Showing papers on "Birefringence published in 1970"

Pretransitional Phenomena in the Isotropic Phase of a Nematic Liquid Crystal

Abstract: We have observed a divergence of the magnetic birefringence, and a critical increase and slowing of the fluctuations in order in the isotropic phase of a nematic liquid crystal. Our results are quantitatively described by a mean-field model except for a critical region close to the ordering temperature where the fluctuations are so large that the meanfield approximation fails.

Effect of birefringence on the performance of linearly polarized YAG:Nd lasers

Abstract: The thermally induced stress birefringence has been known to severely affect the performance of a solid-state laser system when the beam is linearly polarized. This paper refines the theory of the interaction of a linearly polarized wave with a continuously pumped YAG : Nd crystal; in conjunction with experimental evidence this paper illustrates the angular independence of the birefringence, describes the radial dependence, and explains the laser power losses involved.

Isotropic-Nematic Phase Transition in Liquid Crystals

Abstract: The molecular-field description of the isotropic-nematic phase transition in liquid crystals is investigated. It is found that the effects of fluctuations may be observable, and the first corrections due to fluctuations to the light scattering and magnetic birefringence in the isotropic phase are determined. The effects of applied fields on the phase transition are investigated. It is found that if the molecules prefer to line up transverse to an applied field there is a critical field above which the phase transition is second order.

A stress-optic law for photoelastic analysis of orthotropic composites

Abstract: The feasibility for utilizing transparent filament-resin composites for photoelastic stress analysis was investigated. Satisfactory photoelastic stress patterns were demonstrated in simple models with undirectional and bidirectional fiber orientations. A stress-optic law was formulated, based on the concept that the birefringence components contributed by each component of plane stress are combined according to a Mohr circle of birefringence. Applying this concept, the difference of the physical and optical principal directions was accounted for, and a general method of photoelastic solution for the plane-stress problem in orthotropic sheets was developed. The method of analysis is little more complex than the well-known procedures for isotropic materials, but at least three experimental measurements are required to characterize the optical response of the material to plane stress. Partial confirmation of the proposed stress-optic law was obtained by comparison of the theory to limited experimental data obtained in uniaxial-stress samples. It remains to establish a more positive verification by experiments in a variety of biaxial-stress conditions.

Room‐Temperature Ferromagnetic Materials Transparent in the Visible

Abstract: The only known crystals which have a spontaneous ferromagnetic moment at room temperature and are transparent well into the visible spectrum are ferric borate, FeBO3, and ferric fluoride, FeF3. Each of these rhombohedral materials is a canted antiferromagnet with a small moment lying in the plane perpendicular to its unique axis. The Curie temperatures are 348° and 363°K and the room‐temperature moments are 115 and 40 G, respectively. Both materials contain ferric ions in a slightly distorted octahedral environment. Both are green with a series of absorption bands in the visible, a fundamental absorption edge in the violet (FeBO3) or ultraviolet (FeF3), and a large Faraday rotation per unit absorption at room temperature, as high as 14°/dB at 5250 A for FeBO3 and 16°/dB at 4050 A for FeF3. Recent measurements of the magnetic, optical, magneto‐optical, and microwave resonance properties of these materials are discussed. Potential applications of room temperature transparent ferromagnets include microwave magneto‐optical modulation of visible lasers with very low modulation power per unit bandwidth, optical deflection and isolation, magneto‐optic displays, and holograms. Each of these applications is discussed along with the limitations imposed by the optical absorption and birefringence of the available materials. Birefringence interferes with the Faraday effect in most noncubic crystals. Methods for minimizing this interference are described. Finally, requirements for useful transparent ferromagnets are defined and guidelines for finding new materials are suggested.

Growth of High‐Quality LiNbO3 Crystals from the Congruent Melt

Abstract: The use of LiNbO3 crystals in nonlinear optical experiments has been limited by the lack of long, high‐quality crystals. We have shown that crystals grown from a congruent composition melt are significantly less susceptible to birefringence variations which in the past have limited the useful phase‐matchable lengths of LiNbO3 crystals. Both a birefringence test which directly measures the spatial birefringence of the crystal and a second harmonic generation test were used to evaluate crystals grown from melts of various compositions. Crystals with less than 10−5 variation in birefringence over a length of 4 cm have been obtained repeatably when grown from congruent melts.

