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.     

The factors affecting the polarization transformations of X-rays in Laue diffraction from crystals with dislocations

The transformations of X-ray polarization with coherent σ-and π-components have been studied for the Laue diffraction from LiF crystals with a high dislocation density (10⁴–10⁵ mm^?2). The dependence of the parameter characterizing the diffraction-induced birefringence and the degree of coherence of the transmitted beam on the squared reciprocal-lattice vector (H ² = 4sin²θ/λ²) is determined. It is shown that the parameter characterizing the diffraction-induced birefringence in crystals with dislocations is determined by the static Debye-Waller factor (exp(-L)) and the effective thickness Δ of the crystal. The exponent L increases and the parameter Δ decreases with the dislocation density.     

Size effects in [N(CH3)4]2Zn0.58Cu0.42Cl4 crystals

The temperature dependences of birefringence in thin [N(CH₃)₄]₂Zn_0.58Cu_0.42Cl₄ crystals in the thickness range 20 × 10^?6 ≤ d ≤ 500 ? 10^?6 m have been investigated. An increase in the temperatures of the parent phase-incommensurate phase and incommensurate phase-ferroelectric phase transitions has been found. The reasons for the shift in the phase transition temperatures with a decrease in the thickness of [N(CH₃)₄]₂Zn_0.58Cu_0.42Cl₄ crystal and the size effect in crystals with an incommensurate superstructure are discussed.     

Bestimmung der OH−-Konzentration in LiNbO3-Kristallen durch Protonenresonanz-Untersuchungen

Using calibration samples it was possible to evaluate the concentration of OH⁻-ions in LiNbO₃ crystals from investigations of the nuclear magnetics resonance. The relation between the OH⁻ ion concentration and the absorption coefficient at 2,87 μm was found to be N = (3 ± 1) · 10¹⁹ k (2.87 μm) cm⁻³, which permits a quantitative determination of the OH⁻ content by simple optical absorption measurements.     

Increased optical damage resistance of Zr:LiNbO3 crystals

Zr: LiNbO₃ crystals has been grown. The crystal composition and phase are analyzed by X‐ray diffration. The optical damage resistance ability of Zr: LiNbO₃ crystals is studied by the Sénarmont compensation method and the transmitted beam pattern distortion method. The saturated value of the birefringence change of 6 mol% Zr: LiNbO₃ crystal is 1.01×10^‐4, which is seven times smaller than that of congruent pure LiNbO₃ crystal. The results of UV‐Visible and IR absorption spectra of Zr: LiNbO₃ crystals powerfully confirm that the optical damage resistance threshold concentration of the Zr⁴⁺ ions doped in LiNbO₃ crystals is about 6 mol% in the melt. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Infrared absorption of some high-purity chalcogenide glasses

New compouning techniques were devised to prepare high-purity Ge₂₈Sb₁₂Se₆₀ (TI 1173)and Ge₃₃As₁₂Se₅₅ TI 20). The methods were based on the combination of the reactant purification and compounding steps. The goal of the program was to establish the absorption limit for the glasses and to lower the absorption at 10.6 μm. At the present purity level, the GeSbSe glass is found to have an absorption level of about 0.01 cm^?1 at 10.6 μm while the absorption level for the GeAsSe glass is 0.05 cm^?1. Underlying causes for the limits are discussed along with the possibilities for improvement.     

Optical birefringence images of dislocations in large gallium phosphide crystals

The optical birefringence induced by the strain field of a dislocation has been used to follow dislocation lines through large gallium phosphide crystals. Dislocations could be seen whether their lines were parallel to the imaging light beam or were steeply inclined to it. The images obtained so far suggest that it would be possible to resolve individual dislocations in crystals that contain ?5 × 10⁵ dislocations per cm².     

