Context of project
One of the most studied topics in solid state physics of semiconductor nanostructures is the control of single electron spins confined in 2-dimensions (nanowires) and 1-dimension (quantum dots). In particular, for self-ensemble quantum dots (QDs), one main source of electron spin decoherence are the nuclear spins. A single QD one single electron spin can be interacting with tens of thousands of incoherent nuclear spins. One popular approach to diminish the influence of the incoherent nuclear spin bath is known as dynamic nuclear polarisation (DNP). The DNP consists on transferring spin polarisation from the electrons to the nuclei. Ideally, an efficient and fast transfer of spin polarisation would result in a 100% polarisation of the nuclear spin bath, therefore fading this source of decoherence for electron spins.
Dynamic nuclear polarisation with single electron spins ref:
We study experimentally the dependence of dynamic nuclear spin polarization on the power of nonresonant optical excitation in two types of individual neutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show that the mechanism of nuclear spin pumping via second-order recombination of optically forbidden (“dark”) exciton states reported in InP/GaInP quantum dots [E. A. Chekhovich et al., Phys. Rev. B 83, 125318 (2011)] is relevant for material systems considered in this work. In the InGaAs/GaAs dots this nuclear spin polarization mechanism is particularly pronounced, resulting in Overhauser shifts up to ∼80 μeV achieved at ultralow optical excitation power, ∼1000 times smaller than the power required to saturate ground state excitons. The Overhauser shifts observed at ultralow power pumping in the interface GaAs/AlGaAs dots are generally found to be smaller (up to∼40 μeV). Furthermore in GaAs/AlGaAs we observe dot-to-dot variation and even sign reversal of the Overhauser shift which is attributed to the dark-bright exciton mixing originating from electron-hole exchange interaction in dots with reduced symmetry. Nuclear spin polarization degrees reported in this work under ultralow-power optical pumping are comparable to those achieved by techniques such as resonant optical pumping or above-gap pumping with high-power circularly polarized light. Dynamic nuclear polarization via second-order recombination of “dark” excitons may become a useful tool in single quantum dot applications, where manipulation of the nuclear spin environment or electron spin is required.
Dynamic nuclear polarization in InGaAs/GaAs and GaAs/AlGaAs quantum dots under non-resonant ultra-low power optical excitation
J. Puebla, E.A. Chekhovich, M. Hopkinson, P. Senellart, A. Lemaitre, M.S. Skolnick, A.I. Tartakovskii
Phys. Rev. B 88 (4), 9 (2013)
Personal contribution: Device fabrication, spectroscopy experiments, data analysis and manuscript writing