Triple Bragg diffraction in paratellurite crystal

Triple Bragg diffraction in a paratellurite crystal has been considered for the case when the plane of diffraction is oblique to the optical axis of the crystal. It has been shown that effective photoelastic constants for isotropic and anisotropic diffraction depend on the inclination of the plane of diffraction insignificantly. Triple Bragg diffraction of 0.63-μm coherent radiation in paratellurite at a 47.3-MHz slow acoustic wave has been experimentally demonstrated. For an optical power of 0.69 W delivered to a piezoconverter, the relative intensities of diffraction orders equal ~0.4, 0.4, 0.1, and 0.1, respectively.     

Collinear diffraction of a divergent light beam by ultrasound in a paratellurite crystal

The collinear acousto-optical interaction of a divergent light beam with ultrasound along the approximate [110] direction in a TeO₂ paratellurite crystal is investigated theoretically and experimentally. The collinear diffraction is studied at an ultrasonic frequency f ≈ 149 MHz under exposure of the crystal to an uncollimated laser light beam at a wavelength λ = 633 nm and at an angle of divergence as large as 4°. It is shown that the collinear diffraction along the direction forbidden for acousto-optical interactions of plane waves occurs only under conditions where the light beam is uncollimated and the diffraction efficiency increases with an increase in the divergence of the light beam. It is proved that the attenuation of an acoustic wave brings about a decrease in the diffraction efficiency and an increase in the transmission bandwidth of the device used. A model of the collinear acousto-optical filter based on a paratellurite crystal with an interaction length l = 2.7 cm is analyzed. The collinear acousto-optical filter is characterized by a high resolving power (～3000), a high diffraction efficiency (I₁/I₀ ≈ 0.8), and a large angular aperture (Δ? ≈ 4°). This makes collinear diffraction promising for use in acousto-optical filters based on paratellurite crystals.     

Acousto-optic diffraction with allowance for the influence of the acousto-optic cell boundaries

The influence of the dimension of an acousto-optic cell in the plane orthogonal to the plane of acousto-optic diffraction on the acoustic field structure and, consequently, on acousto-optic diffraction is studied theoretically and experimentally. A method is suggested for approximate calculation of the acoustic field with regard to the acoustic column reflection from the crystal faces. Based on this method, the influence of the acousto-optic cell dimensions is analyzed in a wide range of ratios between the height of the piezoelectric transducer and the height of the crystal. A region of acoustic power concentration is predicted to arise both for parallel faces and for slight wedging. A number of statements are demonstrated with paratellurite used as an interaction medium. The results may be useful in designing acousto-optic devices.     

Two-coordinate diffusion of optical beams by an acoustooptic Raman-Nath modulator based on a paratellurite crystal

Acoustooptic Raman-Nath diffraction by a standing acoustic wave in a paratellurite crystal is investigated. An acoustic line is made in the form of a polished cube and serves as a high-Q acoustic resonator. A slow shear wave is excited by a single piezoelectric transducer. Multiple lossless sound reflections lead to two-coordinate light diffusion. When the acoustic intensity introduced into the crystal is about 2 W/cm² at a sound frequency of 7 MHz, there appears a diffraction pattern in the form of a homogeneous light spot with a solid angle of about 0.5 sr. An explanation for the features of the acoustooptic interaction is given. It is shown that this type of diffraction is helpful in designing acoustooptic two-coordinate diffusers of light beams.     

Extension of the angular scanning range of the acousto-optic deflector with a two-element phased-array piezoelectric transducer

The acousto-optic deflector with a two-element phased-array piezoelectric transducer is studied at an almost limiting diffraction efficiency. Significant broadening of the working acoustic frequency band (and angular scanning range) relative to the well-studied low-efficiency regime is theoretically predicted and experimentally demonstrated. For the off-axial deflector based on paratellurite (TeO₂), a radiation wavelength of 1.06 μm, and a two-element transducer with a total length of 6 mm, the working frequency band is 17–48 MHz (with the lower frequency determined by the electromechanical properties of the transducer) at an efficiency of no less than 94% of the maximum value.     

Spatial dependence of the dynamic optical transfer function of a transparent medium perturbed by an acoustic signal

Based on a spatially dependent (SD) dynamic optical transfer function (DOTF) of a medium perturbed by an acoustic signal, a dynamic theory has been developed of the acousto-optical interaction (AOI) of light beams in the field of an arbitrary acoustic signal under conditions of the diffraction distortion of its amplitude profile (DDAP). A system of integro-differential equations is derived for angular and frequency DOTF spectra of coupled waves in the AOI region. An analytical DOTF model has been constructed for a weak acousto-optical coupling. The characteristics of transient processes (TPs) in the diffracted-beam field have been investigated for the spatially dependent DOTF. It is demonstrated that in most practical situations this dependence is manifested rather strongly; however, it does not always affect the TP. Thus, when the light and acoustic beam apertures exceed some characteristic values d^* and L^*, related to the acoustic frequency and speed and the AOI geometry, the effect of the SD DOTF on the transient process can be neglected. Otherwise, the DDAP leads to the asymmetry of the dynamic dependences of the field amplitude of the off-axis angular-spectrum components of the diffracted beam. The parameters d^* and L^* have been calculated for the AOI of several types in lithium niobate and paratellurite crystals. The TP characteristics are illustrated by plots calculated for the DDAP in the case of the AOI in paratellurite. Tomsk State University of Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 40–48, May, 1999.     

