Showing papers on "Birefringence published in 1978"

Applications of linear Dichroism Spectroscopy

Abstract: Linear dichroism is the phenomenon of anisotropic absorption of light. It is shown by materials containing oriented molecules for which the absorption varies with the direction. The absorption intensity is proportional to the square of the scalar product between the electric field vector of the light and a molecule-characteristic transition moment vector, the absorption being maximum when the light vector is polarized parallel to the transition moment and zero when perpendicular to it. Linear dichroism can therefore provide (1) directions of transition moments when the molecule orientation is known (spectroscopic applications), or (2) information on molecular orientation when the transition moments are known (structural applications). Both applications are useful in several chemical systems, but so far the use of linear dichroism has been confined to a relatively small number of specialized laboratories, not least because of a lack of appropriate commercial instruments. Plane-polarized spectra have long been measured on crystals and other well-oriented materials, but systems with less complete orientation have usually been studied by birefringence which has allowed greater sensitivity. Birefringence and linear dichroism are related by the dispersion equations and therefore in principle contain the same basic information. However, linear dichroism is better suited for practical use since it is related in a very simple way to more-or-less well-separated quantal transitions, while birefringence is a complicated average over all transitions in the molecule.

Application Of The Liquid Crystal Light Valve To Real-Time Optical Data Processing

Abstract: This paper reviews the hybrid field-effect liquid crystal light valve (LCLV) and its application to real-time coherent optical data processing. The light valve is basically a high resolution optical-to-optical image converter. The device embodies a CdS photoconductor, a CdTe light-absorbing layer, a dielectric mirror, and a biphenyl liquid crystal layer sandwiched between indium-tin-oxide transparent electrodes deposited on optical quality glass flats. The input image is directed onto the photo-conductor to reduce the impedance of the photoconductor, thereby switching the ac voltage that is impressed across the electrodes onto the liquid crystal to activate the device. The ac operation ensures long operating life for the device. The liquid crystal is operated in a hybrid field-effect mode. It utilizes the twisted nematic effect to create a dark off-state (voltage off the liquid crystal) and optical birefringence to create the bright on-state. The liquid crystal modulates the phase of the coherent readout light. By an additional analyzer an intensity modulation is created.

Laser and Electric Field Induced Birefringence Studies on the Cyanobiphenyl Homologues

Abstract: The optical and d.c. pulsed field Kerr Effects have been used to study the pretransitional behaviour in the isotropic phase for the alkyl cyano-biphenyl homologues (CN φ.φ CnH2n+1) for n = 5–8. From the dynamic and static measurements both the relaxation time (τ) and the Kerr Constant B have been found to be proportional to (T-T*)−1. These results are interpreted in terms of the Landau - de Gennes model and the characteristic parameters of this formalism have been given. The results are discussed in terms of the changing chemical structure.

Linear polarization in birefringent single‐mode fibers

Abstract: Single‐mode fibers which maintain linear polarization are described. The fibers are birefringent and linear polarization is maintained along either of the two principal axes. The birefringence arises from the deliberate enhancement of anisotropic strains.

Physical studies of nematic azoxybenzenes. II. Refractive indices and the internal field

Abstract: The ordinary refractive index and the birefringence are reported for two homologous series of nematic liquid crystals, the p,p′‐di‐n‐alkyl and p,p′‐di‐n‐alkoxy azoxybenzenes. It is found that the high‐frequency dielectric anisotropy is proportional to the anisotropy of the magnetic susceptibility. For axially symmetric molecules this proportionality allows a simplification of the intricate problem of the internal field, which can be taken as independent of the anisotropy of the surroundings of a molecule. Representing a molecule by a homogeneously polarizable spheroid, an equation is derived which relates the anisotropic high‐frequency dielectric permittivity to the anisotropic molecular polarizability, and which gives consistent results for both the solid state and the nematic phase.

Light propagation in twisted anisotropic media: application to photoreceptors.

Abstract: The propagation of light through a slowly twisting anisotropic medium is described by a coupled-mode theory; expressions are derived for the electric field for the case of a birefringent dichroic medium with a constant rate of twist. The method provides a simple and intuitive means for determing the effect of twisting on the linear birefringence and dichroic absorption of the medium, particularly when the light is initially linearly polarized. The theory is well suited to the analysis of light absorption in twisting insect photoreceptors, such as found in bees and ants. We provide full expressions and useful approximations for polarization sensitivity and the initial direction of polarization to give maximum absorption for several types of photoreceptors.

