Femtosecond electron and spin dynamics probed by nonlinear optics

TL;DR

This paper presents a theoretical study of femtosecond-scale electron and spin dynamics in nickel, using nonlinear optical responses like SHG and NOLIMOKE to reveal ultrafast charge and spin behavior and their dependence on material parameters.

Contribution

It introduces a theoretical framework for observing ultrafast spin and charge dynamics via nonlinear optics, highlighting the effects of exchange interaction and interface properties.

Findings

Charge dynamics occur faster than spin dynamics, indicating a spin memory time.

Increasing exchange interaction accelerates spin dynamics.

Spectrally broad excitation sets the intrinsic speed limit of dynamics.

Abstract

A theoretical calculation is performed for the ultrafast spin dynamics in nickel using an exact diagonalization method. The present theory mainly focuses on a situation where the intrinsic charge and spin dynamics is probed by the nonlinear (magneto-)optical responses on the femtosecond time scale, i.e. optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are observable on the time scale of 10 fs. The charge dynamics proceeds ahead of the spin dynamics, which indicates the existence of a spin memory time. The fast decay results from the loss of coherence in the initial excited state. Both the material specific and experimental parameters affect the dynamics. We find that the increase of exchange interaction mainly accelerates the spin dynamics rather than the charge dynamics. A reduction of the hopping integrals, such as present at interfaces, slows down the spin dynamics significantly. Besides, it is found that a spectrally broad excitation yields the intrinsic speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower width prolongs the dynamics. This magnetic interface dynamics then should become accessible to state of art time resolved nonlinear-optical experiments.