Optical orientation of excitons in a longitudinal magnetic field in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

TL;DR

This study investigates exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots with indirect band gap, revealing how magnetic fields and hyperfine interactions influence exciton behavior and enabling a quantitative understanding through a kinetic model.

Contribution

It introduces a kinetic equation model that quantitatively describes exciton population and spin dynamics in these quantum dots under magnetic fields.

Findings

Negligible anisotropic exchange interaction prevents excitonic state mixing.

Dark exciton blockade occurs in magnetic field, reducing nuclear spin fluctuation effects.

The model accurately fits experimental data on exciton recombination and spin dynamics.

Abstract

The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots (QDs) with indirect band gap and type-I band alignment are studied. The negligible (less than $0.2~\mu$eV) value of the anisotropic exchange interaction in these QDs prevents a mixing of the excitonic basis states with pure spin and allows for the formation of spin polarized bright excitons for quasi-resonant circularly polarized excitation. In a longitudinal magnetic field, the recombination and spin dynamics of the excitons are controlled by the hyperfine interaction between the electron and nuclear spins. A QD blockade by dark excitons is observed in magnetic field eliminating the impact of the nuclear spin fluctuations. A kinetic equation model, which accounts for the population dynamics of the bright and dark exciton states as well as for the spin dynamics, has been developed, which allows for a quantitative description of the experimental data.