Direction Of Linear Polarization Research Articles

Constraints onCPTviolation from Wilkinson Microwave Anisotropy Probe three year polarization data: A wavelet analysis

We perform a wavelet analysis of the temperature and polarization maps of the cosmic microwave background (CMB) delivered by the Wilkinson Microwave Anisotropy Probe experiment in search for a parity-violating signal. Such a signal could be seeded by new physics beyond the standard model, for which the Lorentz and CPT symmetries may not hold. Under these circumstances, the linear polarization direction of a CMB photon may get rotated during its cosmological journey, a phenomenon also called cosmological birefringence. Recently, Feng et al. have analyzed a subset of the Wilkinson Microwave Anisotropy Probe and BOOMERanG 2003 angular power spectra of the CMB, deriving a constraint that mildly favors a nonzero rotation. By using wavelet transforms we set a tighter limit on the CMB photon rotation angle {delta}{alpha}=-2.5{+-}3.0 ({delta}{alpha}=-2.5{+-}6.0) at the one (two) {sigma} level, consistent with a null detection.

On X-ray polarization spectroscopy with a Baronova–Stepanenko-type crystal

Certain crystals can split an X-ray's two linear polarization directions by reflecting them equally in two different directions. This paper gives a simple geometrical derivation for the necessary condition identified recently by Baronova and Stepanenko, namely, identical crystal planes under 120○, or three-fold symmetry. An X-ray spectropolarimeter with such polarization-splitting crystals would be particularly useful to diagnose anisotropies in hot, dense plasmas created by intense picosecond lasers.

Spectropolarimetry of the Hα Line in Herbig Ae/Be Stars

Using the HiVIS spectropolarimeter built for the Haleakala 3.7 m AEOS telescope, we have obtained a large number of high-precision spectropolarimetrc observations (284) of Herbig AeBe stars collected over 53 nights totaling more than 300 hr of observing. Our sample of five HAeBe stars, AB Aurigae, MWC 480, MWC 120, MWC 158, and HD 58647, all show systematic variations in the linear polarization amplitude and direction as a function of time and wavelength near the Hα line. In all our stars, the Hα line profiles show evidence of an intervening disk or outflowing wind, evidenced by strong emission with an absorptive component. The linear polarization varies by 0.2%-1.5% with the change typically centered in the absorptive part of the line profile. These observations are inconsistent with a simple disk-scattering model or a depolarization model that produce polarization changes centered on the emissive core. We speculate that polarized absorption via optical pumping of the intervening gas may be the cause.

Valence-band mixing in neutral, charged, and Mn-doped self-assembled quantum dots

We analyze the optical emission of single II-VI quantum dots containing 0 or 1 magnetic atom (manganese) and a controlled number of carriers (0, $\ifmmode\pm\else\textpm\fi{}1$ electron). The emission of these quantum dots presents a large degree of linear polarization. This linear polarization is attributed to a valence-band mixing and we show that in nonmagnetic quantum dots combining both a shape anisotropy and an anisotropic in-plane strain distribution, the linear polarization direction of the exciton are controlled by an interplay between valence-band mixing and electron-hole (e-h) exchange interaction. Similarly, under strong transverse magnetic field, the direction of the linearly polarized emission of the charged excitons is simultaneously controlled by the valence-band mixing and the direction of the magnetic field. In quantum dots containing a Mn atom, the valence-band mixing allows simultaneous hole-Mn spin flips coupling bright and dark excitons. These spin flips are responsible for linearly polarized transitions in the emission of the charged excitons at zero magnetic field.

Excited excitonic states observed in semiconductor quantum dots using polarization resolved optical spectroscopy

We present results on the polarization-resolved photoluminescence emitted from InGaAs/AlGaAs single quantum dots (QDs) grown in inverted tetrahedral pyramids. The emitted light was detected for two mutually perpendicular linear polarization directions in the less conventional cleaved-edge geometry, in addition to the standard top-emission geometry. Whereas the in-plane linear polarization was isotropic, as a consequence of the high symmetry of the system, we found a strong polarization anisotropy of the edge-emitted light revealing QD states of predominantly heavy- or light-hole character. By temperature control of the charge state, several neutral and charged light-hole like exciton complexes were identified. In particular, a biexciton showing a twofold radiative recombination path, leading to two nearly perpendicularly polarized emission multiplets, was identified. These results are also of technological relevance for any design of optoelectronic QD-integrated devices.

