Showing papers on "Circular polarization published in 1994"

Depolarization of multiply scattered waves by spherical diffusers: Influence of the size parameter

Abstract: We study numerically, using the Mie theory, light transmission through a multiply scattering medium composed of a collection of uncorrelated, optically inactive spherical particles The characteristic length over which a plane-wave field is depolarized depends on whether it is initially linearly or circularly polarized and on the size of the particles In a medium containing particles small compared to the wavelength (Rayleigh regime), the characteristic length of depolarization for incident linearly polarized light is found to exceed that for incident circularly polarized light, while the opposite is true in a medium composed of particles large compared to the wavelength (Mie regime) A comparison of numerical results with the data from measurements on suspensions of polystyrene latex spheres in water is made Agreement between these simulations and experiment is good for the range of sizes considered in this paper We also discuss the relevance of the helicity-flip model to the analysis of these data

Second-harmonic generation circular-dichroism spectroscopy from chiral monolayers.

Abstract: The second-harmonic-generation (SHG) efficiency from a monolayer of oriented chiral molecules (R- or S-2,2'-dihydroxyl-1,1'binaphthyl) has a strong dependence upon the handedness of the circularly polarized excitation beam. The SHG circular-dichroism (CD) effect is much larger than in ordinary CD spectroscopy because SHG-CD is electric dipole allowed. The technique allows the study of chiral surfaces. The preference for left or right circularly polarized light is dependent on both the chirality (R or S) and the absolute orientation of the molecules (pointed up or down).

Second‐harmonic generation from chiral surfaces

Abstract: We present a theory of second‐harmonic generation from chiral surfaces including contributions of electric and magnetic dipole transitions to the surface nonlinearity. The nonlinear polarization and magnetization of the surface as well as the second‐harmonic fields that are radiated in the reflected and transmitted directions are expressed in terms of the six possible bilinear combinations of the components of the electric field of the fundamental beam. For the case in which the polarization of the fundamental beam is controlled by means of a quarter‐wave plate between p‐polarized linear and left‐ and right‐hand circular, the second‐harmonic fields can be expanded in terms of only three different functions of the rotation angle of the wave plate. The process exhibits nonlinear optical activity, i.e., it responds differently to the two circular polarizations of the fundamental beam if the phases of certain expansion coefficients are different. The theory is used to explain the results of a recent experiment and excellent agreement is found. The results suggest that in the experiment the largest components of the susceptibility tensors that include magnetic contributions were of the order of ∼10% of those of the electric dipole‐allowed susceptibility tensor.

Dichroic interference effects in circularly polarized soft-x-ray resonant magnetic scattering.

Abstract: Dichroic interference effects were observed in circularly polarized soft-x-ray reflectivity at the Co ${\mathit{L}}_{2,3}$ edges of a magnetic multilayer. The data show a maximum asymmetry ratio of 80%, greater than the degree of circular polarization of the incident light, a changeover in the dichroic characteristics with increasing grazing angle, and an inverted interference pattern in the angular dependence of the asymmetry ratio. These observations are explained successfully by a classical electromagnetic model calculation which includes the complete polarization state of the incident light and spatial interference within the multilayer.

Polarization aberrations. 1. Rotationally symmetric optical systems

Abstract: The polarization in isotropic radially symmetric lens and mirror systems in the paraxial approximation is examined. Polarized aberrations are variations in the phase, amplitude, and polarization state of the electromagnetic field across the exit pupil. Some are dependent on the incident polarization state and some are not. Expressions through fourth order for phase, amplitude, linear diattenuation, and linear retardance aberrations are derived in terms of the chief and marginal ray angles of incidence and the Taylor series expansion coefficients of the Fresnel equations for reflection and transmission at uncoated and thin-film-coated interfaces. Applications to polarization ray tracing are discussed.

