Electron-nuclei spin dynamics in II-VI semiconductor quantum dots

Abstract

We report on the dynamics of optically induced nuclear spin polarization in\nindividual CdTe/ZnTe quantum dots loaded with one electron by modulation\ndoping. The fine structure of the hot trion (charged exciton $X^-$ with an\nelectron in the $P$-shell) is identified in photoluminescence excitation\nspectra. A negative polarisation rate of the photoluminescence, optical pumping\nof the resident electron and the built-up of dynamic nuclear spin polarisation\n(DNSP) are observed in time-resolved optical pumping experiments when the\nquantum dot is excited at higher energy than the hot trion triplet state. The\ntime and magnetic field dependence of the polarisation rate of the $X^-$\nemission allows to probe the dynamics of formation of the DNSP in the optical\npumping regime. We demonstrate using time-resolved measurements that the\ncreation of a DNSP at B=0T efficiently prevents longitudinal spin relaxation of\nthe electron caused by fluctuations of the nuclear spin bath. The DNSP is built\nin the microsecond range at high excitation intensity. A relaxation time of the\nDNSP in about 10 microseconds is observed at $B=0T$ and significantly increases\nunder a magnetic field of a few milli-Tesla. We discuss mechanisms responsible\nfor the fast initialisation and relaxation of the diluted nuclear spins in this\nsystem.\n