Light modulation by acoustic signals for high diffraction efficiency

A theoretical discussion is presented on strong Bragg acoustooptic interaction AOI of light beams in the dynamic field of an acoustic signal. A system of integrodifferential equations is formulated to describe the evolution of the angular and frequency spectra of the beams in the AOI region for a high level of acoustooptic coupling. The third-order approximation in the perturbation method is used to obtain an analytic solution. Calculations are presented on the modulation of monochromatic beams by acoustic pulses having rectangular envelopes and propagating in a lithium niobate crystal, and the same for a signal having linear frequency modulation LFM in a paratellurite crystal, which demonstrate the broadening of the beam spectrum as the depth of the acoustooptic coupling increases, together with the occurrence of an asymmetry specific to strong AOI in the response of the light field to the symmetrical acoustic signal. Tomsk State University for Control Systems and Electronics, Tomsk. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 42, No. 1, pp. 99–106, January, 1999.     

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.     

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.     

Diffraction characteristics of acoustooptic interaction in an acoustic field with a curved wave front

A theoretical model is presented for the strong two-dimensional acoustooptic interaction (AOI) of finite-size beams with arbitrary profiles in an acoustic field with a curved wave front. Transfer functions are derived in a universal normalized form for the AOI. The results of numerical simulation of the diffraction characteristics show that during AOI in an acoustic field with a curved wave front the product of the diffraction efficiency and bandwidth increases at least 1.5 times for a weak interaction and more than five times for a strong interaction. The optimal values of the normalized parameters (of the acoustooptic coupling and the curvature), for which the efficiency and bandwidth are maximum, are determined.Tomsk State Academy of Control Systems and Electronics. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 4, pp. 41–50, April, 1995.     

Acousto-Optic Diffraction in Paratellurite by a Slow Acoustic Mode. Increase of Diffraction Efficiency of Divergent Light

Peculiarities of anisotropic acousto-optic Bragg diffraction in paratellurite by a slow acoustic wave have been considered. The ranges of frequency-angular dependences have been found, in which effective diffraction occurs, and their practical relevance has been formulated. The numerical values for light radiation with a wavelength of 1.06 μm have been obtained. We have proposed the method of enhancement of a diffraction efficiency of divergent optical radiation, which in turn leads to the increase of the response time of an acousto-optical modulator. The method is based on a multifrequency electric control of the acousto-optical modulator. We have shown experimentally that, as distinct from the single-frequency control, a considerable intensity suppression of the zero diffraction order from 30 to 8% is achieved.     

Dispersion des indices de refraction du molybdate de plomb (PbMoO4) et de la paratellurite (TeO2)

An infrared goniometer, shortly described has been used for the determinations of the refractive indices of two interesting acousto-optical materials: lead molybdate (PbMoO₄) and paratellurite (TeO₂). The given results complete those previously available which concern only the visible range. Coefficients of Sellmeier equations for the whole transparency range of both materials are also given and discussed.     

Coherence effects in low energy Na*(3p)+Na+ scattering: Experiment and semiclassical calculations

Pronounced polarization effects have been observed in inelastic collisions of laser state-prepared Na^*(3p,M _L) with Na⁺ leading to Na^*(3d) for the energy rangeE _CM=20–45 eV. Using linearly polarized light the dependence of the inelastic process on the alignment of the electronic charge cloud of the Na^*(3p) prior to the collision has been measured. In studies with left and right hand circularly polarized light the angular momentum transferred in the collision process has been determined. The results are compared with similar data for the 3p→3s deexitation process studied previously [6]. The density matrix of the Na^*(3p) state has been evaluated with respect to the collisional excitation to Na^*(3d). Semiclassical calculations based on the coupled channel impact parameter approximation using pseudopotentials [7] and nonadiabatic rotational coupling elements for the Na ₂ ^* system [12] have been performed. The agreement with the experimental results is good, in particular for the higher collision energies.     

