GaN films studied by near-field scanning optical microscopy, atomic force microscopy and high resolution X-ray diffraction

Spatially resolved photoluminescence measurements from GaN films were achieved using a near-field scanning optical microscope (NSOM). We have studied GaN films grown by metalorganic vapor phase epitaxy on sapphire substrates. Spatial scans of topography, band-edge and yellow luminescence have been performed with submicron spatial resolution. Spatial variations in the photoluminescence characteristics are clearly observed at the submicron scale. Measurements by atomic force microscopy and high resolution X-ray diffraction were also performed and compared with the NSOM measurements.     

In-situ observation of crystal surfaces by optical microscopy

In this experimental course, attendees will learn how to obtain useful information about growth processes of crystals using ordinary optical microscopes, which are usually available in laboratories. We will demonstrate how thicknesses of crystals can be estimated from interference colors. We will also show in-situ observations of spiral steps and strain distributions by differential interference contrast microscopy and polarizing microscopy, respectively.     

Low-voltage scanning electron microscopy of multilayer polymer systems

The possibilities of low-voltage scanning electron microscopy for visualization of specific features of the microstructure in internal layers of multilayer polymer films are demonstrated by the example of “chitosan–polyelectrolyte complex–alginic acid” composite. The process of electron beam interaction with a sample at low electron energies is considered. The key parameters of low-voltage electron microscopy, which make it possible to increase the resolution of SEM images of polymer systems, are discussed.     

Electron and optical microscopy studies of extraterrestrial minerals in NWA4047 meteorite

Analytical electron microscopy and optical microscopy were used to determine the elemental and mineral composition of NWA4047 meteorite. The meteorite was found in 2005 in Morocco, and in 2006 was preliminary classified as H4‐5 ordinary chondrite. The main crystalline meteorite minerals: olivines, pyroxenes, kamacite and taenite, feldspars, as well as troilite identified in our sample represent extraterrestrial minerals typical of the ordinary chondrite of high iron content. Low‐Ca orthopyroxene called bronzite has been revealed. Single enstatite and clinopyroxene crystals have also been identified. Olivines in the meteorite contain 18 mol% of fayalite. The meteorite is a monomict breccia, in which fragments of petrographic types 4, 5, and 6 are present, with the groundmass being type 4. Some of clasts contain large grains of feldspars and high content of feldspars, which is typical of petrologic type 6. Chemical and mineral composition, petrologic types of chondrules, their abundance and sizes, the presence of troilite‐kamacite veins, and nonoxidised iron phases prove that the chondrite belong to H group, i.e. to the olivine‐bronzite group. Our results show that the NWA4047 meteorite is H4‐6 ordinary chondrite. The weathering grade of the meteorite W0/1 is the same as previously established, but the shock grade S3 seems to be higher in comparison with the previous classification. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Diffusion kinetics of some glass systems used for the production of optical fibres

The rate at which the mobile ions in the core and cladding glasses interdiffuse during the drawing of an optical fibre by the double-crucible method, considerably affects the refractive index profile in the final product. In four glass systems suitable for the production of a low-loss graded-index compound glass fibre this quantity has been measured using interference microscopy.     

A study of ferroelectric domain patterns in orthorhobic KNbO3 with optical microscopy

Ferroelectiric domain patterns in natural surface layers of Czochralski as-grown crystals are very complicated. The inner domain patterns are comparatively simple and consist of a regular arrays of so-called thin lens-like 90° domains. Only few inner 60° and 120° domains crossing the 90° domain arrays are observed. The inversion or 180° domains are shape-like separated islands. The 60° microdomains are easily introduced into the surface layers during the specimen processing. There are some elastic interactions between the different kinds of ferroelectric domain walls, the orientations of which are in good coincidence with the theory proposed by Janovec.     

Realization of Fabry-Perot resonators for hard X rays using micro- and nanotechnology

An X-ray Fabry-Perot resonator has been proposed and investigated for more than three decades. The difficulty in realizing such a resonator is mainly due to the strict requirement on coherence for cavity resonance, which is governed by the energy resolution of the incident X-ray photons and the size of the resonator. With the aid of modern micro-and nanotechnology for silicon, such as X-ray lithography, miniature resonators, approximately a few micrometers or smaller, can be prepared. Together with X-ray photons of ultrahigh-energy resolution from synchrotron sources, the required conditions on coherence for cavity resonance are fulfilled. Several X-ray cavities prepared by this means are used in X-ray back-diffraction experiments, where back diffraction acts as mirror reflection for X rays. Resonance interference fringes are observed unambiguously. Attempts to use multilayered materials for cavity fabrication are discussed in relation to X-ray synchrotron experiment. Possible future applications of this X-ray resonator are also mentioned. Published in Russian in Kristallografiya, 2007, Vol. 52, No. 1, pp. 27–31. Dedicated to the 50th Anniversary of the Journal The text was submitted by the author in English.     

