Showing papers on "Optical polarization published in 2002"

Space-variant Pancharatnam-Berry phase optical elements with computer-generated subwavelength gratings.

Abstract: Space-variant Pancharatnam-Berry phase optical elements based on computer-generated subwavelength gratings are presented. By continuously controlling the local orientation and period of the grating we can achieve any desired phase element. We present a theoretical analysis and experimentally demonstrate a Pancharatnam-Berry phase-based diffraction grating for laser radiation at a wavelength of 10.6microm.

Theory of pure dephasing and the resulting absorption line shape in semiconductor quantum dots

Abstract: The pure dephasing of the optical polarization and the corresponding line shape of absorption spectra in small quantum dots due to the interaction of the exciton both with optical and acoustic phonons is calculated. By restricting ourselves to the exciton ground state we obtain a model which is known to be exactly solvable. We study the temperature dependence and the influence of a static electric field. The spectra exhibit strongly non-Lorentzian line shapes including a sharp zero-phonon line. Optical phonons lead to phonon sidebands which may acquire a finite width due to the dispersion of the phonon branch; the width increases with decreasing dot size. Acoustic phonons both due to deformation potential and piezoelectric coupling lead to a broad background in the spectra which is strongly temperature dependent. Typical features of the spectra are qualitatively well reproduced by a perturbative approach based on one-phonon processes. Multiphonon processes, however, give significant contributions in particular in the case of acoustic phonons. Lateral or vertical electric fields lead to an increasing efficiency of the polar interaction mechanisms while deformation potential interaction is much less influenced.

The statistics of polarization-dependent loss in optical communication systems

Abstract: In this letter, we study the statistics of polarization-dependent loss (PDL) in optical systems and evaluate its evolution with system length. We show rigorously that within the range of conceivable parameters in optical links, the distribution of PDL is Maxwellian when it is expressed in decibels. The accumulation of the mean-square PDL with system length stays linear in most systems, but may assume an exponential growth in very long systems with large PDL. The analytical results are compared with numerical simulations and an excellent agreement is observed.

Light propagation characteristics of straight single-line-defect waveguides in photonic crystal slabs fabricated into a silicon-on-insulator substrate

Abstract: Straight single-line defect optical waveguides in photonic crystal slabs are designed by the finite difference time-domain method and fabricated into a silicon-on-insulator (SOI) wafer. By employing an airbridge structure, clear light propagation for both polarizations is observed without any leakage along the waveguide. This experimental result is well explained by photonic bands of pure guided modes. Minimum propagation loss is estimated to be 11 dB/mm. This value is lower than that reported so far for three-line-defect waveguides with an SOI slab structure and almost comparable to that for an index confinement waveguide with a rectangular Si core. This propagation loss is dominated by the scattering loss by some irregularities. However, photonic crystal waveguides have the possibility of an essential lower scattering loss than in the index confinement waveguide because of the inhibition of radiation modes by the photonic bandgap.

Polarized resonance Raman spectroscopy of single-wall carbon nanotubes within a polymer under strain

Abstract: The ${D}^{*}$ Raman band of single-wall carbon nanotubes aligned by shear flow in a polymer matrix has been measured as a function of tensile strain. The Raman intensity varies with the optical polarization direction, an effect which is used here to assess the degree of tube alignment. The strain dependence of the Raman shift depends strongly on the nanotube orientation and the polarization direction. We show that, using polarized light, unoriented nanotubes can be used as strain sensors so that no tube alignment is necessary and the strain can be measured in all directions in a single sample.

Rotors produced and driven in laser tweezers with reversed direction of rotation

Abstract: Micrometer size rotors are produced by light induced polymerization of light curing resin. The propeller shaped rotors trapped in laser tweezers are rotated by the trapping light. A rotor is built that changes the direction of rotation by changing its position relative to the focus. The design is based on the very high numerical aperture of the focused light in the tweezers: the light has a large average component of momentum pointing perpendicular to the optical axis, in the opposite direction before and behind the focus. This perpendicular component drives the rotation and the direction changes when the rotor is moving around the focus.

Semiconductor three-dimensional and two-dimensional photonic crystals and devices

Abstract: Semiconductor three-dimensional (3-D) and two-dimensional (2-D) photonic crystals and their effects on the control of photons are investigated for possible applications to optical chip and functional devices. First we review our approaches creating full 3-D photonic bandgap crystals at near-infrared wavelengths, and also functional devices based on 2-D photonic crystals where the focus is on surface-emitting-type channel-drop filtering devices utilizing single defects in 2-D photonic crystal slabs. Then, we describe the recent progress on 3- and 2-D crystals. On 3-D crystals, the effect of the introduction of a light emitter into the 3-D photonic crystal is investigated, and the design of a single defect cavity is performed. On the 2-D photonic crystals, the photonic states are investigated from the perspective of their polarization properties.

