Scattering of visible light by glasses undergoing phase separation and homogenization

A kinematic theory of light scattering and modern concepts of phase-separation theory were used to examine the most typical experimental results obtained from the study of light scattering by glasses. The examination was made by taking into account the positions of figurative points in the immiscibility diagram. It has been shown that the anomalous phenomena (the predominant light scattering into the backward semi-sphere and the spectral dependence of intensity of type λ^?p, where p > 4) arise in four cases: at the stage of decomposition when the figurative point lies in both the binodal and spinodal regions; after the completion of the decomposition stage when interparticle interference is essential; and in the one-phase region when the concentration fluctuations tend to a higher equilibrium level of development. In each case these phenomena are due to the sign-changed behavior of the dielectric constant and to the existence of a range of negative values in the correlation function. The processes of complete or partial homogenization of the glass are characterized by either the gradual disappearance of anomalous phenomena or the appearance of normal scattering due to the increase in space extension of the inhomogeneity regions at their dissolution.     

X-ray scattering from an irregular surface: Two-scale model

X-ray scattering from an irregular surface, which is described within a two-scale model, has been considered. A method for calculating the differential scattering cross section based on the quasi-classical Kirchhoff approximation is proposed. Coherent scattering from a lateral surface with a periodic motif is analyzed, and the reduction factor is calculated; the latter differs from the well-known expressions of the perturbation theory (first Born approximation, distorted-wave Born approximation). It is shown that the surface profile deviations from a periodic law, edge effects, and the interference of waves reflected from surface areas with different inclination angles significantly affect scattering from lateral structures.     

Enhancement of Raman scattering in Subwave plasmonic nanostructures formed by ion-beam lithography

Subwave gratings with optimized architecture corresponding to the spectral position of resonances at desired wavelengths have been fabricated by ion-beam lithography. The structural and optical properties of the nanostructures have been studied. It is shown that experimental transmission (reflection) spectra are in good agreement with the theoretical spectra calculated within the grating-eigenmode model. An analysis of the Raman scattering of molecules adsorbed on the surface of gratings with optimized architecture demonstrates an increase in the Raman signal intensity from these molecules by four orders of magnitude. Highly sensitive sensors and various elements of optoelectronics can be developed based on these nanostructures.     

Numerical simulation of the recording of diffraction lines with the Bragg-Brentano focusing

The role played by different factors in the broadening of X-ray diffraction lines is evaluated using numerical methods. It is demonstrated that, within the kinematical theory of X-ray scattering, the main factors affecting the linewidth are as follows: the angle of reflection, the number of reflection planes, the focus width, the material of the anode of the X-ray tube, and the angle of misorientation of coherent scattering regions. It is established that, in the case where the number of reflection planes is less than 200, the instrumental component of the linewidth does not exceed 5% of the total linewidth.     

X-ray diffraction analysis of multilayer porous InP(001) structure

Multilayer structures composed of four porous bilayers have been studied by high-resolution X-ray diffraction using synchrotron radiation, and the photoluminescence of these structures has been investigated at 4 K. The porous structures were formed by anodic oxidation of InP(001) substrates in aqueous HCl solution. The structural parameters of the sublayers were varied by changing the electrochemical etching mode (potentiostatic/galvanostatic). The X-ray scattering intensity maps near the InP 004 reflection are obtained. A model for scattering from such systems is proposed based on the statistical dynamical diffraction theory. Theoretical scattering maps have been fitted to the experimental ones. It is shown that a mathematical analysis of the scattering intensity maps makes it possible to determine the structural parameters of sublayers. The reconstructed parameters (thickness, strain, and porosity of sublayers and the shape and arrangement of pores) are in satisfactory agreement with the scanning electron microscopy data.     

Nontrivial role of the resolution function in the formation of double-crystal X-ray rocking curves

A theoretical analysis of the shape of the resolution function of an X-ray diffractometer in the double-crystal nondispersive scheme for measuring rocking curves has been performed. It is shown that this function, along with the strong narrow central line (corresponding to the characteristic line to which the collimator crystal and slits in the experimental scheme are tuned) always contains an additional weak satellite, corresponding to the neighboring characteristic line. This feature manifests itself in experimental curves as an additional pseudopeak, whose intensity is comparable with the corresponding diffraction scattering intensity from a sample in the fundamental characteristic line, and whose shape and exact position are set by the resolution function in a specific experimental scheme. It is shown that the exact allowance for the angular and spectral distribution of incident radiation in the double-crystal scheme makes it possible to precisely determine the parameters of the structures studied.     

