Ultrafast electron dynamics in gold nanoshells

We present ultrafast optical studies on nanoshells, which consist of a nanoscale ${\mathrm{Au}}_2\mathrm{S}$ dielectric core surrounded by an ultrathin Au shell. The plasmon frequency of these nanoparticles can be controlled by varying the ratio of the core diameter to shell thickness. By tuning this plasmon peak, we have observed both transient bleaching and transient absorption for Au nanoshells embedded in polyvinyl alcohol. We relate the induced change in transmission of gold nanoshell films to the electron dynamics in the thin gold shell. Our results indicate that even when the plasmon frequency is shifted below the onset of interband transitions, interband effects are still of primary importance in determining the nonlinear optical response. An electron relaxation time of $1.65\pm 0.10\mathrm{ps}$ is measured that corresponds to an electron-phonon coupling constant of $2.2\times {10}^{16}{\mathrm{W}/(\mathrm{m}}^3\mathrm{}\mathrm{K}),$ smaller than that observed in bulk Au films.