An optical microscopy study of the biaxial ‐ calamitic nematic lyotropic phase transition

Lyotropic nematic textures are investigated, using optical microscopy, near the reentrant isotropic (I_RE) ‐ discotic nematic (N_D^‐) ‐ biaxial nematic (N_B) ‐ calamitic nematic (N_C⁺) ‐ isotropic (I) phase transitions in a lyotropic mixture of potassium laurate, decanol and D₂O. The N_B, intermediate phase between the two uniaxial nematic ones, is characterized by optical birefringence measurements. In this way, using a polarizing microscope and a colour CCD digital camera, the N_B – N_C⁺ phase transition is identified as well as the domain of the N_B phase and N_D^‐ – N_B transition point in accordance with optical birefringence data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electrical elements of the optical systems based on hydrogel - electrochromic polymer composites

Abstract

The results of studies of the electrically controlled elements of optical systems based on hydrogel - electrochromic polymer with conjugated π-electron system are shown. One important feature of conjugated polymers is the ability to change their optical characteristics under an external electric field. The electrochromic effect of these polymers may find applications in optical gating and filtering systems, optical sensors, ‘‘smart windows’’, optical memories, IR-switching, and electrochromic displays. Such electrochromic polymers were embedded in the matrix of a macroporous polymer hydrogel by the oxidative chemical polymerization method. The obtained samples based on hydrogel - polyaniline and hydrogel - polyorthotoluidine composites demonstrate the electrochromic behavior. In these composites, the electrolyte, as well as the electrochromic polymer, are located directly in the volume of hydrogel.     

Adaptation of in-situ microscopy for crystallization processes

In biotechnological and pharmaceutical engineering, the study of crystallization processes gains importance. An efficient analytical inline sensor could help to improve the knowledge about these processes in order to increase efficiency and yields. The in-situ microscope (ISM) is an optical sensor developed for the monitoring of bioprocesses. A new application for this sensor is the monitoring in downstream processes, e.g. the crystallization of proteins and other organic compounds.This contribution shows new aspects of using in-situ microscopy to monitor crystallization processes. Crystals of different chemical compounds were precipitated from supersaturated solutions and the crystal growth was monitored. Exemplified morphological properties and different forms of crystals could be distinguished on the basis of offline experiments. For inline monitoring of crystallization processes, a special 0.5 L stirred tank reactor was developed and equipped with the in-situ microscope. This reactor was utilized to carry out batch experiments for crystallizations of O-acetylsalicyclic acid (ASS) and hen egg white lysozyme (HEWL). During the whole crystallization process, the in-situ microscope system acquired images directly from the crystallization broth. For the data evaluation, an image analysis algorithm was developed and implemented in the microscope analysis software.     

Investigation for ferroelectric domains by scanning electron microscopy

Examples for the direct observation of domain structures in ferroelectric crystals by means of scanning electron microscopy are given. The results for TGS and AFB crystals with direction of the polarization vector perpendicular to the observed surface are presented with respect to temperature changes, to domain enlargement under the influence of an external electric field and to the influence of X-ray- and γ-radiation on the domain formation.     

Determination of the types and densities of dislocations in GaN epitaxial layers of different thicknesses by optical and atomic force microscopy

The change in the dislocation density on the surface of GaN epitaxial layers, which were grown by hydride vapor-phase epitaxy on sapphire substrates with c and r orientations, has been investigated by optical and atomic force microscopy (AFM). It is shown that the observed decrease in the density of threading dislocations with an increase in the layer thickness is related to the annihilation of mixed dislocations. The experimental and theoretical data on the change in the density of mixed dislocations with an increase in the epitaxial-layer thickness are in good correspondence.     

High-resolution transmission electron microscopy for crystallographic study of nanomaterials

The potential of high-resolution transmission electron microscopy (including the quantitative computer processing of images and computer simulation) in a local analysis of nanomaterials is discussed. A number of examples of the application of fast Fourier transform and simulated high-resolution electronmicroscopy images in the identification of nanophases and the crystallographic study of nanocrystals and nanoparticles are considered. The role that B.K. Vainshtein played in the development of a unified approach for determination of the structure of materials using short-wave diffraction is indicated.     

Thermal variation of attenuation for optical fibers

Thermal variation of attenuation for multi-component and high-silica glass fibers has been measured at 0.63 μm wavelength in a temperature range from 20°C to 600°C. The multicomponent glass fibers have shown no increase in loss below 400°C, whereas the high-silica glass fibers have shown a drastic, exponential increase in loss with increasing temperature. The observed change of attenuation has been explained on the basis of the shift of the ultra-violet absorption band calculated from temperature change of refractive index. The increase in optical loss, calculated from the obtained data of refractive index and dispersion parameters at each temperature, shows good agreement with the observed increase in loss.     

