2003 Investigation of transient linear and circular birefringence in metallic thin films

Optical pump–probe measurements have been performed on three materials with pump pulses of 120 fs duration and variable helicity. The samples were probed in a reflection geometry. We compare the transient intensity and rotation signals induced in intrinsic GaAs wafers and sputtered polycrystalline Ni and Al thin films. An initial peak is observed in the transient rotation signal. The dependence of the peak height on the polarization of the pump is found to be qualitatively different for the three materials, indicating different amounts of linear and circular birefringence in each case. For ferromagnetic Ni the peak is superimposed on a longer-lived ultra-fast demagnetization signal. For GaAs and Al we also observe a tail in the rotation signal that decays on time-scales of several picoseconds. In the case of GaAs, we attribute this to the relaxation of a spin polarization that is induced by the pump beam within the thermal electron population. In the case of Al, the dependence of the amplitude of the tail on the pump beam’s polarization is characteristic of linear birefringence. We suggest that this signal is associated with an ultra-fast excitation of the lattice rather than with optical orientation of spin. 1