Spin–orbit coupling in bulk GaAs

We study the spin–orbit coupling in the whole Brillouin zone for GaAs using both the sp³s^*d⁵ and sp³s^* nearest-neighbor tight-binding models. In the Γ-valley, the spin splitting obtained is in good agreement with experimental data. We then further explicitly present the coefficients of the spin splitting in GaAs L- and X-valleys. These results are important to the realization of spintronic device and the investigation of spin dynamics far away from equilibrium.     

Density correlation function and dynamic transient exponents for liquid helium at and aboveT λ

The critical dynamics of liquid helium are studied by means of renormalized field theory on the basis of the symmetric planar-spin model of Halperin, Hohenberg, and Siggia. The stability problem of the dynamic fixed point is discussed in detail. Two-loop results suggest, but do not establish, the stability of the dynamic scaling fixed point. The previously found small fixed point valuew ^*~O(0.15) is tentatively confirmed which implies a small ratio of relaxation rates of the order parameter and the entropy. The ensuing dynamic transient exponents are calculated. The density correlation function is determined toO(ε=4?d) at and aboveT _λ. Its properties in the casew ^*?1 provide quantitative support for the recently proposed explanation of the discrepancy between theory and light scattering experiments. A small value ofw ^* implies pronounced peaks of the frequency spectrum at finite frequencies at and aboveT _λ. It also suppresses the temperature dependence of finite-frequency properties over an enlarged critical region as found in light scattering measurements. The quantitative relation between the value ofw ^*>0 and observable properties of the frequency spectrum is computed.     

The S1−S0 transitions of 1,2,3-trimethylbenzene and 3-fluoro-1,2-dimethylbenzene observed by two-photon spectroscopy

Two-photon (TP)-induced transitions to the ¹L_b state of 1,2,3-trimethylbenzene and 3-fluoro-1,2-dimethylbenzene were studied by resonant multiphoton ionization in a supersonic jet beam. In both molecules the TP S₁ ← S₀ transition has very strong Franck-Condon components and components vibronically induced by ν₁₄. Seven fundamental vibrations of the ¹L_b state are detected in each molecule. A band, which appears near 14₀¹ due to a Fermi resonance, is proposed to be 1₀²15₀¹. The ¹L_b intensity of the heterosubstituted compound is in agreement with the TP orientation intensity rules.     

Filtration of partially coherent optical radiation by means of multiphonon Bragg diffraction

The filtering properties of three-phonon acousto-optical Bragg diffraction for increasing the degree of coherence of a partially coherent optical field are investigated by the example of multiphonon acousto-optical interaction in a TeO₂ single crystal. Two possible mechanisms of coherent scattering upon the formation of the highest orders are taken into account. It is shown that the three-phonon interaction is more efficient by at least a factor of two than any mode of one-phonon diffraction realized at the same frequency and the same acousto-optical interaction length. The experimental investigation, using partially coherent radiation of a He-Ne laser (λ = 0.63 μm) diffracted from a transverse acoustic wave propagating in a TeO₂ single crystal, on the whole confirmed the basic theoretical foundations.     

On the orientation effect of the neutron yield increase in <Emphasis Type="Italic">d</Emphasis>(<Emphasis Type="Italic">d,n</Emphasis>)<Superscript>3</Superscript>Hе reactions at an energy of 7–12 keV in TiD<Subscript>2</Subscript> crystals

The orientation dependence of the d(d, n)³He reaction probability in a TiD₂ crystal at a deuteron energy of 7–12 keV is investigated. The BCM-1.0 code developed for calculating the trajectories of channeled particles within classical mechanics is used to simulate deuteron trajectories at (200)-planar channeling in a 0.15-μm-thick Ti crystal with the angular divergence of the beam taken into account. The enhancement of the reaction probability in the computer experiments is 2.1 in the case of a parallel deuteron beam and near-zero crystal entry angles relative to the (200) planes. In the case of a deuteron beam with its angular divergence equal to 1/5 of the critical channeling angle, the maximum reaction-probability enhancement is 1.5. The results of calculations agree qualitatively with recent experiments performed at Tomsk Polytechnic University.     

