Mutual transformation of light waves by reflection holograms in photorefractive crystals of the $$\bar 43m$$ symmetry

The mutual transformation of light waves in the case of their simultaneous diffraction from a bulk reflection phase hologram, which was formed in a cubic photorefractive crystal of the [`4]3m\bar 43m symmetry class, has been studied. The indicator surfaces of the polarization-optimized values of the relative intensity of the object wave, which make it possible to determine the amplification of this wave for any crystal cut, are constructed. The linear polarization azimuths at which the energy exchange between the light waves reaches a maximum are found numerically for crystals of different cuts.     

Excitation of intense acoustic waves in hexagonal crystals

Resonant excitation of an intense elastic wave using reflection of a pump wave from a free surface of hexagonal crystal is described. A resonance arises in the case of specially chosen propagation geometry where the reflecting boundary slightly deviates from symmetric orientation and the propagation direction of an intense reflected wave is close to that of an exceptional bulk wave, which satisfies the free boundary condition in unperturbed symmetric orientation. It is shown that, in crystals with elastic moduli c ₄₄>c ₆₆, a resonance arises when the initial boundary is chosen parallel to the hexagonal axis 6, whereas in crystals characterized by the relation c ₄₄<c ₆₆ a resonance occurs when the initial boundary is orthogonal to this axis. The fraction of the pump energy transferred to the excited beam depends on the specific relations between the elastic moduli and can be rather significant for specially chosen crystals. Examples of crystals are presented in which the beam intensity can be increased by a factor of 5–10 at sufficiently high frequencies, with beam divergence remaining acceptable.     

X-ray focusing by bent crystals in Bragg backscattering

Focusing of a spherical X-ray wave in Bragg backscattering from weakly and strongly bent crystals is considered theoretically. The analytical formula describing the dimensions of the diffraction backscattering region is obtained. It is shown that, along with the well-known Johann scheme, the use of the backscattering scheme allows one to increase the aperture ratio of the crystal optics by two to three orders of magnitude. The spectral characteristics of bent crystals providing diffraction backscattering (θ = π/2) are discussed. It is shown that the spectral resolution can attain a value of the order of 10^?11.     

Positron annihilation in BaF2 crystals

Positron lifetime measurements in BaF₂ were performed in the temperature range 10 to 292 K. Three lifetime components were resolved, the third component having a very low intensity. The intensity of the second component increases between 12 and 80 K. For further increase of the temperature, up to 290 K, the intensity of this component falls from about 63% to 37%. An increased positronium formation probability is suggested for explaining this observation. Some alternative explanations are discussed, as well.     

Conoscopic figures of optically active uniaxial crystals

Conoscopic figures of optically active crystals are described by the formulas for the intensity of light transmitted by a plate located between arbitrarily oriented polarizer and analyzer. The differences in the behavior of optically active and inactive crystals and also of right-and left-handed crystals are established. The corresponding computations and the simulation of conoscopic figures are performed using the Mathematica-4.1 package of computer programs.     

Photoluminescence study of annealing effects on CuI crystals grown by evaporation method

Cuprous iodide (CuI) is the ultrafastest inorganic scintillation crystal at present. But the low intensity of its ultrafast component luminescence limits the wide application of CuI at room temperature. In this paper, the photoluminescence (PL) characteristics of different quality CuI crystals before and after annealing in various conditions have been investigated in terms of peak position and peak intensity. The origin of different emission band peaked around 426 nm, 680 nm, 718 nm and 820 nm is discussed and the excitation spectra of two mainly emission bands is obtained. Meanwhile, the relative peak intensity of the ultrafast luminescence component to slow lumiescence component of CuI crystals has been studied with respect to the defect concentration of I vacancies. Especially, the method of improving the intensity of ultrafast compentent luminescence of CuI crystals is concluded. These results can provide an important reference for optimizing the luminescence performance of CuI crystals.     