Antenna array of radiators with plural orthogonal ports

Abstract: A phased array antenna (10A, 10B, 10C) is constructed of an array of radiators (24), each of which has a radiating aperture, a first port (26) and a second port (26). The first port introduces a first radiation with a first polarization, and the second port introduces a second radiation with a second polarization which is orthogonal to the first polarization. Individual transmitting amplifiers, in the case of a transmitting array, or individual receiving amplifiers (16A), in the case of a receiving array, are connected to the ports of each of the radiators. The amplifiers associated with the first ports of the respective radiators are connected, in turn, to phase shifters (18A) and attenuators (20A) which constitute a first beamformer for forming a set of one or more beams of radiation. The amplifiers (16A) associated with the second ports of the respective radiators are connected, in turn, to phase shifters (18A) and attenuators (20A) which constitute a second beamformer for forming a set of one or more beams of radiation. The two beamformers operate independently of each other so as to permit separately weighted polarization signals of the antenna to be programmed electronically for various polarizations such as right and left circular polarization or horizontal and vertical polarization. Also, the separately polarized waves associated with the first ports and the second ports permit dual polarization frequency reuse transmission.

Generalization of complex Snell–Descartes and Fresnel laws

Abstract: Generalized Snell–Descartes and Fresnel laws are derived for harmonic inhomogeneous plane waves that are incident upon a static interface between two continuous absorbing dielectric media that are macroscopically characterized by their electric and magnetic permittivities and their conductivities. A coordinate-free formalism based on complex vector algebra is used to carry out all discussions. Surprisingly, the usual complex Snell–Descartes laws for reflection and refraction and Fresnel laws for polarization are recovered only in the special case in which the vector characterizing the direction of inhomogeneity is in the plane of incidence. In the more general case a new deflection angle between planes of incidence and refraction has to be introduced. An experiment is proposed to test this prediction. A generalized form of the TE and TM modes (with respect to the interface), which are elliptically polarized and which are called parallel electric and parallel magnetic modes, also emerges.

Energy-Dispersive Phase Plate for Magnetic Circular Dichroism Experiments in the X-ray Range

Abstract: A 220 diamond phase plate was combined with an energy-dispersive absorption spectrometer to convert linearly polarized X-rays into circularly polarized photons and to detect circular magnetic X-ray dichroism (CMXD) from ferromagnetic samples. In these experiments, carried out at LURE, the energy-dispersive spectrometer was equipped with a bent Si (311) polychromator and vertically collimating slits accepting essentially a linearly polarized incident beam. The quarter-wave plate was operated in the Bragg geometry but well outside the range of quasi total reflection, with the consequence that the forward-diffracted beam was circularly polarized with a polarization rate approaching 80% over the whole energy bandpass of the polychromator. CMXD spectra of GdFe2 and GdCo2 intermetallic compounds were recorded at ca 7.2 keV near the Gd LIII absorption edge: they are essentially identical to the spectra commonly recorded with elliptically polarized X-ray photons collected out of the orbit plane of the storage ring. It is suggested that the energy-dispersive phase plate will be very useful to detect CMXD spectra with energy-dispersive spectrometers exploiting the well collimated linearly polarized emission of standard undulators installed on the storage rings of the third generation.

Polarization domain walls in diffractive or dispersive Kerr media.

Abstract: A novel type of vector dark solitary wave is shown to exist in Kerr materials These solitary waves consist of localized structures separating adjacent regions (or domains) of different polarization eigenstates of the Kerr medium

Introductory Lecture The Theoretical Background to Vibrational Optical Activity

Abstract: The difference in the response of a chiral molecule to left and right circularly polarized light is proportional to d/λ times the total response, where d is a typical internuclear distance and λ the wavelength of the light. Hence, while circular dichroism in electronic transitions in chiral molecules has been studied for many years, vibrational circular dichrosim (VCD) and differential Raman optical activity (ROA) have only been observed in the last 20 years. Major advances in experimental techniques have brought VCD and ROA to the point where differential vibrational spectra of large molecules can be recorded in a reasonable time. The basic theory of the rotational strength for vibrational transitions and of ROA is reviewed. The sum of the rotational strengths of all vibrational transitions from the ground state is shown to be zero. The differential absorption of left and right circularly polarized IR radiation by chiral molecules can also be attributed to the different forces acting on the nuclei that are proportional to the nuclear charge times the local electric field. The prospects for future applications of VCD and ROA are briefly discussed.

