Exciton spin relaxation in quasiresonantly excited $\mathrm{Cd}\mathrm{Te}∕\mathrm{Zn}\mathrm{Te}$ self-assembled quantum dots

We study exciton spin-relaxation in CdTe self-assembled quantum dots (QDs) by using polarized photoluminescence (PL) spectroscopy in magnetic field. Experiments on single CdTe QDs and on large QD ensembles show that by combining LO phonon-assisted absorption with circularly polarized excitation, spin-polarized excitons are photoexcited directly into the ground states of QDs. We find that for single symmetric QDs at $B=0\mathrm{T}$, where the exciton levels are degenerate, the spins randomize very rapidly, so that no net spin polarization is observed. In contrast, when this degeneracy is lifted by only a fraction of a Kelvin through the application of small magnetic fields, optically created spin-polarized excitons maintain their polarization on a time scale much longer than the exciton recombination time. Similar behaviors are found in a large ensemble of CdTe QDs. Finally, due to strong electronic confinement in CdTe QDs, the large spin polarization of excitons shows no dependence on the number of LO phonons emitted between the virtual state and the ground state of the exciton.

Figure 1

Low temperature $(T=6\mathrm{K})$ PLE spectrum of a single CdTe quantum dot. Broad resonances are attributed to the LO phonon-assisted absorption whereas sharp intense lines correspond to direct excited state–ground state excitations.

Figure 2

Low temperature PL spectra measured of single CdTe quantum dots obtained with linearly polarized detection. The excitation was nonresonant, above the ZnTe barrier. The behavior characteristic for (a) asymmetric and (b) symmetric quantum dot is shown. Schematic energy diagrams of exciton levels in QDs are displayed as insets. (c) Statistical distribution of the exchange splitting obtained for approximately 100 single quantum dots.

Figure 3

Quasiresonantly excited macro-PL spectra of CdTe QDs obtained for linearly polarized excitation and detection at $B=0\mathrm{T}$: (a) ${\pi }_X$-polarized excitation, (b) ${\pi }_Y$-polarized excitation.

Figure 4

Quasiresonantly excited macro-PL spectra of the CdTe QD ensemble measured for all four circular polarization configurations at $B=0\mathrm{T}$.

Figure 5

(a) Nano-PL of CdTe QDs measured at $B=0\mathrm{T}$. The shaded areas show emission lines from QDs for which magnetic field dependencies are presented later. (b) Polarized spectra at zero magnetic field for the $2.064\mathrm{eV}$ QD emission line which was excited through 2-LO phonon-assisted absorption. The spectra measured for all four circular polarization configurations are presented.

Figure 6

Quasiresonantly excited macro-PL spectra of the CdTe QD ensemble measured for all four circular polarization configurations obtained at $B=4\mathrm{T}$.

Figure 7

Nano-PL spectra at $B=2.5\mathrm{T}$ obtained with circularly polarized excitation and detection for three single symmetric QDs marked in Fig. 4(a). These dots are populated through LO phonon-assisted absorption with the emission of (a) one-, (b) two-, and (c) three-LO phonons. Solid lines show the expected intensity of the single dot emission if with each next LO phonon replica the 10% depolarization of the emission would take place.

Figure 8

Comparison between measured quasiresonantly excited PL spectrum of CdTe QDs ensemble (squares) with the fitted one obtained by assuming that the emission consists of QD resonantly excited through LO phonon-assisted absorption (solid lines) and broad recombination associated with excited state–ground state relaxation (dashed line).

Figure 9

Polarization, $P$, of the CdTe QDs emission plotted as a function of magnetic field. The data obtained from macro-PL and nano-PL (stars) is shown. For the former the dependencies measured for the first (squares), second (circles), and third (diamonds) LO phonon replicas are presented. Values obtained for ${\sigma }^{+}\left({\sigma }^{-}\right)$-polarized excitation are represented by solid (open) symbols.