Exciton spin relaxation in strongly confining semiconductor quantum dots

Abstract

We study the phonon-induced flip of the exciton spin in single strongly confining quantum dots. The considered two-phonon process contributes to the exciton spin relaxation ͑longitudinal relaxation time T 1 ͒ within the radiative doublet of the exciton ground state. The respective effective matrix element is driven by an interplay of the short-range exchange interaction and the lattice deformation induced by acoustic phonons. The two-phonon process involves the participation of the dipole-forbidden dark states. The here considered relaxation channel is of paramount importance for symmetrical dots and may be several orders of magnitude faster than the previously studied transition between bright states in asymmetrical quantum dots. The calculated relaxation rates depend on the dot composition and shape, and decrease very strongly upon reduction in the dot size. For various individual dots belonging to a large quantum-dot ensemble the respective relaxation times may differ by a several orders of magnitude. For a typical ensemble of InAs/GaAs self-organized quantum dots at low temperatures, the numerical estimates range from few hundreds of microseconds for the largest dots to few tens of nanoseconds for the smallest dots involved.

Complete quantum control of exciton qubits bound to isoelectronic centres

“…The population created in |YS is partially transferred at a rate g yx to the cross-polarized state |XS from which the PL intensity is measured using a fast-gated singlephoton avalanche diode (SPAD) 24 . Hyperfine interaction is believed to be the dominant transfer mechanism between |YS and |XS as spin relaxation due to spin-orbit interaction or the Bir-Pikus hamiltonian are quenched for strongly confined excitons 25,26 . This mechanism does not couple |YS and |XS directly and g yx will thus depend on all possible transfer rates between bright excitons, |XS and |YS, and dark excitons, |ZS and |DS (see Supplementary Note 2).…”

Mapping the Local Density of Optical States of a Photonic Crystal with Single Quantum Dots

“…The lattice parameter (a) ranges from 200 to 385 nm in steps of 5 nm, and the r/a ratio is fixed at 0.30, where r is the radius of the air hole. The QDs are excited with a PicoQuant PDL-800 pulsed diode laser at 781 nm with varying repetition rate (5,10,20,40 MHz). Under a weak excitation condition, we identify neutral excitons by their excitation power and polarization dependence, i.e., multi-exciton complexes are excluded due to the observed linear power dependence before saturation, while single-charge excitons are found to be mono-exponential and have a very weak polarization dependence.…”

“…We note that spin-flip processes coupling bright excitons, i.e. |X b ⇆ |Y b are slow compared to the other decay processes and therefore can be abandoned in the analysis, as theoretically predicted [19,20] and experimentally confirmed from the large anisotropy in the decay rate for X and Y states observed in a PC [21]. Consequently, the five-level scheme can be simplified to the three-level system indicated in Fig.…”

Spin–orbit-induced hole spin relaxation in InAs and GaAs quantum dots

“…The above paradox has prompted a number of theoretical works trying to understand which factors determine the relaxation dynamics of single holes under magnetic fields [29][30][31][32][33] and that of holes forming excitons [15,22,34,35] in quasi-2D InAs/GaAs QDs. For the relaxation to take place one needs a source of energy relaxation, which in these systems is provided by the acoustic phonon bath [23,24], plus a source of spin admixture.…”