Magnetic-field-induced reduction of the exciton polarization splitting in InAs quantum dots

By the application of an in-plane magnetic field, we demonstrate control of the fine structure polarization splitting of the exciton emission lines in individual InAs quantum dots. The selection of quantum dots with certain barrier composition and confinement energies is found to determine the magnetic field dependent increase or decrease of the separation of the bright exciton emission lines, and has enabled the splitting to be tuned to zero within the resolution of our experiments. Observed behavior allows us to determine $g$ factors and exchange splittings for different types of dots.

Figure 1

(a) Energy level schematic of a single quantum dot. (b) Fine structure of the exciton state (X) in a single quantum dot as a function of in-plane magnetic field $B_x$. The size of the black (grey) points represents the fraction of horizontal (vertical) polarization. The areas of the points corresponding to darker states has been multiplied by 5 for clarity.

Figure 2

Top panels show vertically and horizontally polarized photoluminescence spectra as dashed and solid lines respectively for the neutral exciton states of dots $A$, $B$, and $C$ defined in the text. Middle panels show the same with an in-plane magnetic field of $5\mathrm{T}$. Bottom panels show separation of dominant horizontally polarized emission line relative to dominant vertically polarized emission line as a function of magnetic field. Lines show fit to observed behavior.

Figure 3

(a) Coefficient $K$, characterizing the change in optically dominant exciton splitting $S$ as a function of magnetic field, as a function of the splitting at zero field $S_0$, for different dots. Shaded regions show dots for which brighter exciton states cross for fields below 5 and $10\mathrm{T}$. (b) Extrapolated bright-dark exchange splitting $D_0$ as a function of the confinement energy $E_c$ for different quantum dots. Line shows linear best fit to data. (c) $D_0$ as a function of $S_0$. Solid line show best fit linear dependence for dots with ${\mathrm{Al}}_{0.33}{\mathrm{Ga}}_{0.67}\mathrm{As}$ barriers. Dashed line indicates average $D_0$ for dots with GaAs barriers.