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 $B>1\mathrm{T}$.