Exciton spin–flip processes in single quantum dots

Abstract

We study the relaxation of the exciton spin (longitudinal spin-relaxation time T 1 ) in single asymmetrical quantum dots (QD) at low temperatures. The main relaxation mechanism is due to the exciton spin-acoustic phonon coupling via the strain-dependent exchange interaction; piezoelectric coupling is less efficient. For zero magnetic field, relaxation within the radiative exciton-doublet for typical QDs is very slow compared to the exciton lifetime. Relaxation to nonradiative states becomes important for QDs with large singlet-triplet splitting of a few meVs. The calculated relaxation times strongly decrease in high magnetic fields.1 Introduction Most concepts for quantum information processing involving quantum dots (QDs) are based on optical generation, manipulation and read-out of spins. One has therefore to investigate the dynamics of the excitonic spin. Studies of the longitudinal spin-relaxation time T 1 are rare since they require strict resonant excitation conditions and/or high magnetic fields. Recent resonant excitation experiments on InAs QDs indicate no exciton-spin relaxation at low temperatures and magnetic fields up to 8 T [1]. Similar results are reported for CdSe QDs with large lateral dimensions (comparable to twice the exciton Bohr radius of $5 nm in bulk CdSe crystals) [2,3]. However, longitudinal spin-relaxation times comparable to the exciton lifetime were deduced from high magnetic field experiments for much smaller CdSe QDs [4].In a previous communication [5], we studied the exciton-spin relaxation in QDs within the radiative doublet of the ground (heavy-hole) exciton state at low temperatures. In this paper we carry on these studies and investigate additionally the spin relaxation to near by nonradiative states. We discuss explicitly quantum dots based on (In, Ga)As and (Cd, Zn)Se semiconductor compounds. Besides, we consider the contribution of piezoelectric coupling to the excitonic relaxation processes which is considered as an important mechanism for electron spin relaxation in QDs [6].

Exciton spin relaxation in strongly confining semiconductor quantum dots

“…Furthermore, an increase in the splitting between these states leads to an enhancement of the respective relaxation rate 1 / s ͑1͒ as compared to the zero-field case. 8 In contrast, the indirect process becomes practically forbidden at large fields, since now each of the dark states ͉͑+2͘ or ͉−2͒͘ is coupled by a phonon to only one of the bright states ͉͑+1͘ or ͉−1͘, respectively͒, as discussed above and demonstrated by Eq. ͑A9͒.…”

“…For quantum dots in a strong confinement regime, i.e., with a large splitting between the heavy-hole and light-hole exciton ͑labeled below by E lh ͒ at the scale of the finestructure energies, and for a model one-particle ͑lateral͒ wave function ⌿͑x , y͒ϳe −͑x 2 +y 2 ͒/2 2 ͑where is the localization length of the particle movement͒ the decay time id is given by 8,9,18,19 …”

One and Two Phonon Assisted Transitions between Exciton Spin States in a Quantum Dot

“…One is a transition to dark states due to carrier-phonon interactions and the spin-orbit coupling [4]. The other two include a transition between bright states [5] and a transition to dark states [6,7] and are caused by the interplay of the Bir-Pikus Hamiltonian and the short-range exchange interaction.…”

“…Apart from presenting a summary of the results concerning single-phonon assisted transitions [5][6][7], we concentrate on a new two-phonon assisted transition from bright to dark states. We compare the transition rates resulting from all three processes for typical GaAs/InAs self-assembled QD parameters.…”

Exciton spin decay in quantum dots: single and double phonon ssisted transitions

“…One is a transition to dark states due to carrier-phonon interactions and the spin-orbit coupling [4]. The other two, including a transition between bright states [5] and a transition to dark states [6,7], are caused by the interplay of the Bir-Pikus Hamiltonian and the short-range exchange interaction.…”