Polarization-induced size control and ablation dynamics of Ge nanostructures formed by a femtosecond laser

We report a method for controlling the size of a Ge (germanium) nanostructure by changing the angle between the ultrafast laser polarization and the crystal axis of Ge. The nanostructure size dependence on the laser polarization with respect to the Ge crystal axis exhibits a sinusoidal function with a minimum size at (100) axis. Moreover, the measurement of transient reflection reveals the presence of large anisotropies in both its amplitude and its relaxation dynamics with a minimum at (100) crystal axis. This implies that the observed anisotropic dependence of nanostructure size of Ge is followed by a different carrier density as well as its relaxation process, depending on the orientation of the Ge crystal axis only at near and above threshold fluence.