Ultrasonic diffraction of Bessel light beams in uniaxial gyrotropic crystals

Specific features of the acousto-optic diffraction of Bessel light beams propagating in the vicinity of the optical axis of a uniaxial gyrotropic crystal have been investigated. The dependences of the diffraction efficiency on the acousto-optic interaction length, ultrasound power, and polarization state of the incident Bessel light beam have been analyzed using the coupled-wave equations and the overlap-integral method. It is shown that polarization-independent diffraction of Bessel light beams is observed in paratellurite crystals, when the Bragg diffraction efficiency is independent of the polarization state of the incident beam. The physical reason for this diffraction has been established (both theoretically and experimentally) to be simultaneous implementation of two processes of anisotropic scattering, at which the Bragg synchronism conditions are satisfied for orthogonal polarized Bessel beams with elliptical polarization.     

Acousto-optic modulator of depolarized laser radiation on the paratellurite crystal

An original acousto-optic modulator of depolarized laser radiation is based on a paratellurite crystal in the regime of anisotropic diffraction by a slow acoustic wave. Two acoustic waves with different frequencies are simultaneously excited in a single acousto-optic cell. Two diffraction orders of orthogonal polarizations at the exit from the cell are diffracted in opposite sides relative to the zero order. A polarization prism that is placed immediately behind the cell transforms the diffraction orders into a single output depolarized beam. The total efficiency is 96%.     

Dispersion polaritons on metallized surfaces of optically uniaxial crystals

We have constructed a theory of dispersion polaritons (localized electromagnetic waves) on arbitrarily oriented metallized surfaces of optically uniaxial crystals. The domain of existence of polaritons is defined by the following inequalities for permittivities ε_o and ε_e of the crystal and the angle θ between the optical axis and the surface: ?ε_etan²θ < ε_o < 0. Thus, polaritons exist only in the range of wave frequencies ω ensuring negative values of ε_o(ω) for ε_e > 0. The frequency boundaries of this region are specified for the case when the ε_o(ω) dependence corresponds to the model of a single polar excitation. The azimuthal orientation φ of the optical axis projection onto the surface does not appear in the criterion for polariton existence, but affects (together with angle θ) its main dispersion characteristics, such as the refractive index and partial wave localization parameters. This effect is analytically described in detail. Anomalies in the behavior of polariton parameters are studied in the vicinity of the boundaries of the domain of its existence, where the wave fields are especially sensitive to variations in the angles θ and φ. It is shown that a polariton in the plane of propagation (sagittal plane) passing through the optical axis is transformed into a one-partial bulk wave satisfying the boundary conditions. Accordingly, the wave branch under investigation for close orientations (when the optical axis forms a small angle with the sagittal plane) describes deeply penetrating (quasi-bulk) polaritons.     

Propagation of acoustic beams in a paratellurite crystal

A general solution to the problem of acoustic beam diffraction in an anisotropic medium has been obtained. The effect of acoustic anisotropy of paratellurite crystal on the structure of beams propagating in the XY crystallographic plane has been analyzed. Ray spectra are calculated for different propagation directions of ultrasound. The effects of beam focusing, defocusing, and autocollimation caused by acoustic anisotropy have been analyzed.     

Optical nuclear polarization for sensitivity enhancement of2H NMR single-crystal studies

A gain in detection sensitivity of more than three orders of magnitude is achieved in high-resolution solid-state²H nuclear magnetic resonance of monocrystalline fluorene-d₁₀ by applying optical nuclear polarization via excited triplet states of acridine-h₉ guest molecules. The sensitivity gain is utilized to measure the angular dependence (rotation pattern) of the²H nuclear magnetic resonance lines. In this way the principal values and orientations of all²H quadrupolar tensors are determined. Except for the methylene deuterons, all tensors belonging to the same molecule have one principal axis in common, namely the axis perpendicular to the molecular plane, showing that in the crystal lattice the fluorene molecule is in a planar configuration.     

High-aperture binary axicons for the formation of the longitudinal electric field component on the optical axis for linear and circular polarizations of the illuminating beam

Diffraction of uniformly polarized laser beams with vortex phase singularity is theoretically analyzed using the plane wave expansion. It is shown that for a high numerical aperture, an intense longitudinal electric field component is formed on the optical axis in this case. It is numerically demonstrated that an analogous effect is ensured for diffraction of a conventional Gaussian beam from asymmetric binary axicons. The field intensity on the optical axis can be varied either by rotating the optical element or by changing the direction of polarization of radiation.     

