Nuclear spin cooling by helicity-alternated optical pumping at weak magnetic fields in $n$-GaAs

TL;DR

This paper investigates nuclear spin cooling in n-GaAs using helicity-alternated optical pumping at weak magnetic fields, revealing the dynamics of nuclear polarization and its dependence on modulation frequency.

Contribution

It introduces a model explaining nuclear spin polarization behavior under modulated optical pumping and estimates nuclear spin system equilibration time in n-GaAs.

Findings

Nuclear spin polarization results from competition between cooling and warm-up processes.

The model explains the dependence of polarization on modulation frequency.

Estimated nuclear spin system equilibration time is shorter than T2 relaxation time.

Abstract

The spin dynamics of localized donor-bound electrons interacting with the nuclear spin ensemble in $n$-doped GaAs epilayers is studied using nuclear spin polarization by light with modulated circular polarization. We show that the observed build-up of the nuclear spin polarization is a result of competition between nuclear spin cooling and nuclear spin warm-up in the oscillating Knight field. The developed model allows us to explain the dependence of nuclear spin polarization on the modulation frequency and to estimate the equilibration time of the nuclear spin system that appears to be shorter than the transverse relaxation time $T_2$ determined from nuclear magnetic resonance.