We report the results of comprehensive studies of the piezo-optic and elasto-optic effects in lithium tetraborate crystals, Li₂B₄O₇. The full matrices of piezo-optic and elasto-optic coefficients are obtained using a recently developed method based on four-point bending. On this basis, we have estimated the acousto-optic figure of merit for Li₂B₄O₇.

Complete matrices of piezo-optic and elasto-optic tensors are experimentally determined for TlAsS crystals. It is revealed that the piezo-optic coefficients are very high, ∼10 N/m in the order of magnitude. This implies that TlAsS can be referred to the best piezo-optic materials. The same concerns the elasto-optic coefficients, of which absolute values are in the interval 0.28-0.54. It is also found that, at the anisotropic and isotropic interactions with the slowest transverse and longitudinal acoustic waves, the acousto-optic figure of merit reaches extremely high values (1.99×10 s/kg and 9.45×10 s/kg, respectively). In other words, the TlAsS crystals can be referred to as one of the best acousto-optic materials for the visible and infrared spectral ranges.

Abstract. We have developed analytical approach to determine the orientations of cross sections of optical indicatrix (OI) around the optic axes (OAs) in biaxial crystals. It has been found that the angular distribution of cross sections of the OI by the planes perpendicular to the directions close to the OA reveals a topological defect of OI orientations with the strength equal to ½. When a conical circularly polarized wave with cone's axis coinciding with the OA in a biaxial crystal propagates through a sample, a singly charged optical vortex is generated. We have shown that splitting of a single OA in optically uniaxial crystals into two OAs due to electrooptic effect is accompanied by the topological reaction that involves dividing a single defect with the unit strength into two defects with the strengths equal to ½. We have experimentally discovered topological dipoles that consist of topological defects with the strengths of each defect within the pair equal to +½ and -½.

Abstracts.We study the behaviour of topological defects (TDs) of optical indicatrix orientation under the conditions when electrooptic Pockels and Kerr nonlinearities coexist in the crystals belonging to cubic, hexagonal, trigonal and tetragonal systems and a conically shaped electric field is applied along the principal crystallographic and/or optic axes. The topological reactions accompanying the processes of birth, addition, division and annihilation of the TDs of optical indicatrix orientation are observed under electric field variations. It is shown that the conservation law for the strength of TDs holds true under these topological reactions and the behaviours of the TDs can be exhaustively described by four different scenarios.Анотація. У роботі досліджено поведінку топологічних дефектів (ТД) орієнтації оптичної індикатриси при співіснуванні електрооптичних нелінійностей керівського і покельсівського типів у кристалах, що належать до кубічної, гексагональної, тригональної і тетрагональної сингоній, за умови прикладання конічного електричного поля вздовж головних кристалографічних і/або оптичних осей. Виявлено, що за згаданих умов реалізуються топологічні реакції, які супроводжуються народженням, додаванням, поділом і анігіляцією ТД орієнтації оптичної індикатриси. Показано, що за цих топологічних реакцій виконується закон збереження сили ТД, а поведінку ТД можна вичерпно описати чотирма різними сценаріями.

Abstract.We have revealed experimentally a movement of the topological defects of optical indicatrix orientation, which takes place in CaB 4 O 7 glass with residual mechanical stresses under application of external compressive mechanical stresses. The effect is explained basing on the equation of optical indicatrix perturbed by the combination of both residual and external mechanical stresses. The conditions for the appearance of the topological defects with the strength module |1/2| in glasses are formulated. Finally, possibilities for the appropriate topological reactions caused by the applied stresses are discussed.