Voltage-Induced Vorticity and Optical Focusing in Liquid Crystals

Abstract: We have observed that liquid crystals of the $p$-azoxyanisole type exhibit macroscopic rotational motion above a threshold voltage. The vortex motion of the birefringent liquid results in a lattice of cylindrical lenses whose focal lengths are voltage variable. These experiments are in excellent agreement with the dynamic predictions of the continuum theory.

Tunable Dye Laser with Narrow Spectral Output

Abstract: A tunable dye laser with narrow‐banded spectral output in the order of 0.01 A or smaller is described. This narrow spectral output is obtained by means of a birefringent filter (Lyot filter) which was inserted into the laser cavity. The properties of this laser setup have been investigated and are described.

Effect of Optical Inhomogeneities on Phase Matching in Nonlinear Crystals

Abstract: Experimentally, we have observed variations in the phase‐matching temperature (Tpm) for second harmonic generation (SHG) along the pull axis of lithium niobate (LiNbO3) crystals. We have related these to variations in the extraordinary index of refraction caused by changes in crystal stoichiometry occurring in the growth process. We have established a technique, for filtering pathological crystals, based upon a point‐by‐point mapping of the birefringence variations along the direction of interest. This provides information about the details of the large and small‐scale spatial variations. Empirically we can get toleration limits on the birefringence variations for application to parametric oscillator design. We have studied several physical models (single step, multiple step, and linear ramp) for the manner in which monotonic changes in index, appropriate for LiNbO3, are introduced as a function of position. We have found that the usual quality criterion based upon the width of the central peak in an SHG ...

Visible and Infrared Faraday Rotation and Birefringence of Single‐Crystal Rare‐Earth Orthoferrites

Abstract: Faraday rotation and birefringence for propagation parallel to the magnetization, if single‐crystal Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu orthoferrite have been measured from 0.6 to 1.8 μ at room temperature. The parameter δ, the relative phase per unit length, and χ, a parameter that relates the strength of the Faraday rotation to the birefringence, have been measured using transmission techniques. From these parameters θ, the intrinsic Faraday rotation, is calculated and found to be nearly the same for all of the orthoferrites except for samarium. In the wavelength region examined the intrinsic Faraday rotation is about three times greater than that in yttrium iron garnet. The optical axis for Eu, Tb, and Yb orthoferrite have been measured at a wavelength of 0.6 μ. The difficulties in using a material with both Faraday rotation and birefringence in devices is discussed.

Analyses and Corrections of Instrumental Errors in Ellipsometry

Abstract: Expressions for the errors produced in an ellipsometer due to an imperfect compensator, due to birefringence in windows of a cell containing the sample, and due to tilting of the sample are derived. Methods of analyzing the ellipsometer readings to eliminate these errors are given.

Optical properties of KNbO3

Abstract: Measurements of birefringence and refractive indices in KNbO3 were performed in the orthorhombic, tetragonal and cubic phases. KNbO3 is phase matchable and collinear phase matching is possible for a Nd-laser. Absorption measurements reveal an exponential law for the absorption coefficient (Urbach tail). The dispersion of the refractive indices is discussed in terms of a single-oscillator model. The parameters obtained are in good agreement with those of other perovskite materials. Relations for the electrooptic effect in an orthorhombic oxygen-octahedron ferroelectric are derived and it is found that this effect can explain the observed temperature dependence of refractive indices. Finally, the coefficients for the linear electrooptic effect are calculated and discussed.

