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

Article

关键词: CENTER-DOT-P;    ELECTRONIC-STRUCTURE;    POLARIZATION ANISOTROPY;    OPTICAL-PROPERTIES;    QUANTUM;    STRAIN;   

DOI  :  10.1103/PhysRevB.92.165301

来源: SCIE

【 摘 要 】

The heavy-hole (HH) and light-hole (LH) components of the valence states in three-dimensional (3D) bulk semiconductors can mix quantum mechanically as the dimensionality is reduced in forming low-D nanostructures, such as 2D quantum wells, 1D quantum wires, and 0D quantum dots (QDs). This coupling controls the tuning of the excitonic fine-structure splitting, provides an efficient channel for the spin coherence, and leads to polarization anisotropy of light emission, central to several quantum-information schemes. The current understanding is that the mixing scales with the square of delta V-HL/Delta(HL), where delta V-HL and Delta(HL) are the coupling matrix elements of the crystal potential and the energy separation between the primary HH0 and LH0 states, respectively. We discuss two classes of HH-LH coupling mechanisms. First, coupling factors occurring through the numerator dVHL, referred to as direct coupling, including the well-known (i) quantum confinement, (ii) built-in strain, and (iii) shape elongation, as well as three additional direct coupling mechanisms discussed here: (iv) the intrinsic C-2v crystal-field effect, (v) the local symmetry of the interface, and (vi) the alloy disorder. We quantify these six direct HH-LH coupling effects by performing atomistic pseudopotential calculations on a range of strained and unstrained QDs of different morphologies. We find that in unstrained self-assembled QDs such as GaAs/AlGaAs, effects (i)-(vi) contribute 0%, 0%, 0%, 0%, 40%, and 60%, respectively, whereas in strained self-assembled QDs such as InGaAs/GaAs they contribute 0%, 0%, 78%, 0%, 8%, and 14%, respectively, to the direct HH-LH coupling delta V-HL. These relative contributions to direct HH-LH coupling differ significantly from what was previously believed. Second, we discover an unexpected HH-LH supercoupling that effectively reduces the denominator Delta(HL) by the presence of a dense ladder of intermediate states between the HH0 and LH0 states (analogous to superexchange in magnetism). Supercoupling amplifies and propagates the HH-LH interaction and is the dominant source of HH-LH mixing in strained nanostructures where Delta(HL) is fairly large, so by the direct coupling mechanism alone delta V-HL/Delta(HL) would be expected to be rather small. Supercoupling explains a number of outstanding puzzles, including the surprising fact that in strained (InAs/GaAs) QDs the mixing is very strong despite the fact that Delta(HL) is large, and it offers a way to manipulate HH-LH mixing and hence associated properties in nanostructures.

【 授权许可】

Free