Showing papers on "Circular polarization published in 2000"

Generation of circularly polarized high-order harmonics by two-color coplanar field mixing

Abstract: An efficient method is investigated for the generation of circularly polarized high-order harmonics by a bichromatic laser field whose two components with frequencies $\ensuremath{\omega}$ and $2\ensuremath{\omega}$ are circularly polarized in the same plane, but rotate in opposite directions. The generation of intense harmonics by such a driving-field configuration was already confirmed by a previous experiment. With the help of both a semiclassical three-step model as well as a saddle-point analysis, the mechanism of harmonic generation in this case is elucidated and the plateau structure of the harmonic response and their cutoffs are established. The sensitivity of the harmonic yield and the polarization of the harmonics to imperfect circular polarization of the driving fields are investigated. Optimization of both the cutoff frequency and the harmonic efficiency with respect to the intensity ratio of the two components of the driving field is discussed. The electron trajectories responsible for the emission of particular harmonics are identified. Unlike the case of a linearly polarized driving field, they have a nonzero start velocity. By comparison with the driving-field configuration where the two components rotate in the same direction, the mechanism of the intense harmonic emission is further clarified. Depending on the (unknown) saturation intensity for the bichromatic field with counter-rotating polarizations, this scheme might be of practical interest not only because of the circular polarization of the produced harmonics, but also because of their production efficiency.

Depth-resolved two-dimensional stokes vectors of backscattered light and mueller matrices of biological tissue measured with optical coherence tomography.

Abstract: Mueller matrices provide a complete characterization of the optical polarization properties of biological tissue. A polarization-sensitive optical coherence tomography (OCT) system was built and used to investigate the optical polarization properties of biological tissues and other turbid media. The apparent degree of polarization (DOP) of the backscattered light was measured with both liquid and solid scattering samples. The DOP maintains the value of unity within the detectable depth for the solid sample, whereas the DOP decreases with the optical depth for the liquid sample. Two-dimensional depth-resolved images of both the Stokes vectors of the backscattered light and the full Mueller matrices of biological tissue were measured with this system. These polarization measurements revealed some tissue structures that are not perceptible with standard OCT.

Determining the absolute carrier phase of a few-cycle laser pulse

Abstract: In a strong laser field, electrons tunnel from an atom at a rate determined by the instantaneous field. If the pulse is only a few cycles in duration, the highly nonlinear nature of tunnel ionization ensures that the resultant electron wave packet is primarily formed in less than one period. Measuring the direction of above-threshold-ionization electrons produced by circularly polarized light provides a direct method of measuring the absolute carrier phase of a single pulse. The method is robust, surviving spatial and temporal integration as well as intensity fluctuations.

Imaging system using polarization effects to enhance image quality

Abstract: The quality of images produced by confocal microscopy, and especially scanning laser confocal microscopy, is enhanced especially for images obtained in turbid mediums such as many biological tissue specimens, by reducing speckle from scatterers that exist outside (above and below) the focal plane region which is being imaged by utilizing sheared beams, both of which are focused to spots in the focal or image plane (region of interest) and polarizing the beams to have opposite senses of circular polarization (right and left handed circular polarization). The return light from the image plane of certain polarization is detected after passing through the confocal aperture of the confocal microscope. Light from scatterers outside the region of interest, which are illuminated by both of the sheared beams, interfere thereby reducing speckle due to such scatterers, and particularly scatters which are adjacent to the image plane. Sheared beams having orthogonal linear polarization, as may be obtained from a Wollaston or Nomarski prism are converted into circularly polarized beams of opposite polarization sense by a quarter wave plate. The optical signals representing reflections from the focal plane are derived by polarizing optics which may either, be a polarizing beamsplitter in the incident beam path or with a retarder and analyzer. The retarder may be selected to provide different polarization phase shift of the return light, and with the analyzer, detects the degree of elliptical polarization representing the optical activity and circular dichroism producing the optical signal representing the image.