Milestones in the Observations of Cosmic Magnetic Fields

Abstract: Magnetic fields are observed everywhere in the universe. In this review, we concentrate on the observational aspects of the magnetic fields of Galactic and extragalactic objects. Readers can follow the milestones in the observations of cosmic magnetic fields obtained from the most important tracers of magnetic fields, namely, the star-light polarization, the Zeeman effect, the rotation measures (RMs, hereafter) of extragalactic radio sources, the pulsar RMs, radio polarization observations, as well as the newly implemented sub-mm and mm polarization capabilities. The magnetic field of the Galaxy was first discovered in 1949 by optical polarization observations. The local magnetic fields within one or two kpc have been well delineated by starlight polarization data. The polarization observations of diffuse Galactic radio background emission in 1962 confirmed unequivocally the existence of a Galactic magnetic field. The bulk of the present information about the magnetic fields in the Galaxy comes from analysis of rotation measures of extragalactic radio sources and pulsars, which can be used to construct the 3-D magnetic field structure in the Galactic halo and Galactic disk. Radio synchrotron spurs in the Galactic center show a poloidal field, and the polarization mapping of dust emission and Zeeman observation in the central molecular zone reveal a toroidal magnetic field parallel to the Galactic plane. For nearby galaxies, both optical polarization and multifrequency radio polarization data clearly show the large-scale magnetic field following the spiral arms or dust lanes. For more distant objects, radio polarization is the only approach available to show the magnetic fields in the jets or lobes of radio galaxies or quasars. Clusters of galaxies also contain widely distributed magnetic fields, which are reflected by radio halos or the RM distribution of background objects. The intergalactic space could have been magnetized by outflows or galactic superwinds even in the early universe. The Zeeman effect and polarization of sub-mm and mm emission can be used for the study of magnetic fields in some Galactic molecular clouds but it is observed only at high intensity. Both approaches together can clearly show the role that magnetic fields play in star formation and cloud structure, which in principle would be analogous to galaxy formation from protogalactic clouds. The origin of the cosmic magnetic fields is an active field of research. A primordial magnetic field has not been as yet directly detected, but its existence must be considered to give the seed field necessary for many amplification processes that have been developed. Possibly, the magnetic fields were generated in protogalactic plasma clouds by the dynamo process, and maintained again by the dynamo after galaxies were formed.

The Optical Polarization of Near-Infrared-selected Quasi-Stellar Objects

Abstract: Optical broadband polarimetry is presented for near-infrared color-selected active galactic nuclei (AGNs) classified as quasi-stellar objects (QSOs) based on their Ks-band luminosity. More than 10% of a sample of 70 QSOs discovered in the Two Micron All Sky Survey (2MASS) with J-Ks > 2 and M 3%), and values range to a maximum of P ~ 11%. High polarization tends to be associated with the most luminous objects at Ks and with QSOs having the highest near-IR-to-optical flux ratios. The 2MASS QSO sample includes objects possessing a wide range of optical spectral types. High polarization is seen in two of 22 broad emission-line (type 1) objects, but ~ of the QSOs of intermediate spectral type (type 1.5-1.9) are highly polarized. None of the nine QSOs classified as type 2 exhibit P > 3%. It is likely that the unavoidable inclusion of unpolarized starlight from the host galaxy within the observation aperture results in reduced polarization for the narrow emission-line objects. The high polarization of 2MASS-discovered QSOs supports the conclusion inferred from their near-IR and optical colors: that the nuclei of many of these objects are obscured to some degree by dust. Correlations between optical polarization and near-IR luminosity and color imply that the dominant polarizing mechanism in the sample is scattering of AGN light into our line of sight by material located in relatively unobscured regions near the nucleus. The broadband polarization properties of the 2MASS QSO sample are compared to those of other, predominantly radio-quiet, QSOs and are found to be consistent with the idea that the orientation of AGNs to the line of sight plays a major role in determining their observed properties.