Effect of the ultrasonic frequency on the multiple X-ray scattering from a LiNbO<Subscript>3</Subscript> crystal modulated by a surface acoustic wave

The effect of multiple scattering on the formation of the θ-2θ scan curves for a crystal modulated by a surface acoustic wave (SAW), depending on the ultrasonic frequency, has been investigated in the frame-work of the dynamical theory of X-ray diffraction. A model of a Rayleigh surface wave has been analyzed as applied to X-ray diffraction with allowance for the transverse and longitudinal elastic lattice strains. Using the example of the 127° Y′ cut of the LiNbO₃ crystal, it is established that the effects of multiple scattering can be neglected for ultrasonic frequencies above 650 MHz; this finding significantly simplifies the numerical calculations of X-ray diffraction from a crystal modulated by a short-wavelength SAW. A comparative quantitative analysis of the experimental data on synchrotron scattering from the 127° Y′ cut of a LiNbO₃ crystal modulated by a 952-MHz SAW have been performed, both taking into account and neglecting the effects of multiple scattering. It is shown that the computation time can be reduced by 2 to 3 orders of magnitude.     

Studies on interface roughness scattering effects in molecular beam epitaxy grown resonant tunneling structures

Interface roughness (IR) scattering effects in the resonant tunneling (RT) process through molecular beam epitaxy (MBE) grown double barrier (DB) structures have been investigated both theoretically and experimentally. The time dependent perturbation theory was applied to an elastic scattering event which is caused by the IR potential within the dwelling time period of an incident electron inside a DB. The calculated result indicates that the peaked structure of the tunneling transmission coefficient can be degraded drastically when the IR has lateral Fourier components in a specific spatial frequency range, and that the upper and lower limits of the range are related to such structural parameters as the well width and the cavity finesse of the DB. It has been found that the above prediction is possibly supported by the experimental results obtained in tunneling spectroscopy measurements performed on AlAs-GaAs DB structures; measured peak widths of tunneling spectra in samples grown with growth interruption periods are broadened by a factor of more than 2.     

The relationship between the second and fourth rank order parameters for liquid crystals composed of non-rigid molecules

The values of the fourth rank order parameter P₄ determined by Raman scattering experiments are invariably lower than those predicted by the Maier-Saupe theory of nematic liquid crystals. However this theory is only applicable to rigid particles whereas the nematogenic molecules studies contain flexible alkyl chains. We have estimated the influence of the alkyl chain on P₄ with the aid of a model which account successfully for the variation of the second rank order parameter P₂ along the chain. It is found that the values of P₂ and P₄ calculated for the CN direction in the 4-n-alkyl-4′-cyanobiphenyls are indistinguishable from those predicted by the Maier-Saupe theory. It would appear therefore that the low values observed for P₄ cannot be attributed to the non-rigidity of the alkyl chain.     

Study of the dynamic structure factor of strong glasses

The dynamic structure factors (DSFs) of several strong glasses (SF6, SF10, BK7, SUPRASIL) measured by Brillouin light scattering spectroscopy are reported. Spectra have been collected, at and above room temperature, at two scattering angles, θ=90° and θ=180° corresponding to exchanged wavevector q values ranging from 0.0256 to 0.0448 nm⁻¹. In particular we find that the isotropic spectral lineshapes are in all cases well described by the simple hydrodynamic theory of an amorphous solid. The width of the Brillouin peaks are found to be consistent with the predicted q² dependence at both investigated temperatures. This damping is however found to account only partially for the strong asymmetry of the Brillouin line clearly visible on a logarithmic intensity scale. As a matter of fact there is an excess intensity in the very low frequency plateau underlying the central component. The height of this plateau and hence the entire lineshape is well reproduced if a relaxation process is taken into account in the hydrodynamic equations. Owing to the intense elastic scattering we are able to determine unambiguously only the ratio between amplitude and characteristic time of this process which quantifies the sound dispersion to be of the order of a few percent in all samples. The temperature dependence of the parameters indicates that this relaxation cannot be attributed to thermally activated relaxation phenomena. These general findings favorably compare with molecular dynamics simulation results on similar systems.     

