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 Reχ(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 Reχ(3) [(4.80±0.96)×10-8 cm3 J-1] and Imχ(3) [(1.1±0.2) × 10-8> cm3 J-1] observed in experiments can be caused by the surface enhancement.