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 (X⁺) 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>10.3em0exT.