Ultrafast structural transformations in femtosecond laser-excited graphite

Summary form only given. We present here the results of a femtosecond time-resolved study of fast structural changes in laser-excited crystalline graphite exploiting the large optical anisotropy (birefringence) of this material. If a linearly polarized electromagnetic wave is reflected from the surface of an anisotropic medium, the polarization state changes, even at normal incidence. in uniaxial crystals, like graphite, this effect is strongest for reflection from surfaces containing the anisotropy axis, the situation realized in our experiments. Experimental results obtained by time-resolved polarization microscopy on femtosecond excited graphite are summarized. Within a few hundred femtoseconds after laser excitation an increase in the intensity of the p-component (isotropic part) is observed, for the surface perpendicular to the anisotropy axis. In contrast, the anisotropic s-component drops sharply within 300-500 fs. This behavior indicates a very rapid loss of crystalline order, faster than the usual electron-lattice equilibration time, thus confirming the non-thermal nature of the transition.