The spontaneous appearance of hexagon patterns in photorefractive crystals is treated as mirrorless coherent four-wave mixing oscillation. It is shown experimentally that the system has a well-defined coupling-strength threshold and behaves as other coherent optical oscillators near the threshold. A simple relation for the angle of the hexagon sideband as a function of the distance between the end mirror and the sample is derived and verified experimentally.
[Optical Society of America ]
(050.7330) Diffraction and gratings : Volume gratings
(070.5040) Fourier optics and signal processing : Phase conjugation
(190.2620) Nonlinear optics : Harmonic generation and mixing
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing
(190.5330) Nonlinear optics : Photorefractive optics
Y. Uesu, A. Ueno, M. Kobayashi, and S. Odoulov, "Hexagon formation in photorefractive crystals as mirrorless coherent oscillation," J. Opt. Soc. Am. B 15, 2065-2069 (1998)