Showing papers on "Birefringence published in 1971"

Short Range Order Effects in the Isotropic Phase of Nematics and Cholesterics

Abstract: We assume that (1) the local state of order in the isotropic phase is a symmetric traceless tensor Qαβ, proportional to the anisotropic part of a tensor property such as the magnetic susceptibility; (2) the free energy may be expanded in powers of Qαβ and of its gradients. This allows a unified description covering the anomalous magnetic birefringence, the intensity of light scattering, and the properties of the nematic/isotropic interface. For a cholesteric, although the optical rotation is huge in the ordered phase, we predict that it should not be anomalous just above the transition point Tc. We also investigate the dynamics of fluctuations of Qαβ, and discuss the flow birefringence, the frequency width of the Rayleigh scattering, and the attenuation of ultrasonic shear waves, in terms of 3 viscosity coefficients.

LINEAR AND NONLINEAR OPTICAL PROPERTIES OF ZnGeP2 AND CdSe

Abstract: The refractive indices of ZnGeP2, point group 42m, have been determined from 0.64 to 12 μ. ZnGeP2 has positive birefringence with reasonable temperature tunability and a band gap in the visible. Its nonlinear optical coefficient d14 for second harmonic generation (SHG) measured relative to d14 of GaAs is 0.83 ± 15%. The birefringence of ZnGeP2 is not large enough to allow phase‐matched SHG, but nondegenerate phase‐matched three‐frequency mixing is possible and the parametric oscillator threshold power is calculated for several situations. A redetermination of the coefficient d31 in CdSe is made and the materials compared. It is concluded that ZnGeP2 may be a most promising material for parametric generation in the 0.8– to 12‐μ region.

Optical Distortion by Heated Windows in High-Power Laser Systems,

Abstract: In a typical long‐focal‐length laser system, the distortion of the laser beam by the heated exit window can significantly reduce the intensity at the target at power levels well below those required to melt or fracture the window. The spatially inhomogeneous incident laser intensity causes a temperature gradient which changes the thickness and the index of refraction n of the window, causing it to become a lens having, in general, aberration and birefringence in addition to a finite focal length. Expressions are derived for the thermally induced optical distortion in terms of measurable parameters such as n, dn/dT, the strain‐optical coefficients p11 and p12, and the thermal‐expansion coefficient α. Since the values of all these parameters are not known for most materials of interest, alternate expressions are derived for use in obtaining rough estimates of the amount of distortion. The temperature differential ΔTcrit across the radius of the window which causes a factor of 1/q reduction in target intensity is derived and tabulated (for the case of q=2). Figures of merit for rating materials are derived and tabulated for various materials. The distortion is smaller in general for ionic crystals than for covalent crystals. The briefringence is expected to be larger for the ionic materials than for the covalent ones. In typical systems, rather large increases, say an order of magnitude, in the value of ΔTcrit can be obtained by changing the focal length of the optical system. Other methods of reducing the optical distortion are discussed briefly. Since the optical distortion is expected to be the factor which determines the limit of the target intensity in long‐focal‐length systems, a large heat capacity and small values of α, dn/dT, p11, p12, and n are important, while the melting point, tensile strength, and Young's modulus are less important, within obvious limits. Experiments are suggested for studying the thermally induced optical distortion.

Cuticle reflectivity and optical activity in scarab beetles: the rôle of uric acid.

