Negative group velocity in C60 due to RSA effect

We experimentally observed negative group velocity in C60 toluene solution at the wavelength of 532 nm for the first time. The observed time advancement of the modulated signal appears to be due to the RSA (reverse saturable absorption) effect of the C60 molecule. The largest time advancement of 71.65 ms was obtained with a C60 sample of 1.5-mm length with the concentration of 1.39×10-3 mol/l; the corresponding group velocity was -0.021 m/s. We could control the group velocity by adjusting the modulation frequency and the input intensity.