Polarization independent light modulation means using birefringent crystals

Abstract: A first optically birefringent crystal divides an incident randomly polarized light beam into two linearly polarized orthogonal components, namely, an ordinary ray and an extraordinary ray. The incident light beam is so related to the optic axis of the birefringent crystal that ordinary ray is undeviated but the exgraordinary ray is deviated by a selected amount, both of the rays emerging from the birefringent crystal parallel to each other. The amount of deviation is a function of the index of refraction and the length of the path through the crystal. The spaced rays are then directed through an electro-optical polarization modulator which electronically converts the linearly polarized components to elliptically polarized beams with the eccentricity dependent upon the magnitude of an applied electric field. For a particular magnitude of applied electric field, the polarization modulator will cause a 90 DEG rotation of the orthogonal components. The two emerging rays are then passed through a second birefringent crystal which recombines the two rays with the intensity along the emerging axis proportional to the magnitude of the electric field applied to the polarization modulator. In the unenergized state of the polarization modulator maximum transmittance is obtained. In the energized state the transmittance varies according to T = sin2 KE where E is the magnitude of the applied electric field and K is a proportionality constant. Thus, the light modulation system can be controlled electronically to make it selectively transmissive for incident light of all polarizations. The system is also described as applied to a resonant optical cavity such as a Q-switching device for randomly polarized stimulated emission of radiation devices (lasers).

Ferroelectric ceramic light gates operated in a voltage-controlled mode

Abstract: Plates of lead zirconate-lead titanate ferroelectric ceramic can have 1) low-loss optical transmission in thin, polished sections and 2) uniaxial birefringence dependent upon remanent polarization. These properties are potentially useful in electrically variable optical retarders, modulators, and latching light gates. This paper reports the results of measurements of basic light-gate devices using ferroelectric ceramic plates. A number of characteristics of the devices are reported; e.g., dependence of absolute light phase retardation on ceramic remanent polarization; dependence of ON-OFF ratio on exit aperture, switching pulse duration, and light wavelength; switching speed; and the dc hysteresis characteristic of the dependence of remanent polarization upon applied field. In the past, the use of ferroelectric devices under conditions producing partial switching has been discussed exclusively from the point of view of "charge-limited switching." This paper proposes a new mode of operating ferroelectric ceramic light gates using "voltage-controlled switching." Charge-limited switching results naturally when voltage pulses of short duration are used (appreciable ON-OFF ratios can be obtained from a light gate switched with pulses as short as 10 ns). As a result of the hysteresis in the dc switching characteristic, pulses with durations of the order of milliseconds or longer result in operation of the light gate in a voltage-controlled mode. Practical advantages resulting from this mode of operation are discussed.

Linear Magnetic Birefringence in the Ferrimagnetic Garnets

Abstract: Light propagating in a ``cubic magnetic crystal'' perpendicular to M is subject to a linear magnetic birefringence. The origin and representation of this birefringence is discussed. In the ferrimagnetic garnets several mechanisms may be identified, but the overwhelming part is apparently associated with the magnetostrictive distortion. Measurements are presented for each of the rare‐earth iron garnets at 77° and 300°K.

Temperature Dependence of Birefringence in Liquid Crystals

Abstract: The temperature dependence of birefringence in the liquid crystal methoxybenzylidene-$n$-butylaniline has been obtained by measuring the birefringence of a thin sample placed between crossed polarizers. Measurements of transmitted laser light yield minima and maxima which can be related to the difference in refractive indices. A converging beam displays the orientation of the optic axis and is used in determining $\ensuremath{\Delta}n$. Wavelength dependence is measured in a similar way using white light and spectrograph.

Light beam polarization modulator

Abstract: A parallel cell electro-optical phase modulator is used in combination with two birefringent crystals and a half wave plate to produce polarization modulation of an incident light beam. A first birefringent crystal provides a lateral relative displacement between the two orthogonal components of the incident light beam to form two spaced light paths, one through each of the respective electro-optical crystals. The half wave plate rotates the linear polarization of the orthogonal components to allow a second birefringent crystal to recombine the orthogonal components into a single light beam.

Tunable acousto-optic method and apparatus

Abstract: Polarized light waves propagate collinearly with acoustical waves in an optically anisotropic media. For nearly matched momentum vectors of the ordinary optical wave, extraordinary optical wave, and acoustic wave, diffraction of light into the orthogonal polarization occurs in a bandwidth determined by the birefringence and length of the media. The diffracted light may be selected by an output analyzer to form a band-pass filter. The operating point of the filter is varied by varying the frequency of the applied acoustical waves. A theory of operation is set forth together with example embodiments.

