RAPID COMMUNICATIONS PHYSICAL REVIEW B 83, 121302(R) (2011) 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 J. D. Plumhof, 1,* V. Kˇ r´ apek, 2 F. Ding, 1,3 K. D. J ¨ ons, 4 R. Hafenbrak, 4 P. Klenovsk´ y, 2 A. Herklotz, 5 K. D¨ orr, 5 P. Michler, 4 A. Rastelli, 1,† and O. G. Schmidt 1 1 Institute for Integrative Nanosciences, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany 2 Institute of Condensed Matter Physics, Masaryk University, Kotl´ a ˇ r sk´ a 2, CZ-61137 Brno, Czech Republic 3 Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China 4 Institut f ¨ ur Halbleiteroptik und Funktionelle Grenzfl¨ achen, University of Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany 5 Institute for Metallic Materials, IFW Dresden, Helmholtzstrasse 20, D-01069 Dresden, Germany (Received 7 February 2011; published 9 March 2011) 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 μeV 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. DOI: 10.1103/PhysRevB.83.121302 PACS number(s): 78.67.Hc, 78.20.hb, 81.07.St, 81.07.Ta Semiconductor quantum dots (QDs) obtained by epitaxial growth are receiving much attention because of their potential use as building blocks for quantum information processing and communication devices. 1–7 QDs confine the motion of charge carriers in three dimensions and are thus referred to as artificial atoms. Similar to real atoms, external electric and magnetic fields can be used to manipulate the properties of bound states in QDs. 8–13 In addition, the solid-state character of QDs allows for engineering methods which are not available for atoms. Dynamic stress fields 14–17 represent an example, whose wide potential is only recently being recognized. 18,19 The emission of neutral excitons confined in QDs with symmetry lower than D 2d is typically split by sev- eral tens of μeV because of the anisotropic electron-hole exchange interaction. 9,20,21 This broken degeneracy of the bright excitonic states, referred to as fine structure splitting (FSS), prevents the use of QDs as sources of entangled photon pairs on demand. 4–7,22 External electric or magnetic fields have been applied to restore the QD symmetry and achieve FSS values comparable to the radiative linewidth. 10,11,13,14 Seidl et al. 15 showed that also uniaxial strain can in principle be used to reduce the excitonic FSS. Due to the limited tuning range available, it has, however, remained unclear whether strain is suitable to reach sufficiently low values of FSS 5 and what the mechanisms behind the observed FSS changes are. Based on atomistic model simulations for InGaAs/GaAs QDs, Singh and Bester 19 predicted that uniaxial stress generally leads to an anticrossing of the bright excitonic states. Thereby, the magnitude and phase of the mixing of the bright excitonic states are modified, which results in a change of the FSS and in a rotation of the linear polarization of the emitted photons. 18 Such an anticrossing behavior has been recently observed for QDs under strong vertical electric fields. 13 Here we present the first experimental proof of the pre- dictions in Ref. 18 for three different kinds of QDs under anisotropic biaxial stress. A continuum model based on eight- band k · p and configuration interaction theory qualitatively reproduces the observations, highlights their physical origin, and shows how the minimum reachable FSS depends on the angle between strain axis and dot orientation. The measurements are performed on two different samples grown by solid-source molecular beam epitaxy (MBE). The active structures consist of QDs embedded in thin mem- branes, which are released from the underlying substrate and transferred onto a piezoelectric actuator. The first membrane sample, with total thickness of about 150 nm, contains GaAs/AlGaAs QDs 23 and quantum well (QW) potential fluctuations (QWPFs). 20,23 The latter, which are produced by local thickness or alloy fluctuations in a narrow QW, act as QDs with low confinement potential. The second sample contains standard InGaAs/GaAs QDs embedded in 200-nm-thick membranes. 24 The external stress is applied using a piezoelectric [Pb(Mg 1/3 Nb 2/3 )O 3 ] 0.72 − [PbTiO 3 ] 0.28 (PMN-PT) crystal. By applying a voltage V between the front and the back surface of the crystal [i.e., along the x axis in Fig. 1(b)] the side faces, such as the top x -y plane, expand (or contract) parallel to the direction of the electric field F , for positive (negative) applied voltage. Simultaneously, the side faces contract (or expand) perpendicular to the electric field [i.e., along the y axis in Fig. 1(b)]. We denote the strain parallel to the x and y axes as ε and ε ⊥ , respectively. The relation between these strain components is ε ⊥ ≈−0.7 × ε (see Ref. 25). By placing membranes with QDs on the side faces of the PMN-PT we can thus apply strongly anisotropic biaxial stress on the QDs. According to previous results, 16,26 we expect values of ε of the order of a few ‰ in the explored range of the electric field F . Photoluminescence (PL) spectroscopy measurements are performed at a temperature of 8 K in a standard micro-PL setup with a spectral resolution of about 70 μeV. The linear polariza- tion of the luminescence is analyzed by combining a rotatable achromatic half-wave plate and a fixed linear polarizer. 24 Figure 1(a) shows a color-coded PL-intensity map for a neutral exciton (X) confined in a GaAs/AlGaAs QWPF as a function of the emission energy and polarization angle for different values of the electric field F applied to the PMN-PT 121302-1 1098-0121/2011/83(12)/121302(4) ©2011 American Physical Society