Destruction mechanism of color centers and recombination luminescence in NaCl-Cd crystal phosphors

The authors have studied the origin of integral thermoluminescence peaks and the destruction mechanism of color centers during thermal relaxation of X-irradiated NaCl-Cd phosphors. The luminescence spectra have also been studied, and previous similar results are discussed.     

Dislocation interaction with radiation defects in alkali-halide crystals

A mechanism of room-temperature irradiation-induced strengthening of alkali-halide crystals (AHCs) is suggested. It is shown that the deformation-stimulated luminescence is a result of the destruction of hole color centers, which strengthen AHCs, by dislocations.     

Influence of the degree of plastic deformation on the thermal photostability of F-centers in NaCl crystals

The effect of plastic deformations on the thermal stability and thermal photostability of F-centers in NaCl crystals was examined. The thermal photostability of F-centers and the thermal stability of some of these centers increases in proportion to the degree of plastic deformation. The concentration of the less stable F-centers increases due to plastic deformation. The effect was studied in crystals which were deformed before and after x-ray excitation. The results were interpreted from the viewpoint of an ionic activation mechanism in the thermal and photothermal decomposition of the color centers in alkalihalide crystals.The authors are indebted to Yu. L. Lukantsever for an evaluation of the results of this work.     

Microscopic investigation of the creation and destruction of color centers in alkali-halide crystals

A new microscopic method for the study of color-center creation in alkali-halide single crystals is proposed. The creation and destruction of F- and M-color centers in NaCl single crystals and also in threadlike NaCl crystals have been studied on topographically different microareas of the samples.The authors are deeply indebted to Dr. Ch. B. Lushchik, M.A. Elango, and K. I. Gindina for valuable discussions of the problems encountered and also for providing a number of crystals for the investigation.     

The interaction of photoexcited color F-centers with microdefects in the crystal lattice of alkali-halide crystal phosphors. II

The interaction of different microdefects with excited and nonexcited F-centers was studied, using the methods of thermal decolorization, thermal luminescence, photothermal decolorization and photothermal luminescence.A comparison of experimental data with the results of theoretical calculations of the corresponding curves enables an estimate to be made of certain kinetic characteristics of the processes in questions.     

Investigation of the disintegration mechanism of color centers,and recombination luminescence of the (NaCl-Ag,Ca)-crystal phosphor. II

The investigation of relaxation processes in alkali-halide phosphor crystals excited by X-rays is continued.The mechanism of disintegration of color centers, the occurrence of thermoluminescence peaks, as well as the spectra of thermally stimulated and photo-stimulated luminescence, are investigated for a (NaCl-Ag, Ca)-crystal phosphor.     

Interaction of excited F centers with lattice defects in NaCl(Ag) phosphor

Theoretical curves are derived for thermal and photothermal bleaching on the basis of an ionic mechanism for F-center destruction.     

Effect of microdefects in crystal lattice on certain luminescence relaxation processes in alkali halide phosphor crystals

A further development of ideas on an ionic mechanism for the activation of certain relaxation processes in recombination luminescence of alkali halide phosphor crystals is presented. It is shown that each stage of the thermal discoloration process in the phosphor crystals corresponds to disruption of the F-band in an isolated microsection of the specimen. The effect of the degree of plastic deformation on the relative proportions of F-center concentrations on various microregions of the crystal was investigated. The effect of impurity ions, of their concentration, arid of the crystal plastic deformation on the photothermal stability of the F-centers and on the thermal quenching of luminescence during photoexcitation of activator color centers in NaCl-Ag phosphor crystals was also studied.     

Phase Transitions in Ionic Crystals under Ultrahigh Pressures

Structural phase transitions induced in alkali-halide crystals by an ultrahigh pressure are investigated. The pressure of the B1–B2 transition in an infinite crystal is calculated with the use of ion pair interaction potentials obtained by the self-consistent method in the context of nonuniform electron gas theory. In the approximation of seven coordination spheres, relative changes in volumes of B1 and B2 modifications of a number of alkali-halide crystals are calculated.     

