Showing papers on "Circular polarization published in 2006"

Hybrid-Polarity SAR Architecture

Abstract: A synthetic aperture radar (SAR) often is constrained to transmit only one polarization. Within this constraint, two aggressive measurement objectives are 1) full characterization and exploitation of the backscattered field, and 2) invariance to geometrical orientations of features in the scene. Full characterization implies coherent dual-polarization to support the four Stokes parameters. These are rotationally invariant with respect backscatterer orientation if and only if the transmission is circularly polarized. Given that the data products are the Stokes parameters, the receivers can use any orthogonal polarization basis. A SAR in hybrid-polarity architecture (CL-pol) transmits circular polarization and receives two orthogonal mutually coherent linear polarizations, which is one manifestation of compact polarimetry. The resulting radar is relatively simple to implement, and has unique self-calibration features and low susceptibility to noise and cross-channel errors. It is the architecture of choice for two lunar radars scheduled for launch in 2008. Data from a CL-pol SAR yield to decomposition strategies such as the m-delta method introduced in this paper.

Detection of magnetic circular dichroism using a transmission electron microscope

Abstract: The electron microscope, already a powerful research instrument, could become even more powerful following the discovery that magnetic circular dichroism can be detected with a conventional transmission electron microscope. Materials display magnetic circular dichroism if the absorption of left and right circularly polarized light differs in the presence of an applied magnetic field. Application of this effect using synchrotron X-ray photons is a powerful tool for the investigation of magnetic phenomena. The new technique — EMCD or energy loss magnetic chiral dichroism — exploits the similarities between X-ray absorption and inelastic electron scattering to give a TEM capabilities normally associated with expensive synchrotrons. EMCD could be useful in many fields including spintronics and nanomagnetism. Comparison of measurements of electron energy-loss magnetic chiral dichroism with X-ray magnetic circular dichroism spectra obtained from the same specimen, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. A material is said to exhibit dichroism if its photon absorption spectrum depends on the polarization of the incident radiation. In the case of X-ray magnetic circular dichroism (XMCD), the absorption cross-section of a ferromagnet or a paramagnet in a magnetic field changes when the helicity of a circularly polarized photon is reversed relative to the magnetization direction. Although similarities between X-ray absorption and electron energy-loss spectroscopy in a transmission electron microscope (TEM) have long been recognized, it has been assumed that extending such equivalence to circular dichroism would require the electron beam in the TEM to be spin-polarized. Recently, it was argued on theoretical grounds that this assumption is probably wrong1. Here we report the direct experimental detection of magnetic circular dichroism in a TEM. We compare our measurements of electron energy-loss magnetic chiral dichroism (EMCD) with XMCD spectra obtained from the same specimen that, together with theoretical calculations, show that chiral atomic transitions in a specimen are accessible with inelastic electron scattering under particular scattering conditions. This finding could have important consequences for the study of magnetism on the nanometre and subnanometre scales, as EMCD offers the potential for such spatial resolution down to the nanometre scale while providing depth information—in contrast to X-ray methods, which are mainly surface-sensitive.

Pancharatnam-Berry phase optical elements for wave front shaping in the visible domain: Switchable helical mode generation

Abstract: We report the realization of a Pancharatnam-Berry phase optical element [Z. Bomzon, G. Biener, V. Kleiner, and E. Hasman, Opt. Lett. 27, 1141 (2002)], for wave front shaping working in the visible spectral domain, based on patterned liquid crystal technology. This device generates helical modes of visible light with the possibility of electro-optically switching between opposite helicities by controlling the handedness of the input circular polarization. By cascading this approach, fast switching among multiple wave front helicities can be achieved, with potential applications to multistate optical information encoding. The approach demonstrated here can be generalized to other polarization-controlled devices for wave front shaping, such as switchable lenses, beam splitters, and holographic elements.

Properties of circularly polarized vortex beams

Abstract: The properties of circularly polarized vortex beams in cylindrical polarization bases are studied. A circularly polarized vortex beam is decomposed into radial and azimuthal polarization. With the proper combination of vortex charge and the handedness of the circular polarization, a focal field with an extremely strong longitudinal component as well as a flat-topped profile can be obtained. The cylindrical decomposition also sheds light on the connections between orbital angular momentum and the spin of the light beams.