Polarization from Aligned Atoms as a Diagnostic of Circumstellar, Active Galactic Nuclei, and Interstellar Magnetic Fields. II. Atoms with Hyperfine Structure

We show that atomic alignment presents a reliable way to study the topology of astrophysical magnetic fields. The effect of atomic alignment arises from modulation of the relative population of the sublevels of the atomic ground state pumped by anisotropic radiation flux. As such aligned atoms precess in the external magnetic field, this affects the properties of the polarized radiation arising from both scattering and absorption by the atoms. As a result, the polarizations of emission and absorption lines depend on the three-dimensional (3D) geometry of the magnetic field as well as the direction and anisotropy of incident radiation. We consider a subset of astrophysically important atoms with hyperfine structure. For emission lines, we obtain the dependencies of the direction of linear polarization on the directions of the magnetic field and the incident pumping radiation. For the absorption lines we establish when the polarization is perpendicular and parallel to the magnetic field. For both emission and absorption lines we find the dependence on the degree of polarization on the 3D geometry of the magnetic field. We claim that atomic alignment provides a unique tool for studying magnetic fields in circumstellar regions, active galactic nuclei (AGNs), and the interplanetary and interstellar media. This tool allows one to study the 3D topology of magnetic fields and establish other important astrophysical parameters. We consider polarization arising from both atoms in the steady state and also those undergoing individual scattering of photons. We demonstrate the utility of atomic alignment for studies of astrophysical magnetic fields by considering a case of sodium alignment in a comet wake.

Build up and pinning of linear polarization in the Bose condensates of exciton polaritons

The linear polarization degree of the photoluminescence from a nonresonantly pumped CdTe microcavity exhibits a dramatic increase from about 0 to more than 85% once the threshold of stimulated scattering of the exciton polaritons toward the Bose-Einstein condensate is passed. The linear polarization direction is pinned to one of the crystallographic axes of the cavity. These effects are well accounted for within a model of localized condensate of interacting bosons.

An inverse method for designing RF phased array coils in MRI—theoretical considerations

Phased array coils afford a number of advantages over RF volume coils, including a high SNR over a large field of view. In this paper, a time-harmonic inverse method is presented for the theoretical design of RF phased arrays. The method allows any array size to be considered, for which a homogeneous magnetic field is desired within some spherical target region within the coils. An ill-conditioned integral equation is solved using regularization, whereby the error between induced and target magnetic fields is minimized along with an additional constraint related to the curvature of the coil windings. The ability to focus the RF field to arbitrary locations within the coil volume is incorporated into the model and a number of examples are displayed. Attention is also given to the polarization of the induced field, with the direction of linear polarization investigated along with circular polarization. Phased arrays comprising smooth windings that are able to induce highly homogeneous magnetic fields are displayed and discussed for a range of array sizes, and for several different field focusing and polarization considerations.

Valence band mixing in charged self‐assembled quantum dots

AbstractWe analyze the optical emission of neutral, positively and negatively charged excitons in single II‐VI quantum dots presenting a high degree of linear polarization. This linear polarization is attributed to a valence band mixing and we show that in QDs combining both a shape anisotropy and an anisotropic in‐plane strain distribution, the linear polarization direction of the exciton are controlled by an interplay of the valence band mixing and e‐h exchange interaction. Similarly, under strong transverse magnetic field, the direction of the linearly polarized emission of the charged excitons are simultaneously controlled by the valence band mixing and the direction of the magnetic field. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Polarization-resolved optical absorption in single V-groove quantum wires

Optical transitions associated with all three linear polarization directions were investigated in single GaAs V-groove quantum wires of different wire thicknesses. This was accomplished by combining absorption measurements in V-groove waveguide geometry with surface-excited photoluminescence excitation spectroscopy. The observed transitions were identified with the aid of model calculations. It is shown that excitonic intersubband coupling should be accounted for in order to explain the optical transitions associated with confined light-hole-like states. The results are relevant for the design of efficient quantum wire waveguide modulators and lasers.

S-matrix theory of inelastic vibronic ionization of molecules in intense laser fields

Inelastic vibronic ionization (IVI) in the presence of an intense laser field refers to the process of ionization of a target molecule accompanied by vibrational excitation of the residual molecular ion. The recently proposed intense-field many-body $S$-matrix theory of IVI is presented and applied to investigate the distributions of the vibrational states of the residual ${\mathrm{H}}_{2}^{+}$ ion from ionization of ${\mathrm{H}}_{2}$ molecule, and the results are compared with the experimental data. The characteristic features of the calculated distributions are found to agree well with those observed. The shift of the IVI distributions toward the lower vibrational states compared to the Frank-Condon distributions, the positions of the maxima of the distributions, as well as the reversal of the maximum from a lower vibrational state to a higher vibrational state (peak-reversal) are analyzed. The results of the ``adiabatic nuclei'' theory and the ``frozen nuclei'' approximation are compared to assess the latter. Influences of the choice of the vibrational wave functions on the calculated distributions, the dependence of the distributions on the alignments of the molecular axis with respect to the linear polarization direction of the laser, and the effect of circular polarization of the laser on the IVI distribution are discussed. Possible origins of the remaining discrepancy in the individual heights of the calculated vs. measured yields are pointed out. Finally, an isotopic shift of the IVI distributions toward the higher vibrational states, for the heavier isotopes, is predicted and the corresponding distributions for the two isotopes $\mathrm{HD}$ and ${\mathrm{D}}_{2}$ are given.

