Increasing the lasing efficiency in cholesteric liquid-crystal photonic structures

Two types of lasing in cholesteric liquid crystals (LCs) in the range of luminescence of laser dye molecules have been investigated. The first type belongs to the Bragg modes at the photonic band edge, which propagate along the normal to the LC layer. The second type of lasing is related to the modes leaking into the substrate and propagating at small angles to the LC layer. It is shown that the Bragg lasing efficiency can be significantly increased under wide-aperture optical pumping. The method proposed for increasing the lasing efficiency is based on suppressing the excitation of leaky laser modes using partially absorbing thin films as the coatings for LC-orienting substrates. Both experimental results and the theoretical model of the effect using the numerical simulation data are discussed.     

The Slow Precipitation of Sparingly-soluble Metal Salt Powders from Aqueous Solution: Mathematical Relations for the Kinetics of the Crystal Growth Processes in Different Types of Precipitation

The following types of slow precipitation, of sparingly-soluble metal salts from aqueous solution, have been analysed: precipitations without an induction period – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites; precipitations with significant induction periods – (i) with very few microcrystallites or seeds, (ii) with microcrystallites and first/second order surface or diffusion rate-controlled growth, (iii) with very many microcrystallites. Generally, in the early stages of precipitation, growth rates depend on the rate of addition of metal salt ions to the system and on the rate constant for crystal growth; in this stage, rates increase from low values to optimum values: in the later stages of precipitation, the metal salt ions are used up as rapidly as they are added to the system and growth rates then depend only on the rate of addition and on the number of microcrystallites or seeds; in this range, rates gradually decrease to low values. Mathematical relations, for the variation of crystal size with precipitation time at different stages of growth, have been derived for each of the above types of precipitation.     

On the surface mechanism of crystal melting

A crystal melting mechanism is proposed. The melting is caused by enhanced thermal expansion of the lattice on the surface and around structure defects. For the microcrystallites formed this way an approximate calculation of parameters is made. The microcrystallites surface area per mole is shown to be thousands of m², the radius within 1.5 to 10 nm, the gaps between microcrystallites — by 30 percent more than the lattice interatomic distance. The mean thermal linear expansion coefficients determined on the crystal surface are roughly ten times as much as the bulk values.     

Lasing in low-dimensional ZnO objects: Interrelation between the crystallite morphology and feedback formation mechanisms

Specific features of UV lasing in low-dimensional zinc oxide materials have been investigated. Two main types of lasers, differing in the feedback formation mechanism-classical random lasers and microlasers—have been selected in active disordered zinc oxide materials. In the first case, feedback is provided by strong light backscattering, and a large number of scattering particles are involved in the cavity formation. In the second case, each crystallite operates as an individual microlaser. The main lasing spectral parameters are reported for the distinguished types of lasers.     

The Microcrystallite Model of Liquids for Noble Gases and Other Non-Metals

Some main parameters of the microcrystallite model of liquids had been estimated for liquid noble gases and other non-metals by earlier proposed method. The obtained data are in good accordance with the results for liquid metals. Thus, the radii and, especially, the relative distances between the microcrystallites in liquid non-metals are similar to those for liquid metals. This confirms a universal feature of the proposed microcrystallite model.     

Luminescence behavior of Er ions in glass-ceramics containing CaF2 nanocrystals

The upconversion luminescence and near infrared luminescence of the Er³⁺ ions in transparent oxyfluoride glass-ceramics containing CaF₂ nanocrystals have been investigated. The formation of CaF₂ nanocrystals in the glass-ceramics was confirmed by XRD. The oscillator strengths for several transitions of the Er³⁺ ions in the glass and glass-ceramics have been obtained and then the Judd-Ofelt parameters were calculated. The split near infrared emission peaks of the Er³⁺ ions in the glass-ceramics can be observed because the Er³⁺ ions have been incorporated into crystalline environment of the CaF₂ nanocrystals. The upconversion luminescence intensity of Er³⁺ ions in the glass-ceramics increased significantly with increasing heat treated time. The transition mechanism of the upconversion luminescence has been ascribed to a two-photon absorption process.     

