Showing papers on "Circular polarization published in 1989"

Above-threshold ionization in the long-wavelength limit

Abstract: In the long-wavelength limit, above-threshold ionization (ATI) is primarily the result of the interaction of a newly freed electron with the laser field. Classical physics requires that linearly and circularly polarized light produce very different ATI spectra. Measurements performed using both linearly and circularly polarized, picosecond, 10-\ensuremath{\mu}m pulses confirm these conclusions.

Polarization memory of multiply scattered light.

Abstract: Light backscattered from an optically dense random medium is shown to exhibit a pronounced polarization dependence. An unexpected memory of the incident circular polarization of multiply scattering light arises because the wave's helicity is randomized less rapidly than is its direction. A simple model is developed to account for the observed polarization dependence of the intensity and temporal correlations of the intensity fluctuations of backscattered light.

Optical molasses and multilevel atoms: theory

Abstract: A theoretical analysis is given for laser cooling of a two-level atom with magnetic sublevels in the presence of polarization gradients. The optical Bloch equations for the multilevel system are solved numerically for four combinations of polarizations in one-dimensional optical molasses. The light-pressure force on the atom as given by a simple two-level theory is recovered in the absence of polarization gradients, whereas a spatial variation of the polarization is found to lead to a strong cooling force for slow atoms. The increased cooling force is responsible for the recent observations of atoms cooled in optical molasses to temperatures an order of magnitude below the Doppler limit.

Planar microstrip Yagi array antenna

Abstract: The Yagi concept is introduced for the planar microstrip array. With parasitic director and reflector patches located on the same plane of the driven element, the beam peak, by the effect of mutual coupling, can be tilted toward the endfire direction. With this capability, the power divider and phase delay transmission lines in a conventional array are no longer needed, and hence the array efficiency is increased. Both linear and circular polarization microstrip Yagi arrays are described. >

Circular dichroism in the angular distribution of photoelectrons from oriented CO molecules.

Abstract: Theoretical predictions of a new phenomenon arising in photoelectron emission from oriented molecules by circularly polarized light have been experimentally verified for CO. For a special geometry photoelectron-intensity differences occur upon reversal of photon helicity. The measured asymmetries (up to 80%) show good agreement with an ab initio calculation at photon energies between 20 and 40 eV. Theoretically, the new manifestation of circular dichroism is already obtained in the pure electric dipole approximation without inclusion of spin-orbit interaction.

Redevelopment of Kennaugh's target characteristic polarization state theory using the polarization transformation ration formalism for the coherent case

Abstract: The Kennaugh target characteristic polarization theory for the monostatic reciprocal coherent case is developed in greater detail. Emphasis is on the transformation of the scattering matrix under the change of polarization basis via the unitary transformation matrix formulated in terms of the polarization ratio rho . Six characteristic polarization state are determined, and displayed on the Poincare sphere and on power and phase plots. Several simple target cases for demonstrating the applicability of this useful concept to radar target classification, imaging, and identification are considered. >

Accurate calibration of the four-detector photopolarimeter with imperfect polarizing optical elements

Abstract: The first three columns of the instrument matrix A of the four-detector photopolarimeter (FDP) are determined by Fourier analysis of the output current vector I(P) as a function of the azimuth angle P of the incident linearly polarized light. Therefore 12 of the 16 elements of A are measured free of the imperfections of the (absent) quarter-wave retarder (QWR). The effect of angular beam deviation by the polarizer is compensated for by taking the average, (1/2) [I(P) + I(P + 180°)], of the FDP output at 180°-apart, optically equivalent, angular positions of the polarizer. The remaining fourth column of A is determined by the FDP’s response to the right- and left-handed circular polarization states. Because these states are impossible to generate with an imperfect QWR, a novel procedure is developed. In particular, the response of the FDP to the unattainable right- or left-handed circular polarization state is found by taking the average of the responses of the FDP to an elliptical near-circular state and that state rotated in azimuth by 90°. This calibration scheme is applied to measure A of our prototype FDP of four Si detectors at λ = 632.8 nm. A is determined, in external and internal reference frames, free of imperfections in the polarizing optical elements. The FDP, with its uncontaminated A matrix, is used subsequently to evaluate the imperfections of the QWR with the help of an appropriate model.

