# Signatures of relativistic spin-light coupling in ultrafast magneto-optical pump-probe experiments

## TL;DR

This paper explains the polarization-dependent ultrafast magneto-optical Kerr response in pump-probe experiments through relativistic light-spin coupling, revealing how different configurations induce distinct opto-magnetic fields.

## Contribution

It introduces a relativistic Hamiltonian framework showing how light-spin coupling causes polarization-dependent effects in ultrafast magneto-optical measurements.

## Key findings

- Different light-induced opto-magnetic fields for parallel and perpendicular polarizations.
- Relativistic light-spin interaction explains observed ultrafast Kerr response differences.
- Predicts similar effects for circularly polarized pump and linearly polarized probe configurations.

## Abstract

Femtosecond magneto-optical pump-probe measurements of ultrafast demagnetization show an intriguing difference in the first 100 fs of the magneto-optical Kerr response depending on whether the polarization of the pump and probe beams are in parallel or perpendicular configuration [Bigot et al., Nature Phys. 5, 515 (2009)]. Starting from a most general relativistic Hamiltonian we focus on the ultra-relativistic light-spin interaction and show that this coupling term leads to different light-induced opto-magnetic fields when pump and probe polarization are parallel and perpendicular to each other, providing thus an explanation for the measurements. We also analyze other pump-probe configurations where the pump laser is circularly polarized and the employed probe contains only linearly polarized light and show that similar opto-magnetic effects can be anticipated.