Abstract: The iridescent cuticle of certain Rutelino scarab beetles, which is a form optically active and selectively reflects circularly polarized light, incorporates an NH 4 OH -extractable component The ultraviolet absorption spectrum of this component, together with its chromatographic and refractive properties, identify it as uric acid (2,6,8-trihydroxypurine). All species of Plusiotis examined have uric acid in their reflecting layers, as do several species of Anoplognathus. Plusiotis resplendens has a reflecting layer with a uric acid volume fraction of 0.7, P . optima a volume fraction of 0.6. The reflecting layer of P . resplenden s has an anticlockwise helicoidal architecture, the optical thickness of the helicoidal p itch being such that it constructively interferes with visible light wavelengths. An anticlockwise helicoid constructively interferes with only the left circularly polarized component of incident light, right circularly polarized light being transmitted without attenuation. P. resplendens has a 1.8 /xm thick unidirectional layer embedded within the helicoid which functions as a perfect halfwave retardation plate for wavelength 590 nm . This halfwave plate enables the helicoidal reflector in this species to reflect both left and right circularly polarized components of incident light. After passing through the halfwave plate, transmitted right circularly polarized light becomes left circularly polarized ; this light is now reflected and emerges from the cuticle right circularly polarized, after passing back through the halfwave plate. Consequently the total reflectivity of circularly polarized incident light is greater in P. resplendens than in any other species examined; the plate also reduces multiple internal reflexions. Interferometric analysis of the refractive properties of the helicoidal reflectors in species of Plusiotis showed that the ordered incorporation of uric acid increases the birefringence of the system by a factor of five times, e.g. the in tact birefringence of the unidirectional layer of P . resplendens is 0.166 at wavelength 560 nm ; after uric acid extraction the birefringence is reduced to 0.034. As the coefficient of reflexion of a helicoidal reflector is directly proportional to the birefringence of the individual planes comprising the helicoid, beetles incorporating uric acid into their reflecting surfaces reflect circularly polarized light far more efficiently than beetles lacking uric acid. Refractive index values for a single multicomponent plane of the helicoid have been summarized as a biaxial indicatrix, with the Z axis tilte dat 45° to the plane of the epicuticle. Beetle reflecting layers which incorporate uric acid have twenty times greater optical rotatory power compared with reflecting layers lacking this component. Mathematical treatments dealing with helicoidal reflectors predict the form optical rotatory power to be a function of the square of the birefringence, which is in agreement with the experimental observations. To enable uric acid to have the optical effects mentioned above, an epitaxial incorporation into the helicoidal framework is necessary. Although uric acid is a common cytoplasmic reflecting material in arthropods, this is the first record of its presence in an extracellular (cuticular) reflector.

BIREFRINGENCE COMPENSATION AND TEM00 MODE ENHANCEMENT IN A Nd: YAG LASER

Abstract: A Nd: YAG laser has been operated under strong thermal focusing and birefringent conditions and has yielded both linearly polarized output with negligible power loss and TEM00 output of 50–70% of the multimode power. The laser consisted of two Nd: YAG rods separated by a 90° crystal quartz polarization rotator together with a convex mirror and used no special cooling techniques. The performance of this laser, which agreed with theoretical predictions, indicates that this design approach may be generally applicable to the problem of thermally induced aberrations in such systems.

Ellipsometry of Anisotropic Films

Abstract: The macroscopic description of the optical behavior of (thin) films adsorbed on a polished solid surface is extended by allowing for uniaxial anisotropy in the adsorbate. The theoretical results are applied to Langmuir–Blodgett layers of absorbing and nonabsorbing surface-active compounds.

CdGeAs2—A New Nonlinear Crystal Phasematchable at 10.6 μm

Abstract: In this paper, we present the optical and nonlinear optical properties of a new nonlinear crystal CdGeAs2 The chalcopyrite structure (42 m) crystal is positive birefringent with Δn equal to 01 It is phasematchable for second harmonic generation (SHG) and parametric interactions over most of its transparency range from 24 to 18 μm In addition, it has the largest known nonlinear coefficient for a phasematchable crystal with the exception of tellurium The measured nonlinear coefficient is 34 times GaAs The calculated doubling efficiency for SHG of a CO2 laser is 10 times that of tellurium The crystal promises to be a very useful nonlinear material for parametric interactions between 3 and 18 μm

Refractive index, dispersion, and birefringence of silicon carbide polytypes.

Abstract: The refractive index of each of the four common silicon carbide polytypes has been measured over the visible range. The data were analyzed in an attempt to relate the birefringence to the relative hexagonal character of the polytype. A general relationship exists, namely, that the birefringence increases with increasing hexagonal character of the polytype. This relationship is not sufficiently precise to use for the identification of polytypes.

