The generation of the second harmonic of laser radiation in polydomain $Ba_{2}NaNb_{5}O_{15}$ crystals is investigated. Diffuse scattering of the second harmonic is observed in crystals with disordered domain structure, consisting of microdomains. It is concluded that the dimension of acicular microdomains in the direction of the polar axis of the crystal is about 10μm, whereas their transverse dimensions range from 1 to 3 μm. Crystals with periodic laminar domian structure (with a period of 12 μm) exhibit quasisynchronous, noncolinear second harmonic generation with angular width of generation peak of 22'. Such a process can be employed for laser frequency transformation. The second harmonic generation in polydomain barium-lithium and potassium-lithium niobates was also investigated. The results obtained for these materials are analogous to those for $Ba_{2}NaNb_{5}O_{15}$. The reasons for similarity of domain structures of the investigated crystals are discussed.
Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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