# Optical orientation and alignment of excitons in direct and indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment

## TL;DR

This study investigates the spin structure and dynamics of excitons in (In,Al)As/AlAs quantum dots with type-I band alignment, revealing differences between direct and indirect band gap dots using optical orientation and alignment techniques.

## Contribution

It provides the first detailed comparison of exciton spin properties in direct and indirect (In,Al)As/AlAs quantum dots, highlighting the role of exchange interaction and hyperfine effects.

## Key findings

- Direct QDs exhibit polarization influenced by anisotropic exchange splitting.
- Indirect QDs show spin dynamics dominated by hyperfine interactions.
- Exchange splitting in direct QDs matches theoretical models.

## Abstract

The spin structure and spin dynamics of excitons in an ensemble of (In,Al)As/AlAs quantum dots (QDs) with type-I band alignment, containing both direct and indirect band gap dots, are studied. Time-resolved and spectral selective techniques are used to distinguish between the direct and indirect QDs. The exciton fine structure is studied by means of optical alignment and optical orientation techniques in magnetic fields applied in the Faraday or Voigt geometries. A drastic difference in emission polarization is found for the excitons in the direct QDs involving a $\Gamma$-valley electron and the excitons in the indirect QDs contributed by an $X$-valley electron. We show that in the direct QDs the exciton spin dynamics is controlled by the anisotropic exchange splitting, while in the indirect QDs it is determined by the hyperfine interaction with nuclear field fluctuations. The anisotropic exchange splitting is determined for the direct QD excitons and compared with model calculations.