Acousto-optic Bragg diffraction in paratellurite by the sidelobes of the spatial radiation spectrum of an acoustic transducer

Acousto-optic Bragg diffraction in paratellurite is investigated within the two first diffraction orders for the case of diffraction by the sidelobes of the spatial radiation spectrum of an acoustic transducer. One of the diffraction orders is due to anisotropic diffraction, and the other, to isotropic diffraction. Such a diffraction regime is achieved when the diffraction plane is inclined toward the optical axis of the crystal. For light with a wavelength of 0.63 × 10^–4 cm diffracted by a “slow” sound wave with a frequency of 26 MHz, the effect manifests itself when the angle between the acousto-optic diffraction plane and the optical axis of paratellurite is ~3°. The effect is experimentally verified. The diffraction efficiency is 20% for each of the diffraction orders for a microwave signal of 8 V at the transducer.     

Acousto-Optic Diffraction in an Ultrasonic Field of a Width-Profiled Transducer

Acoustical Physics - Acousto-optical diffraction in an ultrasonic field of a slow acoustic mode in paratellurite excited by a hexagonal-shaped piezoelectric transducer has been simulated in a...     

Inclusive production of neutral vector mesons in hadronic Z decays

A data sample of about 3.0 million hadronicZ decays collected by the ALEPH experiment at LEP in the years 1991 through 1994 is used to make an inclusive selection of B hadron events. In this event sample 4227±140±252B* mesons in the decayB*→Bγ and 1944±108±161B** _u,d mesons decaying into a B meson and a charged pion are reconstructed. Here and in the followingB** _u,d denotes the eightL=1(bū) and (bd) states and their charge conjugate. For the well establishedB* meson the following quantities are obtained:ΔM=M _B*−M_B=(45.30±0.35±0.87) MeV/c² andN _B*/(N _B+N _B*)=(77.1±2.6±7.0)%. The angular distribution of the photons in theB* rest frame is used to measure the relative contribution of longitudinalB* polarization states to beσ _L/(σ _L+σ _T)=(33±6±5)%.     

Ⅲ-V族化合物半导体Λ轴光学声子形变势的研究

本文在LMTO-ASA能带计算中,采用冻结声子模型,系统地研究了Ⅲ-V族化合物半导体的Λ轴光学声子形变势随k值的变化关系,提供了9种Ⅲ-V族化合物在Λ轴不同k值情况下的光学声子形变势d₃₀(Λ), d₁₀(Λ,val)和d₁₀(Λ,con)的第一原理计算结果。     

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.     

A generalization of the classical moment problem on *-algebras with applications to relativistic quantum theory. II

We discuss some properties of a non-commutative generalization of the classical moment problem (them-problem) previously introduced. It is shown that there is a connexion between the determination of the problem and the self-adjointness properties in the corresponding Hilbert space. This generalizes the well-known connexion between the determination of the measure in the classical moment problem and the self-adjointness properties of the polynomials as operators in the correspondingL ²-space. The dependence of them-problem on the choice ofC*-semi-norms and on the action of *-homomorphisms is also investigated. As an application, it is shown that if a quantum field (in a very general sense) is essentially self-adjoint then them-problem for the Wightman functional is determined on the quasi-localizableC*-algebra and that the corresponding representation of the localizable algebra generates the bounded observables of the field. It is pointed out that (ultraviolet and spatially) cut-off fields fall in this class and, therefore, are in one to one correspondance with states on the quasi-localizableC*-algebra.Laboratoire associé au Centre National de la Recherche Scientifique.     

复杂流场的超声-激光测量原理研究

本文从复杂流场—旋涡场参量的超声—激光测试方法的需要出发,论述了超声波产生的声相位光栅对激光产生的偏转效应.并研制了适用于产生空气超声相位光栅的大功率高频超声换能器、位移灵敏接收器、数字相位差测定仪等设备,采用了超声发射的匹配技术等,从而获得了明显的空气超声-激光偏转效应,并且测定了两光束的偏转时间差.本文的结果为利用超声-激光的空气声光偏转效应测量空气旋涡流场参量提供了实验依据.