Photoelectron circular dichroism of the randomly oriented chiral molecules glyceraldehyde and lactic acid

Abstract: The differing interaction of left and right circularly polarized light with chiral molecules is shown to lead to different angular distributions of the photoelectrons created by photoionization of a given enantiomer, even when the target molecules are randomly oriented Numerical calculations are presented to demonstrate the magnitude of this effect for the C3H6O3 structural isomers lactic acid and glyceraldehyde, including two different conformations of the latter Circular dichroism in the angular distributions (CDAD) of the valence electrons of these biomolecules is most pronounced close to threshold, but tends to vanish as the electron kinetic energy approaches 20 eV and above CDAD signals are predicted to range, typically, from 10% to 40% and sometimes to more than 60% of the differential cross section

Polarized light propagation through tissue phantoms containing densely packed scatterers.

Abstract: We demonstrate that polarized light is maintained differently in densely packed versus dilute suspensions of polystyrene microspheres. The degrees of linear and circular polarization were measured versus scatterer concentration in aqueous suspensions of 0.48-, 0.99-, 2.092-, and 9.14‐µm-diameter polystyrene microspheres. The results indicate that, for dilute suspensions of microspheres where independent scattering is assumed, the degrees of linear and circular polarization decrease as the scatterer concentration increases. For dense suspensions, however, the degree of polarization begins to increase as the scatterer concentration increases. The preferential propagation of linear over circular polarization states in dense suspensions is similar to results seen in biological tissue.

Spectropolarimetric measurements of magnetic Ap and Bp stars in all four Stokes parameters

Abstract: In this paper we begin an exploration of the potential of spectral line Zeeman linear and circular polarization signatures for reconstructing the surface magnetic field topologies of magnetic Ap and Bp stars. We present our first observational results, which include the very first high-quality measurements of stellar Zeeman spectral line linear polarization ever obtained. Using the new MuSiCoS spectropolarimeter at the Pic du Midi Observatory, over 360 spectra were obtained, in circular or linear polarization, of 14 magnetic Ap/Bp stars and six calibration objects. Zeeman circular polarization signatures are detected in most single lines in essentially all spectra of magnetic Ap stars, with typical relative amplitudes of a few per cent. Linear polarization Zeeman signatures are unambiguously detected in individual strong, magnetically sensitive lines in outstanding spectra of five objects. However, linear polarization is generally not detected in individual strong lines in our much more common moderate signal-to-noise ratio (S/N) spectra, and is essentially never detected in weak lines. In order to overcome the limitations imposed by the S/N ratio and the inherent weakness of linear polarization Zeeman signatures, we exploit the information contained in the many lines in our spectra by using the least-squares deconvolution (LSD) technique. Using LSD, mean linear polarization signatures are consistently detected within the spectral lines of 10 of our 14 programme stars. These mean linear polarization signatures are very weak, with typical amplitudes 10–20 times smaller than those of the associated mean circular polarization signatures. For 11 stars full or partial rotational phase coverage has been obtained in the Stokes I and V or the Stokes I, V, Q and U parameters. The rotational modulation of the LSD mean signatures is reported for these objects. Measurements of the longitudinal field and net linear polarization obtained from these LSD profiles show they are consistent with existing comparable data, and provide constraints on magnetic field models which are at least as powerful as any other data presently available. To illustrate the new information available from these data sets, we compare for four stars the observed Stokes profiles with those predicted by magnetic field models published previously in the literature. Important and sometimes striking differences between the observed and computed profiles indicate that the Zeeman signatures presented here contain important new information about the structure of the magnetic fields of Ap and Bp stars capable of showing the limitations of the best magnetic field models currently available.

Dual polarization modulation: A real-time, spectral-multiplex separation of circular dichroism from linear birefringence spectral intensities

Abstract: A real-time, spectral-multiplex method for the complete separation of circular dichroism (CD) spectra from linear birefringence (LB) spectra is presented. The method, called dual polarization modulation (DPM), involves the introduction of a second source of polarization modulation after the CD sample. The first source of polarization modulation, as in conventional CD spectrometers, is located before the sample. Intensity signals at the detector in phase with each of the two polarization modulation frequencies are demodulated simultaneously in parallel and combined electronically in opposition to eliminate the LB spectrum by real-time cancellation. The accuracy of the cancellation can be adjusted electronically without the need to change the optical alignment of the instrument. The DPM method permits baseline-corrected CD spectra to be measured without the need for a subsequent CD background measurement.