High-Resolution Nuclear Magnetic Resonance Spectroscopy in a Circularly Polarized Laser Beam

Abstract: Nuclei in a fluid subjected to a continuous wave circularly polarized light beam are predicted to experience a static magnetic field proportional to E± x E±, where E± is the electric vector of the right or left circularly polarized wave and the dot denotes a time derivative. The field strongly depends on the local electronic structure and is present in all atoms. For an intensity of 10 watts per square centimeter propagating in the direction of the field of a magnetic resonance spectrometer, the general theory presented here predicts shifts of ± 4 x 10-8 hertz for protons and ± 10-5 hertz for fluorine-19. Larger shifts are predicted if the laser frequency is near an optical absorption.

Mueller-matrix ellipsometry using the division-of-amplitude photopolarimeter: a study of depolarization effects

Abstract: A fully automated Mueller-matrix ellipsometer with a division-of-amplitude photopolarimeter as the polarization-state detector is described. This device achieves Mueller-matrix ellipsometry by measuring the Stokes parameters of reflected light as a function of the fast axis C of a quarter-wave retarder, which, in combination with a fixed linear polarizer, determines the polarization state of incident light. The reflected Stokes parameters were Fourier analyzed to give the 16 elements of the Mueller matrix. We investigated depolarization of polarized light on reflection from rough, heterogeneous, and anisotropic surfaces by obtaining measurements on rolled aluminum and plant leaves. The results demonstrate (1) a variation of degree of polarization of reflected light with the input polarization state, (2) the precision with which the measured matrices describe the depolarization results, (3) effects of surface anisotropy (rolling direction) on depolarization and cross polarization by reflection from aluminum surfaces, and (4) large values and differences in the depolarization effects from conifer and deciduous leaves. Depolarization of light reflected by the aluminum surfaces was most sensitive to the angle between the plane of incidence and the rolling direction when the incident Stokes parameters S1, S2, and S3 were equal.

Measurements of light scattering by a series of conducting surfaces with one-dimensional roughness

Abstract: Measurements of the polarization dependence of the coherent and diffuse scatter from a set of seven conducting surfaces with strictly one-dimensional roughness are presented. The surfaces have been fabricated in gold with photoresist techniques and have been accurately characterized with stylus profilometry. The standard deviation of surface height σ varies between 0.25 and 1.73 μm throughout the series of surfaces, but all have height statistics that are close to Gaussian and a correlation length that is nearly fixed at 3.3 μm. The polarization dependence of the scattered intensity is fully specified by the four unique elements of the Stokes matrix, which are determined from six intensities measured with different polarization conditions. In studies of the coherent scatter for wavelength 3.392 μm, large differences are found between the p- and s-polarized intensities, and it is shown that the relative phase of the p and s coherent amplitudes is strongly dependent on σ. In the case of diffuse scatter, the wide range of behavior exhibited by the scattered intensities and matrix elements is demonstrated for wavelengths 1.152 and 3.392 μm. The rise of backscattering enhancement and associated polarization effects are seen as σ increases, and it is shown that surfaces with surprisingly modest slopes may produce backscattering enhancement. At high angles of incidence, large differences between the p- and s-polarized diffuse intensities are observed.

