Subpicosecond time response of third-order optical nonlinearity of small copper particles in glass

Femtosecond pump-probe experiments with a time resolution of 100 fs have been performed for copper particles with a radius of 4 nm. Differential absorption spectra for a pump centered at 2.05 eV indicates the broadening of the absorption band due to the surface plasmon. The nonlinear response time derived from the recovery time of the nonlinear absorption is dependent on the pumping laser fluences, and is as short as 0.7 ps for 210 μJ/cm2. The relaxation dynamics of nonequilibrium electrons can be described by the usual electron-phonon coupling model.