resolution of spin displacement. Finally, the ability to generate ballistic currents using purely optical techniques allows us to generate transverse Hall-like currents, with transverse charge currents generated from pure spin currents and transverse spin currents generated from pure charge currents. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Polarization control of ultrashort mid-IR laser pulses for transient vibrational circular dichroism measurements,CHIRALITY, Issue 1E 2009Mathias Bonmarin Abstract Linear dichroism and birefringence artifacts are a major source of concern in transient circular dichroism measurements. They mainly arise from interaction of an imperfectly circular polarized probe beam with a nonisotropic sample. We present in this article a procedure to generate mid-IR pulses of highly symmetric left and right handed circular or elliptical polarization states for transient VCD measurements. An infrared femtosecond laser source is synchronized to the natural frequency of a photo elastic modulator. Residual static birefringence of the modulator and the sample cell can be largely compensated by carefully controlling the arrival time of the ultrashort probe pulses at the modulator. Chirality 21:E298,E306, 2009. © 2009 Wiley-Liss, Inc. [source] | ||||||||||