Transform relationship between Kerr-effect optical phase shift and nonuniform electric field distributions

Abstract: Electric field distributions measured using the Kerr effect cause a phase shift between light components polarized parallel and perpendicular to the electric field, proportional to the magnitude squared of the electric field components in the plane perpendicular to light propagation integrated over the light path length. One wishes to recover the electric field distribution from measurements of light phase shifts. For axisymmetric geometries where the electric field depends only on the radial coordinate and whose direction is constant along the light path, as is the case along a planar electrode, the total phase shift for light propagating at a constant distance from the center of symmetry and the electric field distribution are related by an Abel transform pair, a special case of Radon transforms typically used in image reconstructions with medical tomography and holography. The more general Radon transform relates the optical phase shift to non-axisymmetric electric field distributions but is restricted to cases where the applied electric field is perpendicular to the plane of light propagation. If the applied electric field direction changes along the light path, it becomes necessary to account for the change in direction of the light components parallel and perpendicular to the applied electric field and the light polarization equations are generalized. >

Polarization multistability and instability in a nonlinear dispersive ring cavity

Abstract: We study the role of polarization in modulational instabilities in a synchronously pumped ring resonator that is filled with an isotropic nonlinear dispersive medium. To describe nonlinear propagation of the polarized field through the ring, we introduce two coupled driven and damped nonlinear Schrodinger equations. These equations, which result from averaging propagation and boundary conditions over each circulation through the ring, permit a simple stability analysis. This analysis predicts polarization multistability in the steady state as well as the emergence of stable pulse trains whose polarization state is either parallel or orthogonal to a linearly polarized synchronous pump beam. The analytical predictions are confirmed and extended by numerical simulations of polarized wave propagation in the cavity.

Electronic variable polarization antenna feed apparatus

Abstract: Electronically adaptable polarization antenna feed apparatus for receiving any sense of linear polarization or for receiving either sense of circular polarization. The circuit makes use of variable gain amplifiers and/or variable attenuators in each of two orthogonally polarized paths. The signal to noise ratio of a desired polarization sense is maximized by adjusting the amplitude or gain in each path to a desired ratio. If desired, polarization components from the two probes may be combined so that orthogonal polarization senses from two frequency reuse signals be used simultaneously. The apparatus may be used with linearly polarized signals and with circularly polarized signals.

Efficient excitation of a mesospheric sodium laser guide star by intermediate-duration pulses

Abstract: Calculations of backscatter emission of mesospheric sodium atoms in a laser guide star that is excited by pulses ranging from 30-ns to 0.9-μs duration are described. The efficient use of such pulses at saturating irradiance values is shown to require ~3 GHz of spectral broadening to provide access to the full absorption spectrum of the D2 line. The broadening is provided by frequency modulation. A set of density matrices was used to account for all 24 hyperfine states and inhomogeneous Doppler broadening. At the broadband (3-GHz) saturation irradiance of 4 W/cm2, both linearly and circularly polarized laser beams are shown to produce emission rates exceeding 60% of the maximum possible rate-equation rate for the 0.9-μs pulses. As expected, circular polarization produced more backscatter than did linear polarization, but the enhancement never exceeded 1/3 in the calculations that are reported. A brief estimate of state precession in the Earth’s magnetic field suggests that achieving even this enhancement will require that the time scale for optical pumping be held to less than 1 μs, which will require the use of irradiances greater than 0.7 W/cm2 and spectral coverage of the full 3-GHz hyperfine-plus-Doppler absorption profile, at least until most of the population is pumped out of the F = 1 ground states.

Intensity-dependent circular polarization and circumstellar magnetic fields from the observation of SiO masers

Abstract: A new aspect of the propagation of astrophysical maser radiation in the presence of a magnetic field is described in which circular polarization is created. The resulting antisymmetric spectral line profile for this circular polarization resembles that produced by the ordinary Zeeman effect when the Zeeman splittings are much less than the spectral line breadth. It is caused by the change, with increasing maser intensity, in the axis of symmetry for the molecular quantum states from a direction that is parallel to the magnetic field to a direction that is parallel to the direction of propagation. When the maser is radiatively saturated, and the rate for stimulated emission is within an order of magnitude of the Zeeman splitting in frequency units, this 'intensity-dependent circular polarization' is greater than that due to the ordinary Zeeman effect by factors as large as 1000. The circular polarization that is observed in the spectra of circumstellar SiO (J = 1-0) masers associated with late-type giants and supergiants may then be caused by magnetic fields as weak as about 10 mG. With the standard Zeeman interpretation of the observations, magnetic fields of 10-100 G are indicated. The lower fields are similar to the limits obtained from the observation of the 22 GHz water masers which are typically somewhat further from the central star. The observed tendency for the fractional linear polarization of SiO masers to increase with increasing angular momentum of the molecular state is shown to be a likely result of anisotropic pumping. Errors are identified that invalidate a recent conflicting claim in the literature about the basic theory of maser polarization in the regime that is relevant here.