Thermal conductivity and refractive indices of Nd:GdVO4 crystals

The thermal conductivities of Nd:YAG, M(Y,Gd)VO₄ crystals were measured at 298 K. The value of Nd:GdVO₄ crystal along <001> direction was 11.4 W/mK, which was higher than that of YAG crystal measured to be 10.7 W/mK. The principal refractive indices of Nd:GdVO₄ crystal in the temperature range from 20 °C to 170 °C were determined by auto‐collimation method. Based on the measured values of refractive indices, the Sellmeier equation and expression of temperature dependence of refractive indices have been obtained. The measured results show that the birefringence Δn is 0.22007 at 20 °C and temperature coefficient of birefringence is 4.33 × 10⁻⁶/°C for 1.064 μm. These results prove that the GdVO₄ crystal is a laser crystal with excellent thermal and birefringence properties. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-frequency dielectric properties of DMAAS crystals in the vicinity of 110 K

Dielectric properties in “pure” and partly deuterated DMAAS crystals have been studied within the frequency range 40^?2–10⁷ Hz in the vicinity of 110 K. It is established that the crystals possess relatively high conductivity, 10^?4–10^?7 Ω^?1 m^?1, which is explained by their crystal structure. The frequency dependence of the complex dielectric constant has two linear segments, which indicate the change in the charge-carriers motion in the vicinity of 110 K. It is revealed that at low temperatures, conductivity increases at higher frequencies.     

Multicomponent glass optical fibres with a lowered water content and reduced loss

A technique has been developed to reduce the ?OH content of multicomponent glasses. The glasses were treated with perfluoropropane and the linear absorption coefficient of ?OH reduced from typically 0.08 to as low as 0.01 cm^?1, which corresponds to a reduction of ?OH content to approximately 2 ppm. A fibre pulled from treated glasses had a reduced loss at 1.3 μm, and a loss of 2.9 dB km^?1 at 1.12 μm which is the lowest reported loss for a fibre prepared by a double crucible technique.     

Electron irradiation‐induced effects on optical spectra of (NH4)2ZnCl4: x Sr2+ single crystals

The effect of electron‐beam irradiation with different doses on optical constants of (NH₄)₂ZnCl₄: x Sr²⁺ crystals with x=0.000, 0.020, 0.039, 0.087 or 0.144 wt% has been studied. The optical transmission in the energy range 3.4‐6.4 eV was measured hence the absorption coefficient was computed as a frequency function. The absorption coefficient was also calculated as a function of electron‐beam dose. Irradiation with e‐beam did not affect the allowed indirect type of transition responsible for interband transitions of (NH₄)₂ZnCl₄: x Sr²⁺ crystals. Values of the optical energy gap E_g and optical moment E_p for electronic interband transition of unexposed and (NH₄)₂ZnCl₄: x Sr²⁺ crystals after e‐beam exposure were deduced. The area under the absorption band at 5.30 eV was used to evaluate the effect of e‐irradiation on optical parameters of samples with x=0.00, 0.020 or 0.039. A shift in the position and a nonmonotonic change in the intensity of this band with increasing e‐beam dose was observed. Changes in the E_g value were used to evaluate the effect of e‐beam exposure dose on (NH₄)₂ZnCl₄: x Sr²⁺ samples with x=0.087 or 0.144. The obtained results were compared with those obtained for the same crystals after irradiation with different γ‐doses.     

Nature of large-scale oscillations in oxygen concentration along the growth axis in Czochralski-grown silicon crystals

Oxygen distribution in a Si crystal (100 mm in diameter) has been studied by the absorption method in the range of the absorption band of interstitial oxygen, λ = 5.81 μm. Large-scale fluctuations (～1 cm) of the oxygen concentration (N ₀) along the growth axis were determined. Depending on the melt height, the regions of the chaotic and quasiperiodic changes were established, as well as the region of the constant N ₀ value, and their relation to turbulent, quasiperiodic, and stationary modes of melt convection in crystallization. The values of the critical Rayleigh number for the melt transition from stationary to quasiperiodic (3 × 10³) and from quasiperiodic to turbulent (1.7 × 10⁴) convection modes are determined for growth of silicon crystals by the Czochralski method. The dominating modes of N ₀ concentration oscillations at two incommensurable frequencies, f ₁ = 1.3 × 10^?3 and f ₂ = 6 × 10^?4 Hz, are assumed to be related to the oscillatory transfer of oxygen from the walls of the quartz crucible to the crystallization front and restructurization of the convective flow pattern of the melt in the course of crystal growth.     

The Absorption,Fluorescence and Phosphorescence and Phosphorescence of Single Crystals of 1,2,4,5-Tetrachlorobenzene and 1,4-Dichlorobenzene at Low Temperatures

Abstract

The emission spectra of ultra pure single crystals of 1, 2, 4, 5. tetrachlorobenzene (TCB) and 1, 4-dichlorobenzene (DCB) at temperatures from 4.2°K upwards are reported. In addition, by use of the phosphorescence excitation technique the singlet-triplet absorption spectrum at 4.2°K has been obtained.

