Analysis of the photoelastic effect in ionic crystals

An analysis of the photoelastic effect in ionic crystals has been presented within the framework of Clausius-Mossotti theory of the dielectric constant. The values of the strain derivative of the electronic dielectric constant have been calculated in alkali halides and MgO crystals by taking into account the variation of electronic polarizabilities with compressive stress. The results obtained are found closer to the experimental values. The photoelastic behaviour of MgO crystal is predicted to be of opposite nature to that of alkali halides, in conformity with the experimental observations.     

Photoelasticity in polycrystalline aggregates

A theory for the photoelastic behaviour of transparent polycrystalline aggregates consisting of randomly oriented anisotropic crystallites has been developed. Such an aggregate is isotropic but it becomes birefringent under the influence of a uniaxial load. The photoelastic constants of the aggregate are given by the components of the spatial average of the photoelastic tensor of the single crystal, and are worked out by assuming either the strain to be continuous (Voigt approximation) or the stress to be continuous (Reuss approximation). The components of the average photoelastic tensor are very different for these two limits. The elastic and the photoelastic constants of alkali halide aggregates have been evaluated for both the stress continuity and the strain continuity conditions. The maximum variation of the elastic constants in going from the Voigt to the Reuss condition is 50 per cent while the photoelastic birefringence can vary by as much as 300 per cent in alkali halides. In the case of KI and rubidium halides even the sign of the photoelastic birefringence is different for the two limits.     

Photoelastic constants in silver and thallium halides

In the present communication the expressions for the photoelastic constants of the solids crystallizing in cesium chloride structure have been derived on the basis of lattice theory. These expressions have been utilized to predict the magnitudes of photoelastic constants in thallium halides. On the basis of these constants the validity of the Clausius-Mossotti model of electronic polarization in silver and thallium halides has been discussed.     

Calculation of the components of the photoelastic effect in alkali-halide crystals

Calculations are made to determine the contribution of Lorentz-field and Coulomb-field anisotropy in alkali-halide crystals to the photoelastic coefficients p₁₁–p₁₂ and p₄₄ as well as to the piezopolarizability constants K, K, K_x, K_z, and K₀. These calculations are based on Mueller's theory of the photoelastic effect, with data on iron polarizability and photoelastic coefficients. The applicability of Mueller' s theory to the complete series of alkali-halide crystals is analyzed. Criteria are established for the ionicity of bonds in these crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 28–34, December, 1975.     

Effect of three-body interactions on electronic polarizabilities and photoelastic behaviour of partially covalent crystals

Summary We have investigated the effect of three-body interactions on the photoelastic behaviour and the electronic polarization of silver, thallium and copper halides, which are partially covalent in character and possess three different crystall structures. The cation, anion and molecular polarizabilities calculated in their crystalline state seem to be reliable as they follow a systematic trend. Our calculated values of the strain derivatives of the electronic dielectric constants are much closer to their experimental data than those obtained by earlier workers. To speed up publication, the authors of this paper have agreed to not rceive the proofs for correction.     

Analysis of the pressure derivatives of the shear modulii in ionic solids

Suitably modified theory based on Lundqvist potential of ionic cohesion, to explain the pressure derivatives of the solids, has been applied to evaluate the pressure derivatives of second order elastic (SOE) constants and the values of third order elastic (TOE) constants of ionic solids. The calculated values of the pressure derivatives of SOE constants are in good agreement with the experimental values.     

Enhancement of photoelastic constant of optical thin film waveguide assisted by surface acoustic wave

In the present research, we proposed a method of controlling the photoelastic constant using surface acoustic waves, which had not previously been reported, and carried out experimental studies thereof. A Bragg diffraction was carried out to determine the photoelastic constants of Ta2O5. As a result, it is confirmed that the photoelastic constant of a Ta2O5 thin film undergoing a sputtering process, during which surface acoustic waves were excited on the substrate, was about 2.19-2.27 times larger than those of thin films on which surface acoustic waves were not excited.     

Two-fold interferometric measurements of piezo-optic constants: application to β-BaB2O4 crystals

We present the interferometric technique which allows to measure piezooptical and photoelastic characteristics of crystal materials of any symmetry. The offered two-fold interferometric method enables to determine all independent non-zero piezooptic and photoelastic constants by measuring pressure induced changes of optical path. As advantage to known acoustooptical techniques this method allows to measure both the absolute magnitude and sign of photoelastic constants. In general case the determination of 36 components of piezooptic tensor needs to carry out 57 measurements on 16 samples. The corresponding relationships are derived. As an example we apply here the interferometric technique to measure the piezooptic and photoelastic constants in trigonal β-BaB₂O₄ crystals.     

Evaluation of the photoelastic constants of ionic crystals

A theoretical model, for evaluating the photoelastic constants of ionic crystals, has been developed by considering the short range repulsive interactions between nearest and next nearest neighbours, the van der Waals potentials and three body interactions as well as taking into account the electronic polarizabilities of cations and anions. The model has been applied to calculate the photoelastic constants of RbC1, RbBr and RbI crystals. The results obtained in the present study are in good agreement with experimental data.     

Elastic and photoelastic constants of NaCl,KBr and LiF by Brillouin scattering

Elastic and photoelastic constants of NaCl, KBr and LiF were determined from Brillouin frequency shifts and intensities. Reported differences between the hypersonic and ultrasonic elastic constants for KBr and LiF were not confirmed. For each material the constants were in good agreement with the ultrasonic values. Scattering from longitudinal, transverse and mixed phonon modes was clearly observed for each material.     