Optical property analysis of CZT:In single crystals after annealing by a two-step method

Indium-doped Cd_1−xZn_xTe (CZT:In) single crystals were annealed by a two-step method, including a high-temperature step and a low-temperature step in sequence. IR transmittance spectrum, I–V curve and PL spectrum were used to characterize the CZT single crystals. After annealing, the opto-electrical properties of the CZT:In crystals were improved obviously. The average IR transmittance was remarkably increased by about 23%, and the resistivity was enhanced by as high as four orders of magnitude. In the PL spectra, the intensity of the (D⁰, X) peak prominently increased, and the full-width-at-half-maximum was reduced. Meanwhile, the intensity of the DAP peak decreased greatly, and the structure became practically indistinguishable from the background. Moreover, the intensity of the D_complex peak also decreased. The investigation shows that these improvements in the physical properties after annealing are due to variations in the micro-structures. The two-step annealing method can eliminate precipitates/inclusions, remove impurities, compensate Cd vacancies, decrease dislocations and reduce internal stress.     

Group-theoretical representation of holohedral forms of crystals by conjugated simple forms. Intergrowth of crystals

The schemes for division of holohedral simple forms of crystals into conjugated simple forms are derived by decomposition of the symmetry group of the primitive sublattice into double cosets. The number of equivalently oriented simple forms in the intergrowth of crystals, whose primitive space sublattices are parallel to one another, is equal to the number of holohedral permutational conjugated simple forms, which has the value 992 for all the 32 symmetry classes.     

Light propagation in stratified chiral media. The 4 × 4 matrix method

Propagation of electromagnetic waves in stratified bianisotropic chiral structures is described by the 4 × 4 matrix method. At arbitrary layer parameters, the amplitude and the polarization characteristics (intensity, polarization azimuth, and ellipticity) of reflected and transmitted electromagnetic waves are studied as functions of the angle of the wave incidence onto the structure.     

Photoconductivity in Tl4S3 layered single crystals

Photoconductivity measurements are carried out in this work for single crystals of Tl₄S₃ compound by using both pulsed excitation (a.c) and steady state (a.c) methods in order to elucidate the nature of photoconductivity (PC) in this compound. Results are reported in the temperature range from 77 to 300 K, excitation intensity range from 1800 to 3300 Lux, applied voltage range from 8 to 14 V, and wavelength range from 840 to 1450 nm. Both of the ac‐photoconductivity (ac‐PC) and the spectral distribution of the photocurrent are studied at different values of light intensity, applied voltage and temperature. Dependencies of carrier lifetime on light intensity, applied voltage and temperature are also investigated as results of the ac‐PC measurements. By using the results of the dc‐photoconductivity (dc‐PC) measurements, the temperature dependence of the energy gap width is described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Dispersionless polaritons on a twist boundary in optically uniaxial crystals

It has been shown that the so-called dispersionless localized waves (polaritons) can exist in a bicrystal formed by two identical crystals. The optical axes of these crystals lie at the interface at a certain angle to one another. The exact analytic expression is derived for the velocity of the wave propagating along the bisector of the angle between the optical axes in the upper and the lower parts of the bicrystal. The wave structure is analyzed for several limiting cases.     

X-ray topographic study of quartz crystals with induced growth striations

The method of “induced growth striations” was applied to the hydrothermal growth of quartz crystals. Striations were due to a periodical modulation of the growth temperature induced and investigated by means of the X-ray Lang topography and plane wave reflection topography. The results show that the X-ray topographic methods are a sensitive tool for the characterization of the growth striations, which are produced generally by Al impurities. The application of the method of induced growth striations to quartz crystals and the characterization of these crystals with optical and X-ray topographical methods allow a deeper understanding of the crystal growth behaviour.     