Kinetics of recombination interaction of impurity centers in crystal phosphors

Various nonlinear regimes of stationary emission of crystal phosphors with several types of impurity centers are investigated. In the case of two types of centers (radiative and nonradiative recombination), general equations are derived which connect the emission yield with the volume density of the excitation, given the temperature and the parameters of these centers. The degree of nonlinearity K and the temperature sensitivity R_T are obtained for various ratios of the parameters of the centers and for definite ranges of excitation conditions. The conditions needed to obtain anomalously high values of K and R T are established, and their limiting values upon saturation of the effective nonradiative-recombination centers are estimated. The diverse roles of the quenching processes in practical applications of crystal phosphors is discussed.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva, Vol. 117, pp. 3–54 (1980).     

Photoinduced second harmonic generation studies on Tris(thiourea)copper(I) perchlorate Cu(SC(NH2)2)3(ClO4)

We have established a possibility to obtain the photoinduced optical second harmonic generation in the initially centrosymmetric single crystals of tris(thiourea)copper(I) perchlorate crystallized in the monoclinic system with the space group P2₁/c. The second‐order susceptibility of the investigated crystal was measured by inducing the crystal by external laser field. The results obtained are interesting and discussed in detail. We have established that the optical second harmonic generation achieves its maximum (about 0.84 pm/V) at photoinducing power density about 1430 kW/cm² for the fundamental wavelength 1064 nm. The maximal output second harmonic generation (SHG) was achieved at an angle of about 35° between the pumping and fundamental power beam's polarizations. Such maximum may be a consequence of a competition between the so formed anharmonic photothermal expansions and the thermal destruction that occurred because of the heating of the crystal surfaces. Our independent verifications have shown that the photoinduced surfaces were heated up to 8–12° C. This may cause occurrence of photothermal relaxation processes. The output SHG exists only immediately after the act of photoinduction. The obtained dependences are caused by specific features of the Raman spectra of the investigated crystals. Copyright © 2008 John Wiley & Sons, Ltd.     

Photostimulated luminescence and its capabilities in studies of recombination in phosphors

An analysis is made of the effects of various factors on the temperature dependence of the brightness of photostimulated luminescence. A detailed study is made of the thermal dissociation of hole centers, the thermal ionization of one type of electron center, and repeated localization of electrons. It is shown that the brightness changes observed in photostimulated luminescence as a sample is heated reflect not only the mechanism of photostimulated electron liberation, but also several fundamental processes governing the mechanism for the thermal deexcitation of phosphors.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 2, pp. 50–56, February, 1971.     

Luminescence of AgHal phosphors containing a silver sulfide impurity

In a study of the luminescence spectra of AgHal single crystals containing various levels of a silver sulfide impurity, the mechanism for the luminescence is established. The thermal quenching of the steady-state luminescence and of the light sum of the luminescence of AgHal (Ag₂S) phosphors is due to ionic processes, as in pure silver halides. The activation energy for the thermal quenching of the light sum is found. The ratio of the cross sections for electron recombination with holes localized at the luminescence centers responsible for the various luminescence bands is found in a kinetic study of the decay of the optical emission stimulated in these bands.     

Study on thermal diffusivity of materials by laser photothermal reflection technique

A method of measuring thermal diffusivity of materials at room temperature by photothermal reflection technique is described. An intensity-modulated Ar+ laser beam is used as incident light. The beam is focused to about 1 mm diameter spot and illuminates the sample surface. HgCdTe infrared detector is used to receive photothermal signal. Using this technique, the photothermal signals are experimentally measured as the function of different frequencies. The thermal diffusivities can be obtained by fitting the experimental data. On the other hand, the thermal diffusivities of one-way composite and orthogonal symmetric arranged composites Al2O3/Al are measured in transverse, longitudinal and arbitrary directions. The results show that the diffusivity of one-way material decreases with the increase of the measurement angle; the diffusivity of orthogonally arranged material almost keeps the same when measurement angle changes.     

Effective thermal conductivity of condensed polymeric nanofluids (nanosolids) controlled by diffusion and interfacial scattering

Thermal properties of polymeric nanosolids, obtained by condensing the corresponding nanofluids, are investigated using photothermal techniques. The heat transport properties of two sets of polyvinyl alcohol (PVA) based nanosolids, TiO₂/PVA and Cu/PVA, prepared by condensing the respective nanofluids, which are prepared by dispersing nanoparticles of TiO₂ and metallic copper in liquid PVA, are reported. Two photothermal techniques, the photoacoustic and the photopyroelectric techniques, have been employed for measuring thermal diffusivity, thermal conductivity and specific heat capacity of these nanosolids. The experimental results indicate that thermal conduction in these polymer composites is controlled by heat diffusion through the embedded particles and interfacial scattering at matrix–particle boundaries. These two mechanisms are combined to arrive at an expression for their effective thermal conductivity. Analysis of the results reveals the possibility to tune the thermal conductivity of such nanosolids over a wide range using the right types of nanoparticles and right concentration.     

