Broadband measurements of the transient optical complex dielectric function of a nanoparticle/polymer composite upon ultrafast excitation

Abstract

We determined experimentally the complex transient optical dielectric function of a well-characterized polyelectrolyte/gold-nanoparticle composite system over a broad spectral range upon short pulse laser excitation by simultaneously measuring the time-dependent reflectance and transmittance of white light pulses with femtosecond pump-probe spectroscopy. We extracted directly the ultrafast changes in the real and imaginary parts of the effective dielectric function, ${\ensuremath{\epsilon}}_{r}^{\text{eff}}(\ensuremath{\omega},t)$ and ${\ensuremath{\epsilon}}_{i}^{\text{eff}}(\ensuremath{\omega},t)$, from the experiment. This complete experimental set of information on the time-dependent complex dielectric function challenges theories modeling the transient dielectric function of gold particles and the effective medium.