Optical orientation and alignment of excitons in self-assembled $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ quantum dots: The role of excited states

We studied the optical orientation of excitons in an ensemble of self-organized $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ quantum dots (QDs) via photoluminescence (PL) under quasiresonant excitation. The observed PL spectra clearly show the high efficiency of phonon-assisted emission processes. The results demonstrate unambiguously that the QD symmetry is lower than $D_{2d}$, which results in the splitting of optically active exciton states into two linearly polarized dipoles. The corresponding QD axes have different directions, defined by shape and strain anisotropies. It is shown that the spin-relaxation time of excitons in the ground state noticeably exceeds the exciton lifetime. We have further observed that the degree of optical orientation is small at zero magnetic field, but dramatically increases as the field is increased. The experimental results are interpreted in terms of a cascade model, in which the exciton is initially created in the excited state (which is believed to be a coupled state with phonons) and emits from the ground state after energy relaxation. This model provides an explanation for the experimentally observed conversion of the linear polarization from one set of axes to another. It was shown that the excited states have a significant influence on the polarization of the ground-state emission, and thus it is possible to measure their short characteristic times. We were able to estimate the lifetime ${\tau }_2$ of the exciton in the excited state to be $\sim 1\mathrm{ps}$ and the value of the anisotropic exchange-splitting ${\mathrm{\Omega }}_{x,y}$ to be $\sim 0.5\mathrm{meV}$.

Figure 1

Total PL intensity (solid lines) and the degree of polarization of the PL emission (circles) observed for the $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ QDs. The exciting light was at a wavelength $\lambda =5320\mathrm{\AA }$ $\left(2.3305\mathrm{eV}\right)$. Circularly polarized PL observed under circularly polarized excitation (optical orientation) at $B=0$ (closed circles) and at $B=4\mathrm{T}$ (open circles) is shown in panel (a). Panels (b) and (c) show linearly polarized PL observed under linearly polarized excitation (optical alignment) for $e\Vert \left[110\right]$ and $e\Vert \left[100\right]$, respectively.

Figure 2

(a) Dependence of the degree of circular polarization of the PL emission on a magnetic field under circularly polarized excitation. Open circles correspond to data detected at $\lambda =5455\mathrm{\AA }$; full circles to detection at $\lambda =5600\mathrm{\AA }$. (b) The same as (a) but observed with unpolarized excitation, with detection at $\lambda =5600\mathrm{\AA }$.

Figure 3

Dependence of the degree of linear polarization of the PL emission on a magnetic field. Data for the $(x,y)$ frame (where the $x$ axis coincides with the direction defined by the unit vector $e$) are shown by open circles. Data for the $(x^{\prime },y^{\prime })$ reference frame [rotated with respect to the $(x,y)$ frame by 45° around the $z$ axis] are given by full circles. Panel (a) of the figure corresponds to excitation with $e\Vert \left[100\right]$; panel (b) corresponds to excitation with $e\Vert \left[110\right]$.

Figure 4

A schematic picture of cascade excitation of excitons through a coupled state with optical phonons. (a) Corresponds to the case without anisotropic exchange interaction, $B_{\mathit{ex}}=0$; (b) corresponds to $B_{\mathit{ex}}\ne 0$.

Figure 5

Schematic representation of exciton pseudospin precession in the cascade process for QDs of the ⟨100⟩ type $({\Omega }_y\ne 0)$. $S_0$ is the initial pseudospin created under [110] linearly polarized excitation, and $\Omega$ and $\omega$ are the Larmor precession frequencies at the first and second stage of the cascade process, respectively. During the first stage of the cascade, $S_0$ turns by a mean angle $\phi =\Omega \tau <2\pi$. During the second stage $(\omega {\tau }_1⪢1)$, the pseudospin lies along the direction of $\omega$, so it has projections $S_y$ and $S_z$. For QDs of the ⟨110⟩ type $({\Omega }_x\ne 0)$, the components $S_x$ and $S_z$ survive. Note that if $\Omega {\tau }_2⪢1$, the precession of ${\mathbf{S}}_{\mathbf{0}}$ around $\mathbf{\Omega }$ results in the destruction of the pseudospin, since ${\mathbf{S}}_{\mathbf{0}}\perp \mathbf{\Omega }$.