Fine Structure of Excitons in $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ Coupled Quantum Dots: A Sensitive Test of Electronic Coupling

Exciton fine structure in $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ coupled quantum dots has been studied by photoluminescence spectroscopy in magnetic fields up to 8 T. Pronounced anticrossings and mixings of optically bright and dark states as functions of magnetic field are seen. A theoretical treatment of the mixing of the excitonic states has been developed, and it traces observed features to structural asymmetries. These results provide direct evidence for coherent coupling of excitons in quantum dot molecules.

Figure 1

(a) Photoluminescence spectra of a molecule (QDM1) consisting of two InAs QDs separated by a 8 nm GaAs barrier for varying magnetic field. (b) Same as (a) for a molecule (QDM4) with 7 nm barrier. (c) Circularly polarized spectra of QDM4 at $B=7\mathrm{T}$.

Figure 2

Exciton transition energies versus magnetic field for several $\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ QDMs. Symbols give experimental data, and shaded regions indicate widths of emission bands at $B=0$. Lines are for uncoupled single QDs as described in text. Nominal barrier widths $d$ are noted.

Figure 3

(a),(b) Photoluminescence spectra of single QDMs with 8 nm wide barriers (upper traces) at $B=0$ and at 8 T in the Voigt configuration compared to single QDs (lower panels). (c) Diamagnetic shift for two QDMs and a QD versus magnetic field (Voigt configuration).

Figure 4

(a) Calculations for a QDM having two identical dots with lateral offset $b=2\mathrm{n}\mathrm{m}$. The inset gives a sketch of QDM geometry. (b) Calculations for a QDM composed of dots having lateral offset $b=4\mathrm{n}\mathrm{m}$ and a combined electron and hole lateral confinement energy difference $\sim 2\mathrm{m}\mathrm{e}\mathrm{V}$. Labels $1,2,\dots ,$ are described in text. The widths of the lines are proportional to their oscillator strengths. The separation between the dots is $d=8\mathrm{n}\mathrm{m}$ (a) and $d=7\mathrm{n}\mathrm{m}$ (b).