Showing papers on "Circular polarization published in 1980"

Theory of free-electron lasers

Abstract: Abstract : A general analysis is presented of free electron lasers in which a static periodic magnetic pump field is scattered from a relativistic electron beam. The steady state formulation of the problem is fully relativistic and contains beam thermal effects. Growth rates associated with the radiation field, efficiencies, and saturated field amplitudes are derived for various modes of operation. Effects of space charge on the scattering process are included and shown to play a dominant role in certain situations. Scaling laws for the growth rates and efficiencies at a fixed radiation frequency as a function of the magnetic pump amplitude are obtained. The shear in beam axial velocity due to self fields is discussed and various methods of reducing it are suggested. Finally, a detailed illustration of a far infrared (lambda = 2mu m) two-stage free electron laser using a 3 MeV electron beam and a 2 cm wavelength magnetic pump field is presented.

The Optical Activity of Nucleic Acids and their Aggregates

Abstract: This review includes experimental and theoretical work on absorption and scattering of light by chiral macromolecules. All molecules absorb light, but only chiral (handed) molecules show a preferential absorption for right or left circularly polarized light. This phenomenon of circular dichroism (CD) has been very useful in characterizing any chiral aggre­ gate of chromophores-including proteins, nucleic acids, and their complexes. We discuss theoretical methods that relate the absorption and circular dichroism of a polymer or aggregate to the optical proper­ ties of its constituent parts. The experimental data reviewed is limited essentially to nucleic acids, for lack of space. Two new experimental methods that are particularly useful for mac­ romolecules or for systems that scatter a significant fraction of the incident light are fluorescence-detected circular dichroism and circular intensity differential scattered light. Fluorescence-detected circular di­ chroism (FDCD), as the name implies, used the intensity of the fluores­ cence emitted to monitor the intensity of the light absorbed (167). This method provides two main advantages: (a) The spectrum of a complex system containing many chromophores, but only a few fluorophores is greatly simplified, and (b) scattering artifacts, which plague circular dichroism studies using transmitted light measurements, are partly avoided by use of fluorescence detection. The theory (159, 160) and

Circular intensity differential scattering of light by helical structures. I. Theory

Abstract: General expressions for the fields and intensities of scattered electromagnetic radiation by helical structures whose optical properties are described by a uniaxial polarizability along the tangent to the helix are obtained Analytical expressions for the circular intensity differential of scattering (CIDS) as a function of the helix parameters and the wavelength of light are derived It is shown that a sufficient condition for the existence of differential scattering for right and left circularly polarized light is the existence of an asymmetric polarizability The choice of a uniaxial polarizability is found to give rise to form CIDS For incident plane‐polarized light, the scattered fields are found to be generally elliptically polarized The Stokes parameters describing the state of polarization of the scattered radiation are derived These results are expected to be relevant both as a new method to characterize chiral regions in macromolecular structures as well as to describe the light scattering pr

A model for sunspot associated emission at 6 CM wavelength

Abstract: Two-dimensional maps of total intensity and circular polarization of a sunspot region at 6 cm have been calculated using a simple model for the chromosphere-corona transition region and observations of the longitudinal component of the photospheric magnetic field. The calculations are in good agreement with the high resolution observations of the same sunspot region at 6 cm, obtained with the Westerbork Synthesis Radio Telescope. It is shown that the 6 cm radiation is predominantly due to gyroresonance absorption process at the second and third harmonics of the gyrofrequency (H = 900-600 G). Estimates of the conductive flux and the electron density in the transition region above the sunspot are also given.

Statistical summaries of polarized pulsar radiation.

