Photoinduced gratings in a Sn$_2$P$_2$S$_6$ ferroelectric crystal with the period depending on the optical pump power

The light scattering in the form of ring structures in Sn$_2$P$_2$S$_6$ crystals at the propagation of intense laser pump radiation along the $b$ crystallographic axis is detected. Radiation passing through a crystal is completely scattered into a cone whose angle increases with the pump power and decreases reversibly with a decrease in this power. The observed effect can be attributed to a spontaneous increase in the amplitude of photoinduced bulk diffraction gratings and to the scattering of light on them in the directions where the phase increments of the photorefractive and diffraction natures compensate each other. A similar type of scattering was observed previously in other photorefractive crystals, but the appearance of gratings with the period easily varied by varying the pump power is demonstrated for the first time.