Large infrared nonlinear optical response of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">C</mml:mi></mml:mrow><mml:mrow><mml:mn>60</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

${\mathrm{C}}_{60}$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 \ensuremath{\mu}m. The magnitude of the nonlinear susceptibility per ${\mathrm{C}}_{60}$ molecule is \ensuremath{\Vert}\ensuremath{\gamma}\ensuremath{\Vert}=1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}42}$ ${\mathrm{m}}^{5}$ ${\mathrm{V}}^{\mathrm{\ensuremath{-}}2}$ 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.

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