Dynamic nuclear polarization in InGaAs/GaAs and GaAs/AlGaAs quantum dots under non-resonant ultra-low power optical excitation

TL;DR

This study demonstrates efficient dynamic nuclear polarization in InGaAs/GaAs and GaAs/AlGaAs quantum dots using ultra-low power optical excitation, revealing a mechanism involving dark excitons that could benefit quantum information applications.

Contribution

It identifies and characterizes a dark exciton-mediated nuclear spin polarization mechanism in quantum dots at ultra-low optical powers, expanding understanding of spin control in these systems.

Findings

Achieved Overhauser shifts up to ~80 micro-eV at low power in InGaAs/GaAs dots.

Observed dot-to-dot variation and sign reversal of Overhauser shifts in GaAs/AlGaAs dots.

Demonstrated nuclear polarization degrees comparable to high-power techniques.

Abstract

We study experimentally the dependence of dynamic nuclear spin polarization on the power of non-resonant 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 recently reported in InP/GaInP quantum dots [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 micro-eV achieved at optical excitation power ~1000 times smaller than the power required to saturate ground state excitons. The Overhauser shifts observed at low-power optical pumping in the interface GaAs/AlGaAs dots are generally found to be smaller (up to ~40 micro-eV). Furthermore in GaAs/AlGaAs we observe dot-to-dot variation and even sign reversal of the Overhauser shift which is attributed to dark-bright exciton mixing originating from electron-hole exchange interaction in dots with reduced symmetry. Nuclear spin polarization degrees reported in this work under ultra-low 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.