Femtosecond nearly degenerate four-wave mixing in C60 films between 0.55 and 0.70 μm

We used femtosecond nearly degenerate four-wave mixing (nDFWM) to measure the instantaneous third-order nonlinear optical susceptibility function χ1111[−(ω−δω),ω,ω,−(ω+δω)] near the absorption edge of C60 films with absorption reaching up to 2.0×104 cm−1. The frequency shift was set to ℏδω=0.07±0.01 eV, smaller than the width of the spectral feature studied but large enough to suppress non-instantaneous signal contributions. Our data are orders of magnitude below previous DFWM results but connect smoothly to data at longer wavelengths where absorption is low (<≈100 cm−1).