Anisotropic optical properties of crystals with the Ca-gallogermanate structure

The optical properties of inactivated and activated crystals with the structure of Ca-gallogermanate have been studied. The components of the gyration pseudotensor for Sr₃Ga₂Ge₄O₁₄ crystals are determined. In the ultraviolet range of the spectra of all the crystals studied, the bands of circular and linear dichroism and the anomalous behavior of the linear birefringence are observed irrespective of the activating dopant. The formation of these bands is not associated with the electron transitions of doping ions and can be caused by structural defects formed in the growth process.     

Topological Soft Matter for Optics and Photonics

Liquid crystal colloids are interesting for a variety of mechanisms—including self-assembly, optical-tweezers assisted assembly, topology, and material flow—that can be used to create various complex optical and photonic structures. Here, we present a brief overview of liquid crystal colloidal structures, as recently achieved by numerical modeling and experiments. Central to the structures are complex conformations of topological defects, as they can bind, stabilize, or distort the structure. Using topological and geometrical arguments, we show that the defects can be controllably rewired and imprinted, for example by using optical tweezers. We show that 3D colloidal crystals can be assembled from elastic dipoles of spherical beads in nematic liquid crystals or via inherently inhomogeneous order profiles in bulk and confined cholesteric blue phases. Colloidal crystals are generalized to close-packed colloidal lattices, which we show can serve as natural templates for defect networks. Finally, photonic bands are calculated for selected structures and possible defects in the structure are discussed.     

Color centers and charge state change in Ce:YAG crystals grown by temperature gradient techniques

Color centers and impurity defects of Ce:YAG crystals grown in reduction atmosphere by temperature gradient techniques have been investigated by means of gamma irradiation and thermal treatments. Four absorption bands associated with color centers or impurity defects at 235, 255, 294 and 370 nm were observed in as-grown crystals. Changes in optical intensity of the 235 and 370 nm bands after gamma irradiation indicate that they are associated with F⁺-type color center. Charge state change processes of Fe³⁺ impurity and Ce³⁺ ions take place in the irradiation process. The variations of Ce³⁺ ions concentration clearly indicate that Ce⁴⁺ ions exist in Ce:YAG crystals and gamma irradiations could increase the concentration of Ce³⁺ ions. Annealing treatments and the changes in optical density suggest that a heterovalent impurity ion associated with the 294 nm band seems to be present in the crystals.     

Natural crystallization

Natural mineral crystals grow under a broad spectrum of conditions; from vapors, from hydrothermal solutions, from magmas (high temperature solutions), or through metasomatic or metamorphic reactions. In understanding kinetic problems involved in natural crystallization, there are two ways of approach; (1) experimentally stimulating textures of rocks, and (2) decoding the paragenetic information contained in natural crystals. The latter approach is especially important, since in situ observation is impossible. Key Key features which aid in deciphering natural growth processes and conditions include external forms, surface microtopographs of crystal faces, internal inhomogeneity (growth bands, growth sectors, inclusions, twin or exsolution textures), lattice defects (plane defects, dislocations) and impurities (precipitations). Mainly based on the observations of surface microtopographs of natural crystals, characteristics of crystallization in magma, in hydrothermal solution, in vapor phase, in hydrothermal metasomatism and in regional metamorphism are analysed and reviewed in this paper. The difference and similarity between natural and synthetic crystals are also discussed.     

Influence of free convection on the formation of growth defects in potassium bichromate crystals grown from aqueous solutions

The influence of free convection on the formation of inclusions, growth bands and dislocations in potassium bichromate crystals grown by self-nucleation and on freely hanging seeds from aqueous solutions in a wide range of temperature is studied. It is found that in free convection all the three types of defects appear mainly in the top sectors but in stirred solutions the slow-growing faces are more defective.     

Geometrical properties of dislocations in potassium bichromate crystals

The geometry of growth dislocations present in potassium bichromate crystals grown from aqueous solution has been studied by etching. Etch topographs composed of dislocation etch grooves and grooves representing sector boundaries and growth bands have been analysed. It was found that most of the dislocation lines are straight and have well-defined directions in each sector. Refractions of dislocations at growth sector boundaries and at other defects have been observed.     

光子晶体多组元缺陷态问题研究

缺陷态光子晶体可以用于制作良好的谐振器、偏振器、滤光器等光学器件,具有重要的应用价值。本文发展了光子晶体缺陷态问题的PG有限元界面问题计算方法,有效地处理了各种不同组元体系、几何结构、界面形状、材料属性以及模态的光子晶体缺陷态问题。数值结果表明,二组元结构单点缺陷对带隙的影响较小,只是使局部范围内的波继续传播而产生一条缺陷带,多点缺陷使一些特定范围内的波可以传播而产生多条缺陷带,线缺陷产生的影响较大,可以使整个禁带消失。结合线缺陷与点缺陷,波导结构中的侧点缺陷可以有效地应用于光子晶体阻带内诱导窄通带或在波导的通带内诱导非常窄的阻带。三组元结构引入了不均匀介质、复杂介质形状以及不同几何结构的缺陷态。通过计算与分析发现Ω₃区域的介质形状对结果影响比较有限,表面层越不光滑禁带越窄,n型缺陷态在TM模中的高频区域更容易产生禁带。对于TE模来说,n型与v型的缺陷态更容易产生禁带。     