Mueller matrix approach for determination of optical rotation in chiral turbid media in backscattering geometry.

Abstract: For in vivo determination of optically active (chiral) substances in turbid media, like for example glucose in human tissue, the backscattering geometry is particularly convenient. However, recent polarimetric measurements performed in the backscattering geometry have shown that, in this geometry, the relatively small rotation of the polarization vector arising due to the optical activity of the medium is totally swamped by the much larger changes in the orientation angle of the polarization vector due to scattering. We show that the change in the orientation angle of the polarization vector arises due to the combined effect of linear diattenuation and linear retardance of light scattered at large angles and can be decoupled from the pure optical rotation component using polar decomposition of Mueller matrix. For this purpose, the method developed earlier for polar decomposition of Mueller matrix was extended to incorporate optical rotation in the medium. The validity of this approach for accurate determination of the degree of optical rotation using the Mueller matrix measured from the medium in both forward and backscattering geometry was tested by conducting studies on chiral turbid samples prepared using known concentration of scatterers and glucose molecules.

Aperture fed wideband circularly polarized rectangular stair shaped dielectric resonator antenna

Abstract: A circularly polarized rectangular stair shaped dielectric resonator antenna (DRA) is presented. The DRA is excited by a narrow rectangular slot and rotated 45/spl deg/ with respect to the sides of the DRA to generate circular polarization. A parametric study of the length to width ratio to optimize the axial ratio bandwidth is given. A 3 dB axial ratio bandwidth of 10.6% is achieved when the length to width ratio is 1.9.

Atmosphere models of magnetized neutron stars: QED effects, radiation spectra and polarization signals

Abstract: Observations of surface emission from isolated neutron stars (NSs) provide unique challenges to theoretical modelling of radiative transfer in magnetized NS atmospheres. Recent work has demonstrated the critical role of vacuum polarization effects in determining NS spectra and polarization signals, in particular the conversion of photon modes (due to the 'vacuum resonance' between plasma and vacuum polarization) propagating in the density gradient of the NS atmosphere. Previous NS atmosphere models incorporated the mode conversion effect approximately, relying on transfer equations for the photon modes. Such treatments are inadequate near the vacuum resonance, particularly for magnetic field strengths around B ∼ B l ≃ 7 × 10 13 G, where the vacuum resonance occurs near the photosphere. In this paper, we provide an accurate treatment of the mode conversion effect in magnetized NS atmosphere models, employing both the modal radiative transfer equations coupled with an accurate mode conversion probability at the vacuum resonance, and the full evolution equations for the photon Stokes parameters. In doing so, we are able to quantitatively calculate the effects of vacuum polarization on atmosphere structure, emission spectra and beam patterns, and polarizations for the entire range of magnetic field strengths, B = 10 12 -10 15 G. In agreement with previous works, we find that for NSs with magnetic field strength B ≥ 2B l , vacuum polarization reduces the widths of spectral features, and softens the hard spectral tail typical of magnetized atmosphere models. For B ≤ B l /2, vacuum polarization does not change the emission spectra, but can significantly affect the polarization signals. Our new, accurate treatment of vacuum polarization is particularly important for quantitative modelling of NS atmospheres with 'intermediate' magnetic fields, B ≃ (0.5-2) B l . We provide fitting formulae for the temperature profiles for a suite of NS atmosphere models with different field strengths, effective temperatures and chemical compositions (ionized H or He). These analytical profiles are useful for direct modelling of various observed properties of NS surface emission. As an example, we calculate the observed intensity and polarization light curves from a rotating NS hotspot, taking into account the evolution of photon polarization in the magnetosphere. We show that vacuum polarization induces a unique energy-dependent linear polarization signature, and that circular polarization can be generated in the magnetosphere of rapidly rotating NSs. We discuss the implications of our results to recent observations of thermally emitting isolated NSs and magnetars, as well as the prospects of future spectral and polarization observations.

