Simultaneous slow and fast light in C60 solution

Recently, many researchers have reported their work about sub- and superluminal propagation. And many experiments have demonstrated the group velocity transition between slow and fast light. In this letter, the authors reported simultaneous slow and fast light in a single light beam which could be explained by the photorefractive lens-like effect and the spatial dispersion qualitatively. In our experiment, a focused light beam at 532 nm normally incident upon a thin quartz cell full of C60 solution and spatial diffraction rings were observed behind the cell. Recurring to a pinhole diaphragm, we measured the group velocity of light of different position on the diffraction rings. The experimental results demonstrated that slow and fast could exist in a single beam at the same time. We could acquire slow or fast light just by adjusting the position of the detector and keeping the experimental parameters, such as: modulation frequency and incident intensity, unchanged. Our experiment put forward a new way to control the group velocity in nonlinear optical material.