Showing papers on "Circular polarization published in 1993"

The opposition effect of the moon: the contribution of coherent backscatter.

Abstract: The opposition effect, the sharp surge in brightness of an astronomical object observed near zero phase angle, which has been known for more than a century, has generally been explained by shadow hiding. The reflectances of several Apollo lunar soil samples have been measured as a function of phase angle in linearly and circularly polarized light. All samples exhibited a decrease in the linear polarization ratio and an increase in the circular polarization ratio in the opposition peak. This provides unequivocal proof that most of the lunar opposition effect is caused by coherent backscatter, not shadow hiding. This result has major implications for the interpretation of photometric observations of bodies in the solar system, including the Earth.

Optical reversibility theorems for polarization : application to remote control of polarization

Abstract: Using Jones’s formalism, we prove three optical reversibility theorems that relate the polarization ellipticity at the output of an optical system to the polarization of the retroreflected light at the input. We describe how these theorems can be used to measure the ellipticity of a polarization remotely and thus to control it remotely. As an example, we use this method to create a linear or a circular polarization after a total internal reflection inside a prism, and the impurity of polarization is found to be better than 10−3. Finally we describe the use of this remote control to create polarization configurations that are useful for laser cooling of atoms.

Spectropolarimetry of SN 1993J in NGC 3031

Abstract: We have obtained low-resolution optical spectropolarimetry of SN 1993J which shows a high continuum linear polarization and a 05% drop in polarization across the Ha emission line accompanied by a ∼15° position angle rotation From these data we infer the presence of two distinct polarization components and conclude that at least part of the polarization is intrinsic to the supernova We propose that the polarization observed at Hα is due to interstellar polarization within NGC 3031, while the continuum polarization is intrinsic to the object After correcting the observed Stokes parameters for interstellar polarization, we find that the intrinsic continuum polarization is high, P=16%±01% at θ=49°±3°, and is wavelength independent

Strong Circular Dichroism in Photoelectron Diffraction from Nonchiral, Nonmagnetic Material—Direct Observation of Rotational Motion of Electrons

Abstract: Strong circular dichroism is found in 2-dimensional angular distribution patterns of the Si 2p photoelectrons from the Si(001) surface, which has no chirality and magnetism. The forward focusing peaks in the pattern rotate clockwise or counterclockwise when the helicity of the incident circularly polarized light is reversed. These rotations of the pattern are explained by rotational motion of photoelectrons around the nuclei. This is the first direct observation of the rotational motion of the electrons and clarifies the correspondence between the classical and the quantum mechanical ideas of angular momentum.

WKB solution for wave propagation in a time-varying magnetoplasma medium: longitudinal propagation

Abstract: The interaction of a right circularly polarized electromagnetic wave of frequency omega /sub 0/ with a switched-on-time-varying plasma medium, in the presence of a static magnetic field, is considered. Longitudinal propagation is assumed and ion motion is neglected. The electron density is assumed to vary slowly with time, and the solution is obtained through a WKB approximation. The main effect of switching the time-varying medium is the splitting of the original (incident) wave of frequency omega /sub 0/ into three new waves with time-varying frequencies. An exponentially increasing electron density profile is considered to illustrate the solution. The distinguishing feature of the presence of the static magnetic field is the creation of the third wave. The initial value of the instantaneous frequency omega /sub 3/ of this wave is equal to the gyrofrequency omega /sub b/, and the final value depends on omega /sub b/, omega /sub 0/, and the final value of the plasma frequency. omega /sub 3/ decreases with time for the profile under consideration. >

NMR calibration of optical measurement of nuclear polarization in 3He

Abstract: We have performed precise NMR measurements in order to calibrate a standard optical technique for measuring the polarization of 3He nuclei. This optical technique requires knowledge of the pressure-dependent relationship between the nuclear polarization and the degree of circular polarization of the 668-nm light emitted from a 3He discharge. We determined the nuclear polarization by comparing the adiabatic fast-passage NMR signal from samples of polarized 3He at pressures between 0.1 and 5 torr to an identical sample of water. For water, the polarization is known from statistical mechanics. The 3He is polarized using the metastability exchange technique for optical pumping of 3He. The accuracy of the calibration is ±2%, which is required for applications in nuclear physics.

Variable-temperature magnetic circular dichroism.