Abstract: This paper considers the polarization behavior of 18 pulsars as observed at 430 MHz in the Arecibo Polarization Survey. The results following from a statistical analysis are presented as probability of occurrence histograms at a series of pulse longitudes, with the signal being characterized by its total intensity, fractional linear polarization and angel, and fractional circular polarization. The statistical summary displays then provide a much more comprehensive delineation of polarization characteristics than the usual average Stokes parameters.The displays leave no room for doubt as to the central role of the polarization (states or) modes: two modes are active at one longitude or another in virtually every object, and their vacillating dominance with longitude seriously disrupts the average angle behavior in about half the cases. This is to say that, while the average angle behavior is often inconsistent with the rotating-vector (Radhakrishnan and Cooke) model, the individual modes generally exhibit rotations which accord rather closely with the model.We further consider the angular separation of the two modes and encounter several apparently significant cases of nonorthogonality. Evidence for weak intensity-dependent polarizations is encountered in both the linear and circular histograms. The angle displays provide a basis for investigating the nature of themore » depolarization known to exist at 430 MHz. We finally consider two objects which exhibit complex angle behavior and speculate on the properties of a more general rotating-vector model consistent with them.« less

The position and polarization of Type III solar bursts

Abstract: Observations of the position and polarization of Type V solar radio bursts and their preceding Type III bursts are presented. The polarization, frequency range, source position, source movement, source size and brightness temperature of the bursts were measured using a 24-220 MHz spectropolarimeter, an 8-8000 MHz spectrograph and a three-frequency radioheliograph. Type V radiation is frequently found to have the opposite sense of circular polarization from that of the preceding Type III burst, with a degree of polarization similar to that of harmonic Type III radiation. A reversal of polarization is not observed when the accompanying Type III burst has no fundamental-harmonic structure, or when the Type V radiation is poorly developed. Possible mechanisms for the reversal are examined, including opposite magnetic field directions in Type III and V bursts, changes in mode coupling and a change in the mode of emission from o-mode for Type III to x-mode for Type V, and conditions needed for the mode change which is considered the most likely mechanism, are determined.

Circular Polarization Observed in Bioluminescence

Abstract: While investigating circular polarization in luminescence1,2, and having found it in chemiluminescence3,4, we have studied bioluminescence because it is such a widespread and dramatic natural phenomenon5,6. We report here that left and right lanterns of live larvae of the fireflies, Photuris lucicrescens and Photuris versicolor, emit circularly polarized light of opposite sense.

Microwave oven cavity excitation system employing circularly polarized beam steering for uniformity of energy distribution and improved impedance matching

Abstract: A microwave oven cavity excitation system which introduces circularly-polarized electromagnetic wave energy into a cooking cavity through a plurality of feed points appropriately phased to provide a concentrated beam. The relative phasing of the feed points is varied as a function of time to steer the concentrated beam to sweep the interior of the cavity, improving the time-averaged energy distribution. One form of phase shift element disclosed is a dielectric vane rotated by airflow through a feed waveguide within which the dielectric vane is rotatably mounted. As a result of the circular polarization, standing waves in the direction of one of the cavity dimensions are minimized, and the amount of energy reflected back to the generator may be reduced.

Circular intensity differential scattering of light by helical structures. II. Applications

Abstract: Analysis and numerical calculations were made of the total scattering and differential scattering of circularly polarized light by helices as models of chiral structures. The differential scattering patterns are much more sensitive than the total scattering to helical parameters. For large helices the angular dependence of these patterns show lobes which alternate in sign. The number of lobes and the positions of the zeros directly measure the ratios of radius and pitch to wavelength. The signs depend on the sense of the helix. The results are compared with measured circular intensity differential scattering of membranes from the bacterium Spirillum serpens. Good qualitative agreement is obtained.

Whistler mode wave packets in the Earth's foreshock region

Abstract: Measurements of the velocity of discrete whistler-mode wave packets in the region upstream of the earth's bow shock are presented. Data from the dual magnetometers aboard the ISEE 1 and 2 spacecraft was used to determine the time delay between the appearance of a wavefront at each spacecraft in order to obtain the wave velocity in the spacecraft frame, and from it the intrinsic wave phase velocity. Results from the 11 events characterized by sufficiently large velocities reveal the wave packets to exhibit a nearly perfectly circular polarization with the packet field rotating in the left-hand sense about the ambient field direction in the spacecraft frame. In the plasma rest frame, these waves are found to be right-handed polarized waves with frequencies several times the proton gyrofrequency which are attempting to propagate upstream against the solar wind but are in fact being carried towards the earth by the solar wind flow.