Structure of water in microemulsions: electrical, birefringence, and nuclear magnetic resonance studies.

Abstract: Microemulsions, which are optically transparent oil-water dispersions, were produced by mixing hexadecane, hexanol, potassium oleate, and water. As the amount of water is increased, the microemulsion exhibits a clear to turbid to clear transition. In contrast to the clear regions, the turbid region possesses birefringence. The development of birefringence is also accompanied by a sharp decrease in the electrical resistance. The high-resolution (220 megacycle) nuclear magnetic resonance data suggest that water exists in two distinct molecular environments in the birefringent region; the first environment is characterized by relatively less mobile water molecules than oil molecules, whereas in the second environment oil molecules are less mobile than water molecules. The electrical, birefringence, and nuclear magnetic resonance data are in agreement with the proposed mechanism of change in the structure of water from water spheres to water cylinders to water lamellae. The chemical shift of water protons suggests that the molecular environment of water in spheres is different from that of water in cylinders or that of water in lamellae.

Strain-biased ferroelectric-photoconductor image storage and display devices

Abstract: A device suitable for image storage and display applications is described in which the image is stored as a spatially varying birefringence and can, by means of suitably polarized light, either be observed directly or projected onto a viewing screen. The basic device consists of a sandwiched structure of uniform thickness layers, comprising transparent electrodes, a photoconductive film, and a plate of fine-grained ferroelectric ceramic. In order to establish a preferred orientation of the polarization in the plane of the plate, a uniform strain is induced in the plate by placing it either in tension or compression ("strain biasing"). The plate then becomes birefringent, with the principal axes of the optical indicatrix along the strain axes. The magnitude of this birefringence can be controlled by an electric field applied in the thickness direction, and this forms the basis of the device operation. In operation, the image to be stored is projected or scanned onto the photoconductive film. A voltage applied to the transparent electrodes develops, in the ceramic, a transverse field having an intensity modulated by the photoconductive film. When the field is removed, the desired image is stored as a spatial modulation of the birefringence of the ceramic plate. To erase the image the entire structure is flooded with light in the presence of an electric field in the reverse direction, and the plate regains its initial state of uniform birefringence. A large number of experimental devices have been fabricated and tested in the laboratory. In addition to describing the basic device structure, experimental results are presented showing the level of performance obtained from present devices, and a physical interpretation of device operation in terms of domain switching processes is provided.

Birefringence of YAG: Nd Laser Rods as a Function of Growth Direction

Abstract: The thermally induced stress birefringence has been known to affect severely the performance of a solid-state-laser system when the beam is linearly polarized. This study analyzes the birefringence as a function of the YAG:Nd rod-axis orientation; this investigation illustrates the angular dependence of birefringence for directions other than the [111] crystal direction and describes the losses for various rod-axis and polarizer orientations.

Nematic liquid crystal device

Abstract: A device characterized by an electronically-tunable optical birefringence over the range of 0.0 to 0.2 for applied voltages below 25 volts rms includes a thin film of a nematic liquid crystal. The tunable birefringence occurs below the threshold for dynamic scattering and is the result of electric-field-induced spatially-uniform molecular reorientation in a well-aligned nematic liquid crystal. Applications for such a cell include display elements, tunable retarders, color modulators and variable density filters.

Interferometric Studies of Fibers: Part II: Interferometric Determination of the Refractive Indices and Birefringence of Acrylic Fibers

Abstract: Interferometric determination of the principal refractive indices for light vibrating parallel and perpendicular to the fiber axis and the birefringence for Acrilan® have been carried out. Multiple-beam Fizeau fringes in transmission were used for fibers of irregular cross sections. A new method suitable for evaluating small birefringence in fibers was applied. Variation of refractive index with temperature was also determined. The fringes of equal chromatic order were applied for determining the birefringence of the fibers with diameters of 15-40 microns.