Acoustogyration diffraction of optical waves: case of SiO<Subscript>2</Subscript> and TeO<Subscript>2</Subscript> crystals

Interaction of acoustic and light waves with accounting for elastooptic and elastogyration effects is theoretically described. Collinear acoustogyration diffraction in quartz and paratellurite crystals is experimentally investigated and thoroughly analyzed. Piezogyration effect is experimentally studied for TeO₂ crystals. The acoustogyration efficiency and the acoustogyration figure of merit calculated for a number of crystals (GaAs, TeO₂ and SiO₂) are shown to be too small for experimental detection. On the other hand, we demonstrate that the light diffraction at periodical distribution of the imaginary part of dielectric permittivity related to the piezogyration effect should, in principle, be observed for the case of interaction of optical waves with enantiomorphous ferroelastic domain structure and in cholesteric liquid crystals.     

Magnetostriction of monocrystalline URu2Si2

We report magnetostriction measurements on a monocrystalline sample of the heavy-fermion compound URu₂Si₂, below (at 4.2 K) and above (at 20 K) the Néel temperature. For a field direction along the tetragonal axis the magnetostriction is strongly anisotropic; the volume effect amounts to 1.4 × 10^-6T^-1 at 8 T and 4.2 K. For a field direction in the tetragonal plane the magnetostriction is nearly two orders of magnitude smaller, and less anisotropic.     

Vibration-insensitive reference cavity for an ultra-narrow-linewidth laser

We report a novel mounting of the reference cavity used for stabilization of the clock laser in an optical frequency standard. The cavity axis is oriented horizontally and the cavity is supported in its horizontal symmetry plane on four support points. The positions of the points were optimized by finite-element analysis. A sensitivity to accelerations of 1.5 kHz/(m/s²) in the vertical and 14 kHz/(m/s²) in the horizontal direction was measured, which is a reduction in the vertical sensitivity by two orders of magnitude compared to the usual support from below. PACS 42.60.Da; 46.70.Lk; 42.62.Fi     

155Gd Mössbauer investigation of the magnetic order and spin-reorientation in Gd3Ag4Sn4

In a recent study of the magnetic order in Gd₃Ag₄Sn₄ by neutron powder diffraction and ¹¹⁹Sn Mössbauer Spectroscopy we showed that both the Gd(2d) and Gd(4e) sublattices order antiferromagnetically at 28.8(2) K. We also demonstrated that the ‘magnetic event’ around 8 K is in fact a ‘plane to axis’ spin-reorientation of the Gd magnetic structure. Here, we extend our study with ¹⁵⁵Gd Mössbauer Spectroscopy. The initial magnetic ordering at 30(2) K is clear for both sites and substantial changes in the hyperfine fields are observed at 8 K when the magnetic structure reorients.     

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.     

Magnetic-field-induced changes in the photorefractive sensitivity of lithium niobate

The effect of a magnetic field on the average photorefractive sensitivity of an undoped LiNbO₃ crystal is studied by phase-mismatched second-harmonic generation. The experimental data obtained show the photorefractive sensitivity to reverse sign as the external magnetic field exceeds B ₁=?0.38±0.04 T. The magnetic field is oriented perpendicular to the crystal optical axis and to the plane of laser radiation polarization. The variation of the photorefractive sensitivity is associated with paramagnetic iron centers, whose photoionization probability depends on the direction of their magnetic moment relative to the optical axis.     

Acousto-optic diffraction in hematite due to the magnetoelastic modulation of the polarizations of optical modes

Acousto-optic diffraction in the Raman-Nath regime is observed in the α-Fe₂O₃ easy-plane antiferromagnet. This diffraction is caused by the magnetoelastic modulation of the polarizations of normal optical modes propagating along the trigonal axis of the crystal. The diffraction parameters are analyzed as functions of the static magnetic field applied in the basal plane. The intensity of the diffraction wave in fields on the order of the monodomainization field of the samples (H _D ≈ 1.8 kOe), at an acoustic flux power of I _S ～ 7.0 W/cm², was approximately 0.11% of the incident light intensity and its polarization was perpendicular to the polarization of the linearly polarized incident wave. It is shown that the observed intensity is comparable with the intensity of the acousto-optic diffraction in nonmagnetic materials. The results are well described by the theory of acoustooptic diffraction in antiferromagnets {E. A. Turov, Zh. Éksp. Teor. Fiz. 112, 1464 (1997) [JETP 85, 797 (1997)]} and confirm its basic conclusions.     

Acousto-optic deflector of depolarized laser radiation

An original acousto-optic deflector is based on the anisotropic diffraction in the paratellurite crystal. The deflector is characterized by a relatively high diffraction efficiency for depolarized laser radiation. The deflector consists of two sequential acousto-optic cells. Each cell deflects one of the orthogonally polarized components of the originally depolarized radiation. The first and second cells scan the low- and highfrequency parts of the angular interval, respectively. The simultaneous and independent operation of the cells makes it possible to use the entire optical (laser) power. A frequency band of 32 MHz is almost reached for depolarized radiation with a wavelength of 1.06 µm and the absolute angular interval is 50 mrad at a total efficiency of no less than 70%.