Polarization memory in single quantum dots

Abstract

We measured the polarization memory of excitonic and biexcitonic optical transitions from single quantum dots at either positive, negative or neutral charge states. Positive, negative and no circular or linear polarization memory was observed for various spectral lines, under the same quasiresonant excitation below the wetting layer band-gap. We developed a model which explains both qualitatively and quantitatively the experimentally measured polarization spectrum for all these optical transitions. We consider quite generally the loss of spin orientation of the photogenerated electron-hole pair during their relaxation towards the many-carrier ground states. Our analysis unambiguously demonstrates that while electrons maintain their initial spin polarization to a large degree, holes completely dephase.

Spin polarization of the neutral exciton in a single InAs quantum dot at zero magnetic field

“…Up to now, a very low c were recorded at B ext = 0 for the neutral exciton in experiments with QD ensembles 6,9 or individual QDs. 3,8,[10][11][12] In the present report, a high degree ͑ c Ϸ 60%͒ of spin polarization is achieved for the neutral exciton at B ext =0 in individual InAs QDs, studied by micro-photoluminescence ͑ -PL͒. The anomalously high c is explained by a separate capture of e's and h's into the QD.…”

Radiative cascades from charged semiconductor quantum dots

“…Since spin blockading prevents the relaxation of the second level hole to its first level, they provide an estimate for the rate by which the hole spin's scatters. 30 In Figs. 3͑a͒ and 3͑c͒ we probe possible transitions from the S ‫ء‬ singlet state to the triplet T 0 and T 3 intermediate states, respectively.…”

“…The spectral lines observed were identified by various means such as excitation intensity-dependent PL, polarization memory measurements, and detailed comparison with a many carriers theoretical model. 29,30 The various lines are conventionally marked on Fig. 1͑b͒.…”

“…This means that the holes' spin projection on the QD's growth axis is conserved during the relaxation while their in-plane spin projection scatters. 30 The difference between the scattering rates from the singlet to triplet ͑S ‫ء‬ → T 0 ͒ and that from the triplet to singlet ͑T 0 → S ‫ء‬ ͒ is due to the energy difference between these two states ͑ϳ4 meV͒, which is larger than the ambient thermal energy ͑ϳ0.5 meV͒.…”

“…30 The first transition requires complete reversal of the p-shell hole's spin, from parallel to antiparallel to the spin state of the s-shell hole. The second transition involves transverse spin scattering, resulting in relative phase reversal between the spins of the two holes.…”