HIGH AVERAGE POWER OPERATION AND NONLINEAR OPTICAL GENERATION WITH THE Nd:YAlO3 LASER

Abstract: Using a Nd: YAlO3 rod 3 in. long, we have obtained 75‐W continuous output at 1.08 μ and 35 W at 1.0645 μ, both linearly polarized. Laser action has been obtained at five wave‐lengths between 1.06 and 1.10 μ and at two lines near 1.34 μ at room temperature. The preferred polarization for each of these lines has been determined. Intracavity second harmonic and parametric generation with LiNbO3 in this laser have produced a 3‐W average in a single beam at 5400 A and a 775‐mW average at 2.16 μ. The laser is free from depolarizing thermal birefringence at pump powers up to 4.5 kW.

Optical constants of far infrared materials. I: analysis of channeled spectra and application to mylar.

Abstract: A method is presented for analyzing channeled spectra to determine optical constants, using fourier spectroscopy. The use of fourier spectroscopy is integral to the method, for a single signature of the channeled spectrum in the interferogram may be used to yield both real and imaginary parts of the refractive index. The method employed for handling the data permits use of the position of zero crossings to calculate the real refractive index rather than maxima and minima which cannot be located with the same precision. Determination of the order number of the zero crossings is discussed, as is the correction for the convergence of the illumination. Results are given for Mylar, a birefringent material, in the 50–250 cm−1 region.

Birefringence and polymorphism of liquid crystals

Abstract: The double refraction of 32 liquid crystalline substances and their dependence on temperature and wavelength has been studied. The measurements were made on uniaxially oriented liquid crystalline layers using the principle of Abbe's double prism. Such optically uniaxial layers are obtained from the nematic states and the smectic modifications A and B. The smectic modification D proved to be optically isotropic. The smectic modification C could not be obtained uniaxially. The double refraction at the phase transition between two liquid crystalline modifications changes discontinuously and the several liquid crystalline modifications show a characteristic temperature dependence of the refractive indices.The dispersion of ne, no, ni is normal and increases in the sequence no, ni, ne. The dispersion of ne and no changes in a characteristic manner with the temperature. Within the homologous series an alternating decrease of ne, no and (ne–no) is observed. An attempt is made to explain these regularities on the basis of the structure and the order state in the liquid crystalline modifications.

Effect of ultrasound on a nematic liquid crystal

Abstract: The effects of ultrasound on the optical properties of an aligned layer of nematic liquid crystal are described. It was observed that the birefringence of a normally oriented nematic liquid crystal was a function of the acoustic energy, and that with sufficient incident acoustic energy, a scattering phenomenon similar to dynamic scattering occurred.

Measurement and Correction of First-Order Errors in Ellipsometry

Abstract: Defects in ellipsometer elements are treated in first order in a general formalism that expresses the null conditions of an ellipsometer or simpler system as a series of simultaneous linear equations. The solution of these equations is used to develop experimental procedures for the detection and measurement of these defect parameters, and to determine the effect of these parameters on ellipsometrically determined quantities. Defects that can be detected in first order include leakage and scattering in the polarizing prisms and scattering and strain birefringence in the compensator. Defects that can be detected and measured in first order include birefringence in the polarizing prisms and associated modulation cells, optical activity in the compensator, cell-window birefringence, and sample-surface quality. The experimental procedures also determine the absolute reference-azimuth angles of the polarizer and analyzer prisms and the compensator. Experimental results illustrating various aspects of the theory are presented.

The birefringence induced in spherical molecules by an electric field gradient

Abstract: The birefringence induced in a gas of spherical molecules by an electric field gradient is shown to be proportional to the Verdet constant of the gas (the Faraday optical rotation per unit length induced by unit magnetic flux density). This relationship, which is rigorous for atoms in S states at low optical frequencies, is used to predict the field-gradient-induced birefringence in the inert gases. The observed temperature-independent part of the birefringence in molecular hydrogen is approximately equal to the value deduced from the Verdet constant but for methane it is nearly twice as large, indicating the importance of the departure of tetrahedral CH4 from a spherical structure. An exact value for the field-gradient-induced birefringence of the H-atom is derived.

