Optical orientation of electron spins in GaAs quantum wells

TL;DR

This paper investigates how electron spins in GaAs quantum wells can be optically oriented and how their polarization depends on well width and excitation energy, combining experimental measurements with theoretical modeling.

Contribution

It provides a comprehensive experimental and theoretical analysis of electron spin polarization in GaAs quantum wells, highlighting the preservation of spin orientation during relaxation.

Findings

In wide wells, measured polarization closely matches theoretical predictions.

Electron spin orientation is preserved during relaxation in wide wells.

In narrow wells, initial spin polarization is reduced by a constant factor.

Abstract

We present a detailed experimental and theoretical analysis of the optical orientation of electron spins in GaAs/AlAs quantum wells. Using time and polarization resolved photoluminescence excitation spectroscopy, the initial degree of electron spin polarization is measured as a function of excitation energy for a sequence of quantum wells with well widths between 63 Ang and 198 Ang. The experimental results are compared with an accurate theory of excitonic absorption taking fully into account electron-hole Coulomb correlations and heavy-hole light-hole coupling. We find in wide quantum wells that the measured initial degree of polarization of the luminescence follows closely the spin polarization of the optically excited electrons calculated as a function of energy. This implies that the orientation of the electron spins is essentially preserved when the electrons relax from the optically excited high-energy states to quasi-thermal equilibrium of their momenta. Due to initial spin relaxation, the measured polarization in narrow quantum wells is reduced by a constant factor that does not depend on the excitation energy.