Polarization characteristics of noncircular core single-mode fibers

Abstract: It is well known that geometrical or dielectric imperfections in conventional graded-index single-mode fibers depolarize light after a few centimeters. A slight improvement in the polarization performance of these fibers is achieved by introducing noncircularity in the core shape. This is evident from the measurements on borosilicate fibers with dumbbell shaped cores. This result is correlated with Marcatili’s analysis, which shows that changing the core geometry, from square to rectangular, does not appreciably alter the difference in the propagation constants of the two fundamental modes with orthogonal polarizations. Thus, the noncircular geometry and the associated increase in stress-induced birefringence introduced during the manufacturing process alone are not sufficient to improve the polarization performance, and the enhancement of the anisotropic birefringence is necessary to achieve single-polarization fibers.

Single polarization optical fibers: Exposed cladding technique

Abstract: A simple method is described for fabricating fibers with large strain birefringence starting with standard MCVD preforms. The method is illustrated using borosilicate fibers. The birefringence and polarization properties are measured in a long fiber and the birefringence is also measured using a fiber slice and polarizing microscope.

Spontaneous birefringence in boracites. Measurements and applications

Abstract: The techniques and results of measurements of the spontaneous birefringence versus temperature are discussed for the fully ferroelectric/fully ferroelastic phases of Mg-Cl, Cr-Cl, Mn-Cl, Mn-Br, Mn-I, Fe-Cl, Fe-Br, Fe-I, Co-Cl, Co-Br. Co-I, Ni-Cl, Ni-Br, Cu-Cl, Cu-Br, Zn-Cl, Zn-Br, Zn-I, Cd-Cl, Cd-Br and Cd-I boracites. Three types of phase sequence occur: 43m↔mm2↔m↔3m (Fe-Cl, Fe-I, Co-Cl, Zn-Cl), 43m↔mm2↔3m (Fe-Br) and 43m↔mm2 (all other compositions). All transitions are of first order. The magnetic ones are excluded from discussion.Crystallophysical and potential technological applications of the spontaneous birefringence are commented:-Light gates (display, page composers, holographic storage): the favorable symmetry species 43mFmmm2, 43mFm, 43mF3m and the high spontaneous birefringence of boracites allow light gate devices with longitudinal electrode configuration. Feasibility studies on single crystalline Fe-I and of epitaxial growth of Ni-Cl boracite show that the development of zero shear compositi...

X-ray polarization phenomena

Abstract: A novel X-ray polarimeter has been constructed from two-fold Bragg-reflecting channel-cut germanium crystals. The well-known 333 Bragg reflection of copper Kx 1 radiation was used. In addition, elliptically polarized X-rays were produced with a 220 Bragg-reflecting silicon-crystal phase plate placed between the germanium polarizer and analyser. In a series of experiments the optical properties of various non-diffracting materials were measured. Upper bounds were determined for linear dichroism in sheet polaroid and for circular dichroism in quartz. No evidence has so far been obtained for birefringence in calcite but significant indication of optical rotary power was found in quartz. All of these optical effects have been known for some time in diffracting crystals for which case they can be described by the conventional dynamical diffraction theory.

Orientation development and relaxation in injection molding of amorphous polymers

Abstract: Orientation development in amorphous polymer melts being vitrified in an injection molding process is described. Predictions of orientation development are based on the argument that the stress-optical laws are valid in the molten stare and birefringence values appropriate to the stress levels at the time of vitrification remain. This theory is compared to birefringence distributions determined in injection molded parts.

