Showing papers on "Optical polarization published in 2007"

Synthesis and optical properties of silver nanobars and nanorice.

Abstract: Silver nanobars with rectangular side facets and an average aspect ratio of 2.7 have been synthesized by modifying the concentration of bromide added to a polyol synthesis. Subsequent rounding of nanobars transformed them into nanorice. Due to their anisotropy, nanobars and nanorice exhibit two plasmon resonance peaks, scattering light both in the visible and in the near-infrared regions. With a combination of discrete-dipole approximation calculations and single-nanoparticle spectroscopy, we explored the effect of nanostructure aspect ratio and corner sharpness on the frequency of plasmon resonance. Near-field calculations and surface-enhanced Raman scattering measurements on single particles were performed to show how local field enhancement changes with both the wavelength and polarization of incident light.

Adaptive subwavelength control of nano-optical fields.

Abstract: The size of mechanical tools limits their spatial resolution. In the case of a drill, for instance, its diameter determines the size of the smallest hole that it can make, with 1-mm holes from 10-mm drills an obvious impossibility. But in the world of optics, the 'impossible' can happen. Aeschlimann et al. demonstrate a light-based tool that not only 'drills holes' smaller than its own size (its wavelength), but does so in selectable positions that can be changed at the speed of light. The experiment combines adaptive control with nano-optics, to control interactions between light and matter with sub-wavelength resolution and femtosecond timing. Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems1,2,3,4,5,6,7,8,9,10. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated11,12 that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion13. Shaping of the polarization of the laser pulse14,15 provides a particularly efficient and versatile nano-optical manipulation method16,17. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses14,15 and then probing the lateral field distribution by two-photon photoemission electron microscopy18. In this combination of adaptive control1,2,3,4,5,6,7,8,9,10 and nano-optics19, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions11,12,13,16,17,20 opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.

Circular dichroism of planar chiral magnetic metamaterials.

Abstract: We propose, fabricate, and study a double-layer chiral planar metamaterial that exhibits pronounced circular dichroism at near-infrared wavelengths. The antisymmetric oscillation modes of the two coupled layers allow local magnetic-dipole moments and enhanced polarization effects compared with similar single-layer systems where only electric-dipole moments occur. Experiment and rigorous theoretical calculations are in good agreement.

Generation of arbitrary vector beams with a spatial light modulator and a common path interferometric arrangement

Abstract: We describe a convenient approach for generating arbitrary vector beams in a 4-f system with a spatial light modulator (SLM) and a common path interferometric arrangement. A computer-generated hologram is introduced onto SLM for performing the beam conversion. Optical realization of a variety of polarization configurations confirms the reliability and flexibility of our method.

Devising genuine spatial correlation functions

Abstract: The choice of the mathematical form of spatial correlation functions for optical fields is restricted by the constraint of nonnegative definiteness. We discuss a sufficient condition for ensuring the satisfaction of such a constraint.

Efficient extracavity generation of radially and azimuthally polarized beams

Abstract: We demonstrate an efficient transformation of a linearly polarized Gaussian beam to a radially or an azimuthally polarized doughnut (0,1)* Laguerre-Gaussian beam of high purity. We use a spatially variable retardation plate, composed of eight sectors of a lambda/2 retardation plate, to transform a linear polarization distribution to radial/azimuthal distribution. We transformed an Nd:YAG Gaussian beam with M(2)=1.3 to a radially and azimuthally polarized (0,1)* Laguerre-Gaussian beams with M(2)=2.5 and degree of radial/azimuthal polarization of 96-98%.