Abstract: Publisher Summary A magnetic circular dichroism (MCD) spectrum is a measurement of the difference between the absorption of left and right circularly polarized light as a function of the wavelength of the measuring beam when a sample is placed in a magnetic field applied parallel or antiparallel to the direction of propagation of the light beam. There are three possible consequences to the application of a magnetic field—namely, Zeeman splitting of ground and/or excited degenerate states, field-induced mixing of states, and a change in the population of molecules over the Zeeman sublevels of a paramagnetic ground state. These give rise to contributions to the intensity of an MCD spectrum called, respectively, the A-term, the B-term, and the C-term. This chapter describes in detail for the first time the methodology used in the Norwich laboratory (University of East Anglia), which has become that almost universally employed, for the measurement of MCD spectra in the low-temperature and high-field regions. The analysis of magnetization curves is discussed and examples given briefly of the spectra of some of the main classes of transition metal centers.

Notch filters with cholesteric polarizers with birefringent film and linear polarizer

Abstract: An optical notch filter component in a color LCD display comprises an electronically controllable variable retarder interposed between circularly polarizing elements. Entering light in a particular spectral region is circularly polarized by a first of the circularly polarizing elements, and selectively retarded by zero or a half wavelength by the retarder so as to controllably yield right or left handed circular polarization. The second circular polarizer transmits only one of the circular polarizations of the light. By controlling the circular polarization of the light with the retarder, the light is either transmitted or attenuated. More uniform stopband attenuation is provided when the optical notch filter component comprises oppositely handed circular polarizing elements and a variable retarder which applies zero retardation in one of its states. A direct view display system substantially without parallax effects is formed from a stacked assembly of the notch filter components with thin substrate construction. Higher contrast is provided by doubly analyzing each color band.

Possibility of Observing Spin-Polarized Tunneling Current Using Scanning Tunneling Microscope with Optically Pumped GaAs

Abstract: A scanning tunneling microscope (STM) experiment demonstrating the spin-polarized tunneling effect was performed. A GaAs thin-film sample pumped by circularly polarized light, and a ferromagnetic polycrystalline Ni tip are used. The tunneling current is perturbed by modulating the power and polarization of the pumping light. The perturbation arises due to three dominant effects: the thermal expansion of the tip or sample, the variation in excited carrier concentration in GaAs and the spin-polarized tunneling effect. The spin-polarized effect can be distinguished by observing the current dependence on the bias. Well-adjusted pumping optics allows the separate detection of the spin-polarized signal, which depends upon the circular polarization of the light and the tip magnetization. The present experiment indicates the feasibility of a spin-polarized STM with a non-magnetic tip.

Magnetic spectromicroscopy from Fe(100)

Abstract: Magnetic domains on an Fe(100) surface have been imaged by means of energy‐resolved photoemission microscopy. We excited the photoelectrons with circularly polarized synchrotron radiation in the soft x‐ray region, and employed the effect of magnetic circular dichroism in the emitted photoelectrons in order to obtain contrast between differently oriented magnetic domains. This new approach offers a surface sensitive way to combine chemical and magnetic information on a microscopic scale.

Chiral electron pairs from double photoionization

Abstract: Double photoionization of a randomly oriented free atom by circularly polarized light produces photoelectron pairs with the ability to distinguish a right-handed coordinate frame from a left-handed one. This chirality, a new manifestation of electronic correlation, causes a circular dichroism in multiply differential cross sections. Necessary kinematical conditions for the observability of a finite chirality of a photoelectron pair are worked out and discussed. Results of numerical calculations for He and H- show the amount of circular dichroism to depend strongly: (i) on the choice of wavefunctions used, especially on the description of correlation, (ii) on the energy distribution of the photoelectrons, and (iii) on the light frequency. In particular, the authors have found a critical frequency where the dichroism changes sign. The threshold behaviour has been analysed within the frame of the Wannier-Peterkop-Rau theory.

Detection of OH in flames by using polarization spectroscopy.

Abstract: The potentials of polarization spectroscopy in combustion studies are reported. This well-established technique of nonlinear laser spectroscopy is applied to the detection of OH molecules in a premixed acetylene-oxygen and a propane-air flame. The polarization spectrum is recorded in the A (2)Sigma-X(2)Pi(0, 0) band. The spatial distribution of OH is measured in the flames. The saturation of the signal as a function of the laser pulse intensity is also investigated.

Circularly polarized luminescence.

Abstract: Publisher Summary Various spectroscopic techniques are based on the differential interaction of left versus right circularly polarized light with molecules or molecular systems. By far, the most widely known and most often used experimental technique is that of circular dichroism (CD) in which the difference in absorption between the two different circular polarizations is measured, and for which high-quality commercial instruments for UV-visible studies are available. No commercial instruments are currently available that are specifically designed to measure the differential scattering of circularly polarized light, or the difference in intensity of left versus right circular polarization in the emission spectrum. The latter has at various times been referred to as “circularly polarized emission,” “emission circular intensity differentials,” “circularly polarized luminescence,” or, more specifically, as “circularly polarized fluorescence or phosphorescence.” This experimental technique is referred to as “circularly polarized luminescence (CPL).”