Infrared light scattering and absorption microscopy investigations at dislocations in GaAs crystals

In recent years a number of experiments have been published dealing with the light scattering at small particles. All of these experiments have been made in the visible spectral range. In this paper we report an experimental arrangement for infrared light scattering by dark-field illumination. Preliminary results of the observations of dislocations in GaAs crystals are given. The scattering images are correlated with results obtained by means of absorption microscopy with infrared light. Helix-like fine structures of dislocations are found mainly arranged in the 〈110〉 directions.     

X-ray scattering by porous silicon modulated structures

A multilayered porous structure formed as a result of the anodization of a Si(111)(Sb) substrate in an HF:C₂H₅OH (1: 2) solution with a periodically alternating current has been investigated by high-resolution X-ray diffraction. It is established that, despite 50% porosity, a thickness of 30 μm, and significant strain (4 × 10⁻³⁾, the porous silicon structure consists mainly of coherent crystallites. A model of coherent scattering from a multilayered periodic porous structure is proposed within the dynamic theory of diffraction. It is shown that the presence of gradient strains of 5 × 10⁻⁴ or higher leads to phase loss upon scattering from porous superlattices and the suppression of characteristic satellites in rocking curves.     

Overtone Raman Scattering in Lithium Niobate Single Crystals Doped with Terbium

Crystallography Reports - Overtone bands in the spectral range of 1300?1950 cm–1 have been found in Raman scattering spectra of lithium niobate single crystals doped with terbium. The...     

Modification of the crystal structure of gadolinium gallium garnet by helium ion irradiation

The structure of gadolinium gallium garnet (GGG) single crystals before and after implantation by He⁺ ions has been investigated using high-resolution X-ray diffraction methods and the generalized dynamic theory of X-ray scattering. The main types of growth defects in GGG single crystals and radiation-induced defects in the ion-implanted layer have been determined. It is established that the concentration of dislocation loops in the GGG surface layer modified by ion implantation increases and their radius decreases with an increase in the implantation dose.     

Generalization of the nonstandard approach in the dynamic theory of diffraction for deformed crystals

The nonstandard theory of X-ray scattering in a deformed crystal has been generalized. The vector of atomic-plane displacement is introduced into the crystal polarizability model like in the generalized Takagi dynamic theory. The solution to the wave equation is sought for using the procedure of expanding the field amplitude and vector operators in the Fourier components of polarizability χ _H in a series according to the multiscale method. It is shown that considering lattice strain generally calls for introducing various characteristic spatial regions for the diffraction equation, which is in complete agreement with the main concept of the multiscale method. A particular case of a strain field depending on one scale is considered. If a relative change in strain occurs at a length on the order of the extinction length, one can obtain equations generalizing the Takagi equations to the case of arbitrary diffraction geometries.     

New method for calculating the intensities of X rays and thermal neutrons diffracted in single crystals with defects

A new method is proposed to calculate the intensities of X rays and thermal neutrons diffracted in single crystals with defects. This method is based on comparing the dynamic theory of the Bragg diffraction of X rays and thermal neutrons in perfect single crystals with the small-angle scattering theory.     

X-ray scattering from a material under an external load

The differential X-ray scattering cross section of a polycrystalline sample placed in a high-pressure chamber under an arbitrary load has been calculated. It is shown that the broadening and displacement of most diffraction lines under shear load are, generally speaking, of the same order of magnitude, except for the reflections on the vectors parallel to threefold and higher order axes (these reflections do not undergo broad-ening). The scattering line shape contains information about the components of the compressibility tensor; to derive it, one needs a diffractometer resolution satisfying the condition δθ/θ < τ/G (τ is the shear stress and G is the typical shear modulus of the material studied). An experiment under hydrostatic load is analyzed, and it is shown that the differential bulk compressibility modulus of a polycrystal under pressure, which are derived from X-ray data, coincides with the value given by the natural strain theory.     

Porosity and structural parameters of Karelian shungites according to the data of small-angle synchrotron radiation scattering and microscopy

The nanoporosity and structure of natural carbons has been investigated on the example of Karelian carbon-rich shungites by comparing the data of small-angle synchrotron radiation scattering and highresolution microscopy. The analysis of small-angle scattering data is based on the model of scattering spheres with lognormal size distribution. It is found that the structure of samples from the Maksovo and Zazhogino deposits subjected to high temperatures in the geological medium and (also to a lesser extent) a sample from the Shunga deposit can be described as an aggregation of polydisperse scattering spheres with lognormal size distribution; the characteristic scatterer size is determined for them. A comparison with microscopy data shows that these scatterers are mainly associated with pores, and the character of their size distribution is similar to that previously established for nanoglobules in schungites.