Photonic crystals for the visible range fabricated by autocloning technique and their application

Abstract: We fabricate photonic crystals for the visible range by the ‘autocloning’ technique, in which multilayers are stacked by an appropriate combination of sputter deposition and sputter etching. TiO2/SiO2 and Ta2O5/SiO2 are chosen as materials since they are transparent in the range and give a high contrast of refractive indices. The fabrication technique has flexibility regarding materials and size and is very reliable and reproducible even if the pitch is less than 0.2 μm. We also study the application of photonic crystals to birefringent elements such as waveplates and polarization selective gratings and experimentally verify that they are useful for optical pick-up systems.

Multimode interference photonic switches (MIPS)

Abstract: The switching characteristics of multimode interference photonic switches (MIPS) are analyzed by wide-angle finite difference beam propagation method (FD-BPM). As a result, it is found that the MIPS with a multimode interference (MMI) length of 599 /spl mu/m can switch the output light polarization insensitively with a crosstalk of less than -20 dB for a wavelength range of 70 nm. These analyses show that the MIPS can also have a 3-dB coupler function. Experimentally, an InGaAsP/InP 1/spl times/2 MIPS with a thicker InP cladding layer was fabricated and exhibited switching operation with crosstalks of -8 dB and -10 dB at 0 and 20 mA current injections, respectively. For a 2/spl times/2 MIPS with a thinner InP cladding layer, better switching characteristics with a crosstalk of -13 dB and an extinction ratio of 17 dB were realized.

Tunable acoustooptic filters and equalizers for WDM applications

Abstract: We describe a high performance acoustooptic tunable filter for add-drop application and for signal equalization in WDM telecommunication crossconnects. It results from a thorough investigation in TeO/sub 2/ of bulk collinear interaction, the geometry of which, particularly the direction of propagation of the acoustic wave, has been chosen in order to obtain the best compromise between the spectral resolution of the device and the acoustooptic figure of merit. Less than 40 mW of electric power is needed either to deviate 100% of a selected light wavelength /spl lambda/ at resonance, or to induce a 30-dB attenuation of its intensity. The sidelobes practically vanish for this configuration and the resolution is equal to 0.75 nm (or 94 GHz) for /spl lambda/=1.55 /spl mu/m. Polarization splitters combined with half-wave plates allow to completely get rid of polarization sensitivity problems. The use of optical fibers to collect the signal at the filter outputs, actually contributes to the high performance of the device as a whole. Experiments have been performed by multiplexing three signals in the input fiber, separated by 4, 2, and 1 nm. The transmission of the filter has been examined through the bar and cross state.

Formation of three-dimensional periodic microstructures by interference of four noncoplanar beams

Abstract: A newly reported method of making three-dimensional microstructures or photonic crystals by holographic lithography has some obvious advantages over other techniques with the same purpose. A systematic and comprehensive analysis of interference of four noncoplanar beams (IFNB) is provided. It shows that all 14 Bravais lattices can be formed by means of IFNB and gives explicit relationships between each lattice and the corresponding recording geometry. The concept of pattern contrast is extended to the case of IFNB, and it is indicated that a uniform contrast for each interference term can be obtained by properly choosing the beam ratio and polarization. A calculation algorithm is then developed to optimize the direction of polarization of each beam to ensure maximum uniform contrast. These results, verified by computer simulations, may lay a theoretical foundation for fabrication of photonic crystals with the approach of IFNB.

Elliptic critical points: C-points, a-lines, and the sign rule.

Abstract: The critical points of generic paraxial ellipse fields consist of singular points of circular polarization, called C -points, and azimuthal stationary points, i.e., maxima, minima, and saddle points. We define these stationary points here and review their properties. The sign rule for ellipse fields requires that the sign of the singularity indices I(C)=+/-1/2 of the C -points on non-self-intersecting lines of constant azimuthal ellipse orientation (modulo pi/2), i.e., a -lines, alternate along the line. We verify this rule experimentally, using a newly developed interferometric technique to measure C -points and a -lines in an elliptically polarized random optical field.

Influence of anisotropies on transverse modes in oxide-confined VCSELs

Abstract: We present a comprehensive fully vectorial model for the cavity eigenmodes of oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with the details of their complex structure. It includes device-inherent symmetry-breaking mechanisms like noncircular geometries and material anisotropies related to the elasto-optic and electro-optic effect. The latter is accounted for in the model starting from the material and doping profiles. We compare these theoretical results with experimental findings of spectrally and polarization-resolved transverse mode nearfields of oxide-confined VCSELs with two different aperture diameters. Within a parametric study of the influence of aperture anisotropies, we are able to calculate frequencies and gains of all transverse mode families, their polarization dependence and their spatial mode profiles which are in good agreement with the experimental findings.