Linear and circular dichroism in angle resolved Fe 3p photoemission.

Abstract: Using a recently developed spin-polarized, fully relativistic, multiple scattering approach based on the layer Korringa-Kohn-Rostoker Green function method, we have reproduced the Fe $3p$ angle resolved soft x-ray photoemission spectra and analyzed the associated large magnetic dichroism effects for excitation with both linearly and circularly polarized light. Comparison between theory and experiment yields a spin-orbit splitting of 1.0-1.2 eV and an exchange splitting of 0.9-1.0 eV for Fe $3p$. These values are 50%-100% larger than those hitherto obtained experimentally.

High brightness solid-state laser with zig-zag amplifier

Abstract: A solid-state laser architecture producing a beam of extremely high quality and brightness, including a master oscillator (10) operating in conjunction with a zig-zag amplifier (16,30), an image relaying telescope (17) and a phase conjugation cell (20). One embodiment of the laser architecture compensates for birefringence that is thermally induced in the amplifier (16), but injects linearly polarized light into the phase conjugation cell (20). Another embodiment (19) injects circularly polarized light into the phase conjugation cell (20) and includes optical components that eliminate birefringence effects arising in a first pass through the amplifier (16,30). Optional features (26) permit the use of a frequency doubler assembly to provide output at twice optical frequencies, and an electro-optical switch or Faraday rotator (71) to effect polarization angle rotation if the amplifier material (16,30) can only be operated at one polarization. The zig-zag amplifier (16,30) is cooled by flow of cooling liquid, preferably using longitudinal flow to minimize temperature gradients in a vertical direction, and has cooling channel seals (50,52) disposed in dead zones that receive no light, to minimize optical damage to the seals (50,52). Light is input to the amplifier (16) at a near normal angle of incidence, to minimize polarization by reflection and to permit a polarizer (14) to be used to extract an output beam from the amplifier (16,30). Antireflective coatings (60) on edges and on sides of the amplifier (16,30) eliminate parasitic oscillations, and wedge-shaped windows (36) provide uniform pumping by eliminating gaps between diode arrays (32).

Stabilization in relativistic photoionization with circularly polarized light

Abstract: Relativistic ionization rates and related phenomena are calculated for ground-state hydrogen atoms in the presence of a circularly polarized electromagnetic field A Dirac formalism is used, with spin effects fully included A primary purpose of this work is to explore the effects of relativity on atomic strong-field stabilization A wide range of frequencies is covered, and in all cases relativistic effects first dampen the stabilization phenomenon as the field intensity increases, and then strongly enhance it in the intensity domain where the ponderomotive potential exceeds the electron rest energy Relativistic effects can produce major changes in the photoelectron energy spectrum, but perhaps the most easily observable effect is the major shift that can occur in the photoelectron angular distribution

Interferometric enhancement of chiral asymmetries: Ellipsometry with an optically active Fabry-Perot interferometer

Abstract: Light propagation through an optically active Fabry–Perot interferometer, consisting of an isotropic chiral layer separating two homogeneous achiral media, is analyzed with an interest in the manifestations of chiral asymmetry in the reflected wave. It is shown that the differential reflection of circularly polarized light and the optical rotation and the ellipticity of incident linearly polarized light can be enhanced by several orders of magnitude (depending on reflection geometry, layer thickness, and optical parameters) under conditions of moderate to high levels of reflectance. This would provide a practical experimental approach to investigating chiral structure and interactions in systems not easily amenable to study, such as weakly chiral thin films and layers, both transparent and absorbing. (Previously proposed and confirmed enhancement techniques, based on multiple total external reflection, could be usefully applied only to absorbing chiral media.) Two experimental configurations of particular potential interest employ (1) total internal reflection and (2) metallic reflection at the second chiral–achiral interface. For a sufficiently thick chiral layer the reflectance of polarization orthogonal to that of the incident wave should be directly observable even under conditions of ordinary reflection. It is also shown that, in contrast to wide belief, reflection at normal incidence from an isotropic chiral medium can manifest optical activity (under appropriate circumstances when the ambient achiral media are birefringent).