Effect of Fast Electron Irradiation and Annealing on the Luminescence in GaAs(Zn) crystals. Irradiation-induced 1.26 and 1.39 eV Emission Bands

The effect of 2.2 MeV electron irradiation and subsequent annealings on the photoluminescence in zinc-doped p-type GaAs crystals is studied and analyzed. Rather strong emission bands peaked at hv_m (77 K) near 1.26 eV (induced by electron irradiation) and 1.39 eV (induced by annealing of irradiated crystals) are observed. Evidence is presented that the 1.26 and 1.39 eV emission bands occur due to radiative electronic transitions in As_iZn_Ga and V_AsZn_Ga pairs induced by irradiation and annealing of irradiated crystals, accordingly. The observed variations in the intensities of the 1.26 and 1.39 eV emission bands upon irradiation and subsequent annealings of GaAs(Zn) crystals are explained in terms of irradiation and annealing-induced variations in the amount of 1.26 and 1.39 eV radiative centres resulting from: a) the effective interaction of mobile radiation-induced defects in the arsenic sublattice with zinc atoms leading to the formation of As_iZn_Ga and V_AsZn_Ga pairs; b) the thermal dissociation of As_iZn_Ga and V_AsZn_Ga pairs on individual components.     

Micromorphology of pure and PbI2‐doped CdI2 dendritic single crystals

Undoped and PbI₂‐doped dendritic single crystals were grown by vapour growth technique. The basal surfaces of the as grown crystals were examined by optical and electron microscopy to observe wide variety of growth and defect features. Apart from typical features of dendritic growth, features of overgrowth, slip bands, growth steps and their bunching, etc. were observed. The basal surfaces of the crystals were then etched by controlled condensation of water vapour, after optimizing the etching condition, and the microscopic studies were repeated. Etch pits of hexagonal and triangular shape, both symmetric and asymmetric, and of different density, were observed in the case of undoped and doped crystals, respectively. In some cases, crystallographic hillocks were also observed. The crystals were also examined by X‐ray diffraction for their polytypism and related behaviour. The results are analyzed to elicit information on the correlation of structure, defects and surface features of the crystals. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

坩埚下降法生长CaF2单晶的研究

采用坩埚下降法生长了CaF2单晶体,研究了不同条件生长的单晶缺陷和光谱性能.结果表明:当晶体生长过程中进入水等含氧杂质时,所生长的晶体不仅在1500nm附近产生非常宽的OH-两倍振动吸收带,而且在可见-紫外波段也形成强烈的色心吸收带.同时,杂质离子Ce3+的存在也导致晶体出现306nm的吸收带.     

Influence of cluster defects of variable composition on the optical and radiative characteristics of oxide crystals

It is shown that oxide crystals contain cluster defects of variable composition, which cause absorption of light in the transparency region of crystals. The model based on the presence of cluster defects in oxide crystals explains well the experimental data on the thermal and radiative coloring of these crystals. It is noted that cluster defects accumulate oxygen in oxide crystals. These defects are responsible also for the photochromic effect in them. Application of the noted model made it possible to fabricate lead tungstate scintillators at North Crystals Company for the ALICE project (CERN) with almost 100% reproducibility of their operating characteristics.     

Quenching by Copper Atoms of the EL2-induced Luminescence in GaAs

It is shown that the introduction of copper atoms into GaAs crystals containing antistructure defects EL2 (isolated arsenic atoms on gallium sites As_Ga) leads to a practically complete disappearance of the EL2-induced luminescence bands peaked at 0.63 and 0.68eV. This effect is connected with the passivation of the EL2 defects (i.e. with the substantial decrease in their concentration) because of their interaction with copper atoms (they become bound by copper atoms) resulting in an appearance of electrically inactive AS_GaCU_Ga complexes.     

Peculiarities of reoriented bands formation in crystals

The peculiarities of the formation and the development of reorientated bands in crystals are considered. Reorientated bands are investigated experimentally in MgO single crystals under the compression testing. It is shown that the misorientation between the material in the band interior and the surrounding volume can reach some degrees. The data of electron microscopy investigations of dislocation structure of such bands are performed. The disclination model is proposed for the propogation of the reorientated bands. This model permits to explain the laws of the band development, for example, the formation of discrete areas with reoriented crystal lattice ahead the terminated band.     

Photoacoustic Spectroscopy of Defect States in Etched and Air-annealed CuInSe2 Single Crystals

Photoacoustic spectra of cleaved, polished and etched, and air-annealed n-type CuInSe₂ single crystals are measured at different frequencies between 30 and 312 Hz. The spectra related to the bulk of the crystals exhibit five structures due to defects that are also present in p-type crystals. Polishing and etching as well as subsequent air annealing at 100, 200 and 300 °C reveal rather complex changes of the defect equilibrium in the near-surface region of the crystals which include both relative concentration changes of existing defects and creation of new defects. The results for polished and etched crystals correspond to trends expected from etching induced local modifications of the composition and structure as revealed by electron spectroscopies and ion channeling. Air annealing is found to affect all existing defects and to create up to five new defects which cannot be explained in terms of the related point defect model proposed by CAHEN and NOUFI.     

