Strain-induced anticrossing of bright exciton levels in single self-assembled GaAs/Al${}_x$Ga${}_{1-x}$As and In${}_x$Ga${}_{1-x}$As/GaAs quantum dots

We study the effect of elastic anisotropic biaxial strain, induced by a piezoelectric actuator, on the light emitted by neutral excitons confined in different kinds of epitaxial quantum dots. We find that the light polarization rotates by up to $~$80${}^{°}$ and the fine structure splitting (FSS) varies nonmonotonically by several tens of $\mu$eV as the strain is varied. These findings provide the experimental proof of a recently predicted strain-induced anticrossing of the bright states of neutral excitons in quantum dots. Calculations on model dots qualitatively reproduce the observations and suggest that the minimum reachable FSS critically depends on the orientation of the strain axis relative to the dot elongation.

Figure 1

Behavior of a neutral exciton confined in a GaAs/AlGaAs QWPF under anisotropic biaxial stress. (a) Color-coded PL intensity vs polarization angle and energy for different values of electric fields applied to the piezoelectric actuator [the field values for the panels 1–4 are indicated in (f)]. The dashed lines are guides for the eye showing the rotation of the polarization direction. The $x$ direction in (b) corresponds to polarization angles of 0${}^{°}$, 180${}^{°}$, 360${}^{°}$, 540${}^{°}$, and 720${}^{°}$ and coincides with the polarization direction of the high-energy component at $F=-20$ kV/cm (panel 4). (b) Sketch of the device consisting of a membrane (sample) glued on a side of a PMN-PT crystal. (c),(d),(e) Polarization dependence in polar coordinates of the relative emission energy for panels 1, 2, and 4, respectively, of (a). The mean emission energy $E(F)$ for each value of $F$ is subtracted. (f) Polarization angle of the high-energy component of $X$ with respect to the direction of the electric field vs $E(F)$. The dots marked by red circles correspond to the data shown in (a). (g) FSS vs average emission energy.

Figure 2

(a),(b) Polarization and FSS behavior of a GaAs/AlGaAs QD and (c),(d) of an InGaAs/GaAs QD. The polarization angle $\varphi$ is defined as the angle of the higher energetic emission line with respect to the $x$ direction in Fig. 1b. For both QDs shown here the $x$ direction roughly corresponds to the [1$\stackrel{̲}{1}$0] direction of the GaAs membrane.

Figure 3

Theoretical study of the influence of anisotropic biaxial strain on the light emitted by a excitons in model GaAs/AlGaAs QDs. The left inset in (a) shows the shape of the artificial structure. (a) Polarization of the high-energy excitonic component with respect to the $x$ direction ([1$\stackrel{̲}{1}$0]) in Fig. 2b for different values of $\alpha$ (i.e., angle of the elongation direction with respect to the [110] crystal direction; see right inset). (b) Corresponding values of the FSS, the left (right) inset displays the direction of the applied stress for compressive (tensile) strains $\varepsilon$. (c)–(e) Density map of the ground-state hole wave function for a QD with $\alpha =0$ for different strain values. The white ellipse indicates the shape of the QD. (f) Orientation of the hole wave function with respect to the [110] direction vs strain. See text for more details.