Kinetics of luminescence of CsI single crystals scavenged by Y3+

CsI single crystals were grown from the melt scavenged by Y³⁺ (YCl₃) addition in 6.7·10⁻⁴–6.7·10⁻³ mol·kg⁻¹ range. The addition of the scavenger amounts comparable with the total concentration of the oxygen‐containing admixtures in molten CsI results in complete destruction of the latter. Because of this, the intensity of the band with a maximum at 2.8 eV in radioluminescence spectra caused by the oxygen‐containing admixtures (anion vacancies) considerably decreases, and the fraction of the slow 2μs‐component corresponding to these admixtures becomes lower than 0.01 (0.007). The addition of larger quantities of YCl₃ leads to the appearance of a wide band with a maximum at 2.8 eV caused by cation vacancies, and the intensity of the slow 2μs‐component increases to 0.02. The maximum ratio of two faster components with the decay constants equal to 7 and 30 ns reaches 0.65:0.33 at Y³⁺ concentration in CsI melt equal to 6.7·10^‐3 mol·kg^‐1, the effective luminescence time of fastest components is ca 14 ns. The dependence of the ‘Fast/Total ratio’ on Y³⁺ concentration passes through its maximum (0.81) corresponding to the equivalence of Y³⁺ and O²⁻ concentrations in the growth CsI melt.     

X-ray diffraction in polydomain crystals modulated by transverse waves of atomic displacements. 2. Two-wave modulation of crystals

X-ray diffraction in an orthorhombic crystal modulated by two transverse-displacement waves propagating along one of the crystallographic directions has been numerically studied within the framework of the kinematic approximation. Crystal models are considered that allow the superposition of the displacement waves or the coexistence of domains modulated by the displacement waves with different parameters. It is shown that the formation of peaks additional to the satellite ones is possible not only due to the superposition of the displacement waves, but also due to a certain correlation of the displacement waves in each domain.     

Mechanoluminescence of halogenate crystals

Mechanoluminescence (ML) has been studied in twenty halogenate crystals. All piezoelectric crystals show ML similar to the emission from second positive group of molecular nitrogen, and all the crystals not exhibiting ML are non-piezoelectric. Some of the non-piezoelectric crystals also exhibit nitrogen emission ML which is comparable in intensity to that of piezoelectric crystals. The ML appears only during the different steps developing in the fracture region of the force versus compression curve of the crystals. No considerable change in the ML activity is found due to the number of crystallization or due to the addition of different impurities from 100 to 10.00 ppm. The ML activity per mole of the halogenate crystals varies around four orders of magnitude. It is concluded that in addition to the piezoelectrification of the newly created surfaces there exists some other process of electrification which may cause the ML exitation in halogenate crystals.     

Effect of Annealing on the Luminescene Property of PbWO4 Crystals Grown by the Czochralski Method

PWO Crystals grown by the Czochralski method were annealed under different conditions. The transmittance of PWO crystal was decreased when it was annealed in anoxygen‐rich environment; where as it was in creased when the crystal was annealed in vacuum. A mechanism on the transmittance change was proposed. The main peaks of X‐ra y exite demission spectra showed that both as‐grown and vacuum‐annealed crystals fit into blue light region with the latter one showing higher intensity. The oxygen‐rich annealed crystals hifted to longer wave length of green light region at lower intensity. The faster components in the light yield of crystals annealed in vacuum were higher correspondence to its higher intensity of the blue light.     

Photon and phonon velocity operators in dispersive crystals

Some particular examples of velocity operators in absorbing and excited crystals having dielectric, magnetic, and elastic anisotropy and frequency dispersion are considered. Expressions for the velocities of polarized normal waves are given in the vector form for any directions of phase normals. These expressions are valid for geometric analysis of velocities in the case of continuous or discrete changes in the sets of 3D propagation directions and the sets of initial polarizations of bulk waves and wave beams in crystals.     

Growth and properties of dyed KDP crystals

Potassium dihydrogen phosphate (KDP) crystals doped with xylenol orange (XO) and methylthymol blue (MTB) are grown from aqueous solutions by the method of solvent evaporation at room temperature under the conditions of natural convection and by the method of temperature lowering. Studied is the influence of the mother solution acidity on the character of the crystal coloration. The color and coloration intensity of the grown crystals are shown to strongly depend on the solution's pH. It is revealed that the crystal habit changes in the presence of organic dyes. The optical transmission spectra and the luminescence spectra of KDP:XO solutions and of the grown crystals are measured. The effect of thermal treatment and UV‐irradiation on the coloration stability of the crystals is studied. It is found that the laser damage threshold in the prismatic impurity rich colored sectors of KDP:XO is the same as that in the prismatic sectors of pure KDP crystals. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)