Spin polarized photoelectrons from atoms and molecules

Abstract: This paper reviews the experimental activities since 1979 for the study of the spin polarization of photoelectrons emitted by atoms and molecules exposed to VUV radiation. Two series of studies have been performed: First, using unpolarized and linearly polarized radiation the photoionization of the rare gases Ar, Kr, and Xe has been studied in an angle-, energy-, and spin-resolved photoemission experiment. Second, the spin polarization of photoelectrons produced by circularly polarized VUV radiation has been measured for some atoms and molecules. This was performed for most targets by use of the synchrotron radiation in Bonn.

Feed system for microwave oven

Abstract: The heating uniformity of a microwave oven is significantly improved by a feed that radiates rotating elliptically polarized electromagnetic waves. A pair of crossed slots in the feed rectangular waveguide has directional characteristics and radiates energy with right-hand and left-hand circular polarization. A variable phase shifter in the waveguide changes the phase of one of the polarizations. Both polarizations interfere in the oven cavity and produce rotating elliptically polarized waves. The circular polarizing element can be a 3-dB hybrid coupler or a turnstile junction.

Effect of incoherent scattering on attenuation and cross-polarization of millimeter waves due to rain: preliminary calculations at 34.8 and 82 GHz for spherical raindrops

Abstract: A numerical solution of the radiative transport equation is given for a plane-parallel rain medium in connection with the estimation of the incoherent effect on attenuation and cross-polarization of a millimeter wave due to rain The method of solution consists of an extension of the spherical harmonics method An assembly of spherical raindrops is considered From the analysis, it is found that the exact expression of Mie scattering can be incorporated into the integral term of the transport equation, thus allowing the integral to be evaluated without approximation Preliminary calculations are performed at 348 and 82 GHz Circular polarizations are considered The results include the directional patterns of reflected and transmitted incoherent intensity, both for left-hand and right-hand circular polarizations, the effect of drop size on the directional patterns, the incoherent signal intensities within the rain medium, and so on

Theory of optical polarization due to electron capture from localized target states by fast ions

Abstract: The excited-projectile state density matrix is derived from the impact-parameter version of the Oppenheimer-Brinkman-Kramers approximation for a one electron-capture process from localized target states The result is used to determine the Stokes parameters of the emitted light General conditions for the production of atomic orientation are investigated and the connection to a recently published classical model is established Large values of atomic orientation and alignment are found for capture into a hydrogenic 2p state of the projectile A simple model for the beam-tilted-foil interaction yields a sinβ dependence of the circular polarization on the tilt angleβ Model calculations for beam-surface scattering show good agreement with experimental data for the relative Stokes parameters

A lossless, passive isolator for optically pumped far‐infrared lasers

Abstract: A lossless, passive technique which isolates the pump and the far‐infrared cavities in an optically pumped far‐infrared laser system has been applied successfully. The isolator consists of a polarizer and a quarter‐wave plate. Pumping is accomplished with a circularly polarized beam, and the resulting circular polarization of the far‐infrared output stability without sacrifice of either power or mode quality.

Selectable linear or circular polarization network

Abstract: This specification discloses a polarization network for selectively converting between linear and circular polarization. The network includes a twistable waveguide having a generally rectangular cross section. Coupled to the twistable rectangular waveguide is a transducer waveguide which acts as an interface between the rectangular waveguide and circular waveguide. A polarizer having a generally circular cross section is coupled to the transducer waveguide and can convert transmitted electromagnetic waves between linear and circular polarization. A coupling means permits relative rotation between the transducer waveguide and the polarizer and selective securing at any of three rotational positions. A first position permits a linearly polarized signal to pass through the polarizer remaining linearly polarized. A second position converts between a linearly polarized signal and a right hand circularly polarized signal. A third position converts between a linearly polarized signal and a left hand circularly polarized signal. Second coupling means between the polarizer and circular horn waveguide permits arbitrary orientation of a linearly polarized signal within the cross-sectional plane.