Flows of Nematic Liquid Crystals

Abstract: The first liquid crystal was discovered about a century ago by Reinitzer (1888), who observed that the organic crystal cholesteryl benzoate melted at a temperature of 145°C yielding a turbid fluid which, upon the further addition of heat, became clear at a temperature of 179°C. The turbid fluid was found to exhibit optical birefringence, so the intermediate state apparently possessed the mechanical pro­ perties of a liquid and the optical properties of a crystal. Further research on the melts of organic crystals composed of relatively long organic molecules indicated that there was often a succession of liquid crystalline states between the crystal and the isotropic fluids. Mixtures of an isotropic liquid and either amphiphilic molecules as in soaps, or the rodlike molecular configurations of synthetic poly­ peptides, were also found to exhibit birefringence over ranges of concentration and/or temperature. The liquid crystalline states can be broadly classified into two types, smectic and nematic, on the basis of the molecular order that is characteristic of each. In smectic liquid crystals the molecules are arranged in parallel layers, often with their long axis oriented normal to the layers. Usually the molecules of each layer possess the mobility of a fluid within the layer. The orientation of the long axis with respect to the layer and the degree of mobility of the molecules within the layers provide the basis for further subclassifications of the smectics. Nematic liquid crystals are less ordered than smectics. In nematics the centers of mass of the molecules are unordered, as in isotropic fluids ; but the relatively long molecules exhibit an orientational ordering and, locally, seek to remain parallel to one another. In an ideal sample of nematic there is uniform parallel alignment of the molecules, and short-range molecular forces resist any departures from this homogeneous state. Chiral molecules with a liquid crystalline phase exhibit a variation of the nematic ordering. In these materials, called cholesteric liquid crystals, the molecules, locally, lie on parallel planes in which nematic order prevails ; how­ ever, as the normal to the planes is traversed, the direction of the molecules rotates with a pitch that is comparable to optical wavelengths.

Rigid electro-optic device using a transparent ferroelectric ceramic element

Abstract: A rigidly bonded electro-optic phase retardation device comprising a transparent, electrically induced ferroelectric ceramic element bonded with a rigid adhesive in sandwich-like configuration between transparent sheet elements, such as glass, or polarizer elements is described. The electro-optic phase retardation device, provided with means for applying an electric field across at least a portion of the induced ferroelectric ceramic element, can be utilized as a birefringent optical device for controllable modulation of polarized light passing through the device. The device is bonded with an adhesive material, such as a cross-linked unsaturated polyester, effective to provide a rigid boundary between the ceramic material and the sheet elements thereof and exhibits rapid optical response speed in the switching from a field-induced birefringent mode to an isotropic zero-field condition. The device is thus suited to application in welding plates, glasses, goggles, helmets and similar devices for controlling the transmission of light radiations to the wearer's eyes, particularly in the prevention of flashblindness.

Observations of the marginal band system of nucleated erythrocytes.

Abstract: The marginal band (MB) of nucleated erythrocytes (thos of nonmammalian vertebrates) is a continuous peripheral bundle of microtubules normally obscured by hemoglobin. Treatment of these elliptical cells with modified microtubule polymerization media containing Triton X-100 yields a semilysed system in which MB, nucleus, and trans-MB material (TBM) are visible under phase contrast. The TBM apparently interconnects structural components, passing around opposite sides of the nucleus and suspending it in native position. In uranyl acetatestained whole whole mounts (goldfish) examined by transmission electron microscopy, the TBM appears as a network. MBs of semilysed cells are relatively planar initially, but twist subsequently into a range of "figure-8" shapes with one of the two possible mirror-image configurations predominant. Nuclei and MBs can be released using proteolytic enzymes, to which the TBM seems most rapidly vulnerable. MBs thus freed are birefringent, generally untwisted, and much more circular than they are in situ. As a working hypothesis, it is prosposed that the flattened, elliptical shape of nucleated erythrocytes is a result of TBM tension applied asymmetrically across an otherwise more circular MB, and that the firure-8 configuration occurs when there is extreme TBM shrinkage or contraction.

Single polarization optical fibers and methods of fabrication

Abstract: Orthogonally polarized waves are more effectively decoupled in a waveguide that is fabricated in a manner so as to deliberately enhance stress-induced birefringence. This characteristic is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. Three methods of preparing the preform are disclosed. Optical waveguides capable of transmitting power with only one direction of polarization are desirable for use with integrated optical devices which are polarization sensitive.

The Radio-Frequency Birefringence of Polar Ice

Abstract: Radio-echo observations have shown that polar ice in situ is birefringent. The most likely explanation of the birefringence is an anisotropy in the radio-frequency dielectric constant of the ice single crystal, combined with the ordering of the orientations of the ice crystals in polar ice. It is possible to calculate the birefringence of ice which has a distribution of crystal orientations using a technique similar to that used to derive the dielectric properties of heterogeneous media. The experimentally observed birefringence may then be shown to be consistent with the crystal orientation fabric at the site of the observations if the anisotropy of the dielectric constant is slightly less than 1%, that is, slightly less than the accuracy of the laboratory measurements which have failed to detect any anisotropy. Further experimental observations might be used to obtain information on not only the level of anisotropy of the single crystal but also on the crystal orientation fabric of the ice.

