Optical pumping and negative luminescence polarization in charged GaAs quantum dots

Optical pumping of electron spins and negative photoluminescence polarization are observed when interface quantum dots in a GaAs quantum well are excited nonresonantly by circularly polarized light. Both observations can be explained by the formation of long-lived dark excitons through hole spin relaxation in the GaAs quantum well prior to exciton capture. In this model, optical pumping of resident electron spins is caused by capture of dark excitons and recombination in charged quantum dots. Negative polarization results from accumulation of dark excitons in the quantum well and is enhanced by optical pumping. The dark exciton model describes the experimental results very well, including intensity and bias dependence of the photoluminescence polarization and the Hanle effect.

Figure 1

(Color online) (a) PL intensity of an individual QD (grayscale) for neutral exciton $\left(X^0\right)$, negative trion $\left(X^{-}\right)$, and positive trion $\left(X^{+}\right)$ as a function of emitted photon energy and applied bias (Ref. 16). The sample was excited by light with energy 1.686 eV. (b) Photoluminescence polarization measured for the excitons shown in (a).

Figure 2

Photoluminescence polarization of the negative trion $\left(X^{-}\right)$ vs applied bias for three values of the incident laser power.

Figure 3

(a) The Hanle effect measured for the negative trion $X^{-}$ with 3.9 V applied bias. The peak width is 35 mT corresponding to an electron spin lifetime of 16 ns. (b) Dependence of PL polarization on the laser power $B_x=0$ and 0.5 T with an applied bias of 4.5 V. Symbols are the experimental data. Solid and dashed lines are the results of kinetic modeling using parameters ${\tau }_b=100\text{ps}$, $W{\tau }_b=0.25$, ${\tau }_b/{\tau }_T=0.2$, ${\tau }_b/{\tau }_H=0.05$, ${\tau }_b/{\tau }_S=0.03$, and $G_d^{\Downarrow }/G_b^{\Uparrow }=0.08$.

Figure 4

(Color online) Pictorial summary of relaxation processes that influence polarization of negative trions, as considered in the kinetic model. Upper panel: Bright excitons are excited in the quantum well continuum, and hole polarization is lost. A mixture of bright and dark “cold” excitons is created at the QW band edge. At the band edge, bright excitons can decay radiatively or be captured in the QDs, but dark excitons can only be captured. Lower panel: excitons are captured by the QDs with resident electron spin antiparallel to the exciton electron spin. The trions created from bright excitons emits ${\sigma }^{+}$ polarized light, while trions created from dark excitons are ${\sigma }^{-}$ polarized. The dark exciton pathway optically pumps the QD resident electron spin because it replaces the original resident electron (spin up) with the photoelectron (spin down).

Figure 5

Negative trion PL polarization degree as a function of average QD electron occupation calculated for three pumping powers. The parameters used in the kinetic model are ${\tau }_b=100\text{ps}$, $W{\tau }_b=0.27$, ${\tau }_b/{\tau }_T=1$, ${\tau }_b/{\tau }_S=0.035$, and ${\tau }_b/{\tau }_H=0.05$. The horizontal axis in this figure scales monotonically with the applied bias in Fig. 2 although the quantitative relation between the two is unknown.

Figure 6

(Color online) (a) Calculated trion PL polarization vs laser intensity for $B_x=0$ (solid lines) and 0.5 T (dashed lines) for two values of the ratio of generation rates $G_d^{\Downarrow }/G_b^{\Uparrow }$ for dark and bright excitons. The ratio $G_d^{\Downarrow }/G_b^{\Uparrow }$ is changed experimentally by changing the laser excitation energy (see Ref. 6). (b) Electron spin polarization in zero magnetic field vs pumping power. Other parameters were ${\tau }_b=100\text{ps}$, $W{\tau }_b=0.27$, ${\tau }_b/{\tau }_T=1$, ${\tau }_b/{\tau }_S=0.035$, and ${\tau }_b/{\tau }_H=0.05$.

Figure 7

(Color online) Hanle peak depth vs electron spin polarization obtained parametrically from the calculations shown in Fig. 6. The Hanle peak depth corresponds to the difference between the $X^{-}$ polarization values for $B_x=0$ and 0.5 T in Fig. 6a.