Concentration and wavelength dependence of the effective third-order susceptibility and optical limiting of C₆₀ in toluene solution

The optical limiting action of C₆₀ in toluene solution is mainly due to reverse saturable absorption (RSA). It is shown that the formalism describing nonlinear optical response due to instantaneous two-photon absorption can be used in the case of sequential two-photon absorption, yielding effective values of the relevant parameters of optical nonlinearity due to RSA. The effective two-photon absorption parameter beta _eff and the effective nonlinear refractive index parameter gamma _eff', which are related respectively to the imaginary and real parts of the effective third-order susceptibility chi _eff⁽³) were measured by the z-scan technique as a function of fullerene concentration and of incident laser intensity and wavelength over the 420-640 nm region. The concentration dependence of these parameters indicates that the solution is optically thin as far as the ground state of C₆₀ is concerned, whereas the wavelength dependence confirms the applicability of the formalism used in the sequential two-photon absorption model. Comparisons are made with other z-scan results on C₆₀.