Gain of circularly polarised arrays composed of linearly polarised elements

Abstract: The gain of circularly polarized (CP) array antennas realized by proper phasing of sequentially rotated linearly polarized (LP) elements is compared to that of arrays using CP elements and demonstrated by calculations for microstrip patch elements. When element spacing is large and array size is small, the advantages of LP elements are offset by the significant reduction in gain due to high cross polarized lobes in the diagonal planes. For large arrays of closely spaced elements, this gain loss reduces to a negligible amount. However, for spacings above a critical value of about 0.7 wavelengths, unacceptably high gain losses will be incurred.

Optical switches using cholesteric or chiral nematic liquid crystals and method of using same

Abstract: Fiber optic switches using layers of cholesteric or chiral nematic liquid crystals (CLCs) are disclosed. The CLC layer acts both as a polarization beamsplitter and as a reflector by decomposing incoming unpolarized light into two orthogonal circularly polarized components. In a number of embodiments, switchable waveplates are employed to controllably switch the polarization direction of each of the polarization components. The components are then reflected back toward the CLC layer where they are recombined into a transmitted unpolarized beam. The direction of transmission depends on the circular sense of the polarized components, and therefore on the state of the switchable waveplates. In other embodiments, the CLCs are used actively by selectively exposing them to electric or magnetic fields or to heat which cause a phase change that temporarily neutralizes the circular polarization effect. In this manner, the CLCs can be used as switch elements themselves.

Polarized light emission after grazing ion-surface scattering due to capture of spin-polarized electrons.

Abstract: We have observed the capture of polarized electrons into excited terms of atoms after the interaction of fast ions with a magnetized Fe(110) surface at grazing incidence. The spin polarization of captured electrons results in a modified circular polarization fraction of fluorescence light. This experiment has considerable potential as a new analytical tool for investigating surface magnetism with extreme surface sensitivity, as a method for detailed studies of ion-surface interaction, and as a means to produce nuclear spin-polarized beams.

Multiple image, single display system and method

Abstract: A multiple image, single display system (2) includes a monochromatic display source (4) which provides first and second rapidly alternating visual images (8, 10) which pass through an active polarizer assembly (14). The polarizer assembly includes an image polarizer (20), which polarizes all the light to a left hand circular polarization, and a polarization changer (22) coupled to the display source by a synchronizer 28). The polarization changer switches the polarization of the first image but not the second image. A first user (34) sees the first image, but not the second image, through a right hand circular user polarizer (16) while a second user (36) sees the second image, but not the first image, through a left hand circular user polarizer (18). The user polarizers are wave-length sensitive so they block light only if the light has the same polarization and if the light is the wavelength of the light produced by the display source; the user polarizers are otherwise transparent to visible light. This permits the invention to be used in critical low light situations.

Reflection type electrooptical device and a projection type display apparatus using the same

Abstract: In a reflection type liquid crystal electrooptical device having a twisted nematic liquid crystal sandwiched between two substrates faced to each other, a linearly polarized incident light enters into the liquid crystal layer and after the reflection in the state of a circularly polarized light, enters into the liquid crystal layer again. As to an outgoing light, twisted angle, Δnd and polarized incident angle are fixed so as to rotate the polarized surface at 90° with respect to that of the incident light (the condition of twist angle of 63°, 0.33λ≦Δnd≦0.47λ, and twist angle of 193°, 0.95λ≦Δnd≦1.15λ (μm) is preferable, wherein the direction of polarizing shaft coincides with the direction of liquid cyrstal molecules.) Whereby, it is possible to use the largest value of Δnd so that the thick liquid crystal layer can be used and a wide margin for manufacturing can be also secured. Due to the outgoing light as linearly polarized light, light loss can be prevented in comparison with the case of where the outgoing light is an elliptically polarized light. Further, the improved liquid crystal display device has a sharp electrooptical constant relative to an electric field effect, and having high contrast display can be provided by selecting an optimum twist condition.

Effect of chirality on the Doppler shift and aberration of light waves

Abstract: The Doppler shift and aberration effects are examined for the case of plane electromagnetic wave propagation in optically active or chiral media. These phenomena are found to separate a single monochromatic plane wave into two waves, as observed in a coordinate system in motion with respect to the source. It is shown that the Doppler effect is manifested as two shifted frequencies, one for each of the chiral eigenmodes, while the aberration results in two apparent directions of propagation, one for each eigenmode. Therefore, when motion is involved (either a moving source or a moving observer), a monochromatic single plane wave in one frame may be observed as two plane waves with unequal frequencies and differing directions of propagation in the other. The resulting double frequency shift is plotted as a function of relative velocity.