Atmosphere Models of Magnetized Neutron Stars: QED Effects, Radiation Spectra, and Polarization Signals

Abstract: Theoretical modeling of surface emission from magnetized neutron stars (NSs) requires proper treatment of QED effects, in particular the effect of photon mode conversion due to the ``vacuum resonance'' between plasma and vacuum polarization. Previous NS atmosphere models incorporated this effect approximately, using transfer equations for the photon modes which are inadequate near the vacuum resonance, particularly for field strengths around $B_l\simeq 7\times 10^{13}$ G, where the vacuum resonance occurs near the photosphere. In this paper, we provide an accurate treatment of the QED-induced mode conversion effect in NS atmosphere models, employing both the modal radiative transfer equations, coupled with an accurate mode conversion probability at the resonance, and the full evolution equations for the photon Stokes parameters. In doing so, we are able to quantitatively calculate the effects of vacuum polarization on the atmosphere emission spectra, beam patterns, and polarizations for the entire range of field strengths, $B=10^{12}-10^{15}$ G. We find that for NSs with $B\ga 2 B_l$, vacuum polarization reduces the widths of spectral features, and softens the hard spectral tail typical of magnetized atmosphere models. For $B\la B_l/2$, vacuum polarization does not change the emission spectra, but can significantly affect the polarization signals. We show that vacuum polarization induces a unique energy-dependent linear polarization signature, and that circular polarization can be generated in the magnetospheres of rapidly rotating NSs. We discuss the implications of our results for recent observations of thermally emitting isolated NSs and magnetars, as well as the prospects for future spectral and polarization observations.

Stokes parameters and polarization contrasts in Young's interference experiment

Abstract: We derive a spectral interference law that governs the behavior of the four Stokes parameters in Young's two-pinhole experiment with a random electromagnetic beam. In addition to the visibility of intensity fringes, we introduce three new contrast parameters that describe the interference-induced changes in the field's state of partial polarization. The polarization modulation depends on the electric field correlations at the pinholes and is closely related to the two-point Stokes parameters. The results are expected to be particularly useful in polarization interferometry and electromagnetic coherence theory. The formalism is demonstrated with specific examples.

Full polarization study of SiO masers at 86 GHz

Abstract: Aims. We study the polarization of the SiO maser emission in a representative sample of evolved stars in order to derive an estimate of the strength of the magnetic field, and thus determine the influence of this magnetic field on evolved stars. Methods. We made simultaneous spectroscopic measurements of the 4 Stokes parameters, from which we derived the circular and linear polarization levels. The observations were made with the IF polarimeter installed at the IRAM 30 m telescope. Results. A discussion of the existing SiO maser models is developed in the light of our observations. Under the Zeeman splitting hypothesis, we derive an estimate of the strength of the magnetic field. The averaged magnetic field varies between 0 and 20 Gauss, with a mean value of 3.5 Gauss, and follows a 1/r law throughout the circumstellar envelope. As a consequence, the magnetic field may play the role of a shaping, or perhaps collimating, agent of the circumstellar envelopes in evolved objects.

Determination of chiral asymmetries in the valence photoionization of camphor enantiomers by photoelectron imaging using tunable circularly polarized light

Abstract: An electron imaging technique has been used to study the full angular distribution of valence photoelectrons produced from enantiomerically pure molecular beams of camphor when these are photoionized with circularly polarized light. In addition to the familiar beta parameter, this provides a new chiral term, taking the form of an additional cosine function in the angular distribution which consequently displays a forward-backward electron ejection asymmetry. Several ionization channels have been studied using synchrotron radiation in the 8.85-26 eV photon energy range. With alternating left and right circularly polarized radiations the photoelectron circular dichroism (PECD) in the angular distribution can be measured and shows some strong dynamical variations with the photon energy, depending in sign and intensity on the ionized orbital. For all orbitals the measured PECD has a quite perfect antisymmetry when switching between R and S enantiomers, as expected from theory. In the HOMO(-1) channel the PECD chiral asymmetry curves show a double maxima reaching nearly 10% close to threshold, and peaking again at approximately 20% some 11 eV above threshold. This is attributed to a resonance that is also visible in the beta parameter curve. Newly optimized CMS-Xalpha photoionization dynamics calculations are also presented. They are in reasonably good agreement with the experimental data, including in the very challenging threshold regions. These calculations show that PECD in such randomly oriented samples can be understood in the electric dipole approximation and that, unlike the case pertaining in core-shell ionization-where a highly localized achiral initial orbital means that the dichroism arises purely as a final state scattering effect-in valence shell ionization there is a significant additional influence contributed by the initial orbital density.

