Optical Pumping of the Electronic and Nuclear Spin of Single Charge-Tunable Quantum Dots

Abstract

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Nuclear spin effects in semiconductor quantum dots

“…Electron Overhauser shifts in excess of 100 μeV can be obtained using optical pumping [22][23][24][25]33,34 , and B nuc up to 3 T have been reported [23][24][25] . Using rough estimates of the dot composition, degrees of nuclear polarization up to 60% are now routinely obtained.…”

“…However, there are a number of ways to overcome the energy mismatch and to induce a preferential pumping direction, both by optical and electrical means. There have been many attempts to find an efficient way of achieving DNP [21][22][23][24][25]33,34,49,52,53 , with the main motivation to achieve a 100% polarization of nuclear spins, which would prevent nuclear-nuclear flip-flops, strongly suppressing the randomness in the nuclear field and concomitant electron spin decoherence 10 .…”

Submillisecond electron spin relaxation in InP quantum dots

“…7,8 At the same time, a fundamental question arises associated with the origin of so long-lived spin polarization. Along with suppression of main spin relaxation processes in QDs, predicted theoretically, 2 persistence of the spin polarization can result from dynamic nuclear polarization [10][11][12][13] capable to conserve the electron spin for a long time. It should be particularly emphasized that the dynamic nuclear polarization is a very slow process which cannot realize fast recording of information.…”

The Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature

“…There, a dominant low-field contribution to spin depolarization curves was observed and was argued to arise from the resident electron left behind after recombination of negative trions. From low-temperature Hanle measurements of single QDs [13], there is evidence that the half-width of Hanle curves is significantly larger for positive trions than for negative trions, indicating a shorter spin lifetime for the former. However, detailed studies of the Hanle effect in positively charged QD ensembles remain few so far and are still lacking at temperatures close to RT.…”

“…Previous studies of the Hanle effect in QDs were conducted at lower temperatures [13][14][15][16][17] and were mainly concentrated on characterization of negatively charged QDs. There, a dominant low-field contribution to spin depolarization curves was observed and was argued to arise from the resident electron left behind after recombination of negative trions.…”