The Bragg Diffraction of Light at Acoustophotorefractive Holographic Gratings in Gyrotropic Cubic Crystals

We investigate diffraction of light at phase holographic-type gratings recorded by a piezoactive ultrasonic wave in gyrotropic cubic photorefractive crystals. We show that the efficiency of diffraction at a holographic grating can be increased substantially by applying a d.c. field to a crystal under the conditions of a longitudinal electrooptical effect; elliptical polarization of diffracted light is determined by electrically induced anisotropy of a crystal in the piezofield of a photorefractive grating, by the external electric field, detuning of phase synchronism, gyrotropy of the crystal, ultrasound frequency, and the time of recombination of charge carriers.     

Effect of dispersion and damping of thermal phonon states on the longitudinal ultrasonic absorption in germanium crystals

A method has been proposed for approximating a phonon spectrum of cubic crystals, which has been obtained from data on inelastic neutron scattering for symmetric directions, over the entire Brillouin zone in the form appropriate for studying relaxation characteristics of phonon systems. The effect of dispersion and damping of thermal phonon states on the longitudinal ultrasonic absorption in anharmonic processes of scattering with the participation of three longitudinal phonons has been investigated for germanium crystals. It has been shown that the inclusion of the dispersion leads to a decrease in the anisotropy of ultrasonic absorption in the LLL relaxation mechanism and makes it possible to fit the results obtained from calculations of the ultrasonic absorption coefficients to the experimental data in the low-temperature range. The temperature dependence and anisotropy of the relaxation rate of longitudinal thermal phonons in germanium crystals have been determined from experimental data on ultrasonic absorption. The performed analysis has refined values of the relaxation parameters obtained from the interpretation of the data on thermal conductivity of germanium crystals with different isotopic compositions in the isotropic-medium model.     

Polarization-independent modulation of light in gyrotropic cubic crystals

Polarization-independent modulation of light in gyrotropic cubic crystals is investigated in the Bragg and intermediate diffraction regimes. It is found that, for crystals with considerable specific rotation, the photoelastic anisotropy is suppressed by the circular anisotropy and the observed diffraction efficiency depends weakly on the polarization azimuth of the incident light. Good agreement is obtained between modulation parameters calculated theoretically on the basis of the coupled wave theory and experimental results for a bismuth silicate crystal in the intermediate diffraction regime.     

Intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals

An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic and cubic crystals in an external electric field is considered. A system of equations of the coupled waves, which describes acoustooptic diffraction in gyrotropic anisotropic crystals with consideration of the electrically induced optical anisotropy for a strong interaction between light and ultrasound, is presented. An intermediate regime for the diffraction of light on ultrasound in gyrotropic anisotropic crystals which is close to the Bragg regime for a weak acoustooptic interaction is studied. It is shown that the diffracted light is elliptically polarized and that the ellipticity and polarization azimuth of the diffracted wave depend on the anisotropy of the photoelasticity, the electrically induced anisotropy of the crystal in the external electric field, the gyrotropy, and the asymmetry of the diffraction structure. Zh. Tekh. Fiz. 67, 80–82 (September 1997)     

Bragg diffraction of light by a hypersound in gyrotropic cubic crystals

Noncollinear acousto-optical interaction of light beams in acoustically gyrotropic cubic crystals is considered. The dependences of the diffraction efficiency of the parameters of the optical and acoustical gyrotropy have been established. The attainability of the optimal efficiency of diffraction by shear ultrasonic waves due to the anisotropy electrically induced by the external field under conditions of longitudinal electro-optical effect is shown.__________Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 1, pp. 80–84, January–February, 2005.     

Specific features of polarization anisotropy in optical reflection and transmission of colloidal photonic crystals

This paper reports on the measurements (in linearly polarized light) of the transmission and reflection spectra of colloidal photonic crystals with three-dimensional and one-two-dimensional photonic band structure, i.e., opal films and Langmuir-Blodgett crystals with a refractive index contrast of ∼1.5: 1.0. It has been shown that the polarization anisotropy is enhanced considerably in the diffraction resonance range both in transmitted and reflected light, and that the anisotropy in the resonance range can be as high as 99%. The interaction of photonic crystal eigenmodes has been found to affect the polarization anisotropy. The assumption has been made that the coincidence of the maxima in polarization anisotropy of the resonant and nonresonant light reflection in colloidal crystals originates from the disorder in their lattices. The generality of the results obtained is confirmed by the fact that the polarization anisotropy manifests itself in the same way in colloidal crystals with different lattice symmetries.     