Porous silicon optical filters for biosensing applications

The surface of nanostructured silicon (porous silicon) was biofunctionalized by the deposition of 3-aminopropyltriethoxysilane from solution, leading to high density of amine groups covering the surface which would promote the further immobilization of biomolecules. In addition, porous silicon Bragg reflectors were developed for their use in the visible range. The optical behavior of these structures was previously designed by the use of a computational program, from which the optical constants and thickness of the individual porous silicon layers were determined. The possibility of using these structures as biosensors has been explored, based on the significant changes in the reflectance spectra before and after exposing the porous silicon optical structures to biomolecules. In particular, it is shown that there is a notable shift of the reflectance maximum associated to the Bragg reflector after immobilization of polyclonal mouse antibodies. Thus, the experimental results open the possibility of developing biosensors based on the variation of the position of the optical spectrum of porous silicon based devices.     

Amino acids-precursors for synthesizing nonlinear optical materials

Some new nonlinear optical materials were synthesized from amino acids l-arginine and l-valine and single crystals were grown from their aqueous solution by solvent evaporation method at constant temperature and slow cooling method. The synthesized materials were identified by carbon, hydrogen, nitrogen (CHN) test and single crystal X-ray diffraction (XRD) analysis. The grown crystals were characterized by measuring their thermal, optical and mechanical properties by Differential Thermal Analysis (DTA), Thermo Gravimetric Analysis (TGA), powder second harmonic generation (SHG) efficiency, damage threshold and microhardness measurements. Our study shows that halides of l-arginine and l-valine are promising Nonlinear Optical (NLO) materials having Second Harmonic Generation (SHG) efficiency, mechanical and thermal stability greater than other semi-organic NLO materials. But among the halide salts, l-valine hydrobromide (LVHBr) emerged as most promising NLO materials as far as SHG efficiency, and damage threshold are concerned.     

Novel Bi-doped glasses for broadband optical amplification

Broadband infrared luminescence is observed in various Bi-doped oxide glasses prepared by conventional melting-quenching technique. The absorption spectrum of the Bi-doped germanium oxide glass consists of five broad peaks at below 370, 500, 700, 800 and 1000 nm. The fluorescence spectrum exhibits a broad peak at about 1300 nm with full width at half maximum (FWHM) of more than 300 nm when excited by an 808 nm laser diode. The fluorescence lifetime at room temperature decreases with increasing Bi₂O₃ concentration. Influence of the glass composition and melting atmosphere on the fluorescence lifetime and luminescent intensity is investigated. The mechanism of the broadband infrared luminescence is suggested. The product of stimulated emission cross-section and lifetime of the Bi-doped aluminophosphate glass is about 5.0 × 10^?24 cm² s. The glasses might be promising for applications in broadband optical fiber amplifiers and tunable lasers.     

Defects in a mixed-habit Yakutian diamond: Studies by optical and cathodoluminescence microscopy,infrared absorption,Raman scattering and photoluminescence spectroscopy

Widespread occurrences in the crystallisation history of natural diamonds are epochs of mixed-habit growth in which normal {1 1 1}-faceted growth is accompanied by non-faceted growth on curved surfaces of mean orientation ∼{1 0 0}, termed ‘cuboid’. This paper analyses mixed-habit-related phenomena in a near-central, (1 1 0)-polished slice of an octahedron from the Mir pipe, previously studied principally by SIMS probes analysing N impurity content and C and N isotope composition. In the present work, newly studied features include dislocation content, fine structure in cathodoluminescence (CL) patterns, refined IR absorption data, Raman and photoluminescence (PL) microspectroscopy and microscopy of internal non-diamond bodies. Topographic imaging and spectroscopic techniques traced the specimen's morphological evolution from a cubo-octahedral core containing complex relative development of {1 1 1} and cuboid sectors, both populated by graphite crystallites, diameters up to ∼5 μm, lying on all diamond host {1 1 1}. Coherently overgrowing the core was a zone of widely but smoothly varying relative development of {1 1 1} and cuboid sectors, both on birefringence evidence dislocation-free, emitting strongly from cuboid sectors the PL spectra associated with Ni–N-vacancy complexes. An enclosing octahedral shell of solely {1 1 1} lamellae terminated mixed-habit growth. High-resolution FTIR absorption measurements of I(B′), the integrated absorption due to {1 0 0}-platelet defects, showed from its absence or weakness that total or substantial platelet degradation had taken place in the mixed-habit zones, indicating that these had undergone conditions close to the diamond–graphite phase boundary in their history.