Optical orientation and control of spin memory in individual InGaAs quantum dots

A high degree of spin polarization (or spin memory) is achieved using quasiresonant optical excitation at zero magnetic field $(B=0)$ for singly positively charged excitons $\left(X^{+}\right)$ in individual quantum dots embedded in a Schottky diode. The high degree of spin memory indicates highly efficient optical excitation (“writing”) of long-lived spin-polarized electrons, determining the $X^{+}$ spin orientation. We demonstrate control of the degree of spin polarization by the applied bias, controlling carrier tunneling rates in the device. In addition, efficient spin-selective optical excitation of neutral excitons is achieved for $B>1\mathrm{T}$.

Figure 1

(a) PL spectra for dot B measured in two orthogonal linear polarizations. The fine structure splitting in $X^0$ emission is indicated. Diagram shows energy structure of the neutral $\left(X^0\right)$ and charged $\left(X^{+}\right)$ excitons. Short arrows show spin orientation of the holes (h) and electron $\left(e\right)$. Long arrows show transitions from initial to final states accompanied by photon emission. (b) PL spectra for dot A excited with ${\sigma }^{+}$-polarized light at $1.274\mathrm{eV}$. Spectra detected in ${\sigma }^{+}$ $\left({\sigma }^{-}\right)$ polarization are shown with black (grey) lines. Bottom (top) spectrum is measured at 0.8 $\left(0.6\right)\mathrm{V}$. The diagram shows the band structure of a Schottky diode in two bias regimes, when the dot (marked QD) is charged with extra holes or empty. The inset shows the bias dependence of the intensity ratio of $X^{+}$ and $X^0$.

Figure 2

PLE spectra measured for dot A for $X^{+}$ at $V=0.6$ and $1\mathrm{V}$ and $X^0$ at $V=1\mathrm{V}$. Spectra measured for ${\sigma }^{+}$ excitation and ${\sigma }^{+}$ $\left({\sigma }^{-}\right)$ detection are shown with black (grey) lines. The numbers mark the most pronounced features in the spectra (see text for details).

Figure 3

Vertical lines and symbols in the top panel show the approximate boundaries of the three bias regimes where $X^{n+}$ (multiply-charged exciton), $X^{+}$ and $X^0$ can be detected in PL. (a) Bias dependence of $X^{+}$ and $X^0$ PL intensity for dot A excited at $1.274\mathrm{eV}$ with ${\sigma }^{+}$-polarized light. The intensity detected for $X^{+}$ in ${\sigma }^{+}$ $\left({\sigma }^{-}\right)$ polarization is shown with black (grey) triangles. Circles show the intensity of $X^0$ measured in ${\sigma }^{+}$ polarization. (b) Bias dependence of the degree of circular polarization of $X^{+}$ calculated using the data in (a).

Figure 4

Unpolarized PL spectra for dot A at $B=2\mathrm{T}$ excited with linearly (grey) and circularly (black) polarized light resonant with feature 1 in Fig. 2.