Polarization-dependent effects in surface-enhanced Raman scattering (SERS)

Abstract: A few key examples of polarization effects in surface-enhanced Raman scattering (SERS) are highlighted and discussed. It is argued that the polarization of the local field, which is felt by an analyte molecule in a location of high electromagnetic field enhancement (hot-spot), can be very different from that of the incident exciting beam. The polarization dependence of the SERS signal is, therefore, mostly dictated by the coupling of the laser to the plasmons rather than by the symmetry of the Raman tensor of the analyte. This sets serious restrictions for the interpretation of both single-molecule SERS polarization studies and for the use of circularly polarized light in techniques like surface-enhanced Raman optical activity.

The magnetic field in the star-forming region Cepheus A. from H_2O maser polarization observations

Abstract: We present linear and circular polarization observations of the H 2 O masers in 4 distinct regions spread over I x 2 arcsec around the HW2 high-mass young stellar object in the Cepheus A star-forming region. We find magnetic fields between 100-600 mG in the central maser region, which has been argued to trace a circumstellar disk. The masers further from HW2 have field strengths between 30-100 mG. In all cases the magnetic field pressure is found to be similar to the dynamic pressure, indicating that the magnetic field is capable of controlling the outflow dynamics around HW2. In addition to several H 2 O maser complexes observed before, we also detect a new maser filament, ≃1" (≃690 AU) East of HW2, which we interpret as a shocked region between the HW2 outflow and the surrounding medium. We detect a linear polarization gradient along the filament as well as a reversal of the magnetic field direction. This is thought to mark the transition between the magnetic field associated with the outflow and that found in the surrounding molecular cloud. In addition to the magnetic field we determine several other physical properties of the maser region, including density and temperatures as well as the maser beaming angles.

Full-Polarization Observations of OH Masers in Massive Star-forming Regions. II. Maser Properties and the Interpretation of Polarization

Abstract: We analyze full-polarization VLBA data of ground-state OH masers in 18 massive star-forming regions previously presented in a companion paper. We confirm results previously seen in the few individual sources studied at milliarcsecond angular resolution. The OH masers often arise in the shocked neutral gas surrounding ultracompact H II regions. Magnetic fields as deduced from OH maser Zeeman splitting are highly ordered on the scale of a source and on the maser clustering scale of ~1015 cm, which appears to be universal. OH masers around ultracompact H II regions live ~104 yr before turning off abruptly, rather than weakening gradually. These masers have a wide range of polarization properties. At one extreme (e.g., W75 N), π-components are detected and the polarization position angles of maser spots show some organization. At the other extreme (e.g., W51 e1/e2), almost no linear polarization is detected and partial depolarization occurs. A typical source has properties intermediate to these two extremes, with no clear pattern in the distribution of polarization position angles. This can be explained if Faraday rotation in a typical OH maser source is large on a maser amplification length but small on a single (e-folding) gain length. Increasing or decreasing Faraday rotation by a factor of ~5 among different sources can explain the observed variation in polarization properties. Pure π-components (in theory, 100% linearly polarized) are seldom seen. We suggest that almost all π-components acquire a significant amount of circular polarization from low-gain stimulated emission of a σ-component from velocity-coherent OH lying along the propagation path.

MOJAVE: Monitoring of jets in active galactic nuclei with VLBA experiments. II. First-epoch 15 GHz circular polarization results

Abstract: We report first-epoch circular polarization results for 133 active galactic nuclei in the MOJAVE program to monitor the structure and polarization of a flux-limited sample of extragalactic radio jets with the VLBA at 15 GHz. We found strong circular polarization (≥0.3%) in approximately 15% of our sample. The circular polarization was usually associated with jet cores; however, we did find a few strong jet components to be circularly polarized. The levels of circular polarization were typically in the range of 0.3%-0.5% of the local Stokes I value. We found no strong correlations between fractional circular polarization of jet cores and source type, redshift, EGRET detections, linear polarization, or other observed parsec-scale jet properties. There were differences between the circular-to-linear polarization ratios of two nearby galaxies versus more distant quasars and BL Lac objects. We suggest this is because the more distant sources have either (1) less depolarization of their linear polarization, and/or (2) poorer effective linear resolution, and therefore, their VLBA cores apparently contain a larger amount of linearly polarized jet emission. The jet of 3C 84 shows a complex circular polarization structure, similar to observations by Homan & Wardle 5 years earlier; however, much of the circular polarization seems to have moved, consistent with a proper motion of 0.06c. The jet of 3C 273 also has several circularly polarized components, and we find that their fractional circular polarization decreases with distance from the core.