Switching a Helical Polymer between Mirror Images Using Circularly Polarized Light

Abstract: A system has been created in which chiral information inherent in circularly polarized light can be used to influence the helical sense characteristics of a polymer. A racemic mixture of a photoresolvable ketone-containing group was appended through different linkage patterns to a polymer having dynamically interconverting equal populations of left- and right-handed helical backbone conformations. Irradiation with circularly polarized light in the ketone's chromophore gave rise to easily measurable circular dichroism signals in the polymer backbone helix, which changed sign with a change in the sense of the circularly polarized light. This demonstrates that the small enantiomeric excess produced by the irradiation, even diluted with large proportions of achiral pendants, is capable of enforcing a disproportionate excess of one helical sense in the polymer. The results, expressed as optical activity as a function of the degree of polymerization, could be analyzed using an approximate solution of a one-dime...

Parameters characterizing electromagnetic wave polarization

Abstract: In this paper, generalizations of the Stokes parameters and alternative characterizations of three-dimensional (3D) time-varying electromagnetic fields is introduced. One of these characteristics is the normal of the polarization plane, which, in many cases of interest, is parallel (or antiparallel) to the direction of propagation. Others are the two spectral density Stokes parameters which describe spectral intensity and circular polarization. The analysis is based on the spectral density tensor. This tensor is expanded in a base composed of the generators of the SU(3) symmetry group, as given by Gell-Mann and Y. Ne'eman [The Eight-fold Way (Benjamin, New York, 1964)] and the coefficients of this expansion are identified as generalized spectral density polarization parameters. The generators have the advantage that they obey the same algebra as the Pauli spin matrices, which is the base for expanding the 2D spectral density tensor with the Stokes parameters as coefficients. The polarization parameters introduced are formulated in the frequency domain, thereby further generalizing the theory to allow for wide-band electromagnetic waves in contrast to the traditional quasi-monochromatic formulation.

Detection and Measurement of Parsec-Scale Circular Polarization in Four AGN

Abstract: We present five epochs of 15 GHz VLBA observations of 13 AGN. These observations were specially calibrated to detect parsec scale circular polarization and our calibration techniques are discussed and analyzed in detail. We obtained reliable detections of parsec scale circular polarization in the radio jets of 4 AGN: 3C84, PKS 0528+134, 3C273, and 3C279. For each of these objects our detections are at the level of ~ 0.3-1% local fractional circular polarization. Our observations are consistent across multiple epochs (and different calibration techniques) in the sign and magnitude of the circular polarization observed. 3C273 and 3C279 both undergo core outbursts during our observations and changes in the circular polarization of both sources are correlated with these outbursts. In general, we observe the circular polarization to be nearly coincident with the strong VLBI cores of these objects; however, in 3C84 the circular polarization is located a full milli-arcsecond south of the source peak, and in the 1996.73 epoch of 3C273 the circular polarization is predominately associated with the newly emerging jet component. Circular polarization may be produced as an intrinsic component of synchrotron radiation or by the Faraday conversion of linear to circular polarization. Our single frequency observations do not easily distinguish between these possible mechanisms, but independent of mechanism, the remarkable consistency across epoch of the sign of the observed circular polarization suggests the existence of a long term, stable, uni-directional magnetic field. Single dish observations of 3C273 and 3C279 at 8 GHz by Hodge and Aller suggest that this stability may persist for decades in our frame of observation.