Modification of the parabolic approximation in the theory of ultrasonic-beam diffraction in a strongly anisotropic crystal

A modification of the parabolic approximation of the diffraction theory is based on the approximation of the wave surface with allowance for the shape of the angular spectrum of the ultrasonic beam. It is demonstrated that such an approach is more accurate in the analysis of the acoustic field at a relatively high diffraction divergence and/or high acoustic anisotropy. An acoustic field is simulated in paratellurite, which is a leading acoustic material with anomalously high acoustic anisotropy.     

The intermediate regime of light diffraction by ultrasound in planar gyrotropic optical waveguides in an external electric field

A theoretical investigation is carried out for an intermediate regime of optoacoustic interaction, close to the Bragg one, by ultasonic Rayleigh surface waves in gyrotropic planar waveguides in an external electric field. A system of equations of associated waves is presented which describes the planar optoacoustic interaction in intermediate, Raman—Nath, and Bragg regimes of diffraction by ultrasonic surface waves in gyrotropic crystals with electrically induced anisotropy. It is shown that the intermediate (transition) regime of optoacoustic interaction, which is characterized by an angular selectivity and by the presence of several diffraction maxima, is the basic regime of diffraction. In this case diffracted light is generally elliptically polarized with an ellipticity and polarization azimuth that depend on the external electric field intensity. Translated from Zhurrnal Prikladnoi Spektroskopii, Vol. 66, No. 1, pp. 30–35, January–February, 1999.     

Polarization anisotropy of optical transmission in opals and Langmuir-Blodgett crystals

The transmission spectra of thin-film colloidal photonic crystals with three-dimensional and one-dimensional-two-dimensional photonic energy band structures, i.e., opals and Langmuir-Blodgett crystals with a refractive index contrast of ∼1.5: 1.0, have been measured in linearly polarized light. It has been demonstrated that the polarization anisotropy in the light transmitted through the crystal is uniquely related to the diffraction resonance and that the degree of polarization can exceed 90%. A higher degree of polarization is provided by lattices that are characterized by a smaller attenuation of light polarized in the plane of incidence. It has been revealed that the diffraction resonances from the crystal planes for which the dispersions are in anticrossing with the dispersion of the growth planes acquire the same anisotropy. The general character of the results obtained has been confirmed by the fact that the polarization anisotropy identically manifests itself in colloidal crystals that have different symmetries and lattice orderings.     

Optical Anisotropy of Photonic Crystals of Cubic Symmetry Induced by Multiple Diffraction of Light

The optical spectra of Bragg reflection from opal-like photonic crystals under conditions of the resonant enhancement of the multiple diffraction of light have been studied experimentally and theoretically using the photonic crystal structures prepared of monodisperse polystyrene globules. It is shown that the reflection signal registered in mutually orthogonal configurations of the polarizer and analyzer is related to the intrinsic optical anisotropy of the crystals and is a specific manifestation of the multiple Bragg diffraction in three-dimensional photonic crystals.     

Intermediate and Bragg acousto-optic interaction in elastically anisotropic medium

The Raman-Nath, intermediate and Bragg regimes of acousto-optic interaction in an acoustically anisotropic medium are theoretically examined in the paper. The general model of the interaction is extended over the case of an elastically anisotropic medium. Basic results of computer modeling of diffraction processes taking place in crystals are presented in the paper. The interaction of light and ultrasound is investigated in media with arbitrary elastic anisotropy in a wide range of the Klein-Cook and Raman-Nath parameters. The influence of the acoustic anisotropy on the process of light diffraction, especially on the light intensity in diffraction maxima, is also discussed.     

Rapid growth of magnetite nanoplates by ultrasonic irradiation at low temperature

Two-dimensional plate-like Fe(3)O(4) nanocrystals were synthesized by a facile method using ultrasonic irradiation in aqueous solution at low temperature without protection from oxygen. The crystals were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier Transform infrared spectroscopy. The products subjected to ultrasound showed a two-dimensional morphology. The results obtained indicate that the morphologies of the magnetite crystals depend more on the ultrasonic irradiation than on the growth temperature. The thickness and width of the crystals increased with increasing temperature of the reaction medium. In addition, the magnetic hysteresis loop of the magnetite nanoplates was obtained at room temperature.     

