Transient optical polarization response of aluminium at an interband transition

Abstract

Femtosecond optical pump–probe measurements have been performed upon a500 Å polycrystalline Al film at an interband transition at a wavelength of 800 nm.The dependence of the intensity and polarization of the reflected probe pulseupon the polarization of the pump pulse has been determined. A sharp peak wasobserved in the polarization signal at zero time delay that contains contributionsfrom both the specular inverse Faraday effect (SIFE) and the specular opticalKerr effect (SOKE). The SIFE and SOKE are associated with the opticalorientation of angular and linear momentum, respectively. The dependence ofthe peak height upon the pump polarization has allowed us to deduce values ofχxxyy = (1+0.6i) × 10−8 rad cm3 erg−1 and χxyyx = (0.5−0.1i) × 10−8 rad cm3 erg−1 for the non-vanishing components of the local cubic susceptibility tensor. A distortion ofthe peak was observed when the pump was elliptically polarized. Its detailed shape showsthat the characteristic relaxation time for the SOKE is about 30 fs longer than that for theSIFE in measurements of ellipticity. This suggests a similar difference in the relaxationtimes for the angular and linear momentum of the optically excited hot electrons. Theinitial peak was generally followed by a tail that decayed with a time constant of severalpicoseconds, which we attribute to an ultrafast anisotropic excitation of the lattice.