Apparatus for High-Resolution Birefringence Measurement in Liquid Crystals

Abstract: A rotating-analyzer technique for measuring the optical birefringence in liquid crystals is described. This apparatus is continuously readable and has excellent sensitivity. For a sample 50 μm thick, the apparatus has a resolution in the change of birefringence of 4 × 10−6, corresponding to a temperature change of 1 mK in the sample tested.

Low-birefringence single-mode optical fibers: preparation and polarization characteristics.

Abstract: A number of applications require single-mode optical fi­ bers in which the state of polarization of the guided light is sustained. The solid-core optical fibers described in the literature generally exhibit strong birefringence and also usually have a depolarizing effect. In this Letter we describe experiments on single-mode fi­ bers that were prepared under carefully controlled conditions and studied for their polarization behavior. The most fa­ vorable results were noted for fibers prepared by chemical vapor deposition (CVD). First, a Si02-B2O3 glass was de­ posited from the gas phase as cladding on the inside wall of a fused-silica tube, followed by pure quartz glass as the core material. The tube was then collapsed into a rod by raising the temperature and thereafter drawn into a fiber. Directly after leaving the furnace, the fiber was coated with silicon resin. Through precise control of the various step of the process, especially during the collapsing of the tube, a circular cross section in preform and fiber were realized. Figure 1 shows a transmission micrograph of a cross section of the preform. A comparison of round and oval fibers made of glass of the same composition showed a circularly symmetrical cross

A new method for measuring the optical activity of crystals and the optical activity of KH2PO4

Abstract: A new and general method has been developed by which the optical activity of crystals can be measured along different directions from the optic axes. The principle of the method is to measure accurately the intensity of emergent light from parallel and crossed polarizers between which the specimen is properly oriented. The experimental apparatus is relatively simple and needs no elaborate machinery. By this method, gyration and birefringence can be obtained simultaneously as a function of wavelength. As a test of the applicability of the method, a component of the gyration tensor g11 and birefringence ne−n0 of quartz have been observed as a function of wavelength, e.g., g11=5.69×10−5, and ne−n0=9.15×10−3 at a wavelength of 550 nm. This method was also applied to the study of the optical activity of KH2PO4. It was found that the nonenantiomorphous D2d phase (paraelectric) of KH2PO4 is optically active, g11=2.28×10−4 at a wavelength of 506 nm.

Optical birefringence of ultrathin AlxGa1−xAs‐GaAs multilayer heterostructures

Abstract: The measured birefringence of ultrathin AlxGa1−xAs‐GaAs multilayers grown by molecular beam epitaxy is shown to result from the difference in the effective refractive indices for transverse electric and magnetic polarizations and from the quantum‐size effect of the one‐dimensionally confined carriers in the GaAs potential wells.

Additive colour properties and colour gamut of cholesteric liquid crystals

Abstract: LIQUID crystals are organic compounds in a state of matter intermediate between the isotropic liquid and crystaline solid. They are fluid but at the same time they exhibit molecular order. Depending on the ordering one distinguishes the smectic, the nematic and the cholesteric liquid crystals1 (CLC). In the latter, the rod-like molecules are arranged in layers with their long axes parallel to each other. In each successive layer the direction of the long axis is rotated by an angle of 10–20 arc min; the molecules then form a helical structure. The spacing between layers differing by an angle of 360° is called the pitch, p. Due to the periodicity of molecular orientation, reflections from all layers separated by p/2 interfere constructively2, if the reflected wavelengths equal the product of the pitch and the refractive indices seen by the wave. This condition is met for a band of wavelength that is relatively narrow and steep and appears as a highly saturated colour2,3. The width of the band equals the product of the pitch and the difference of the two refractive indices of birefringence. Colours produced by such constructive interference, sometimes described as irridescent colours, have also been observed in certain beetles, birds and butterflies4. We report here results of experiments showing that colours of superimposed CLC coatings add like coloured lights and produce a colour gamut greater than that obtained with inks, dyes and pigments.