Multiple-reflection λ4 phase shifter: A viable alternative to generate circular-polarized synchrotron radiation

Abstract: We propose the use of a multiple-reflection polarizer as a highly effective tool to generate circular-polarized synchrotron radiation by utilizing the phase difference in the s- and p-components of multiple-reflected light. In the energy range hν = 8–100 eV, a quadruple reflector can be optimized either in the degree of circular polarization P circ or in the modified throughput TP circ 2 . A linear undulator/quadruple reflector polarizer system is much more effective than the crossed-field undulator concept. In addition to the high throughput, a multiple reflector used as a post-polarizer for monochromatized light provides an easy way to control and modulate the state of polarization P circ . The proposed concept of converting linearly polarized monochromatic light into circular light does not encounter the potential dephasing problems intrinsic to the other alternatives. Extensive model calculations to optimize the figure of merit TP circ 2 are presented, and a mechanical design of a prototype quadruple polarizer presently under construction at SRC is discussed in detail.

High-resolution selective reflection spectroscopy in intermediate magnetic fields

Abstract: We have measured the modifications of frequency-modulated selective reflection spectra of the CsD 2 line recorded with circularly polarized light by a longitudinal magnetic field in the range 120–280 G. The spectra recorded withσ + andσ − polarizations were found to be qualitatively different, but are well described by a theoretical model based on the diagonalization of the hyperfine-Zeeman Hamiltonian. The technique presented here is a simple way for investigating fully resolved complex Zeeman spectra in moderate magnetic fields and may find applications in the investigation of anisotropies in long-range atom-surface interactions.

Directivity of the radio emission from the K1 dwarf star AB Doradus

Abstract: We present measurements of the spectrum and polarization of the flaring radio emission from the K1 dwarf star AB Doradus, together with previously reported single frequency measurements (with no polarization information) on 3 other days. On all 4 days spanning a 6 month period, the emission was strong and, when folded with the stellar rotation period, showed similar time variations with two prominant peaks at phase 0.35 and 0.75. These peaks coincide in longitude with two large starspots identified from the stellar optical light curve and have half-powe widths as small as 0.1 rotations and no larger than 0.2 rotations. The modulated emission shows no measurable circular polarization, and its two peaks have different turnover frequencies.

The complete polarization state of Io-related radio storms from Jupiter: A statistical study

Abstract: We report on the complete polarization state (four Stokes parameters) as a function of both frequency and time, of Io-related radiation from Jupiter as observed with the spectropolarimeter at Nancay, France. Our observations include 37 radio storms, with events of both lefthand (LH) and righthand (RH) elliptical polarization and from all Io-related sources (A,B,C and D), plus two non-Io events. The high degree of circular polarization often observed, particularly for LH emission, is difficult to explain in terms of the standard theory of cyclotron maser radiation. The interpretation of the observations in terms of the locations of the sources in Jupiter's magnetosphere is the subject of a companion paper.

Transmission of light through a slab of cholesteric liquid crystal

Abstract: A theoretical treatment is given of the problem of transmission of light through a slab of cholesteric liquid crystal. The axially symmetric arrangement is considered in which the faces of the crystal are perpendicular to the axis and the light propagation is parallel to the axis. The linear problem, as governed by Maxwell’s equations and the usual boundary conditions, is treated. Explicit solutions for the transmitted fields are obtained, for either circular polarization of the incident wave. Several limiting cases are discussed, in which especially simple formulas apply and in which there are physical pictures of the transmission phenomena.