Electron-positron pair creation by powerful laser-ion interaction

Presently available high-power laser pulses of ponderomotive energy U p ≫ 2mc 2 should permit the fundamental processes of quantum electrodynamics in such fields, in particular, the formation of electron-positron pairs in impacts of laser pulses with highly charged ions, to be observed. We evaluate the highly nonlinear production rates of this process and investigate the most favorable conditions of pair production, in particular, either along the direction of linear polarization or in the propagation direction of the laser pulse. For femtosecond radiation pulses, it is possible to represent the laser beam by a monochromatic and linearly polarized electromagnetic plane wave. This approximation considerably simplifies the calculations required.

Direction controllable linearly polarized laser from a dye-doped cholesteric liquid crystal

We demonstrate a direction controllable linearly polarized laser from a dye-doped cholesteric liquid crystal (CLC) in a homogeneous cell coated with a metallic mirror on the inner side of a glass substrate. Due to coherent superposition of two orthogonal polarization states, the output laser light becomes linearly polarized and its output energy is greatly enhanced. Moreover, the linear polarization direction angle is proportional to the product of the CLC effective birefringence and cell gap. Hence direction tunable laser devices can be demonstrated by controlling the cell gap and the operating temperature.

Interpretation of second solar spectrum observations of the Sr I 4607 Å line in a quiet region: Turbulent magnetic field strength determination

This paper presents and interprets some observations of the limb polarization of 4607 Å obtained with the spectropolarimeter of the French-Italian telescope THEMIS in quiet regions close to the solar North Pole on 2002 December 7–9. The linear polarization was measured for a series of limb distances ranging from 4 to 160 arcsec, corresponding to heights of optical depth unity in the line core ranging from about 330 to 220 km, respectively, above the level. To increase the polarimetric sensitivity, the data were averaged along the spectrograph slit (one arcmin long) set parallel to the solar limb. Since the data show no rotation of the linear polarization direction with respect to the limb direction, the observed depolarization is ascribed to the Hanle effect of a turbulent weak magnetic field, the zero-field polarization being derived from a model. The interpretation is performed by means of an algorithm which describes the process of line formation in terms of the atomic density matrix formalism, the solar atmosphere being described by an empirical, plane-parallel model. The collisional rates entering the model (inelastic collisions with electrons, elastic depolarizing collisions with neutral hydrogen), have been computed by applying fast semi-classical methods having a typical accuracy of the order of 20% or better (see Derouich [CITE]), leading to 6% inaccuracy on the magnetic field strength determination. We assume a unimodal distribution for the intensity of the turbulent field. The computed intensity profile has been adjusted to the observed one in both depth and width, by varying both microturbulent and macroturbulent velocities. The best adjustment is obtained for respectively 1.87 km s-1 (micro) and 1.78 km s-1 (macro). The evaluation of the magnetic depolarization leads then to the average value of 46 Gauss for the turbulent magnetic field strength, with a gradient of -0.12 Gauss/km. Our results are in very good agreement with the value of 60 Gauss determined at large μ, in the volume-filling field case, by Trujillo Bueno et al. ([CITE], Nature, 430, 326), using a 3D magneto-convective simulation. This validates our method.

Kinetics of Anisotropic Scattering and Optical Turbulence upon Laser Irradiation of AgCl:Ag Films

The kinetics of anisotropic scattering and optical turbulence upon formation of quasi-periodic structures (QPSs) in photosensitive composite AgCl:Ag films irradiated by Gaussian beams of a He-Ne laser is studied. The buildup kinetics of the scattering induced by an unfocused beam is used to find the threshold exposures for the formation of QPSs and an additional spatiotemporal instability resulting from interaction between the gratings during their development. It was established that there exists a threshold for the formation of a strong instability (optical turbulence) when the QPSs are induced by a focused beam. The decay curves of the mean frequency of the chaotic intensity oscillations, as well as the dependence of this frequency on the beam power, are obtained under conditions of turbulence. It is shown that both the turbulence and its decay are controlled by a competition between the microgratings in the QPSs and by gradual widening of the area of active competition under the action of the Gaussian beam. The AgCl:Ag films are shown to be capable of rapidly and repeatedly rearranging the QPSs and the spatial orientation of the anisotropic scattering upon variation of the direction of linear polarization of the laser beam.

