Measurements of linear and circular birefringence in metals by femtosecond optical pump-probe spectroscopy

Optically induced transient linear and circular birefringence has been studied in three different materials: ferromagnetic Ni, semiconducting GaAs and the non-magnetic metal Al. A pump-probe experiment with sub-ps resolution was set up for this purpose. The time-resolved reflectivity, rotation and ellipticity of the reflected probe beam were recorded after pumping with light of variable helicity. In the Ni sample an ultrafast demagnetisation effect was observed and the variation of the rotation and ellipticity on sub-picosecond time scales was compared. Rotation and ellipticity were found to have a similar time dependence. In GaAs, optical orientation of spin was achieved and the subsequent spin relaxation was measured for different pump powers. Fitting of the optical rotation data has revealed the power dependence of the various decay constants. In the Al sample a small signal was observed that decays on ps time scales. This was attributed to a linear birefringence effect resulting from a cubic optical nonlinearity. In all three samples the dependence of the birefringence upon the pump polarisation was studied. A sharp peak was observed at zero delay with a width .that was determined by the laser pulse width. The peak observed in GaAs showed a behaviour characteristic of the specular inverse Faraday effect (SIFE, a circular birefringence), while that of Ni and Al is characteristic of a combination of the SIFE and the specular optical Kerr effect (SOKE, a linear birefringence). The Al measurements also showed a tail, that decays within a few ps, and that is characteristic of the SOKE. It is suggested that a lattice excitation provides the most likely mechanism for the long-lived linear birefringence. No clear evidence was found for optical orientation of spin in Al although a small but poorly resolved tail in the ellipticity data gives an indication of the existence of such an effect. (author)SIGLEAvailable from British Library Document Supply Centre- DSC:DXN063958 / BLDSC - British Library Document Supply CentreGBUnited Kingdo