Femtosecond optical nonlinearity of Au nanoparticles under their excitation in nonresonant relative to surface plasmon conditions

Abstract

Nonlinear optical response of Au island films to femtosecond laser pulses is studied in the vicinity but not exactly at the surface plasmon absorption peak (λ _spr = 560 nm). The third-order nonlinear optical susceptibility is shown to be Reχ ⁽³⁾ = +1.7 × 10⁻⁷ esu and Reχ ⁽³⁾ = +1 × 10⁻⁷ esu at the wavelengths 800 and 460 nm, respectively. Kinetics of the optical nonlinear response has been studied for wavelengths 400 and 800 nm. It is believed that the origin of nonlinearity at the wavelengths is related to the free-electron heating in the conduction band and their further thermalization via electron–electron scattering, but at 400 nm the contribution to the nonlinear susceptibility because of interband d → s, p transitions is also possible.