Individual neutral and charged ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}\text{−}\mathrm{Ga}\mathrm{As}$ quantum dots with strong in-plane optical anisotropy

We report neutral $\left(X^0\right)$ and charged excitons $\left(X^{+}\right)$ with high degrees of linear polarization $(⩾70\%)$ in charge-tunable InGaAs quantum dots (QDs). The QD emission exhibits a small Zeeman splitting for magnetic fields parallel to the growth direction, in contrast to “conventional” dots in the same sample, and remains predominantly linearly polarized up to $5\mathrm{T}$. With the aid of in-plane field measurements, the observations are explained in terms of heavy–light-hole mixing due to QD anisotropy. This results in elliptical polarization of the QD emission and strong reduction of the exciton $g$ factor. A combination of the data obtained for magnetic fields in Faraday and Voigt configurations allows a full determination of the electronic properties.

Figure 1

(a) Schematic band diagram of Schottky photodiode. (b), (c) PL spectra of quantum dots $A$ (b) and $B$ (c) recorded for two orthogonal linear polarizations at applied biases of 0.6 and $0.8\mathrm{V}$, respectively. Neutral and positively charged excitons, $X^0$ and $X^{+}$, are observed. (d) Ratio of the integrated intensities of $X^0$ and $X^{+}$ for B (open squares) and A (solid) QDs as a function of reverse bias.

Figure 2

(a) $X^0$ and $X^{+}$ PL spectra for QD $B$ in magnetic field in the Faraday configuration $(B\Vert z)$, for ${\sigma }^{+}$ and ${\sigma }^{-}$ circular polarizations at reverse bias $0.8\mathrm{V}$. (b) $X^0$ and $X^{+}$ PL spectra for QD $B$ in magnetic field in the Voigt configuration $(B\Vert \left[100\right])$ for two crosslinear polarizations $({\pi }_x,{\pi }_y)$. The spectra were measured at $0.4\mathrm{V}$, where emission (line $C$) attributed probably to $X^{2+}$ is observed (see Ref. 15). (c) Schematic transitions of neutral and positively charged excitons.

Figure 3

(a) $X^0$ and $X^{+}$ of PL spectra for QD $A$ in magnetic field in the Faraday configuration $(B\Vert z)$ at $0.8\mathrm{V}$. (b) PL spectra for $X^0$ and $X^{+}$ recorded for crosslinear $({\pi }_x,{\pi }_y)$ and circular $({\sigma }^{+},{\sigma }^{-})$ polarizations at reverse bias $0.8\mathrm{V}$ at $5\mathrm{T}$. (c) $X^0$ and $X^{+}$ PL spectra for QD $A$ in the Voigt configuration $(B\perp z)$ for two crosslinear polarizations $({\pi }_x,{\pi }_y)$ for $B\Vert \left[110\right]$ at $0.7\mathrm{V}$. (d) Summary of electron and hole $g$ factors for $B\Vert z$ and $B\perp z$ for QD $A$ and QD $B$.