Large infrared nonlinear optical response of ${\mathrm{C}}_{60}$

${\mathrm{C}}_{60}$Buckminsterfullerene exhibits a large ultrafast third-order nonlinear optical response. Degenerate four-wave-mixing measurements in ${\mathrm{C}}_{60}$-benzene solutions were performed using 50-psec pulses at 1.064 μm. The magnitude of the nonlinear susceptibility per ${\mathrm{C}}_{60}$ molecule is ‖γ‖=1.5×${10}^{\mathrm{-}42}$ ${\mathrm{m}}^5$ ${\mathrm{V}}^{\mathrm{-}2}$ 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 ${\mathrm{C}}_{60}$ molecule.