Limitations on the measurement of relaxation times using a pulsed Kerr effect method

Abstract: Relaxation times may be measured by observation of the birefringence produced by the application of a pulsed electric field. A brief resume is given of the methods of detection of the birefringence. These may be linear or quadratic according to whether the signal is proportional to the first or second power of the birefringence. A combination (quasi-linear) is also possible. Considerations of signal-to-noise ratio, current limitations of the detector system and signal distortion are fully treated. The analysis when two relaxation times are present simultaneously is given and it is shown that such a case may be confused with the results obtained with one relaxation time if the parameters are not fully known. Errors in the determination of relaxation times are examined. It is concluded that a linear detection method is preferable to a quadratic one because the signal-to-noise ratio and distortion tends to be less but these advantages do not occur when large bandwidths have to be used.

Method and means for compensating thermal lensing in a laser system

Abstract: Birefringence induced by nonuniform heating in a laser rod is compensated for by introducing a compensating material of similar physical properties to the rod and artificially inducing birefringence in the compensating material in a sense opposite to that taking place in the laser rod.

Optical anisotropy determinations for rigid particles from rayleigh light-scattering measurements at optical saturation

Abstract: In suspensions of asymmetric particles, nonlinear changes in the intensity components of light scattered in the presence of an intense laser beam or self-induced changes in scattered laser light components are studied for various polarizations of the laser beam. Quantitative analysis is performed for all four intensity components of the scattered light (vertical V v , horizontal H h , and cross components, V v and H v ) for an arbitrary degree of optical reorientation of the particles, with applications extending from weak reorientation to saturation, when all the particles are aligned in the oscillation direction of the electric vector if the beam is linearly polarized or in the propagation direction of the beam if the latter is unpolarized or circularly polarized. For these two limiting cases, simple relations between the variations δV v , δV h , δII v , and δH h for various polarizations of the laser beam are proposed, making possible direct determinations of the sign and value of the optical anisotropy of particles as well as decisions whether they are cigar-like or disk-like in shape. Moreover, the nonlinear variations in the vertical component V v are shown to depend on the degree of optical reorientation of the particles in the same way as the optical birefringence induced by laser light.

Direct display of light images with picosecond resolution

Abstract: A LIGHT IMAGE SUCH AS A PICOSECOND OPTICAL PULSE GENERATED BY A LASER OR AN OPTICAL ECHO SIGNAL GENERATED BY PICTURE RANGING SYSTEM, IS DISPLAYED DIRECTLY WITH PICOSECOND RESOLUTION. A LASER PULSE, FOR EXAMPLE, IS MADE INCIDENT UPON A SCATTERING MEDIUM TO PRODUCE A SCATTERED FORM OF THE PULSE WHICH IS VIEWED BY THE NAKED EYE, A CAMERA OR OTHER DISPLAY DEVICE THROUGH AN OPTICAL GATE WHICH FUNCTIONS AS A CAMERA SHUTTER HAVING AN EXTREMELY SHORT FRAMING TIME OF THE ORDER OF A FEW PICOSECONDS OR LESS. THE OPTICAL GATE UTILIZES A MATERIAL IN WHICH BIREFRINGENCE IS OPTICALLY INDUCED BY AN OPTICAL CONTROL PULSE OF HIGH INTENSITY AND PICOSECOND DURATION. OTHER APPLICATIONS, INCLUDING OPTICAL READ-OUT OF COMPUTER MEMORIES FORMED BY STACKS OF PHOTOGRAPHS OR HALOGRAMS, ARE DESCRIBED.

The Effect of Convolutions on Orientation Measurements in Cotton Fibers

Abstract: Ten genetic varieties of cotton were examined by x-ray orientation and birefringence techniques. The relationship of an optically measured convolution angle to x-ray and birefringence orientation p...

