We investigate the pulse length dependence of the third-order nonlinear response of a gold film using pulse widths between 325 and 650 fs at λ=1320nm via a sensitive cut-pulse technique that measures background-free self-phase modulation. We find a cubic dependence of the transmitted nonlinear power on incident peak intensity that is independent of the pulse width in this range to within experimental accuracy. We estimate a value of χ3=8±3×1018m2/V2 by fitting experimental data to numerical simulations accounting for both losses and reflections at each film interface. These results provide valuable insights into near-infrared nonlinear plasmonic device designs.

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