Spin dynamics in relativistic light-matter interaction

TL;DR

This paper reviews relativistic spin operators, identifies the Foldy-Wouthuysen operator as the most suitable, and applies it to study electron-spin precession in high-intensity light fields, emphasizing the importance of relativistic effects.

Contribution

It unifies various proposals for relativistic spin operators and demonstrates the experimental relevance of the Foldy-Wouthuysen operator in high-intensity light-matter interactions.

Findings

Only the Foldy-Wouthuysen and Pryce operators qualify as proper relativistic spin operators.

Ground states of highly charged ions can help experimentally identify the correct spin operator.

Relativistic effects are essential for accurate modeling of electron-spin precession in intense light fields.

Abstract

Various spin effects are expected to become observable in light-matter interaction at relativistic intensities. Relativistic quantum mechanics equipped with a suitable relativistic spin operator forms the theoretical foundation for describing these effects. Various proposals for relativistic spin operators have been offered by different authors, which are presented in a unified way. As a result of the operators' mathematical properties only the Foldy-Wouthuysen operator and the Pryce operator qualify as possible proper relativistic spin operators. The ground states of highly charged hydrogen-like ions can be utilized to identify a legitimate relativistic spin operator experimentally. Subsequently, the Foldy-Wothuysen spin operator is employed to study electron-spin precession in high-intensity standing light waves with elliptical polarization. For a correct theoretical description of the predicted electron-spin precession relativistic effects due to the spin angular momentum of the electromagnetic wave has to be taken into account even in the limit of low intensities.