Magnetic Circular Polarization of F Center Emission and the Spin Mixing Processes in Optical Cycle

Abstract: Magnetic circular polarization (MCP) of F centar emission has been measured for KCl, KBr, KI, RbCl, and RbBr. The optical pumping cycle is proposed to include three spin mixing parameters in absoption, radiationless decay ( e r1 ), and emission processes. From the analysis of experimental data using formulae derived from rate equations for the cycle, para- and dia-magnetic components ( Δ p and Δ d ) of MCP, and the spin mixing parameters are determined. Pumping photon energy dependence of Δ p are observed. It shows that e r1 for KBr depends on photon energy and predominates the net spin mixing parameter ( e t ). Temperature dependence of Δ d are shown to be relevant to the spin orbit interaction in the relaxed excited state of F center. The source of the discrepancy found between our determined values of e t and those by other schools is clarified.

Optical polarization of the Seyfert galaxies MRK 3, MRK 231, NGC 3227 and NGC 3516

Abstract: The paper presents intermediate resolution observations of the emission line and continuum polarization of the Seyfert galaxies Mrk 3, Mrk 231, NGC 3227, and NGC 3516. The polarization shows a strong wavelength dependence with the polarization increasing smoothly into the blue for each galaxy. This wavelength dependence, together with the presence of polarized H(alpha) emission, indicates that the polarization of each galaxy is caused by an asymmetric dust envelope surrounding the nucleus. Observations of the polarization of the /O III/ lambda 5007 emission in Mrk 3 and NGC 3227, and the polarization through the nonstellar Na ID line absorption in Mrk 231 are used to place constraints on the extent of the polarizing clouds in these galaxies. No polarization variability was detected with time-bases ranging from a few weeks to three years.

A new helical antenna design for better on-and off-boresight axial ratio performance

Abstract: A new helical antenna design and its measured performance are presented. This modified helical antenna provides significant improvement in the purity of the circular polarization performance over its half-power beamwidth and is most suitable for space communication application where circular polarization purity and minimum-edge-coverage gain are often required.

Microwave oven cavity excitation system providing controlled electric field shape for uniformity of energy distribution

Abstract: A microwave oven cavity excitation system for promoting time-averaged uniformity of microwave energy distribution within the cooking cavity. Circularly-polarized microwave energy is radiated from a feed waveguide into an adjacent cooking cavity by means of an aperture, such as an X-slot, in the feed waveguide properly electrically located laterally within the feed waveguide so as to nominally radiate an electric field having circular polarization properties and, overall, shaped as an approximate hemisphere. A cross-sectional slice of the field, for example in the plane of the food supported on a conventionally-located shelf, is circular in shape. The radiating X-slot is controllably and selectively electrically moved laterally with respect to the waveguide centerline with the result that the sectional shape of the resulting field changes from circular to elliptical, with the degree and orientation of the ellipse depending upon the direction and degree of movement of the coupling aperture with respect to the waveguide centerline. The shape of the field is constantly varied through various elliptical configurations during operation, to provide the desired time-averaged uniformity of energy distribution through a suitably-programmed controller.

Polarization characteristics of an altazimuth sky scanner.

Abstract: A theoretical description of the polarization characteristics of an altazimuth sky scanner optical system based on Mueller-Stokes calculus is presented. This computer-driven optical system was designed to perform laboratory studies of skylight and of celestial objects during day or night, and has no space limitations; however, the two parallel 45 deg tilt mirrors introduce some intrinsic polarization. Therefore, proper data interpretation requires a theoretical understanding of the polarization features of the instrument and accurate experimental determination of the Mueller-Stokes matrix elements describing the polarizing and depolarizing action of the system.

Shifts of Coherent Light Beams on Reflection at Plane Interfaces between Isotropic Media

Abstract: Previous experimental and theoretical work on both longitudinal and transverse shifts of light beams at totally reflecting interfaces is briefly reviewed and the discrepancy between the predictions of the two principal theoretical approaches is discussed. A theoretical treatment, valid for an interface between any two media, is presented. The intensity profile of the reflected beam is the same as that of the incident beam (albeit shifted in the reflecting interface) only for certain polarization states of the incident beam and provided that the reflection parameters of the interface meet certain conditions. If these conditions are not met the reflected beam profile suffers distortion and, possibly, deviation from its expected direction. Because the polarization state of a beam is, in general, altered by reflection, measurements of the shifts over a large range of angles of incidence at a single reflection are needed in order to verify the predictions.