The phosphorescence emission of TCB at 4.2°K occurs predominantly from a defect origin “X” situated 48 cm^?1 below the triplet exciton band. The triplet exciton energy level is at 26676 cm^?1 from both emission and absorption studies. This is the triplet emission origin when the crystal is above 12°K. The temperature dependence of the emission intensity from the defect has an activation energy of 40±8 cm^?1. This value is consistent with our suggestion of thermal depopulation of the traps.

Single crystals of DCB show strong excimer emission and weak triplet exciton emission. There is no evidence for trapping levels. The triplet exciton emission. There is no evidence for trapping levels. The triplet exciton origin is 27890 cm^?1 from both emission and absorption studies.

Weak fluorescence (φ < 10⁰²) is detected from both TCB and DCB.

Vibrational analyses are reported for the absorption, fluorescence and phosphorescence spectra.     

Study on K(DxH1‐x)2PO4 Crystals: Growth Habit,Optical Properties and their Improvement by Thermal‐Conditioning

Phosphite, which often exists in growth solutions obtained directly from commercial P₂O₅ , was found to have significant inhibiting effects on the growth of pyramidal face of KDP crystals. K(D_xH_1‐x)₂PO₄ (referred to as DKDP) crystals with different deuterium fraction x were grown and the optical performances were investigated. The absorption coefficients at 1.05 μm decreases monotonically with the increase of x. The transmission threshold shift from 1.65μm at x=0 to 2.10 μm at x=0.96. The high temperature phase transition temperature and latent heat were measured using the method of differential scanning calorimetry (DSC). Thermal conditioning experiments were carried out at 180°C and 140°C for KDP and DKDP, respectively. After conditioning, a different degree of improvement was observed in the optical homogeneity of the samples, while the laser damage threshold and light absorption coefficient showed no significant change.     

Growth and characterization of photorefractive Bi12TiO20 single crystals

Photorefractive Bi₁₂TiO₂₀ single crystals of high optical quality were grown in a resistive heating furnace from high temperature nonstoichiometric (10:1) solutions of Bi₂O₃ and TiO₂ at pulling rates 0.3–0.8 mm/h at rotation 20–30 rpm along the <001> and <011> axis. Powder X-ray analysis, Laue method, and electron-probe microanalysis were used for characterization. BTO crystals have the bcc structure of sillenite type with a₀ = 10.178(8) Å. The chemical composition of the crystals can be written down as Bi_{12.1 ± 0.2}Tio_{0.96 ± 0.09}O_20.1. Natural optical activity ρ of BTO crystals is 6.3 ± 0.2 deg/mm at λ = 0.633 μm and 11.9 ± 0.2 deg/mm at λ = 0.5145 μm, optical absorption coefficient α = 0.42 ± 0.04 cm⁻¹ at λ = 0.633 μm and linear electro-optic coefficient r₄₁ = r₅₂ = r₆₃ = 5.3 pm/V. Fanning effect in the “fiber-like” BTO sample was studied and double phase conjugation with conversion efficiency up to 8% was observed in a wide range of incidence angles of the pump at λ = 0.633 μm for 2 × 3 mW input light power.     

Optical and electro-optical properties of ZnMgSe single crystals

The optical and electro-optical properties of large-sized (diameter, 20 mm; length, 70 mm) single crystals of Zn_{1 ? x}Mg_xSe (x ～ 0.5) semiconductor solid solutions are investigated. The crystals are grown by the vertical Bridgman-Stockbarger method. It is found that, for ZnMgSe single crystals with a magnesium content up to 25 at. %, the modulus of the difference between the electro-optical coefficients |r₁₃ ? r₃₃| at a wavelength of 0.63 μm is equal to (1.1 ± 0.22) × 10^?12 m/V, which is comparable in order of magnitude to the difference in the electro-optical coefficients for classical hexagonal A^IIB^VI compounds, such as CdS and CdSe. It is shown that single crystals of ZnMgSe solid solutions are promising materials for use in the fabrication of electro-optical modulators, λ/4-and λ/2-wavelength plates, and multifunctional optical elements operating in the range of high-intensity radiation of CO and CO₂ lasers.     