Polar decomposition of 3 x 3 Mueller matrix: a tool for quantitative tissue polarimetry.

Abstract: The polarization properties of any medium are completely described by the sixteen element Mueller matrix that relates the polarization parameters of the light incident on the medium to that emerging from it. Measurement of all the elements of the matrix requires a minimum of sixteen measurements involving both linear and circularly polarized light. However, for many diagnostic applications, it would be useful if the polarization parameters can be quantified with linear polarization measurements alone. In this paper, we present a method based on polar decomposition of Mueller matrix for quantification of the polarization parameters of a scattering medium using the nine element (3 x 3) Mueller matrix that requires linear polarization measurements only. The methodology for decomposition of the 3 x 3 Mueller matrix is based on the previously developed decomposition process for sixteen element (4 x 4) Mueller matrix but with an assumption that the depolarization of linearly polarized light due to scattering is independent of the orientation angle of the incident linear polarization vector. Studies conducted on various scattering samples demonstrated that this assumption is valid for a turbid medium like biological tissue where the depolarization of linearly polarized light primarily arises due to the randomization of the field vector's direction as a result of multiple scattering. For such medium, polar decomposition of 3 x 3 Mueller matrix can be used to quantify the four independent polarization parameters namely, the linear retardance (delta ), the circular retardance (psi), the linear depolarization coefficient (Delta) and the linear diattenuation (d) with reasonable accuracy. Since this approach requires measurements using linear polarizers only, it considerably simplifies measurement procedure and might find useful applications in tissue diagnosis using the retrieved polarization parameters.

Reconfigurable circularly polarized antenna for short-range communication systems

Abstract: This paper describes the design rules of a compact microstrip patch antenna with polarization reconfigurable features (right-handed circular polarization (CP)/left-handed CP). The basic antenna is a circular coplanar-waveguide (CPW)-fed microstrip antenna excited by a diagonal slot and the CPW open end. This device is developed for short-range communications or contactless identification systems requiring polarization reconfigurability to optimize the link reliability. First, experimental and simulated results are presented for the passive version of the antenna excited by an asymmetric slot. A reconfigurable antenna using beam-lead p-i-n diodes to switch the polarization sense is then simulated with an electrical modeling of the diodes. Finally, the efficiency reduction resulting from the diode losses is discussed.

Polarization singularities of focused, radially polarized fields

Abstract: The state of polarization of strongly focused, radially polarized electromagnetic fields is examined. It is found that several types of polarization singularities exist. Their relationship is investigated, and it is demonstrated that on smoothly varying a system parameter, such as the aperture angle of the lens, different polarization singularities can annihilate each other. For example, the evolution of a lemon into a monstar and its subsequent annihilation with a star is studied. Also, the quite rare collision of a C-line and an L-line, resulting in a V-point, is observed.

Polarization properties of Scarabaeidae

Abstract: Beetles of the scarab family are known to reflect circularly polarized light from incident unpolarized light. They are unusual in that there are many animals that use polarized light in some form and several that actually create it, but there are few examples of the creation of circularly polarized light by animals. Scarabs have been measured with a spectropolarimetric reflectometer and are found to reflect light that is generally left-hand circularly polarized. Previous work is summarized, and what is believed to be new measurements of several scarab specimens are presented.

Optical polarization grating induced liquid crystal micro-structure using azo-dye command layer.

Abstract: We create planar-periodic alignment in nematic liquid crystal (LC) cell by using a command layer of azo-dye molecules directly deposited on the cell substrates and exposed with two interfering laser beams of opposite circular polarizations. Permanent high efficiency polarization gratings are thus created. The diffraction efficiency of those gratings is controlled by a uniform electric field applied across the cell. The electro-optical properties of such polarization gratings are studied. Obtained gratings can be used for electrically controlled discrimination and detection of polarized components of light.