Polarimetric characterization of light and media - Physical quantities involved in polarimetric phenomena

Abstract: An objective analysis is carried out of the matricial models representing the polarimetric properties of light and material media leading to the identification and definition of their corresponding physical quantities, using the concept of the coherency matrix. For light, cases of homogeneous and inhomogeneous wavefront are analyzed, and a model for 3D polarimetric purity is constructed. For linear passive material media, a general model is developed on the basis that any physically realizable linear transformation of Stokes vectors is equivalent to an ensemble average of passive, deterministic nondepolarizing transformations. Through this framework, the relevant physical quantities, including indices of polarimetric purity, are identified and decoupled. Some decompositions of the whole system into a set of well-defined components are considered, as well as techniques for isolating the unknown components by means of new procedures for subtracting coherency matrices. These results and methods constitute a powerful tool for analyzing and exploiting experimental and industrial polarimetry. Some particular application examples are indicated.

Quantum-Dot Semiconductor Optical Amplifiers

Abstract: This paper reviews the recent progress of quantum-dot semiconductor optical amplifiers developed as ultrawideband polarization-insensitive high-power amplifiers, high-speed signal regenerators, and wideband wavelength converters. A semiconductor optical amplifier having a gain of > 25 dB, noise figure of 20 dBm, over the record widest bandwidth of 90 nm among all kinds of optical amplifiers, and also having a penalty-free output power of 23 dBm, the record highest among all the semiconductor optical amplifiers, was realized by using quantum dots. By utilizing isotropically shaped quantum dots, the TM gain, which is absent in the standard Stranski-Krastanow QDs, has been drastically enhanced, and nearly polarization-insensitive SOAs have been realized for the first time. With an ultrafast gain response unique to quantum dots, an optical regenerator having receiver-sensitivity improving capability of 4 dB at a BER of 10-9 and operating speed of > 40 Gb/s has been successfully realized with an SOA chip. This performance achieved together with simplicity of structure suggests a potential for low-cost realization of regenerative transmission systems.

Dual-band negative index metamaterial: double negative at 813 nm and single negative at 772 nm.

Abstract: This work is concerned with the experimental demonstration of a dual-band negative index metamaterial. The sample is double negative (showing both a negative effective permeability and a negative effective permittivity) for linearly polarized light with a wavelength between 799 and 818 nm, and the real part of its refractive index is approximately −1.0 at 813 nm. The ratio of −Re(n)/Im(n) is close to 1.3 at 813 nm. For an orthogonal polarization, the same sample also exhibits a negative refractive index in the visible (at 772 nm). The spectroscopic measurements of the material are in good agreement with the results obtained from a finite-element electromagnetic solver for the actual geometry of the fabricated sample at both polarizations.

Spin-polarized light-emitting diodes and lasers

Abstract: Spin-polarized light sources are a new class of devices in which the radiative recombination of spin-polarized carriers results in luminescence exhibiting a net circular polarization. The operation principles and design of spin-polarized light sources are discussed. A comprehensive review of experimental work on spin-polarized light-emitting diodes and surface-emitting lasers is provided, concluding with a discussion of future prospects.

Multiwaveband Polarimetric Observations of 15 Active Galactic Nuclei at High Frequencies: Correlated Polarization Behavior

Abstract: We report on multifrequency linear polarization monitoring of 15 active galactic nuclei containing highly relativistic jets with apparent speeds from ~4c to >40c. The measurements were obtained at optical, 1 mm, and 3 mm wavelengths, and at 7 mm with the Very Long Baseline Array. The data show a wide range in degree of linear polarization among the sources, from 30%, and interday polarization variability in individual sources. The polarization properties suggest separation of the sample into three groups with low, intermediate, and high variability of polarization in the core at 7 mm (LVP, IVP, and HVP, respectively). The groups are partially associated with the common classification of active galactic nuclei as radio galaxies and quasars with low optical polarization (LVP), BL Lacertae objects (IVP), and highly optically polarized quasars (HVP). Our study investigates correlations between total flux, fractional polarization, and polarization position angle at the different wavelengths. We interpret the polarization properties of the sources in the sample through models in which weak shocks compress turbulent plasma in the jet. The differences in the orientation of sources with respect to the observer, jet kinematics, and abundance of thermal matter external to the jet near the core can account for the diversity in the polarization properties. The results provide strong evidence that the optical polarized emission originates in shocks, most likely situated between the 3 and 7 mm VLBI cores. They also support the idea that the 1 mm core lies at the edge of the transition zone between electromagnetically dominated and turbulent hydrodynamic sections of the jet.