Optical Anisotropy of Poly (P-Phenylene Terephthalamide) Fibres

Abstract: SUMMARY The principal polarizabilities of poly(p-phenylene terephthalamide)—PPT—fibres were calculated using the molecular model of Northolt, together with the values of bond polarizabilities given by Denbigh and also by Bunn & Daubeny Following generally accepted practice, the Lorentz-Lorenz expression was utilized to derive the principal refractive indices (and the birefringence) of the PPT fibres The experimental values of the refractive indices, for four different PPT samples, were obtained with the aid of the Pluta polarizing interference microscope; some comments on the relevant procedures being included Finally, discussion on the discrepancy between the calculated and observed values of refractive indices is attempted

Influence of noncircular core on the polarisation performance of single mode fibres

Abstract: Measurements on borosilicate fibres with noncircular cores indicate that the noncircular geometry and the associated stress-induced birefringence alone are not sufficient to maintain polarisation in single-mode fibres. To achieve single polarisation fibres, it is necessary to deliberately enhance the anisotropic birefringence.

Generation of tunable radiation below 2000 Å by phase‐matched sum‐frequency mixing in KB5O8⋅4D2O

Abstract: We have investigated second harmonic generation (SHG) and sum‐frequency mixing (SFG) in deuterated potassium pentaborate using N2 laser pumped dye lasers. The crystal 90° second harmonic generates at 2161.5 A with a power conversion efficiency of 5% at an input power of 15 kW. Tunable phase‐matched VUV SFG is observed from 2161.5 to 1962.4 A. Transmission and birefringence measurements indicate that the crystal can phase match with high conversion efficiencies to below 1700 A.

Optical gyroscope system

Abstract: An optical gyroscope is described of the type which passes light beams in opposite directions through a single mode fiber optic wave guide that extends in a circle or coil, and which enables measurement of rotation rate of the coil by measuring the relative phase shifts of the beams by interferometric techniques, wherein simplification and enhanced accuracy are obtained. Beam splitting and phase shifting of the light is facilitated by utilizing brief pulses of light and by using light-controlling devices which are operated for a brief time only when the light pulse passes in one direction through the device but not at a different time when the pulse is passing in the opposite direction through the device. High accuracy in rotation measurement is achieved at both very slow and very fast rotation rates, by alternately operating the system so that at zero rotation the interfering waves are alternately 90° out of phase and in phase. Linear polarization of the light beams is maintained by coiling the full length of the optic fiber in a single plane so that bending stresses induce a birefringence in the waveguide, thus altering the propagation constants of the two polarized modes and decoupling them.

Characteristic relations of flow birefringence

Abstract: This is the first of two closely related papers on the flow-birefringence response to the velocity vector field of a particular liquid representing a certain class of birefringent bodies.* The flowing material under study was the aqueous solution of the compound known under the name NGS 1828 and commonly known as “milling yellow” or “acid yellow”. This solution appears to exhibit all three major mechanisms of birefringence. The physical parameters characterizing this material depend strongly on temperature, concentration and age and, therefore, it can be considered as representing a typical class of liquids used in flow-model experiments. The paper presents the experimental evidence that the flow birefringence cannot be explained and described by the simple mathematical model of birefringence in solid continuum which relates the changes of the components of dielectric tensor to the components of stress and strain tensors, or their derivatives, and which neglects the influence of the spectral frequency (wavelength of radiation). Results are presented for transmission birefringence (and for scattered-light birefringence in the second paper) in the visible and the infrared bands of radiation. It is shown that: It is further shown that there exists a relation between the absorption bands, the maximum transmittance, the dispersion of birefringence, the spectral dependence of optic-axes direction, and the linear range of optical response. Within the maximum transmittance band and the linear range of mechanical response the linear range of birefringence is maximum and the dispersion of birefringence is minimum with respect to the shear-strain rate; the corresponding dispersion of optic axis is also minimum. Samples of typical recordings are given in the visible and the infrared radiation for typical flow patterns. One of the practical conclusions is that to optimize the flow-birefringence studies of engineering problems it is advisable to choose the radiation in the near-infrared range. The evidence presented shows that the common trend in engineering research toward simplification of the model of the flow-birefringence response is not necessary.

Physical Studies of Homologous trans-4-ethoxy-4′-n-alkanoyloxyazobenzenes: Birefringence

Abstract: The birefringence of a homologous series of nematogenic trans-4-ethoxy-4′-n-alkanoyloxyazobenzenes has been measured and observed trends correlated with the change in molecular structure represented by the increasing length of the terminal alkane chain. Order parameters calculated from the birefringence and density data on the basis of the isotropic and anisotropic internal field models are compared and discussed. Also the theory fitting the birefringence data to a power series in temperature is discussed. Fractional volume changes calculated for the nematic-isotropic transition using refractive index data are compared to experimentally observed values.