Homogeneous Linewidth Narrowing of the Charged Exciton via Nuclear Spin Screening in an InAs/GaAs Quantum Dot Ensemble

摘要

In semiconductor quantum dots, the electron hyperfine interaction with thenuclear spin bath is the leading source of spin decoherence at cryogenictemperature. Using high-resolution two-color differential transmissionspectroscopy, we demonstrate that such electron-nuclear coupling also imposes alower limit for the positively charged exciton dephasing rate, \gamma, in anensemble of InAs/GaAs quantum dots. We find that the dephasing rate issensitive to the strength of the hyperfine interaction, which can be controlledthrough the application of an external magnetic field in the Faradayconfiguration. At zero applied field, strong electron-nuclear coupling inducesadditional dephasing beyond the radiative limit and \gamma = 230 MHz (0.95 \mueV). Screening of the hyperfine interaction is achieved for an external fieldof ~1 T, resulting in \gamma = 172 MHz (0.71 \mu eV) limited only byspontaneous recombination. On the other hand, application of a Voigt magneticfield mixes the spin eigenstates, which increases the dephasing rate by up to75%. These results are reproduced with a simple and intuitive model thatcaptures the essential features of the electron hyperfine interaction and itsinfluence on \gamma.