Cascade spin dynamics of excitons localized in indirect-band-gap (In,Al)As/AlAs quantum dots with type-I band alignment
We investigate the spin dynamics of excitons localized in type I (In,Al)As/AlAs quantum dots with an indirect in momentum space band structure. Polarized selective photoluminescence spectroscopy, i.e. fluorescence line narrowing, under magnetic fields up to 5 T applied in the Faraday geometry is employed. The experiment reveals a cascade spin evolution process of excitons in the indirect band-gap quantum dots: an initial short term spin dynamics associated with excited direct exciton states possessing a large oscillator strength is followed by electron relaxation into the X valley of the Brillouin zone and subsequent long term spin dynamics of indirect excitons. The two step mechanism manifests itself in the distinct features of the magnetic field dependences of photoluminescence: two component recovery of optical orientation, two component linear to circular polarization conversion and the presence of the linear polarization plane rotation. At the same time, suppression of the optical alignment shows one-component behavior governed by the spin dynamics of the indirect exciton states. Within the pseudospin formalism, we derive analytical expressions that quantitatively describe the observed dependences and yield estimates for the anisotropic exchange splitting: 210 {\mu}eV for direct excitons and 1.3 {\mu}eV for indirect excitons. Further analysis using the density matrix formalism agrees well with the pseudospin model calculations and shows that the finite optical orientation at zero magnetic field is due to comparable magnitudes of the anisotropic splitting of the indirect exciton states and the splitting of the X-valley electron states caused by the hyperfine interaction with nuclei.