Nonlinear absorption in Au films: Role of thermal effects

Abstract

The effective nonlinear optical absorption coefficient ${\ensuremath{\beta}}_{\mathrm{eff}}$ is measured for $20\text{\ensuremath{-}}\mathrm{nm}$-thick Au films at $630\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ as a function of pulse width. The $z$-scan measurements show that ${\ensuremath{\beta}}_{\mathrm{eff}}$ increases from $6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ to $6.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}\mathrm{cm}\phantom{\rule{0.2em}{0ex}}{\mathrm{W}}^{\ensuremath{-}1}$ as the pulse width is varied from $0.1\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5.8\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$. To help interpret this $\ensuremath{\sim}100\ifmmode\times\else\texttimes\fi{}$ increase, differential transmission and reflectivity measurements are performed using $775\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ pump and $630\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ probe pulses. All experiments are simulated with a two-temperature model for electrons and lattice. The pulse width dependence of ${\ensuremath{\beta}}_{\mathrm{eff}}$ is consistent with thermal smearing of $d$-band to conduction-band transitions, with ${\ensuremath{\beta}}_{\mathrm{eff}}$ arising from changes in the linear $(\mathrm{Im}\phantom{\rule{0.2em}{0ex}}{\ensuremath{\chi}}^{(1)})$ absorption coefficient.