Spectroscopic study of optically induced ultrafast electron dynamics in gold

Using a supercontinuum pulse as a probe, we have measured the transient reflectivity spectra of a thin film of gold for different values of the pump-probe time delay. The wavelength ${\lambda }_x$ at which the measured transient reflectivity changes sign has been found to depend upon the time delay, leading to bipolar time resolved signals. The time dependence of ${\lambda }_x$ has been shown to be consistent with calculations that take into account the full dependence of the reflectivity upon the electron occupation number, and to contradict qualitatively a model in which the signal is assumed to be directly proportional to the occupation number. The shift of ${\lambda }_{\mathrm{x}}$ has been found to persist at time delays that are much longer than the time required for the electrons to thermalize. Therefore the bipolar reflectivity signals do not necessarily contain a contribution from nonthermalized electrons, as has been previously assumed.

Figure 1

(Color online) The transient reflectivity spectra are shown for different pump-probe time delays. The inset shows the shift of the zero crossing point with time delay. The white light continuum spectrum is shown by the dashed black line.

Figure 2

The time resolved reflectivity signals constructed from the spectra in Fig. 1 are shown for different probe wavelengths.

Figure 3

The fit of the reflectivity signals shown in Fig. 2 is illustrated for various probe wavelengths. The symbols and solid lines represent the data points and the fitted curves, respectively.

Figure 4

The values of the electron-phonon coupling constant extracted from the fits shown in Fig. 3 are plotted against the probe wavelength. The solid line denotes the value of the constant derived in Ref. 16.