Profile morphology and polarization of young pulsars

Abstract: We present polarization profiles at 1.4 and 3.1 GHz for 14 young pulsars with characteristic ages less than 75 kyr. Careful calibration ensures that the absolute position angle of the linearly polarized radiation at the pulsar is obtained. In combination with previously published data, we draw three main conclusions about the pulse profiles of young pulsars. (i) Pulse profiles are simple and consist of either one or two prominent components. (ii) The linearly polarized fraction is nearly always in excess of 70 per cent. (iii) In profiles with two components, the trailing component nearly always dominates, only the trailing component shows circular polarization and the position angle swing is generally flat across the leading component and steep across the trailing component. Based on these results, we can make the following generalizations about the emission beams of young pulsars. (i) There is a single, relatively wide cone of emission from near the last open field lines. (ii) Core emission is absent or rather weak. (iii) The height of the emission is between 1 and 10 per cent of the light cylinder radius.

Two-panel liquid crystal system with circular polarization and polarizer glasses suitable for three dimensional imaging

Abstract: Disclosed herein are systems and methods for three dimensional display that produce stereo images (left and right eye images) each having particular known polarization so that they can be segregated and viewed with suitable polarizer eyeglasses to create the perception in the human mind of a three-dimensional image. According to one or more embodiments, two liquid crystal panels are utilized with the two panels being stacked one in front of the other relative to a light source and two polarizers are used surrounding the panel closest to the light source. In alternative embodiments, that panel and polarizers comprise a typical liquid crystal display (“LCD”) monitor. In preferred embodiments, images produced by the system are then circularly polarized for decoding by eyeglasses having corresponding circularly polarized lenses.

Offset Cross-Slot-Coupled Dielectric Resonator Antenna for Circular Polarization

Abstract: A cross-slot-coupled cylindrical dielectric resonator antenna (DRA) is studied theoretically and experimentally. In previous papers, a cross-slot of unequal slot lengths was centered under the dielectric resonator (DR), resulting in circular polarized operation of the antenna. In the present study, the design is enhanced by setting the centers of the two slots at different positions and taking into consideration the partial independence of the slot modes from the DRA mode. Thus, circular polarization (CP) bandwidth of up to 4.7% is attained experimentally in the broadside direction. It is also shown that a largely asymmetrical structure results in a very high bandwidth, but with the tradeoff of distorted CP operation off-broadside

Integrating polarized light over a planetary disk applied to starlight reflected by extrasolar planets

Abstract: We present an efficient numerical method for integrating planetary radiation over a planetary disk, which is especially interesting for simulating signals of extrasolar planets. Our integration method is applicable to calculating the full flux vector of the disk-integrated planetary radiation, i.e. not only its observed flux (irradiance), but also its state of polarization (linear and circular). Including polarization is important for simulations of the light reflected by a planet, in particular, because this will generally be polarized. Our integration method is based on the expansion of the radiation field of a spherical, horizontally homogeneous planet into generalized spherical functions. With the expansion coefficients, the flux vector of the disk-integrated, reflected starlight can be obtained rapidly for arbitrary planetary phase angles. We describe the theory behind the disk-integration algorithm and results of accuracy tests. In addition, we give some illustrative examples of the application of the algorithm to extrasolar planets.

Polarization modulation of terahertz electromagnetic radiation by four-contact photoconductive antenna

Abstract: Generation and modulation of circularly polarized terahertz electromagnetic radiation have been demonstrated by using a four-contact photoconductive antenna and a total-reflection Si prism. The quality of the circularly polarized terahertz pulsed radiation has been evaluated by using a polarization sensitive terahertz time-domain spectroscopy system. The characteristic of the dynamic modulation between the left and right circularly polarized states of the THz radiation is also evaluated. The ellipticity of the modulated circularly polarized THz radiation without a polarizer is not as good as that of the non-modulated because of the non-uniform bias field distribution and the asymmetric pump laser intensity profile on the photoconductive gap.

