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The theory of nonlinear diffraction of intensive light beams propagatingthrough photorefractive media is developed. Diffraction occurs on a reflectingwire embedded in the nonlinear medium at relatively small angle with respect tothe direction of the beam propagation. It is shown that this process isanalogous to the generation of waves by a flow of a superfluid past anobstacle. The ``equation of state'' of such a superfluid is determined by thenonlinear properties of the medium. On the basis of this hydrodynamic analogy,the notion of the ``Mach number'' is introduced where the transverse componentof the wave vector plays the role of the fluid velocity. It is found that theMach cone separates two regions of the diffraction pattern: inside the Machcone oblique dark solitons are generated and outside the Mach cone the regionof ``ship waves'' is situated. Analytical theory of ``ship waves'' is developedand two-dimensional dark soliton solutions of the equation describing the beampropagation are found. Stability of dark solitons with respect to their decayinto vortices is studied and it is shown that they are stable for large enoughvalues of the Mach number.

Nonlinear diffraction of light beams propagating in photorefractive media with embedded reflecting wire