Parametric instabilities of a large-amplitude circularly polarized Alfvén wave : linear growth in two-dimensional geometries

Abstract: The growth of parametric instabilities, which may lead to the development of a turbulent cascade, is studied using an MHD code that permits nonlinear couplings in the parallel direction to the ambient magnetic field and one perpendicular direction. Compressibility is included in the form of a polytropic equation of state. Parametric instabilities associated with a parallel-propagating decay instability are found to dominate the low-beta case. An obliquely propagating filamentationlike instability dominates the high-beta case. The nonlinear growth of the nth harmonic of a daughter wave growing as a factor of n times the fundamental's growth rate is found in both cases. Nonlinear saturation is caused by the parallel decay instability in the low-beta case and by the oblique filamentationlike instability in the high-beta case.

Uncombed fields as the source of the broad-band circular polarization of sunspots

Abstract: We investigate the production of broad-band circular polarization (BBC) of sunspots using a model inspired by the discovery of small-scale variations of magnetic inclination in sunspot penumbrae. The proposed model is a simple representation of a horizontal magnetic flux tube embedded in an inclined magnetic field, with a flow in either one or both components (Evershed flow). This model is shown to produce a sizeable BBC without requiring substantial net vertical gradients of the magnetic vector. It reproduces the BBC observations in a qualitatively correct and quantitatively acceptable manner for reasonable values of its free parameters. In addition, our model also explains the crossover effect observed in sunspot penumbrae

Three-dimensional relativistic model of a bound particle in an intense laser field

Abstract: We analyze a three-dimensional model of a Klein-Gordon particle in a short-range separable potential and interacting with an intense plane-wave electromagnetic field. In the specific case of the circular polarization of the radiation, we find an exact solution of the Klein-Gordon equation of the system and derive analytic expressions for obtaining the total and partial rates of particle ejection by N-photon absorption, the energy spectrum of the ejected particle, as well as the amplitudes for stimulated bremsstrahlung and its inverse.

Polarization instability in a twisted birefringent optical fiber

Abstract: The fast-axis polarization instability arises in a weakly birefringent fiber as a result of competition between the natural fiber birefringence and the nonlinear ellipse rotation. Direct observation of the fast-axis polarization instability is reported. A full theoretical development of the polarization instability in a twisted, birefringent optical fiber is presented. The theory includes the derivation of and full solutions for the evolution of light in a twisted fiber as well as stability analysis and phase-plane representation of the solutions. The experiment is described in detail; good agreement is obtained between theory and experiment. As a result of the instability, very small variations in either the input power or the input polarization to the fiber result in large changes in the output polarization. A crossed polarizer at the fiber end converts the polarization variation into intensity information. Thus the modulation depth of an input pulse has been increased from 15% to 100%. Modulation gains of as much as 10 times are theoretically possible.

Charge conjugation symmetry and the nonexistence of the photon's static magnetic field

Abstract: It is shown that Evans' proposal of an axial magnetic field associated with a circularly polarized light wave or photon in free space cannot be correct because it would violate charge conjugation symmetry. However, an axial magnetic field induced by a circularly polarized light wave in an atomic or molecular medium conserves charge conjugation symmetry, being observed as the inverse Faraday effect.

On the calibration of line-of-sight magnetograms

Abstract: Inference of magnetic fields from very high spatial, spectral, and temporal resolution polarized images is critical in understanding the physical processes that form and evolve fine scale structures in the solar atmosphere. Studying high spectral resolution data also helps in understanding the limits of lower resolution spectral data. We compare three different methods for calibrating the line-of-sight component of the magnetic field. Each method is tested for varying degrees of spectral resolution on both synthetic line profiles computed for known magnetic fields and real data. The methods evaluated are: (a) the differences in the center of gravity of the right and left circular components for different spectral resolution, (b) conversion of circular polarization, at particular wavelengths, to magnetic fields using model-dependent numerical solutions to the equations of polarized radiative transfer, and (c) the derivative method using the weak field approximation. Each method is applied to very high spatial and spectral resolution circular polarization images of an active region, acquired in the Fei 5250 A Zeeman-sensitive spectral line. The images were obtained using the 20 mA pass-band tunable filter at NSO/Sacramento Peak Observatory Vacuum Tower Telescope. We find that the center-of-gravity separation offers the best way of inferring the longitudinal magnetic field.