Dependence of Second Harmonic Generation on Crystal Inhomogeneity

Abstract: Effect of the nonuniformity of the natural birefringence in a nonlinear crystal on second harmonic generation (SHG) is theoretically and experimentally studied. The second harmonic power at the phase‐matching temperature, the second harmonic power vs temperature, and the temperature bandwidth are calculated in terms of the gradient of the natural birefringence and compared with those from the SHG experiment using a cw Nd:YAG laser and Ba2NaNb5O15 single crystal. It is found that the second harmonic peak power is much depressed by the nonuniformity of the natural birefringence in a crystal. The oscillations in the second harmonic power vs temperature curve become broad and the temperature bandwidth increases with decreasing crystal homogeneity. Satisfactory agreement was obtained between theory and experiment. By analytical calculation the dependence of SH power on crystal length l, is expressed in terms of the inhomogeneity length li, which is defined as a measure of the length beyond which SHG is limited...

A new technique to study photoelastic dispersion in cubic and noncubic crystals.

Abstract: A new technique employing a Soleil-Babinet compensator is described to study the photoelastic behavior of crystals at various wavelengths. The photoelastic dispersion is reported on an isometric crystal, sodium chloride, and a birefringent crystal, barite, for some typical orientations. The values of the stress birefringence obtained at different wavelengths are compared with the values given in the literature. The merits of the present technique over the previous methods are discussed.

Electrooptic effects in ferroelectric ceramics

Abstract: Ferroelectric ceramics are piezoelectric and optically birefringent. Moreover, their coefficients of piezoelectricity and birefringence are electrically variable. Consequently, these ceramics are applicable to a variety of devices: electrically tuned oscillators and transformers; miniaturized high-Q, high-frequency filters; FM discriminators; optical memories; electrically Controlled light shutters and valves; and electrically activated multicolor displays. This article deals with some possibilities for devices that exploit the electrically controllable optical effects. A later article will consider piezoelectric applications.

Studies on biaxial stretching of polypropylene film. IV. Overall orientation during one‐step biaxial stretching and its application to the estimation of the principal refractive indices of isotactic polypropylene

Abstract: Polypropylene film was biaxially stretched in one step in air at 140°C or 152°C, and the deformation was studied optically. A linear relation held between Δnss and vA−½ for vA > 10, at both temperatures, where Δnss is the birefringence with respect to the normal to the film and vA is the degree of stretching expressed as the factor by which the area of the film is increased. Extrapolation of data in this linear region yielded a value of 20 × 10−3 for −Δnss at infinite vA. Since it is presumed that the polypropylene molecules lie completely parallel to the film surface when the film is stretched infinitely, −Δnss at vA−½ = 0 must be just half Δn°, the intrinsic birefringence in the case of completely parallel orientation. Thus, Δn° must be 40 × 10−3. This value was obtained experimentally in uniaxial stretching when the birefringence with respect to the direction of drawing was extrapolated to infinite extension. Similar relations held between np, the average of the refractive indices in the two stretching directions, and vA, and between nss, the index normal to the film, and vA. By similar extrapolations, (1/2)(n′γ + n′β) and n′β = n*α′ were estimated, and thence nα′ was obtained. Here, n′α and n′β are the refractive indices along the c axis (molecular chain axis) and b axis. All these optical parameters refer to a density of 0.900 g/cm3. Hence by applying a density correction to those values, the principal refractive indices and the intrinsic birefringence of polypropylene crystal were evaluated as follows: nα = 1.5522, nβ = n*α = 1.5106 and Δnc° = 4.16 × 10−3, where n*α is the refractive index prependicular to the b and c axes of the crystal.

Threshold Calculations for an Optical Parametric Oscillator Employing a Hemispherical Resonator

Abstract: A calculation of optical parametric oscillator threshold has been carried out for an oscillator which employs a hemispherical resonator. The analysis considers the effects of focusing and double refraction, and makes possible the calculation of threshold as a function of the focusing parameter ξ and double‐refraction parameter B. The analysis is similar to that carried out by Boyd and Kleinman for a different class of optical resonators. Their analysis assumes that the pump beam is centered (longitudinally) with respect to the focus of the oscillator pump beam. In a hemispherical resonator, however, only half of the pump‐beam focal region can be utilized because of the resonator configuration. As one would expect, oscillator threshold is higher for the hemispherical case than for the Boyd and Kleinman case. However, the increase is at most a factor of 2 (when beam walkoff is absent, i.e., when B=0), and gradually goes to unity as B increases in size. Thus by using a hemispherical resonator in an optical p...