Universal Recovery of the Energy-Level Degeneracy of Bright Excitons in InGaAs Quantum Dots without a Structure Symmetry

Abstract

The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.

Statistical properties of exciton fine structure splitting and polarization angles in quantum dot ensembles

“…Introduction.-The fine structure splitting (FSS) of excitons in self-assembled quantum dots (QDs) poses the major obstacle to the realization of entangled photon pairs from biexciton cascade process, thus has been a subject of extensive investigation in the past decade [1][2][3][4][5][6][7][8][9]. Now it is quite clear that the FSS arises from the intrinsic nonequivalence along [110] and [110] directions in zinc-blende crystals, which reduce the symmetry of the underlying lattice from T d to C 2v for pure circular lens-shaped QDs, and the other nonuniform effects such as local strain, shape irregularities, alloys and interface effects [10,11], which further reduce the symmetry to C 1 for alloyed QDs [12].…”

“…A single external field, such as electric field [13][14][15][16][17][18], magnetic field [2,19], or anisotropic stress [12,[20][21][22][23][24], is insufficient to eliminate the FSS because the lower bound of FSS is generally much larger than the homogeneous broadening of the emission line (∼ 1 µeV). To eliminate the FSS, two non-equivalent fields have to be combined [8,9]. Generally, the FSSs depend strongly on the local details of QDs, thus is hard to be predicted in theories.…”

Excitonic fine structure splitting in type-II quantum dots

“…The external strain field allowed to reach FSS below experimental resolution in GaAs/AlGaAs QDs; 14,15 the simultaneous application of electric field allowed for a more powerful symmetry restoration and rather universal recovery of low FSS. 16 Various effects contributing to the FSS can be divided into two classes based on the involved length scale: atomic and macroscopic. Atomic-scale effects are connected with the irregularities of the crystal lattice such as the interfaces, particular elements distribution in alloys, 17 charged defects, 18 etc.…”

The role of phonons for exciton and biexciton generation in an optically driven quantum dot

“…Apart from selecting QDs that happen to exhibit almost zero splitting [16], also specially designed growth [37] or annealing [38] techniques have been developed for that purpose. Alternatively, one can use dots with high symmetry such as, e.g., self-organized In(Ga)As/GaAs QDs grown on (111) substrate which should ideally have no exchange splitting [39], or apply strategies to actively suppress the splitting, such as, e.g., the ac-Stark tuning [40] or application of strain or external electric and/or magnetic fields [41,42,43,44,45,46]. Furthermore, all preparation schemes aim at being as fast as possible without degrading other properties such as the robustness or the spectral selectivity of the scheme.…”