Polarization and Phase-Shifting for 3D Scanning of Translucent Objects

Abstract: Translucent objects pose a difficult problem for traditional structured light 3D scanning techniques. Subsurface scattering corrupts the range estimation in two ways: by drastically reducing the signal-to-noise ratio and by shifting the intensity peak beneath the surface to a point which does not coincide with the point of incidence. In this paper we analyze and compare two descattering methods in order to obtain reliable 3D coordinates for translucent objects. By using polarization-difference imaging, subsurface scattering can be filtered out because multiple scattering randomizes the polarization direction of light while the surface reflectance partially keeps the polarization direction of the illumination. The descattered reflectance can be used for reliable 3D reconstruction using traditional optical 3D scanning techniques, such as structured light. Phase-shifting is another effective descattering technique if the frequency of the projected pattern is sufficiently high. We demonstrate the performance of these two techniques and the combination of them on scanning real-world translucent objects.

Self-collimating photonic crystal polarization beam splitter

Abstract: We present theoretical and experimental results of a polarization splitter device that consists of a photonic crystal (PhC) slab, which exhibits a large reflection coefficient for TE and a high transmission coefficient for TM polarization The slab is embedded in a PhC tile operating in the self-collimation mode Embedding the polarization-discriminating slab in a PhC with identical lattice symmetry suppresses the in-plane diffraction losses at the PhC-non-PhC interface The optimization of the PhC-non-PhC interface is thereby decoupled from the optimization of the polarizing function Transmissions as high as 35% for TM- and 30% for TE-polarized light are reported

Calculation of optical trapping forces on a dielectric sphere in the ray optics regime produced by a radially polarized laser beam.

Abstract: We calculated the optical trapping forces on a microscopic particle in the ray optics regime for the case where a radially polarized laser beam is applied. A higher axial trapping efficiency than for a circularly polarized doughnut beam was predicted due to the large p polarization component. Three-dimensional optical trapping was expected for particles with a larger index of refraction and for objectives with a smaller numerical aperture.

Polarization-selective excitation of nitrogen vacancy centers in diamond

Abstract: The nitrogen vacancy (NV) center in diamond is promising as an electron spin qubit due to its long-lived coherence and optical addressability. The ground state is a spin triplet with two levels $({m}_{s}=\ifmmode\pm\else\textpm\fi{}1)$ degenerate at zero magnetic field. Polarization-selective microwave excitation is an attractive method to address the spin transitions independently since this allows operation down to zero magnetic field. Using a resonator designed to produce circularly polarized microwaves, we have investigated the polarization selection rules of the NV center. We first apply this technique to NV ensembles in [100]- and [111]-oriented samples. Next, we demonstrate an imaging technique, based on optical polarization dependence, which allows rapid identification of the orientations of many single NV centers. Finally, we test the microwave polarization selection rules of individual NV centers of known orientation.

ZnO Doped With Transition Metal Ions

Abstract: Spin-dependent phenomena in ZnO may lead to devices with new or enhanced functionality, such as polarized solid-state light sources and sensitive biological and chemical sensors. In this paper, we review the experimental results on transition metal doping of ZnO and show that the material can be made with a single phase at high levels of Co incorporation (~15 at.%) and exhibits the anomalous Hall effect. ZnO is expected to be one of the most promising materials for room-temperature polarized light emission; but to date, we have been unable to detect the optical spin polarization in ZnO. The short spin relaxation time observed likely results from the Rashba effect. Possible solutions involve either cubic phase ZnO or the use of additional stressor layers to create a larger spin splitting in order to get a polarized light emission from these structures or to look at alternative semiconductors and fresh device approaches

Polarization beam splitter using a binary blazed grating coupler.

