Studies of nonlinear absorption and refraction in C60/toluene solution

Nonlinear absorptive and refractive properties of C60/toluene solution are studied using the Z-scan technique and a 532 nm laser delivering single picosecond (ps) pulses and trains of ps pulses separated by 7 nanoseconds (ns). With single ps pulses, reverse saturable absorption and positive nonlinear refraction are observed and attributed to population transitions among the singlet states. With ps pulse trains, a sign change of nonlinear refraction relative to that of single ps-pulse excitation is observed and is attributed to heat-induced temperature rise and population relaxing to triplet states.