Nematic Defects and Colloids as Photonic Elements

Birefringent structures in liquid crystalline fluids, such as colloidal assemblies or topological defects, show high potential for use as photonic elements. Here, we present a brief overview of two photonic phenomena originating from coupling light fields with complex birefringent nematic profiles: (i) the generation of vector laser beams from simple Gaussian beams by propagating light along nematic discliantions, and (ii) tunable photonic crystals from blue phase colloidal crystals conditioned by the different underlying symmetries of the particle lattice and the blue phase birefringence. The polarization profile of initially simple linearly polarized Gaussian beams is shown to change into a defect structure at distinct distances travelled along the disclination with the topological invariant (winding number) of the light field and nematic director distinctly coupled. Upon pulsed laser illumination, the nematic discliantions are also shown to split the light pulse into multiple intensity regions. Blue phase I face centred cubic colloidal crystals are shown as examples of tunable photonic crystals, where local band-baps can open by differently combining the symmetries of the two components, e.g. by changing the particle size. The spatial profiles of selected photonic bands in the blue phase colloidal crystals are shown, finding the particles and blue phase double twist cylinders as possible carriers of high-light-intensity regions.     

Interference pattern of X-ray waves inside the crystal and its image in vacuum in double-field approximation (II)

The energy flows of the X-ray waves, diffracted in thick crystals in double-field approximation, have been examined. It has been demonstrated that in a symmetric Laue case the energy flows in the upper layers are oriented in the direction of incidence and reflection, whereas in the lower layers they are oriented in the direction of the normal to the entrance surface of the crystal. The formation of pendular bands during diffraction of X-rays in thick parallelepiped and wedge-shaped crystals has been investigated. A method for obtaining pendular bands from such crystals is described.     

Topography and goniometry of very perfect crystals

In order to characterise very perfect crystals such as Si which do not show any visible defects in conventional X-ray diffraction topography, the following topics are reviewed. (1) Intensity distribution of Pendellösung fringes in section topographs. (2) The tails of rocking curves. (3) Borrmann anomalous transmission. (4) Decoration of the defects. (5) Diffuse scattering. The experiments reviewed here indicate that the currently available good crystals are not regarded as ideally perfect crystals. The deviations from ideal perfection in (1), (2) and (3) are due to weak deformation of the crystal matrix surrounding the invisible defects whereas (5) gives us direct information about the atomic structure of the defects.     

水热法KTP晶体生长与宏观缺陷研究

本文报道了水热法KTP晶体的生长工艺及晶体生长形态,系统研究了水热法KTP晶体的宏观缺陷,其宏观缺陷主要为添晶、生长脊线、裂隙和包裹体.提出了晶体生长工艺的改进措施,如提高原材料和试剂的纯度、调整籽晶的悬挂方式、减少籽晶的尺寸等,都可以减少晶体的宏观缺陷,提高晶体的质量.//(011)切向的籽晶生长的晶体质量较高,且能很好地应用于激光器件中.     

Fluorescence and amplified emission properties of single-crystal 2,5-bis(4-biphenylyl)thiophene

Ligth amplification properties are investigated for single crystals of commercially available 2,5-bis(4-biphenylyl)thiophene. Depending on growth methods, three kinds of crystals are obtained having different fluorescence spectra in blue-green color. Under optical pumping, their amplified spontaneous emission (ASE) bands appear at different wavelengths. The solution-grown crystal shows ASE bands at λ = 464 and 494 nm which are assigned to the 0–1 and 0–2 transitions, respectively. By contrast, the vapor- and slide-boat-grown crystals show a red-shifted 0–1 band at λ = 496 and a 0–2 band at 520 nm, respectively.     

Disorder in GeSe2 small crystals and medium-range structure in amorphous GeSe2

The Raman scattering spectra of various forms of GeSe₂, such as amorphous films, single crystals, and small crystals prepared by a gas-evaporation deposition technique, are investigated with an excitation photon energy ranging from 2.1 to 2.8 eV. In the spectra of the crystals, there are two Raman bands, originating from two types of breathing vibrations of the GeSe_4/2 tetrahedra: one is spread over the corner-sharing tetrahedra and the other is quasi-localized on the edge-sharing tetrahedra. In addition to the resonant Raman scattering related to the edge-sharing tetrahedra through the exciton transition observed with 2.71 eV excitation in the single crystals, a new resonant spectrum is found only in the small crystals with 2.54 eV excitation. With increasing disorder in the crystals, the intensity ratio between the two breathing vibration bands increases in the off-resonant excitation region. The Raman spectra in the amorphous states are ascribed to the breathing vibrations of GeSe_4/2 tetrahedra which form a medium-range structure topologically similar to that of crystalline fragments.