Abstract: A novel polarization beam splitter using a two-layer grating coupler is proposed and demonstrated. It can directly couple the normally incident light from fiber into two separate waveguides according to their polarization states while splitting them. It realizes high coupling efficiency and a good extinction ratio by using binary blazed grating couplers. The coupling length is less than 14 microm. The extinction ratio is better than 20 dB for both polarizations over a 40 nm wavelength range, and the coupling efficiencies for the two layers are 58% and 50%.

Forward and reverse product decompositions of depolarizing Mueller matrices.

Abstract: Because of the noncommutativity of the matrix product, the three factors into which a depolarizing Mueller matrix is decomposed, i.e., the diattenuator, the retarder, and the depolarizer, form six possible products grouped into two families, as already pointed out [J. Opt. Soc. Am. A13, 1106 (1996); Opt. Lett.29, 2234 (2004)]. We show that, apart from the generalized polar decomposition generating the first family of products, there exists a dual decomposition belonging to the second family. The mathematical procedure for this dual decomposition is given, and the symmetry existing between the two decompositions is pointed out. The choice of the most appropriate decomposition for a given practical optical arrangement is likewise discussed and illustrated by simple examples.

Modeling optical and UV polarization of AGNs. I. Imprints of individual scattering regions

Abstract: Context. Spectropolarimetry of AGNs is a powerful tool for studying the structure and kinematics of the inner regions of quasars. Aims. We wish to investigate the effects of various AGN scattering region geometries on the polarized flux. Methods. We introduce a new, publicly available Monte Carlo radiative transfer code, Stokes, which models polarization induced by scattering off free electrons and dust grains. We model a variety of regions in AGNs. Results. We find that the shape of the funnel of the dusty torus has a sign ificant impact on the polarization e ffi ciency. A compact torus with a steep inner surface scatters more light toward t ype-2 viewing angles than a large torus of the same half-opening angle, �0. For �0 60 ◦ it is polarized parallel to the symmetry axis. In between these intervals the orientation of the polarization depends on the viewing angle. The degree of polarization ranges between 0% and 20% and is wavelength independent for a large range of �0. Observed wavelength-independent optical and near-UV polarization thus does not necessarily imply electron scattering. Spectropolarimetry at rest-fr ame wavelengths less than 2500 A may distinguish between dust and electron scattering but is not conclusive in all cases. For polar dust, s cattering spectra are reddened for type-1 viewing angles, and made bluer for type-2 viewing angles. Polar electron-scattering co nes are very effi cient polarizers at type-2 viewing angles, whilst the polar ized flux of the torus is weak. Conclusions. We predict that the net polarization of Seyfert-2 galaxies decreases with luminosity, and conclude that the degree of polarization should be correlated with the relative streng th of the thermal IR flux. We find that a flattened, equatorial, e lectronscattering disk, of relatively low optical depth, reproduc es type-1 polarization. This is insensitive to the exact geo metry, but the observed polarization requires a limited range of optical d epth.

Electronic and Optical Properties of ${\rm a}$ - and ${\rm m}$ -Plane Wurtzite InGaN–GaN Quantum Wells

Abstract: Electronic and optical properties of a-(Phi = 0) and m-plane (Phi = pi/6) InGaN-GaN quantum-well (QW) structures are investigated using the multiband effective-mass theory with an arbitrary crystal orientation. These results are compared with those of c-plane or (0001)-oriented wurtzite InGaN-GaN QWs. We derive explicitly the Hamiltonians with their elements and the interband optical matrix elements with polarization dependence for the a-, m-, and c-planes. The bandgap transition wavelength of the QW structure with the m-plane is found to be longer than that of the QW structures with the a-plane. The average hole effective masses of the topmost valence band along k'y for the a-and m-planes are significantly lower than that of the c-plane. Here, the prime indicates physical quantities in a general crystal orientation. In addition, their optical gain and optical matrix element show strong in-plane anisotropy. The optical gain of the y'-polarization is much larger than that of the x'-polarization because the optical matrix element for the y'-polarization is larger than that of the x'-polarization.