Spin polarization of Auger electrons from noble gases after photoionization with circularly polarized light

Abstract: Spin polarization of Auger electrons arising from photoionization of atoms by circularly polarized light is studied theoretically. We present numerical calculations for the spin-polarization and angular anisotropy parameters of Auger decay for the Ar L3M2,3M2,3, Kr M4,5N2,3N2,3, Xe N4,5O1O2,3 and Xe N4,5O2,3O2,3 transitions. Our model employs Dirac-Fock wavefunctions using intermediate coupling for the final ionic states. We give complete data sets including relative intensities, angular distribution and all spin polarization parameters for the transitions considered, that are calculated consistently within one model.

Time reversal and Hermiticity characteristics of polarizability and optical activity operators

Abstract: Exact non-singular effective polarizability operators are introduced that exhibit well defined time reversal and Hermiticity characteristics and can therefore be applied to all forms of (optically active) light scattering including resonance phenomena. Dual circular polarization (DCP) and dual linear polarization (DLP) Raman optical activity (ROA) observables are reformulated in terms of matrix elements of these generalized scattering operators. Employing the fundamental operator symmetry properties, Stokes and anti-Stokes ROA observables are shown to exhibit the same signs for the in-phase DCPI and out-of-phase DLPII modulation technique, and opposite signs for the out-of-phase DCPII and in-phase DLPI modulation approach for the case of diamagnetic chiral molecules.

Computer-aided design of multilayered dielectric frequency-selective surfaces for circularly polarized millimeter-wave applications

Abstract: A procedure for the computer-aided design of dielectric-layered frequency-selective surfaces is introduced. Emphasis is placed on millimeter-wave applications involving circularly polarized incident signals. The design routine incorporates filter theory for high-low impedance structures and analysis concepts known from thin-film optics which include the losses of the materials. For given material constants and performance specifications, the exact thicknesses of the dielectric layers as well as the angle of the incident wave are determined by applying optimization strategies. A 40-GHz design example demonstrates the attractiveness of the design in circularly polarized millimeter-wave frequency-selective surface applications. >

Spin-resolved Auger electron spectroscopy of barium atoms.

Abstract: The angular dependence of the spin polarization of Auger electrons was measured after 5p photoionization of free Ba atoms with circularly polarized light. A substantial polarization transfer from the oriented photoion onto the Auger electrons was observed in confirmation of theoretical predictions by Kabachnik and Lee. The ratio of the dipole transition matrix elements and the orientation and alignment coefficients were extracted from the experimental data for the Auger electron polarization parameters.

Ionization of highly excited hydrogen atoms by a circularly polarized microwave field.

Abstract: Some quantum-mechanical results describing the ionization of initially highly excited hydrogen atoms by a strong, circularly polarized microwave field are presented. A simplified two-dimensional model of the atom is used. Discrepancies between various classical estimates for the low-frequency ionization threshold are resolved.

Analysis and design of a two-octave polarization rotator for microwave frequency

Abstract: The analysis and design of a 45 degrees polarization rotator are presented. A two-octave frequency band is achieved with only three grids of strips, each rotated with respect to the previous one. Over this band the transmission loss is less than 0.5 dB, the circular polarization ratio is less than 2.0 dB, and the major axis of the polarization ellipse forms 45 degrees +or-3 degrees with respect to the linear polarization direction of the incident plane wave. The analysis method is an original scheme based upon the conjugate-gradient fast Fourier transform (CG-FFT) method to analyze periodic structures and the generalized scattering matrix method to study their connection. A comparison between numerical and measured data is presented. >

Spherical helical antenna with circular polarisation over a broad beam

Abstract: A spherical helical antenna is proposed and its radiation characteristics are analysed using numerical methods and experimental measurements. Results for a 10 turn helix are presented. These results indicate that the spherical helix can provide circular polarisation over a wide beamwidth. The radiation patterns have a broad main beam, essentially no sidelobes, and low backlobe level.

Circularly polarized local antenna arrangement for a nuclear magnetic resonance tomography apparatus and method for operating same

Abstract: An antenna arrangement for a magnetic resonance imaging apparatus is composed of an array having at least three linearly polarized local coils which mutually overlap to such an extent that they are geometrically decoupled Each local coil is separately connected to a combination network wherein a complex operation between the output signals of the respective local coils is produced such that a radio-frequency signal corresponding to a circular polarization is obtained