Polarization dependence of optical excitations in anisotropic metal/insulator heterostructures

In the framework of real-time time-dependent density functional theory (RT-TDDFT) we unravel the layer-resolved dynamics of a Fe₁/(MgO)₃(001) multilayer after optical excitations. Short optical pulses with two polarization directions of light and a frequency smaller than the band gap of bulk MgO induce strongy anisotropic dynamic response: Substantial transient changes to the electronic structure, which persist after the duration of the pulse, are only observed for in-plane polarized electric fields. We find the largest effect in the Fe-layer, but time-dependent changes in the occupation numbers are visible in all layers, promoted by the presence of interface states. The time evolution of the layer-resolved occupation numbers indicates transfer from in-plane to out-of-plane orbitals that support the propagation of optically induced excitations into the interface region. We also see a small net charge transfer away from oxygen towards the Mg sites even for MgO layers, which are not directly in contact with the metallic Fe.

[1] M. E. Gruner and R. Pentcheva, Phys. Rev. B 99, 195104 (2019).