溶胶-凝胶法制备CuAlO2微晶及其合成活化能研究

采用溶胶-凝胶法制备了铜铁矿结构的CuAlO2微晶,利用示差扫描量热法(DSC)对其合成活化能进行了初步研究;通过XRD、TEM等分析方法对CuAlO2微晶的物相组成和显微结构进行表征.结果表明,以乙酸铜、硝酸铝和乙二醇为反应原料,采用溶胶-凝胶法可以成功制备CuAlO2微晶.前躯体在烧结过程中,首先生成了CuAl2O4和CuO中间相,然后继续反应再形成了CuAlO2微晶.DSC分析表明CuAlO2的合成温度在1181 ℃左右,计算得到其合成活化能为88.25 kJ/mol.利用紫外-可见光谱分析,CuAlO2微晶的光学带隙约为3.85 eV.     

Characteristic crystal growth from amorphous WO3 film by vacuum heating

Selective growth of WO₂, W and WO_3−x crystals from amorphous WO₃ film by vacuum heating at 400–900°C was clarified. The grown WO_3−x crystals were incommensurate structure based on crystallographic share structure. The growth process of WO₂ crystal in the amorphous film was directly observed at high temperature in the electron microscope. The growth front of the WO₂ crystal consumes WO₃ microcrystallites with various orientations. The growth speed of the WO₂ depended on WO₃ microcrystallites orientation. The origin of the wavy growth front of WO₂ was due to an orientation dependence of the WO₃ microcrystallites.     

Influences of Yb3+ ion concentration on the spectroscopic properties of silica glass

We investigated optical spectroscopic properties of silica glass doped with ytterbium (Yb³⁺) ions to assess their lasing performance in diode-laser pumped systems. The Yb-doped silica glass preforms were fabricated by solution doping technique using the MCVD process. The stimulated emission cross-section and laser performance parameters were determined from the measured absorption spectra using the method of reciprocity. Fluorescence decay characteristics were observed to be deviated from exponential behavior as the Yb³⁺ ions doping level increases and there exists a lifetime quenching behavior related to the cluster effect of Yb³⁺ ions into silica glass. Nevertheless, lasing parameters indicated that clustering of Yb³⁺ ions does not significantly affect the spectral properties relevant to the predicted lasing performance when concentration is low, but becomes predominant at higher concentration.     

Effect of higher protonation on lasing performance of Rhodamine-B in sol-gel glasses

Three types of Rhodamine-B (Rh-B) impregnated sol-gel samples (mentioned as type-A, type-B, type-C) were prepared. After studying photophysical properties, the dried samples were subjected to laser study under nitrogen laser pumping in a transverse dye laser cavity. Type-B and type-C samples were found to be lasing, while type-A did not show any lasing action. The photostability study of these samples under nitrogen laser pumping showed that Rhodamine-B was more photostable in type-C sol-gel host than in type-B. These results have been explained by taking into account the molecular changes in Rh-B in the sol-gel host matrix.     

To the Unified Model of Condensed State

The problem of unified model of condensed state can be solved by using solid-like model of liquids. A liquid arises when in a heated crystal a universal process of microcrack formation takes place that leads to originating a great number of very small crystals: microcrystallites. In the paper the two basic notions: the effective temperature of crystal surfaces and the internal thermal stresses in crystals are discussed. These notions make possible the microcrystallite model of liquids and the unified model of condensed state. Thus, the microcracks, resulted from thermal stresses, below the melting point lead only to decreased strength of crystals but above melting point the microcracks are spread all over the crystal bulk and cause splitting of the crystal into microcrystallites that are moving freely, in short, making up a liquid. The simple relations for these processes are given. In the end of the paper the identical features of the condensed states, solid and liquid, has been considered.     