Black hole mass and binary model for BL Lac object OJ 287

Abstract: Recent intensive observations of the BL Lac object OJ 287 raise a lot of questions on the models of binary black holes, processing jets, rotating helical jets and thermal instability of slim accretion disks. After carefully analyzing their radio flux and polarization data, Valtaoja et al. ([CITE]) propose a new binary model. Based on the black hole mass of estimated with the tight correlations of the black hole masses and the bulge luminosity or central velocity dispersion of host galaxies, we computed the physical parameters of the new binary scenario. The impact of the secondary on the accretion disk around the primary black hole causes strong shocks propagating inwards and outwards, whose arrival at the jet roots is identified with the rapid increase of optical polarization and the large change of polarization angle at about 0.30 yr after the first main optical flare. An increase of optical polarization, a large rotation of positional angle and a small synchrotron flare at 2007.05 between the optical outbursts at 2006.75 and 2007.89 are expected by the model. With the estimated parameters, we predicated an increase of γ -ray flux appearing about 5 days after the first optical/IR peak, which is consistent with the EGRET observations.

Polarization-insensitive quantum-well semiconductor optical amplifiers

Abstract: Theoretical modeling and fabrication of polarization-insensitive semiconductor optical amplifiers that use a multi-quantum-well structure as the gain media are reported. Polarization insensitivity of gain is achieved through the introduction tensile strain into the quantum wells. Gain calculations, using the k/spl middot/p method, were performed to obtain the required amount of tensile strain to obtain polarization insensitivity over a wide energy spectrum. Fabricated amplifiers show a polarization-insensitive (<1 dB) spectral width of 10 nm at 1300 nm in the InGaAsP/InP system, 15 nm at 1300 nm in the AlInGaAs/InP system, and 40 nm at 1550 nm in the AlInGaAs/InP system.

Optical injection dynamics of two-mode vertical cavity surface-emitting semiconductor lasers

Abstract: Two-mode vertical cavity surface-emitting lasers (VCSELs) subject to external optical injection are investigated experimentally. The spectrum of a VCSEL with two parallel-polarized modes demonstrates significantly different behavior compared to that of a VCSEL with two orthogonally-polarized transverse modes. In the latter, external optical injection can be used to achieve single-mode operation. However, in the case of two parallel-polarized transverse modes, VCSEL single-mode operation cannot be obtained via optical injection. In this case, when the injection frequency is close to one of the modes, the coexistence of the injected beam and that mode gives rise to very rich dynamical behavior; however, the other mode is unperturbed by the optical injection.

Spatio-temporal modeling of the optical properties of VCSELs in the presence of polarization effects

Abstract: We develop an optical dynamical model for vertical-cavity surface-emitting lasers (VCSELs) which describes, in an unified way, polarization and spatial effects. The model is based on equations for the lateral dependence of the slowly-varying amplitudes of the optical field in both circular polarizations, and equations for the carrier density in both spin orientations. This provides a natural generalization of the spin flip model for the description of polarization properties of VCSELs extensively used in the literature. In its present form, the model assumes given functional dependence of the guiding mechanisms (built-in refractive index and thermal lensing) as well as the spatial dependence of the current density. We investigate the transverse mode behavior of gain-guided, bottom and top-emitter VCSELs by implementing the model with an analytical approximation to the susceptibility of quantum-well semiconductors. We demonstrate that the stronger the thermal lens, the stronger the tendency toward multimode operation, which indicates that high lateral uniformity of the temperature is required in order to maintain single mode operation in gain-guided VCSELs. We perform analytical calculations of the threshold curves in both types of VCSELs. Also, close-to-threshold numerical simulations show that, depending on the current shape, thermal lensing strength and relative detuning, different transverse modes can be selected.

Optical polarization beam combiner/splitter

Abstract: An optical device for combining two orthogonally polarized beams or splitting a beam into two orthogonally polarized beams is provided that utilizes two collimating/focusing lenses and a thin film wire-grid polarizer. Because the thin film wire-grid polarizer can be fabricated in very thin profile, the provision of a thin film wire-grid polarizer allows the optical polarization beam combiner/splitter device to be highly integrated and simultaneously realize a number of performance advantages of a thin film wire-grid polarizer over other types of polarizers utilized in various prior art polarization beam combiner/splitters.

Polarization conversion in the reflectivity properties of photonic crystal waveguides

Abstract: Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.