Investigation of nonlinear refraction of the C60/toluene solution by a time-resolved pump-probe system based on a 4f nonlinear imaging technique with the phase object

Excited state nonlinear refraction of the C-60/toluene solution is studied using a picosecond time-resolved pump-probe system based on a 4f nonlinear imaging technique with the phase object. Negative nonlinear refraction of the C-60/toluene solution is observed after zero-delay time besides normal nonlinear absorption. By using a five-energy-band model, both the change of refraction index per unit density of the first singlet excited state and the first triplet excited state of the C-60 molecule having a negative sign are determined. The nonlinear refraction, as well as the dynamics of neat toluene and its excited state contribution to the solution, is also investigated.