Geometrical impact on the optical polarization of droplet epitaxial quantum dots

Abstract

We report on experimental and theoretical investigations of the optical anisotropy of GaAs/AlGaAs quantum dots grown by droplet epitaxy. With in situ annealing in the growth, the shape of quantum dots is systematically controlled from a tall and laterally symmetric shape to a flat and laterally elongated one. Photoluminescence spectroscopy demonstrates an uncommon observation: the more elongated the quantum dots, the lower the degree of linear polarization. Theoretical analysis based on a four-band k • p theory reveals a substantial impact of vertical confinement on the valence heavy-hole and light-hole mixing, which leads to the enhancement of polarization anisotropy for taller quantum dots. The influence of Coulomb interactions on polarization anisotropy is studied by using the partial configuration interaction method, and is shown to reduce the polarization anisotropy through the mixing of single-particle configurations with different symmetries.

Supercoupling between heavy-hole and light-hole states in nanostructures

“…This formula is expected from the classical "folded down" perturbation approach [1][2][3][4][5][6][7][11][12][13][14]16 . On the other hand, all data points of strained In(Ga)As/GaAs QDs exhibit a denominator ∆ HL that is effectively reduced by δ = 78.6 meV with respect to the class of unstrained GaAs QDs and fall then close to another curve:…”

“…The perturbation potential δV C 2v lowers the QD symmetry to C 2v and hence introduces HH-LH mixing, which modifies the ground hole state HH0 [1][2][3][4][5][6][7][11][12][13][14]16,19,22 ,…”

“…Whereas in 0D QDs k i is replaced by the operator −i ∂ ∂r i (i = x, y, z) as a result of space confinement, the ground state has an effective finite wavevector which leads to finite R and S and thus δV HL = 0. Such 3D quantum confinement was previously considered as the only mechanism leading to HH-LH mixing in unstrained QDs 5,14,[20][21][22] . For flat GaAs QDs [where a z (height) ≪ L(wide)], 3D confinement within the Luttinger-Kohn Hamiltonian gives rise to λ LH ≃ 0.53a z /L 5 and λ 2 LH = 0.2%, a much smaller value than our determined 3.5%, for a disk-shaped dot with a z = 2 nm, L = 25 nm, as shown in Fig.…”

“…The most popular theoretical approach used in nanostructures is to fold the Luttinger-Kohn k · p or PikusBir strained Hamiltonian of bulk zinc-blende (ZB) semiconductors down to an effective 2 × 2 HH Hamiltonian and taking the admixture of neighboring bands such as LH band into account perturbatively [5][6][7][8][9]18 . In the early days of nanostructures research, the HH-LH mixing was depicted as a result of spatial quantum confinement 5,14,[20][21][22][23][24] , which leads to finite off-diagonal matrix elements within the Luttinger-Kohn k · p Hamiltonian. However, δV HL , and thus HH-LH mixing, was later recognized to be zero by the symmetry in symmetric self-assembled QDs which were assumed (incorrectly) to have the D 2d point group [11][12][13]19 .…”

“…(ii) providing an efficient channel for spin decoherence [5][6][7][8][9][10] , (iii) creating a polarization anisotropy of light emission which is important for quantum information schemes [11][12][13][14][15] , and (iv) giving an additional efficient mechanism for optical initialisation of hole spin qubit 16,17 . The origin of this mixing has fascinated physicists and chemists ever since low-dimensional semiconductor nanostructures such as 2D quantum wells, 1D quantum wires, and 0D QDs have been made [11][12][13]19 , yet, the controlling mechanisms are rather vague.…”