Theory of the photoelastic effect in ZnO,ZnS and CdS crystals

It has been a general opinion that the Clausius-Mossotti dielectric theory is not capable of explaining the observed photoelastic behaviour of partially ionic crystals.However in the present communication we show that by making account of the variation of polarizabilities under compressive stress within the framework of the Clausius-Mossotti theory it is possible to obtain a reasonable agreement with experimental data on the photoelastic behaviour of ZnO, ZnS and CdS crystals.     

Third-order elastic constants and pressure derivatives of the second-order elastic constants of β-tin

The second- and third-order elastic constants and pressure derivatives of second-order elastic constants of tetragonal β-tin have been obtained using the deformation theory. The strain energy density derived using the deformation theory is compared with the strain dependent lattice energy obtained from the elastic continuum model approximation to get the expressions for the second- and third-order elastic constants. Higher order elastic constants are a measure of the anharmonicity of a crystal lattice. The 12 non-vanishing third-order elastic constants and the six pressure derivatives of the second-order elastic constants in tetragonal β-tin are obtained in the present work and are compared with the available experimental values. The second-order elastic constant C₃₃ obtained in the present study is in reasonable agreement with the experimental values. The third-order elastic constants are generally one order of magnitude greater than the second-order elastic constants as expected of a crystalline solid. The third-order elastic constant C₃₃₃ is higher in magnitude than all other values. This shows a greater anharmonicity of β-tin along the c-axis direction of the crystal.     

Effective dielectric and photoelastic tensor components in thin layered superlattices

The effective photoelastic constants of a superlattice composed of thin alternating layers of orthorhombic symmetry are calculated as functions of dielectric, elastic and photoelastic constants of the constituents. In a preliminary step to this calculation, we also obtain the effective dielectric tensor for any symmetry of the layers. It is shown that appropriate combinations of these effective constants are arithmetic averages of the corresponding quantities in the constituents, each layer having a weight equal to its relative thickness.     

Dielectric behaviour and optic mode Gruneisen parameters of the halides of silver,caesium and thallium

The optic mode Gruneisen parameters in silver, caesium and thallium halides are calculated using the Born model for interionic forces and the Szigeti theory of dielectric constants. The strain derivatives of the electronic and static dielectric constants are also evaluated and compared with experimental data. The strain derivative of static dielectric constant reveals the inadequacy of the Born model for the crystals under study. Possible modifications have been suggested to improve the situation. The theoretical values of the optic mode Gruneisen parameters closely agree with recent experimental data. An appropriate process has been adopted to evaluate the average values for the Gruneisen parameter.     

The optical properties and electronic structure of liquid mercury and mercury-indium alloys

Recent experimental values for the optical constants of liquid mercury are shown to differ appreciably from free electron values. An apparent violation of the sum rule by the experimental results can be explained by considering the influence of the core electrons. The optical constants of liquid mercury-indium alloys (4.9–33.4 at. % In) show some agreement with free electron theory with a suitable choice of relaxation time. The significance of these results for Mott's (1966) theory of the electronic structure of liquid mercury is discussed.     

残余应力对光弹实验中超声图像的干扰分析

动态光弹成像技术是观测固体内部超声应力场的重要手段,然而样品在制作过程中会产生残余应力,给观测带来一定干扰,特别是缺陷附近的应力集中效应,使得缺陷散射声场的研究更为困难。本文利用线性应力理论分析了超声应力与残余应力的相互关系,并推导出该叠加应力场在光弹系统中的光强表达式,通过实验验证,证明了该理论的可行性。本文结果可为应力集中区域的散射声场分析提供借鉴。     

Theory of strain derivative of electronic dielectric constant of ionic crysts

A theoretical method for evaluating the strain derivative of the electronic dielectric constant of ionic crystals has been developed. The analysis presented here is based on the shell model and takes account of the exchange charge polarizations. Values of strain derivative of the electronic constant dielectric calculated for 6 alkali halides and MgO show a remarkable agreement with experimental data on photoelastic constants.     

Acoustooptical and elastic properties of laminated KY(MoO4)2 crystals

Data on acoustic (absorption and velocity of sound), optical (refractive index and optical absorption coefficient), and photoelastic (coefficients of acoustooptical quality and photoelastic constants) properties of KY(MoO₄)₂ crystals are obtained. It is shown that, not only does the anisotropy of binding forces lead to a significant anisotropy of acoustic and photoelastic properties, but it also determines anomalously high elastic nonlinearity in the direction of the Y-axis perpendicular to cleavage planes.     

Laser picosecond acoustics in a two-layer structure with oblique probe light incidence

A theory for the analysis of experiments involving laser picosecond acoustics with obliquely incident probe light in a two-layer structure is outlined. The reflectance and phase changes of the reflected light are calculated with a theory that takes into account the effects of multiple optical reflections. The sample consists of a single partially transparent layer on a substrate, both with arbitrary optical constants. We discuss the conditions in which one may discriminate between components of the optical modulation of a probe beam arising from the photoelastic effect and from the displacement of the sample interfaces induced by the acoustic strain.     

Influence of the core‐hole effect on optical properties of magnesium oxide (MgO) near the Mg L‐edge region

The influence of the core‐hole effect on optical properties of magnesium oxide (MgO) is established through experimental determination of optical constants and first‐principles density functional theory studies. Optical constants (δ and β) of MgO thin film are measured in the spectral region 40–300 eV using reflectance spectroscopy techniques at the Indus‐1 synchrotron radiation source. The obtained optical constants show strong core exciton features near the Mg L‐edge region, causing significant mismatch with Henke's tabulated values. On comparing the experimentally obtained optical constants with Henke's tabulated values, an edge shift of ～3.0 eV is also observed. Distinct evidence of effects of core exciton on optical constants (δ and β) in the near Mg L‐edge absorption spectra are confirmed through first‐principles simulations.