Single-feed circularly polarized microstrip ring antenna and arrays

Abstract: An analysis is presented for a microstrip-feed proximity-coupled ring antenna and a four-element array. Interactions between the embedded microstrip feed and the radiating element(s) are rigorously included. Results demonstrate that circular polarization of both senses can be achieved with a ring antenna with proper design of two inner stubs located at angles of /spl plusmn/45/spl deg/ with respect to the feedline. Theory and experiment demonstrate an axial ratio 3-dB bandwidth of 1% and the voltage standing wave ratio (VSWR) <2 bandwidth of 6.1%. The axial ratio bandwidth is typical for a microstrip antenna with perturbations, while the VSWR bandwidth is larger than for the circular or rectangular patch with perturbations. A mutual coupling study between two elements shows that the axial ratio is less than 2 dB for interelement spacing greater than 0.55/spl lambda//sub eff/, while the VSWR <2 for all spacings considered. A comparison between theory and experiment is provided for a 2/spl times/2 element array. The benefits of sequentially rotating the antenna elements in an array environment are presented. The axial ratio and VSWR bandwidths are both increased to 6.1% and 18% for a four-element array. A single-element antenna with two orthogonal feeds to provide both senses of polarization is demonstrated. The ring antenna is small (D//spl lambda//sub 0/=0.325), the substrate thickness is thin (H//spl lambda//sub 0//spl sim/0.035), and the microstrip feed produces a completely planar antenna system, which is compatible with microwave and millimeter integrated circuits (MICs), and monolithic microwave integrated circuits (MMICs).

Polarization properties of light backscattered from a two layer scattering medium.

Abstract: The polarization properties of light backscattered from a two layer scattering medium are investigated. Linear, circular and elliptical polarization states are considered and it is demonstrated that the degree of polarization of the backscattered light is sensitive to the optical properties of both layers and to layer thickness. Furthermore, it is shown that the polarization memory of circularly polarized light enables deeper layers to be probed whereas linearly polarized light is more sensitive to surface layers. This has applications for characterizing burns and melanoma.

Strong, Variable Circular Polarization in PKS 1519–273

Abstract: We find strong (> 1%) circular polarization in the intraday-variable radio source PKS1519-273. The source exhibits ~ 12 hourly variability in all four Stokes parameters at 4.8 and8.6 GHz, and longer timescale variability at 2.5 and 1.4 GHz. The characteristics and frequencydependence of the variability suggest that it is due to interstellar scintillation. VSOP limits on thedistance to the scattering screen constrain the brightness temperature to T B > 5 x 1013 K. Thefluctuations in total intensity are well-correlated with those in circular polarization, implying that thevariable component of the source is -3.8 ± 0.4% circularly polarized at 4.8 GHz. The origin of thecircular polarization is unclear.

Radio circular polarization of active galaxies

Abstract: We present the first results of a circular polarization survey conducted with the AustraliaTelescope Compact Array (ATCA). We demonstrate the ability to make circular polarizationmeasurements with a standard error of only 0.01per cent, and have detections from bothblazar and non-blazar active galactic nuclei (AGN). Our results show that, as a group, BLLac sources and quasars have systematically higher circular polarization than radio galaxies.We demonstrate the association of high levels of circular polarization with total-intensityvariability and flat/inverted spectral index as further evidence that circular polarization isassociated with blazar activity. We also include preliminary circular polarization monitoringdata and the detection of circular polarization from the GHz Peaked Spectrum (GPS) sourcePKS 1934–638, and discuss possible implications.Key words: polarization – galaxies: active. 1 INTRODUCTION1.1 Circular polarization of active galaxiesCircular polarization 1 has long been regarded as a potentiallyimportant tool for the study of extragalactic radio sources. Attemptsto make use of it have generally been frustrated, however, because itis so difficult to measure. Analysis of the collation of circularpolarization data presented by Weiler & De Pater (1983),generally from the 1970s, demonstrates the problem of circularpolarization experiments; out of 50 radio galaxies and 43 quasars,circular polarization was detected at least twice at the 2.5slevelfor only six radio galaxies and 15 quasars. For these detections,circular polarization levels are typically m