What happens to plane waves at the planar interfaces of mirror-conjugated chiral media

Abstract: The plane-wave reflection and transmission characteristics of bimaterial interfaces between chiral and chiral–achiral interfaces have been extensively explored. We report on the curious characteristics of the interface formed by two chiral half-spaces, one of which is the mirror image of the other; this is referred to as problem 1. It is shown that these characteristics are related to the reflection of plane waves on the interface of a chiral half-space and a perfectly conducting one, which constitutes problem 2.

Photodetachment spectra of H- in parallel electric and magnetic fields.

Abstract: Simple analytic formulas are presented for the photodetachment cross section of ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ in the presence of parallel electric and magnetic fields. A rich array of photodetachment spectra are displayed that illustrate the strong dependence of the cross section on the polarization of the photons and on the electric and magnetic fields.

Polarization Aberration Diagrams

Abstract: Polarization aberrations are variations of the polarization associated with different ray paths through an optical system. This paper presents particular polarization aberration expansion that is second order in the object and pupil coordinates and describes polarization effects that are functionally equivalent to the wavefront aberration terms: piston, tilt, and defocus. This polarization aberration expansion is applicable to weakly polarizing systems, such as radially symmetric systems of lenses and coatings, as well as to strongly polarizing systems incorporating polarizers and retarders. A computer simulation of this expansion has been performed, and graphical results are presented displaying the effect of 24 of these polarization aberrations acting on linearly and circularly polarized light.

Optical activity of polarised atoms

Abstract: General expressions for circular dichroism and circular dichroism in the angular distribution of photoelectrons from polarised atoms in the electric dipole approximation are derived. They are characterised by the same parameters which define the spin polarisation of the photoelectrons ejected from unpolarised atoms.

Brillouin gain variation due to a polarization-state change of the pump or Stokes fields in standard single-mode fibers.

Abstract: The effect of pump or Stokes field polarization-state changes on Brillouin gain is investigated in standard (non-polarization-maintaining) single-mode fibers with a modal birefringence of ~10−7. The Brillouin gain change due to polarization azimuth variation of the input pump or Stokes field reaches ±31% of the average gain expected for a completely scrambled polarization state. This polarization effect on the Brillouin gain may lead to cross-talk fluctuation in bidirectional coherent transmission systems and to degradation of Brillouin amplifier performance.

Polarization phenomena in x-ray optics

Abstract: X-ray optics has undergone a veritable renaissance in recent years because of improvement in instrumentation and methods of measurement, which has been partly but not entirely due to the availability of synchrotron sources of radiation. One of the interesting phenomena in the development of X-ray optics has been the study of polarization effects. As far back as 1906, the polarization of X-rays scattered through 90° was used to demonstrate their electromagnetic character [1]. Classical phenomena, such as birefringence, dichroism, and rotation of polarization [2–6], have been observed in X-ray optics, and there has been discussion on the possibility of producing quarter-wave plates capable of transforming linear into circular polarization and vice versa [3, 7–9].

Dual-polarized printed circuit antenna having its elements, including gridded printed circuit elements, capacitively coupled to feedlines

Abstract: A dual-polarization geometry for printed-circuit antennas, in which two layers of radiating elements and corresponsing power dividers are provided, one set (400) of power dividers being disposed orthogonally with respect to the other (200), so as to enable reception of two signals with orthogonal senses of polarization. Either dual linear or dual circular polarization may be achieved through suitable selection of radiating elements. Alternatively, a quadrature hybrid may be coupled, either externally or as an integral part of the antenna, to enable dual circular polariza­tion. Various shapes and types of radiating elements (16b), having patches or slots, may be employed in the various layers. Fur­ther, a gridded radiating element, formed of a grid (300) wherein a metallization region is etched selectively to yield a plurality of metal strips (16a) disposed parallel to each other with a predeter­mined separation, may be used. This element, in contrast to known elements which employ solid metallization regions, finds particular application in dual polarization geometries, wherein two signals are received simultaneously, and enables improved performance over a wide bandwidth. The gridded element also is applicable to single-polarization geometries.