Polarization patterns of thick clouds: overcast skies have distribution of the angle of polarization similar to that of clear skies

Abstract: The distribution of polarization in the overcast sky has been practically unknown. Earlier the polarization of light from heavily overcast skies (when the Sun's disc was invisible) has been measured only sporadically in some celestial points by point-source polarimetry. What kind of patterns of the degree p and angle α of linear polarization of light could develop after transmission through a thick layer of ice or water clouds? To answer this question, we measured the p and α patterns of numerous totally overcast skies on the Arctic Ocean and in Hungary by full-sky imaging polarimetry. We present here our finding that depending on the optical thickness of the cloud layer, the pattern of α of light transmitted through the ice or water clouds of totally overcast skies is qualitatively the same as the α pattern of the clear sky. Under overcast conditions the value of α is determined predominantly by scattering on cloud particles themselves. Nevertheless, the degrees of linear polarization of light from overcast skies were rather low (p≤16%). Our results obtained under overcast conditions complete the earlier findings that the α pattern of the clear sky also appears in partly cloudy, foggy, and smoky skies. Our results show that the celestial distribution of the direction of polarization is a very robust pattern being qualitatively always the same under all possible sky conditions. This is of great importance for the orientation of polarization-sensitive animals based on sky polarization under conditions when the Sun is not visible.

90% Extraordinary optical transmission in the visible range through annular aperture metallic arrays

Abstract: We demonstrate what we believe to be the first experimental extraordinary optical transmission (EOT) of up to 90%, thanks to a well-identified guided mode that propagates through annular apertures engraved into an optically thick silver layer. In spite of the metal losses, high transmission can be obtained by adjusting the geometrical parameters of the fabricated structure, as was already theoretically demonstrated. To our knowledge, this is the first study showing such a large transmission in the visible range.

Multilayer polarizing grating mirror used for the generation of radial polarization in Yb:YAG thin-disk lasers

Abstract: The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disk laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disk laser.

Experimental demonstration of a narrowband, angular tolerant, polarization independent, doubly periodic resonant grating filter.

Abstract: Resonant grating filters are promising components for free-space narrowband filtering. Unfortunately, due to their weak angular tolerance, their performances are strongly deteriorated when they are illuminated with a standard collimated beam. Yet this problem can be overcome by resorting to a complex periodic pattern known as the doubly periodic grating [Lemarchand et al., Opt. Lett.23, 1149 (1998)]. We report what we believe to be the first experimental fabrication and characterization of a bidimensional doubly periodic grating filter. We obtained a 0.5 nm bandpass polarization independent reflection filter for telecom wavelengths (1520-1570 nm) that presents a transmittivity minimum of 18% with a standard incident collimated beam.

Early Optical Polarization of a Gamma-Ray Burst Afterglow

Abstract: We report the optical polarization of a gamma-ray burst (GRB) afterglow, obtained 203 seconds after the initial burst of γ-rays from GRB 060418, using a ring polarimeter on the robotic Liverpool Telescope. Our robust (2σ) upper limit on the percentage of polarization, less than 8%, coincides with the fireball deceleration time at the onset of the afterglow. The combination of the rate of decay of the optical brightness and the low polarization at this critical time constrains standard models of GRB ejecta, ruling out the presence of a large-scale ordered magnetic field in the emitting region.

Ferromagnetism in Transition-Metal Doped ZnO

Abstract: ZnO is an attractive candidate for spintronics studies because of its potential for exhibiting high Curie temperatures and the relative lack of ferromagnetic second phases in the material. In this paper, we review experimental results on transition-metal (TM) doping of ZnO and the current state of theories for ferromagnetism. It is important to re-examine some of the earlier concepts for spintronics devices, such as the spin field-effect transistor, to account for the presence of the strong magnetic field that has deleterious effects. In some of these cases, the spin device appears to have no advantage relative to the conventional charge-control electronic analog. We have been unable to detect optical spin polarization in ZnO.