Circularly polarized circular sector dielectric resonator antenna

Abstract: A circular sector dielectric resonator antenna (DRA) with circular polarization and a single feed is investigated and demonstrated The design utilizes the radius to height ratio and feed position of the circular sector DRA to excite two resonant modes that are spatially orthogonal in polarization and in phase quadrature Experimental results are provided for the design and these demonstrate that the circular sector DRA produces circular polarized radiation with axial ratio less than 3 dB over a 10% bandwidth

Evidence of magnetic accretion in an SW Sex star: discovery of variable circular polarization in LS Pegasi

Abstract: We report on the discovery of variable circular polarization in the SW Sex star LS Pegasi. The observed modulation has an amplitude of ~0.3 % and a period of 29.6 minutes, which we assume as the spin period of the magnetic white dwarf. We also detected periodic flaring in the blue wing of Hbeta, with a period of 33.5 minutes. The difference between both frequencies is just the orbital frequency, so we relate the 33.5-min modulation to the beat between the orbital and spin period. We propose a new accretion scenario in SW Sex stars, based on the shock of the disk-overflown gas stream against the white dwarf's magnetosphere, which extends to the corotation radius. From this geometry, we estimate a magnetic field strength of B(1) ~ 5-15 MG. Our results indicate that magnetic accretion plays an important role in SW Sex stars and we suggest that these systems are probably Intermediate Polars with the highest mass accretion rates.

Circular polarization of the fluorescence from films of poly(p-phenylene vinylene) and polythiophene with chiral side chains

Abstract: Circularly polarized photoluminescence has been observed from chiral -conjugated polymers (e.g., see Figure). It was found that the degree of circular polarization was greater in the absorption than in the emission, which is interpreted in terms of trap sites acting as luminescent centers. These results may be important for the fabrication of LEDs for producing circularly polarized light.

Scintillation-induced Circular Polarization in Pulsars and Quasars

Abstract: We present a physical interpretation for the generation of circular polarization resulting from the propagation of radiation through a magnetized plasma in terms of a rotation measure gradient, or "Faraday wedges." Criteria for the observability of scintillation-induced circular polarization are identified. Application of the theory to the circular polarization in pulsars and compact extragalactic sources is discussed.

Magnetic field effects on coherent backscattering of light

Abstract: We have studied the influence of magneto-optical Faraday rotation on coherent backscattering of light experimentally, theoretically and by computer simulations of Monte-Carlo type. The consistency of these three approaches reveals new aspects of the propagation of vector waves in turbid media with and without Faraday rotation. Experimentally, we have demonstrated that the Faraday rotation may almost completely destroy the reciprocity of light paths. However, as shown by the simulations, the cone of coherent backscattering may not only be destroyed but also shifted off the exact backscattering direction, parallel to the magnetic field, as long as the amount of circular polarization is not completely destroyed by the multiple scattering. The relationship between coherent backscattering, depolarization and Faraday rotation are explained by a simple path model of vector waves. This leads to a new characteristic correlation length required to properly describe the influence of Faraday rotation on multiple light scattering.

A dual‐band circularly polarized stacked elliptic microstrip antenna

Abstract: A novel design of a probe-fed stacked microstrip antenna for obtaining dual-band circular polarization (CP) radiation is presented. By properly selecting two elliptic patches of different axial ratios in the proposed design, and introducing a small air-gap layer between the two patches, dual-band CP radiation can be obtained by exciting the top elliptic patch using a single probe feed. Details of the proposed antenna design and experimental results for the obtained dual-band CP performances are presented and discussed. ©2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 24: 354–357, 2000.