Characterization and luminescence of a-Si:H:Cl films

a-Si:H:Cl films have been deposited by glow-discharge and characterized by infrared transmission, optical absorption and photoluminescence. The influence of growth parameters on the H and Cl content has been investigated. The luminescence spectra show that three different radiative transitions can occur, at 0.75, 0.95 and ～1.3 eV. These bands have been interpreted respectively in terms of the following recombinations: defect to defect, defect to band tail, band tail to band tail.     

Thermally Stimulated Luminescence of Montmorillonite Clay

X-ray excited luminescence (XL) spectra of montmorillonite clay at RT showed a broad emission band around 380 nm with two shoulders at 340, 450 nm eitherside two sharp peaks and two shoulders are seen at 465, 487, and 548, 576 nm, respectively. The 548 and 576 nm peaks enhanced their intensity at LNT. Whereas thermally stimulated luminescence (TSL) revealed an intense glow peak at 355 K and a weak shoulder around 400 K. Room temperature annealing studies indicated that the 355 K glow peak is not as stable as 400 K shoulder which turns out to be a peak on RT annealing. The X-ray excited luminescence has been attributed to radiative recombination of electrons with holes which are associated with SiO^3-₄, AlO^4-₄ radicals and divalent impurities available in the naturally available clay. TSL is also understood on similar lines. Glow peak reactions are proposed tentatively and their trap parameters have been calculated.     

Study of Lasing in Liquid-Crystal Systems with Microgratings

Waveguide lasing in a layer of nematic liquid crystal (NLC) doped with a dye has been experimentally observed. A chromium micrograting with a period of 1.76 μm has been deposited on the surface one of the cell substrates to provide a distributed feedback (DFB) and partially extract laser radiation to a quartz substrate. The DFB provides lasing in the ninth diffraction order for waveguide TM modes. Laser radiation is observed at the output of the substrate end face at an angle of 67.0° ± 1.5° with respect to the normal to the waveguide plane. An increase in voltage across the micrograting electrodes leads to red-shifted multimode lasing. A numerical simulation of a structure imitating the experimental cell has shown good agreement between the calculation results and experimental data.     

Structure‐controlled growth of ZnO nanonails by thermal evaporation technique

Wurtzite ZnO nanonail structures have been grown on sapphire substrate by simple thermal evaporation of Zn powder in oxygen ambient. Growth parameters such as growth temperature and oxygen gas flow have been examined for the growth of nanonail structure. It is found that the nanonail structures repeatedly grow under a certain relation between the growth temperature and the oxygen flow. Also, at higher growth temperature, the nanonails grow in the form of branched‐structures. The grown ZnO nanonails have hexagonally well‐faceted cap and grow mostly perpendicular to the sapphire substrate. Excellent luminescence properties of a strong UV emission peak with negligible green band have been obtained at room temperature. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Texture and surface analysis of thin-film GaAs on glass formed by pulsed-laser deposition

Thin-film GaAs on glass was formed by ablating n-type GaAs with nano-second pulses at 532 nm. The deposition was done in the most straightforward way without heating the substrate. The texture of films has been investigated with X-ray measurements, spatially resolved micro-Raman spectroscopy, and atomic force microscopy. The results reveal that the film texture is of multi-phase nature consisting of randomly oriented GaAs microcrystallites, amorphous parts, and (1 1 1) zincblende migrations in the nano-regime.     

Low ohmic multilayer contacts in lead-Tin-Telluride diode lasers

The preparation and the influence of low ohmic multilayer thin film contacts of lead-salt homo- and heterolasers on the degradation of lasing parameters during recycling processes between low working temperatures and room temperatures storage are described and discussed in detail.     

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.     

The Effect of Microstructure on Thermally Stimulated Luminescence of Y–Ba–Cu–O System

Thermally stimulated luminescence of Y – Ba – Cu – O (YBCO) samples in as-prepared, quenched and water treated conditions have been studied. The TSL spectrum is observed to be different for these samples. Quenched and water-treated samples are expected to decompose into a number of insulating phases. Identification of such phases using thermally stimulated luminescence is attempted.