Amplitude dependence of the internal friction of alkali-halide crystals in an electric field

The influence of an electrical field on the internal friction of an alkali-halide crystal (AHC) was investigated for relative deformation amplitudes corresponding to separation of dislocations from fastening centers, and for higher amplitudes when plastic deformation of the specimen occurs in an ultrasonic wave field. The influence of the electrical field in the deformation domain under consideration reduces to direct action on charged dislocations, re-orientation of dipole fastening centers, and activation of sources in block boundaries.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 5–11, August, 1989.The author is grateful to Dr. N. A. Tyapunina for valuable comments, remarks, and unchanging interest in the research.     

微型光热驱动机构的光热膨胀理论模型与实验研究

提出和发展了一种基于微尺度光热膨胀效应的微型光热驱动机构.建立了膨胀臂的微尺度光热传导与膨胀的物理模型,利用有限元分析和数学物理方法,推导出膨胀臂的一维温度分布表达式及光热膨胀量和偏转量的计算公式.利用准分子激光微加工系统设计制作了一个长度为1200?μm 的微型光热驱动样机,根据激光和样机的相关参数,计算得到光热膨胀量与激光功率之间的理论关系曲线.实验结果表明,微型光热驱动机构的光热偏转量（对应于膨胀量）与照射到膨胀臂上的激光功率近似成线性关系,实验曲线与理论模型及计算结果符合良好.在理论和实验研究基础上,作为光热微驱动的应用实例,设计并加工制作了一个双向微开关型光热驱动机构,并通过实验实现了双向微开关功能. 关键词： 光热膨胀臂 微驱动机构 光热传导模型 激光     

Thermal characterization of dielectric thin films using an improved genetic algorithm

This paper presents theoretical and experimental investigations of the thermal conductivity and the contact resistance of dielectric materials deposited as thin films. Silicon dioxide and poly(p-phenylene) films deposited on alumina substrates are studied in order to determine simultaneously the thermal conductivity of the film and the resistance of the contact between the film and the substrate. Measurements are obtained by using a photothermal technique, and an improved genetic algorithm (GA), especially suitable for thermal characterizations of thin film structures in the sub-micron range. A theoretical study for evaluating the optimal conditions for the photothermal measurements is presented. This is done by studying the sensitivity of the unknown parameters to the thin film thickness and to the properties of the materials. As the photothermal analysis is basically performed in unsteady state conditions, this study highlights the relation between the intrinsic and effective conductivity of the materials.     

Photothermal techniques with a top-hat beam excitation

A flatly topped circular beam (“top-hat beam”) is employed as the excitation beam for photothermal deflection and surface thermal lens techniques. The Fresnel diffraction model is applied to describe the photothermal signals. The theoretical comparison between a Gaussian beam and a top-hat beam excited photothermal signal amplitudes shows that the use of the top-hat beam excitation improves the measurement sensitivity of the photothermal techniques. Experimental results for both photothermal deflection and surface thermal lens are presented. The potential applications of the top-hat beam excited photothermal techniques are highlighted.     

Physics of Solid-State Dielectrics: Superstrong Electric Fields

Electric strengthening with decreasing dielectric thickness allows superstrong electric fields, whose strength exceeds the breakdown one for thick dielectrics, to be created in thin layers of solid-state dielectrics without an electric breakdown. Such fields are called superstrong. In thin dielectric layers of micron thickness, the processes can be investigated which cannot be observed in thicker layers due to the onset of the breakdown. In the present paper, the results of experimental investigations of processes and phenomena taking place in thin monocrystal layers of alkali-halide crystals (AHC) in superstrong electric fields are generalized. Among these processes and phenomena are: electric currents and luminescence (electroluminescence) of AHC layers, impact excitation and electronic ionization of luminescence centers and ions of the host crystal lattice, emission of electrons, accelerated in the layer by the electric field, in vacuum, formation of point and linear defects in AHC under the action of strong and superstrong electric fields, etc. All these phenomena form a new scientific direction – physics of solid-state dielectrics: superstrong electric fields. The results of investigations of superstrong electric fields allow new approaches to the understanding of mechanisms of dielectric breakdown to be realized.