Light pulse group velocity manipulations due to the specific dispersion of a medium (so-called "slow" and "fast" light phenomena) can be obtained on the basis of several mechanisms. One of these techniques is two-wave mixing in a photorefractive crystal. This work presents a modification of this method, exploiting the strong beam fanning in Sb-doped Sn_2P_2S_6 crystals. Our experimental results demonstrate a "fast light" behavior of Gaussian pulses transmitted through a Sn_2P_2S_6:Sb sample. The phenomenon is due to the beam fanning (i.e., the self-diffraction of the incident beam on self-induced noisy photorefractive gratings) that ensures a significant depletion of the input beam. Due to the relatively fast photorefractive response of the Sn_2P_2S_6:Sb crystals this depletion occurs with times in the range of 10–100 ms, depending on the beam intensity, and the "fast light" feature is observed. The temporal and amplitude characteristics of the output beam are measured in function of the intensity and polarization azimuth of the incident beam. Besides, a negative phase shift of the periodical output beam relative to a sinusoidal intensity-modulated input beam is also obtained experimentally. It is shown that the phase and amplitude relation between the input and output periodic signals are described by a simple analytical expression that takes into account the beam fanning strength (depletion factor) and its dynamics (depletion time constant). It is also demonstrated that the pulse advance (or phase shift of the modulated signal) can be regulated by the light polarization azimuth. The advantages of the proposed method are discussed.
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