Wide-aperture acousto-optic interaction in birefringent crystals

Regularities of the wide-aperture mode of light diffraction by acoustic waves in crystals with different values of birefringence are considered. The diffraction has been investigated theoretically and experimentally with the aim of applying it in acousto-optic filters used in processing uncollimated optical beams and images. It is proved that the main filtration parameters depend on the birefringence of crystals used in processing luminous fluxes. By the example of wide-aperture filters based on paratellurite, magnesium fluoride, and potassium dihydrogen phosphate crystals, it is shown that the choice of crystals with high birefringence may increase the maximum attainable angular aperture and throughput of devices. It is also proved that application of crystalline compounds of mercury and tellurium (characterized by high optical anisotropy) in filters increases spectral resolution and improves the quality of filtered images.     

A new concept for the origin of the Schaefer-Bergmann diffraction patterns

The origin of the Schaefer-Bergmann diffraction patterns is interpreted by the concept of a diffuse acoustic field occuring in high Q-factor crystals like the sound field in large low-loss auditoriums. It is shown that the inhomogeneous illumination of the patterns obtained by crystals with high symmetry or with weak anisotropy is due to interference phenomena in the diffuse field.     

Ultrasonic polar scans: numerical simulation on generally anisotropic media

Ultrasonic polar scans are based on the recording of the reflected or transmitted amplitude of sound, impinging a fiber reinforced composite from every possible angle of incidence. The mechanical anisotropy of such materials makes the reflection coefficient direction dependent, whence an ultrasonic polar scan forms a fingerprint of the investigated material. Such scans have already proved to be very valuable in the characterization of composites. Simulations have been performed for single layered and multi-layered systems, for pulsed and harmonic waves. Fiber reinforced composites are mostly orthotropic. The current report presents simulations not only on orthotropic materials but on materials of any kind of anisotropy. These extended numerical simulations are not only valuable in the characterization of highly sophisticated composites, but may also be used to characterize thin slices of crystals and even layered crystals.     

Features of acousto-optic interaction in uniaxial gyrotropic crystals

The features of noncollinear acousto-optic interaction in gyrotropic crystals in the intermediate diffraction regime and the regime of Bragg diffraction were investigated. The dependence of the efficiency of diffraction in gyrotropic paratellurite, tellurium, and quartz crystals on the intensity of the ultrasonic wave, acousto-optic interaction length, and incident-light polarization was investigated. It is shown that the gyrotropy of the crystal is responsible for the appearance of a multiple-peak structure of the Bragg maximum. It has been established that in the case of propagation of incident and diffracted waves in the vicinity of the optical axis of the crystal, the diffraction efficiency is independent of the polarization state of the incident light. The results of theoretical calculations are in agreement with the experimental results obtained for uniaxial crystals. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 67, No. 1, pp. 105–110, January–February, 2000.     

Diffraction properties of four-petal Gaussian beams in uniaxially anisotropic crystal

Propagation properties of polarized four-petal Gaussian beams along the optical axis of uniaxially anisotropic crystals were investigated. Based on the paraxially vectorial theory of beam propagation, analytic expressions of the diffraction light field were obtained. The effects of the anisotropy on the polarization properties of the diffracted four-petal Gaussian beams have also been explained by numerical method. The results elucidate that the linear polarization state and the symmetry of the incident beams cannot be kept during propagation in anisotropic crystals.     

Propagation and polarization properties of hollow Gaussian beams in uniaxial crystals

Based on the paraxial vectorial theory of beams propagating in uniaxially anisotropic media, we have derived the analytical propagation equations of hollow Gaussian beams (HGBs) in uniaxial crystals, and given the typical numerical example to illustrate our analytical results. Due to the anisotropy crystals, the ordinary and extraordinary beams originated by incident HGBs propagate with different diffraction lengths, thus the linear polarization state and axial symmetry of incident HGBs do not remain during propagating in crystals.     

Ultrasound influence upon calcium carbonate precipitation on bacterial cellulose membranes

The effect of ultrasonic irradiation (40 kHz) on the calcium carbonate deposition on bacterial cellulose membranes was investigated using calcium chloride (CaCl₂) and sodium carbonate (Na₂CO₃) as starting reactants. The composite materials containing bacterial cellulose-calcium carbonate were characterized by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and color measurements. The polymorphs of calcium carbonate that were deposited on bacterial cellulose membranes in the presence or in the absence of ultrasonic irradiation were calcite and vaterite. The morphology of the obtained crystals was influenced by the concentration of starting solutions and by the presence of ultrasonic irradiation. In the presence of ultrasonic irradiation the obtained crystals were bigger and in a larger variety of shapes than in the absence of ultrasounds: from cubes of calcite to spherical and flower-like vaterite particles. Bacterial cellulose could be a good matrix for obtaining different types of calcium carbonate crystals.