LARGE INFRARED NONLINEAR OPTICAL-RESPONSE OF C60

C60 buckminsterfullerene exhibits a large ultrafast third-order nonlinear optical response. Degenerate four-wave-mixing measurements in C60-benzene solutions were performed using 50-psec pulses at 1.064-mu-m. The magnitude of the nonlinear susceptibility per C60 molecule is \gamma\ = 1.5 x 10(-42) m5V-2 and of the same size as that observed in polydiacetylene. In contrast to conjugated polymers, however, a dominant positive real part of the nonlinearity is found, 3 times larger than the imaginary component. The nonlinear response can be described within the model of a free electron in a spherical box, confirming the complete delocalization of electrons on the C60 molecule.