First detection of return currents in solar flares by spectropolarimetry with THEMIS

Using THEMIS French-Italien telescope with the MTR mode, the Hydrogen Hα and Hβ lines have been observed to be linearly polarized up to a few percent by impact during the impulsive phase of two solar flares associated with high-frequency radio pulses. Two privileged directions of linear polarization are present, respectively radial (in the disk center to flare direction) and tangential (perpendicular to the radial direction). This 90 degree change in the linear polarization direction is interpreted as due to the chromospheric return current generated by the penetration of a non-thermal electron beam into the chromosphere.

'Six-beam X-ray section topograph' images and computer-simulated images using the new n-beam dynamical theory based on the Takagi-Taupin equations.

A 'six-beam X-ray section topograph' experiment was performed whose images are in excellent agreement with the images computer-simulated using the theory derived by one of the authors [Okitsu (2003). Acta Cryst. A59, 235-244] based on the Takagi-Taupin dynamical theory. X-rays whose wavefront was limited to 0.1 x 0.1 mm were incident on a [1 1; 1]-oriented floating-zone silicon crystal with thickness of 9.911 mm so that a six-beam condition was satisfied. The six reflection indices were 000 (forward diffraction), 440, 484, 088, 4;48 and 4;04. The plane of incidence of the 088 reflection was adjusted so as to be perpendicular to the direction of the incident horizontal linear polarization of the synchrotron radiation, which was monochromated into 18.5 keV (0.670 A wavelength). While the simulations were performed under the assumptions that the incident X-rays were linearly polarized both horizontally and vertically, they were evidently different and only the simulated images under the assumption of horizontally polarized incident X-rays were in excellent agreement with the images obtained by the experiment.

Dissection of rovibronic band structure by polarization-resolved degenerate four-wave mixing spectroscopy

Judicious selection of polarization characteristics in degenerate four-wave mixing (DFWM) spectroscopy is shown to provide a facile and robust means for discriminating rovibronic features according to their changes in rotational angular momentum, ΔJ. Building upon a perturbative (weak-field) treatment of the resonant DFWM response, theoretical analyses are presented for a collinear arrangement of linearly polarized electromagnetic waves that interact with an isotropic ensemble of gas-phase target molecules. The polarization unit vectors for two input fields (E1 and E3) are presumed to be fixed along the Y-axis (φ1=φ3=π/2), while the remaining incident field (E2) has its orientation within the transverse X–Y plane specified by angular coordinate φ2. For φ2=π/4, complete elimination of high-J Q-branch (ΔJ=0) and P-/R-branch (ΔJ=±1) structure is found to occur when the detected direction of linear polarization is switched between the limiting values of φ4,Q=−18.43° and φ4,P/R=+26.57°, respectively. These predictions are corroborated experimentally by probing the tunneling-split origin region of the tropolone à 1B2–X̃ 1A1 (π*←π) absorption system under ambient, bulk-gas conditions. Other polarization-based schemes for rovibronic branch suppression are discussed as are the effects incurred by (strong-field) optical saturation phenomena. The implementations of DFWM suggested by this work should prove useful for dissecting and unraveling the congested spectra often exhibited by massive polyatomic species.

Optical imaging based on time-resolved Stokes vectors in filamentous tissues.

Time-resolved Stokes vector components of light transmitted through filamentous tissues were measured with a view to improving the imaging quality of optical images in such tissues. Temporal profiles of the Stokes vectors and the time-resolved degree of polarization (DOP) were calibrated to produce higher image quality than that of images based on time gating, polarization discrimination, or both. A thin chicken bone inserted into chicken breast tissue with filament orientation in different directions with respect to the direction of input linear polarization was scanned to demonstrate images of higher spatial resolution and contrast based on the measurement of time-resolved DOP.

A polarimetric view of the open cluster Lyngå6 and the Cepheid TW Normæ

We present UBVRI polarimetric observations of stars belonging to the small open cluster Lynga6 and of TW Nor, a Cepheid which can be seen near the center of the grouping. The wavelength of maximum polarization is computed and then analyzed in the context of its relation to the optical properties and characteristic particle size distribution of the grains responsible for the polarization. The amount and direction of linear polarization is also computed. Results indicate a mean polarization efficiency close to the value found for the interstellar medium. Two out of seven members of Lynga6 present indications of intrinsic polarization in their measures. We conclude that part of the polarization detected in the light from members of the cluster comes from a dust cloud located in the Local arm, at a distance of about 1 kpc. In general, polarization measurements do not support the membership of TW Nor to Lynga6.