Influences of self-induced thermal effects on phase matching in nonlinear optical crystals

Abstract: In nonlinear optical crystals useful for second-harmonic generation, index inhomogeneities introduced by the self-induced temperature profile due to the laser beam influence the phase-matching conditions for nonlinear optical interaction. We have found that broadening of the width of the central peak, increase of asymmetry, shift of the maximum towards higher temperature, disappearance of the subsidiary structure in the second-harmonic power versus temperature, and the reduction of the conversion efficiency for second-harmonic generation may be caused. These effects depend upon the total absorbed power in the crystal, the birefringence variations due to temperature, the thermal conductivity, and the wavelength.

Temperature compensation of birefringent optical filters

Abstract: Temperature characteristics of birefringent optical filters were improved by two orders of magnitude by combining calcite and LiTaO 3 crystals in series. Theoretical treatment of the temperature compensation and experimental results are presented.

Two-colour liquid-crystal display

Abstract: A 7-segment numeric display with electrically controlled colour has been realised which uses the field-dependent birefringence in nematic liquid crystals. The colours obtained are very bright, especially the red and green.

Wave propagation in thin-film optical waveguides using gyrotropic and anisotropic materials as substrates

Abstract: Wave propagation in optical waveguides on substrates of magnetic, optically active, or birefringent material is analyzed. The conditions for TE \Leftrightarrow TM mode conversion are derived and computer calculations showing the characteristic of a quartz-substrate mode converter is obtained. A mathematical analysis of a gyrator using two mode converters in series is given and the schemes for an isolator, modulator, optical switch, and optical read-out devices are mentioned.

Nonlinear Optical Properties of Potassium‐Lithium Niobates

Abstract: Crystals of composition (K2O)0.3(Li2O)0.7‐x (Nb2O5)x with x in the range 0.515 to 0.55 have useful and unique nonlinear optical properties. These are transparent ferroelectric crystals having the tetragonal tungsten‐bronze structure. Thus, they do not have to be detwinned, as does NaBa2Nb5O15. The dc halfwave voltage is about 1350 V at 6328 A, and the nonlinear constant d31 is 1.7d31 (LiNbO3). For x=0.515 the birefringence is very large. Thus, the phase‐matching temperatures for harmonic generation without double refraction with the Nd‐YAG laser is 450°C. For x=0.55, 140°C is obtained. With these crystals the GaAs injection laser emitting 0.90 μ can be phase matched above room temperature for the first time without double refraction.

A new method of optically reading domains in Bismuth Titanate for display and memory applications

Abstract: Three previously proposed methods of optically detecting ferroelectric domains in bismuth titanate (Bi 4 Ti 3 O 12 ) are reviewed and the merits of each discussed. A fourth method which is introduced involves a tilt of a natural crystal plate about the b axis and utilizes the difference in birefringence between domains. The major advantages of the new read method are its high efficiency and contrast and its insensitivity to the optical degradation or "depoling" which occurs after repeated polarization reversals. Resolution test patterns written into Bi 4 Ti 3 O 12 crystals via a photoconductor film show up to 90 lines/mm resolution. In addition, photographic images have been stored, demonstrating a gray-scale capability.

On the birefringence of rods and cones

Abstract: Measurements were made of the birefringence of bleached rods and cones of six non-mammalian vertebrates. The variables included fixatives and temperature, and the immersing medium namely ethyl alcohol, urea, type of Ringer solution, and sucrose. The temperature variations of the birefringence of rods and cones were similar in frogs and in goldfish, respectively, but showed a significant interspecies difference. Rods fixed faster than cones, and the birefringence of cones is higher than that of rods. The birefringence of frog rods rose with the osmolarity of sucrose, but was unaffected by urea. Over 30% of the rod volume is occupied by solid: fixation is believed to destroy the two types of intrinsic birefringence which are postulated to exist as a result of the experiment on osmolarity, and to leave only form birefringence intact. A theory of receptor birefringence is also tested.