ACOUSTO-OPTIC PHENOMENA IN CRYSTALS WITH STRONG ANISOTROPY OF OPTIC AND ELASTIC PROPERTIES V.B.Voloshinov

The paper examines regular trends of acousto-optic interactions taking place in crystals possessing strong anisotropy of optic and elastic properties. The major attention is devoted to analysis of light diffraction in media with large birefringence, i.e. with big difference o e n n n − = ∆ between the refraction indexes of ordinary o n and extraordinary e n polarized optical beams. The analysis is also concentrated on peculiarities of light and ultrasound interaction in anisotropic materials that possess strong anisotropy of elastic properties. Single crystals of paratellurite (TeO2), tellurium (Te), calomel (Hg2Cl2), mercury bromide (Hg2Br2) and mercury iodide (Hg2I2) were investigated owing to the large birefringence and due to the extremely wide obliquity angles 0 50 > Ψ between phase and group velocities of ultrasound. The paper considers regular trends of light diffraction in the anisotropic media and examines possible applications of new diffraction phenomena in novel modifications of acousto-optic instruments. Introduction The paper is devoted to investigation of light diffraction by ultrasound propagating in crystalline media possessing strong anisotropy of optic and elastic properties. It is known that materials with the anisotropy of their physical properties have successfully been used in acousto-optic instruments of light beam control [1,2]. Many modifications of acousto-optic devices applying the diffraction have been designed and applied in modern Optics, Spectroscopy, Optical Information Processing and Laser Technology [3,4]. However, new acousto-optic crystalline materials were synthesized in recent times. It is reasonable to expect that the new materials that possess extremely strong anisotropy of physical properties will be considered as candidates for the application in novel modifications of acousto-optic devices such as filters, deflectors and modulators [14]. The major attention during the present investigation is devoted to analysis of light diffraction in the optically anisotropic crystals possessing extraordinary large birefringence. In particular, the acousto-optic interaction is examined in the crystals with the difference o e n n n − = ∆ between the indexes of refraction of the extraordinary e n and ordinary o n polarized optical beams as big as the refractive indexes themselves, i.e. o n n ~ ∆ . It should be noted that efficient acousto-optic crystals have been grown with the relative birefringence b of the order o n n b / ∆ = = 0.3 0.4 [1,5]. In this paper, the crystals characterized by the large birefringence 3 . 0 > b such as tellurium (Te), calomel (Hg2Cl2), mercury bromide (Hg2Br2) and mercury iodide (Hg2I2) are examined with the goals of application in acousto-optic instruments [1-5]. It is likely that novel types of modulators, deflectors and tunable acousto-optic filters (AOTFs) will be developed on base of the unique birefringent materials. Moreover, it may be expected that the new devices, in many aspects, will be superior to the existing acousto-optic systems of light beams control. It was noticed that many of the new acousto-optic materials were characterized not only by the strong anisotropy of their optical properties but also by the strong anisotropy of their elastic properties [4-7]. It is known that the elastic anisotropy manifests itself in the extremely large obliquity angles Ψ between phase and group velocities of ultrasound [4]. The acoustic “walkoff” angle Ψ evaluated between the acoustic wave vector and the Poynting vector may appear in the crystals as large as dozens of degrees. For example, the obliquity angle in the single crystal of tellurium is exceeding 0 50 > Ψ while the walkoff angle in calomel, mercury bromide and mercury iodide is about 0 70 = Ψ [6,7]. As for the commonly used tellurium dioxide single crystals, they are also of undoubted interest for the applications because TeO2 is characterized by the acoustic obliquity angles up to 0 74 = Ψ [7]. The carried out investigation proves that the unusual propagation of the elastic waves in these media results in new effects that may be observed during the interactions of light and ultrasound [6-9]. The regimes of the acousto-optic interactions existing only in the materials possessing the strong optic and elastic anisotropy are examined in the presentation. Many of the interactions have not yet been realized in experiments. On the other hand, there are diffraction regimes that are well known in Acousto-Optics and that have been successfully used in acousto-optic devices [3,7-10]. The paper describes modifications of acousto-optic instruments that may be designed on base of the new effects existing in the optically and elastically anisotropic materials. WCU 2003, Paris, september 7-10, 2003