The Radio Spectrum of TVLM 513-46546: Constraints on the Coronal Properties of a Late M Dwarf

Abstract: We explore the radio emission from the M9 dwarf TVLM 513-46546 at multiple radio frequencies, determining the flux spectrum of persistent radio emission, as well as constraining the levels of circular polarization. Detections at both 3.6 and 6 cm provide a spectral index measurement α (where Sν ∝ να) of -0.4 ± 0.1. A detection at 20 cm suggests that the spectral peak is between 1.4 and 5 GHz. The most stringent upper limits on circular polarization are at 3.6 and 6 cm, with V/I < 15%. These characteristics agree well with those of typical parameters for early- to mid-type M dwarfs, confirming that magnetic activity is present at levels comparable with those extrapolated from earlier M dwarfs. We apply analytic models to investigate the coronal properties under simple assumptions of dipole magnetic field geometry and radially varying nonthermal electron density distributions. Requiring the spectrum to be optically thin at frequencies higher than 5 GHz and reproducing the observed 3.6 cm fluxes constrains the magnetic field at the base to be less than about 500 G. There is no statistically significant periodicity in the 3.6 cm light curve, but it is consistent with low-level variability.

Electromagnetic forces for an arbitrary optical trapping of a spherical dielectric

Abstract: A double tweezers setup was employed to perform ultra sensitive force measurements and to obtain the full optical force curve as a function of radial position and wavelength. The light polarization was used to select either the transverse electric (TE), or transverse magnetic (TM), or both, modes excitation. Analytical solution for optical trapping force on a spherical dielectric particle for an arbitrary positioned focused beam is presented in a generalized Lorenz-Mie diffraction theory. The theoretical prediction of the theory agrees well with the experimental results. The algorithm presented here can be easily extended to other beam geometries and scattering particles.

Ultrafast laser waveguide writing: lithium niobate and the role of circular polarization and picosecond pulse width

Abstract: For the first time to our knowledge, ultrafast laser writing has generated room-temperature stable guided-wave optics in bulk lithium niobate for the telecommunication spectrum. Among a seven-dimensional parameter space for waveguide optimization, two frequently overlooked parameters, pulse duration and polarization, were found to be key in overcoming undesired nonlinear optical responses imposed by this material. Single-mode waveguides were best formed with circularly polarized light having a relatively long pulse duration of approximately 1.0 ps. The waveguides were highly polarization dependent and guided in both telecommunication bands near 1300 and 1550 nm, exhibiting losses as low as 0.7 dB/cm.

Optical image measuring instrument

Abstract: An optical image measuring instrument capable of shorting the measurement time. A flash is outputted from a xenon lamp (2) and converted into a wide-band light by means of an optical filter (2A). The flash is converted into a linearly polarized light by means of a polarizing plate (3). The linearly polarized light is divided into a signal light (S) and a reference light (R) by means of a semitransparent mirror (6). The reference light (R) is converted into a circularly polarized light by means of a wavelength plate (7). The signal light (S) and the reference light (R) are combined by means of a semitransparent mirror (6) to produce an interference light (L). A CCD (23) detects the interference light having the same characteristic as the interference light (L). The interference light (L) is separated into an S-polarized component (L1) and a P-polarized component (L2) by means of a polarized beam splitter (11). These components are detected by means of CCDs (21, 22). A computer (30) creates an image of an object (O) to be measured from the detection signals from the CCDs (21, 22, 23). With such an optical image measuring instrument (1), since two polarized components (L1, L2) of the interference light (L) can be simultaneously obtained, the measurement time can be shortened.

Wavelength conversion based on four-wave mixing in high-nonlinear dispersion shifted fiber using a dual-pump configuration

Abstract: The dual-pump all-optical wavelength conversion based on a four-wave mixing (FWM) in a high-nonlinear dispersion shifted fiber (HNL-DSF) is demonstrated experimentally. The polarization sensitivity of the wavelength converter based on this dual-pump figuration is investigated experimentally and theoretically. The experimental results indicate that the wavelength-conversion configuration with copolarization pumps shows the smallest polarization sensitivity. A model of the beating-wave modulation from the dynamic wave equation is erected to explain the experimental phenomena, and the theoretical analyses agree well with the experimental results.