Nonlinear-optical parameters of thin C60 films at 532 nm

The real and imaginary parts of the nonlinear third-order susceptibility χ⁽³⁾ of thin (100 nm) C₆₀ films are studied by the z-scan method at the 532-nm second-harmonic wavelength of a picosecond Nd : YAG laser emitting 55-ps pulses. The change in the sign of Re_χ⁽³⁾ from negative (at a pulse repetition rate f = 2 Hz) to positive (at f = 0.5 Hz) is explained by the thermal lens effect. The very large values of Re_χ⁽³⁾ [(4.80 ± 0.96) × 10^-8 cm³ J^-1] and Im_χ⁽³⁾ [(1.1 ± 0.2) × 10^-8 cm³ J^-1] observed in experiments can be caused by the surface enhancement.