Photon energy dependence of Kerr rotation in GeTe/Sb₂Te₃ chalcogenide superlattices

We report on pump-probe based helicity dependent time-resolved Kerr measurements under infrared excitation of chalcogenide superlattices, consisting of alternately stacked GeTe and Sb₂Te₃ layers. The Kerr rotation signal consists of the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found to monotonically increase with decreasing photon energy over a sub-eV energy range. Although the dependence of the SIFE can be attributed to the response function of direct third-order nonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear susceptibility resulting from electronic transitions between bulk valence/conduction bands and interface-originating Dirac states of the superlattice.