The optical and near-infrared continuum polarization of five magnetic white dwarf stars - new observations and considerations regarding its origin

Abstract: Linear and circular broadband continuum polarization measurements throughout the 0.35-1.65-micron spectral region were obtained for GrW +70 deg 8247, GD 229, G240-72, G227-35, and LP 790-29. The continuum is found to be characterized by significant Coulomb modification of the Landau-type autoionization thresholds. The spectral characteristics of continuum polarization originating from a centered dipole field distribution are predicted. The magnetobremsstrahlung model can describe several characteristics of the polarization of Grw + 70 deg 8247, suggesting that plasma eigenmodes draw the resultant continuum polarization away from a photoionization edge. 86 refs.

ANSERLIN. a broad-band, low-profile, circularly polarized antenna

Abstract: A technique is presented for improving the performance of low-profile, circularly polarized (CP) radiating-line antennas. The essential elements of the technique pertain to establishing a single traveling wave on an annular sector of conducting strip that is positioned parallel and close to a large conducting surface. The antenna is named the annular sector, radiating-line (ANSERLIN) antenna. Using this technique, the impedance bandwidth becomes so large that it no longer has any bearing on the operating bandwidth. Instead, the pattern shape and axial ratio become the factors that limit the bandwidth. Both senses of circular polarization can be transmitted or received from the same structure. Experimental results for two-port ANSERLIN antennas are presented. The method of exciting a traveling wave on the annular sector is discussed. Design parameters are given for an antenna that will produce a broadside axial ratio that is less than 0.5 dB at a specified frequency. >

Ultrasensitive thermal lens‐circular dichroism spectropolarimeter for small‐volume samples

Abstract: A novel, ultrasensitive crossed‐beam thermal lens‐circular dichroism spectropolarimeter (CBTL‐CD) has been developed. In this instrument, the two excitation beams, left circularly polarized light (LCPL) and right circularly polarized light (RCPL), were derived from the same argon‐ion laser. The chiral sample was sequentially excited by these two beams, and the corresponding thermal lens signals were monitored by a He‐Ne probe laser intersecting perpendicularly with the two pump beams inside the sample. The apparatus is about three orders of magnitude more sensitive than the conventional circular dichroism spectropolarimeter. In addition, it can be used to detect chiral samples having very small volumes. A detection limit of 180 ng of optically active Co(en)3 complexes whose volume was as small as 8 μl has been estimated for this apparatus using 11‐mW excitation laser power modulated at 2.3 Hz. The optics and instrumentation of the apparatus are described in detail.

Calibration of a Mott detector using circularly polarized impact radiation from helium

Abstract: The conventional methods of determining the analyzing power of a Mott detector for electron‐polarization measurement are not very accurate. That is why alternative methods have recently been discussed. The present paper shows how a Mott detector can be calibrated with an uncertainty of ±1% using the circular polarization of impact radiation from helium atoms that are excited by polarized electrons. The retarding‐field‐type Mott analyzer, its performance, and the calibration procedure are described in detail. Accurate data of the circular light polarization that may be used for absolute electron polarimetry are given at electron energies of 23.3 and 26 eV.

Detecting transition regions in magneto‐optical disk systems

Abstract: Several combinations of incident polarization and detection schemes suitable for direct detection of transitions in magneto‐optical disk systems are described. One such combination uses a circularly polarized light with a single split detector in the far field. Another scheme uses linear polarization in conjunction with differential detection. In the first scheme the medium must be optimized for Kerr rotation while the second method requires maximum ellipticity.

Symmetry-dependent alignment of the electron-spin polarization vector due to electronic band hybridization observed in photoemission from Ag(100)

Abstract: We have performed spin- and angle-resolved photoemission experiments in normal emission from a (100) surface of silver using circularly polarized synchrotron radiation at oblique and normal incidence. The observed spin polarization reaches values up to 45%, with the corresponding peaks in the intensity distributions fully consistent with the bulk band structure. In the case of normally incident light the spin polarization vector is aligned with the surface normal, whereas deviations from this behavior are found at oblique incidence of the circularly polarized light. A particular hybridization in the initial states is responsible for this effect, which can be understood on the base of symmetry arguments.