Polarimetry of young stellar objects - III : Circular polarimetry of OMC-1

Abstract: We present the first imaging circular polarimetry of the Orion Molecular Cloud, OMC-1. The observations, taken in the J, H, Kn and nbL bands, reveal a complex pattern of circular polarization. Globally, there is a background circular polarization of the order of ±2 per cent in the Kn band, conforming to the typical quadrupolar patterns that have been observed in other outflow sources. Overlying this pattern are regions of relatively high degrees of circular polarization to the east and west of the source IRc2, with degrees as high as +17 per cent in the Kn band, the highest circular polarization yet measured for any young stellar object. No circular polarization is seen in the J band, indicating that the circular polarization detected at longer wavelengths originates from within OMC-1 and not from scattering off the foreground ionization front associated with the M42 nebula. We demonstrate a correlation between these patches of high circular polarization and regions of enhanced linear polarization, and argue that these observations are best explained using a model that incorporates scattering of radiation off oblate grains, which have been aligned by the local magnetic field. Modelling of the ellipticity (the ratio of circular to linear polarization) suggests that the grains are composed of silicate and/or organic refractory material, and that grains larger than are typically found in the interstellar medium are needed. The lower, background, circular polarization is produced by scattering off randomly oriented grains in the outflow cavities, the grain alignment being destroyed by the passage of shocks. We put forward a morphological model for OMC-1 which has the regions of high polarization separate from, but near to, the main outflow region. Those regions exhibiting high polarization must somehow have a direct view of the illuminating source of the nebula. Implications of this work to the origins of life are briefly discussed.

Coplanar waveguide-fed circularly polarized microstrip antenna

Abstract: Design of a square microstrip antenna for obtaining circular polarization (CP) radiation using a coplanar waveguide (CPW) feed is presented. This CP design is achieved by insetting a slit to the boundary of the square microstrip patch, which makes possible the splitting of the dominant resonant mode into two near-degenerate orthogonal modes for CP radiation and introducing an inclined slot in the CPW feed line for coupling the electromagnetic (EM) energy of the CPW to the square patch. Good impedance matching for CP operation can be obtained by adjusting the inclined slot length and the tuning-stub length of the CPW feed line. Typical experimental results are presented and discussed.

An inverse compton scattering model of pulsar emission. III. Polarization

Abstract: Qiao and his collaborators recently proposed an inverse Compton scattering model to explain radio emission from pulsars. In this paper, we investigate the polarization properties of pulsar emission in the model. First of all, using the lower frequency approximation, we derived the analytical amplitude of the inverse Compton scattered wave of a single electron in a strong magnetic field. We found that the outgoing radio emission of a single relativistic electron scattering off the "low-frequency waves" produced by gap sparking should be linearly polarized and have no circular polarization at all. However, considering the coherency of the emission from a bunch of electrons, we found that the outgoing radiation from the inner part of the emission beam, i.e., that from the lower emission altitudes, preferentially has to have circular polarization. Computer simulations show that the polarization properties, such as the sense reversal of circular polarization near the pulse center, the S-shape of position angle swing of the linear polarization, and a strong linear polarization in conal components, can be reproduced in the ICS model.

Discovery of Circularly Polarized Radio Emission from SS 433.

Abstract: We report the discovery of circularly polarized radio emission from the radio-jet X-ray binary SS 433 with the Australia Telescope Compact Array. The flux density spectrum of the circular polarization, clearly detected at four frequencies between 1 and 9 GHz, is of the form V~nu-0.9+/-0.1. Multiple components in the source and a lack of very high spatial resolution do not allow a unique determination of the origin of the circular polarization or of the spectrum of fractional polarization. However, we argue that the emission is likely to arise in the inner regions of the binary, possibly via propagation-induced conversion of linear to circular polarization, and the fractional circular polarization of these regions may be as high as 10%. Observations such as these have the potential to help us investigate the composition, whether pairs or baryonic, of the ejecta from X-ray binaries.

Faraday rotation at the 2 p edges of Fe, Co, and Ni

Abstract: We present resonantly enhanced magneto-optical Faraday rotation data of linearly polarized soft x rays across the 2p absorption edges in Fe, Co, and Ni thin films using synchrotron radiation. Rotation angles of up to \ifmmode\pm\else\textpm\fi{}90\ifmmode^\circ\else\textdegree\fi{} were measured. These values are more than one order of magnitude larger than so far observed. By linear polarization analysis the dependence of the Faraday rotation on layer thickness, angle of incidence, magnetic field strength, and photon energy was investigated. The dichroism of the dispersion and absorption parts of the refraction index is in agreement with magnetic circular dichroism absorption data of the same films with circularly polarized light.