Investigation of structural perfection and faceting in highly Er‐doped Yb3Al5O12 crystals

The undoped Yb₃Al₅O₁₂ (YbAG) single crystals and doped with 1.5, 10 and 30 at% erbium were grown by the Czochralski method. The YbAG crystals offer efficient emission of laser beam of 2.94 µm and 1.55 µm important for practical applications in communication and medicine. The crystals were investigated by various synchrotron X‐ray diffraction methods including white beam topographic methods, monochromatic beam topography and recording of the rocking curves. The experiments were performed at E2 and F1 experimental stations in HASYLAB. The investigations proved a good crystallographic perfection of the crystals, in most cases revealing only segregation fringes and growth facets. The crystallographic identification of the facets was performed together with direct evaluation of growth front radius from the transmission section topographs. Relative lattice parameter changes connected with erbium segregation were found to be less than 2 × 10^‐5 inside the segregation fringes and 8 × 10^‐5 in the facets. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of doping on OH absorption in LiNbO3 crystals

Undoped, Cr doped and Mg, Cr codoped LiNbO₃ crystals were grown by conventional Czochralski technique. Comparative study was carried out using Fourier transform infrared (FTIR) and UV‐Visible spectroscopy. Infrared optical absorption for OH^– ion has been used to study the effect of dopants on the crystals. The peak position of OH^– shift to 3535 cm^‐1 for Mg, Cr codoped crystals compared to 3484 cm^‐1 for undoped and Cr doped crystals. Prominent absorption bands are found in the visible region centered at 480 nm (20833 cm^‐1) and 653 nm (15313 cm^‐1) in Cr doped crystals. Whereas in Mg, Cr codoped crystals these broad absorption bands are red shifted to 517 nm (19342 cm^‐1) and 678 nm (14749 cm^‐1). UV cutoff in Cr doped crystals shift towards higher wavelength compared to undoped LiNbO₃ crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low-temperature growth and doping of InP:S single crystals from melt-solution in microgravity

The effect of microgravity on the growth of bulk InP:S single crystals from a melt with an initial equilibrium composition (84 at % In, 16 at % P, and ～2.2 × 10¹⁸ at cm^?3 of S) on board the Foton-11 satellite was investigated. The growth of crystals on board the satellite and on Earth (a reference crystal) was carried out by the traveling heater method. The samples of the grown crystals were investigated by metallography, double-crystal X-ray diffractometry, single-and double-crystal X-ray topography, and secondary-ion mass spectrometry. It is shown that the mass transfer in the melt in microgravity is similar to the diffusion mode. Hence, the mass transfer in the melt results in the following: the formation of a nonstationary boundary layer, depleted in phosphorus; the constitutional supercooling at the crystallization front accompanied with the development of a cellular substructure in the early growth stage; and the hypothetical phase structurization of the transition layer with the formation of In-based associates (clusters), which were found in the grown crystals in the form of spherical defects 10–20 μm in diameter. The coefficients of sulfur distribution k₀ = 0.274 and k_eff = 0.43, the sulfur diffusivity in the melt D_S = 4.2 × 10^?7 cm²/s, and the effective thickness of the transition layer δ = 0.07 cm in terrestrial gravity are determined. The data obtained are necessary to develop a mathematical model of crystallization in zero gravity.     

Electrical Signal Induced by a Chemical Reaction in pts

Abstract

Single crystals of monomeric pTS (bis-p-toluene-sulphonate of 2,4-hexadiyne 1,6-diol), when isothermally annealed at 350 K, were found to generate an electrical signal peaking at a time corresponding to that of a maximum conversion rate. The measurements reported here were carried out in the current mode, i.e. the samples were shortcir-cuited through the electrometer insistance (usually 10¹⁰ ohms). The measured signals were of the order of 10^?12 10¹¹ A, and — although differing in details — were found to be satisfactorily reproducible in general features.

It is shown that a steady-state polarization of the order of 10^?4 C/m2 is built into the pTS samples during the polymerization. The effect is probably associated with non-compensated dipolar defects created on polymerizing pTS crystals, and possibly also accounting for the pyroelectricity of poly-pTS, reported earlier in the literature.