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We study experimentally the dependence of dynamic nuclear spin polarizationon the power of non-resonant optical excitation in two types of individualneutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show thatthe mechanism of nuclear spin pumping via second order recombination ofoptically forbidden (''dark'') exciton states recently reported in InP/GaInPquantum dots [Phys. Rev. B 83, 125318 (2011)] is relevant for material systemsconsidered in this work. In the InGaAs/GaAs dots this nuclear spin polarizationmechanism is particularly pronounced, resulting in Overhauser shifts up to ~80micro-eV achieved at optical excitation power ~1000 times smaller than thepower required to saturate ground state excitons. The Overhauser shiftsobserved at low-power optical pumping in the interface GaAs/AlGaAs dots aregenerally found to be smaller (up to ~40 micro-eV). Furthermore in GaAs/AlGaAswe observe dot-to-dot variation and even sign reversal of the Overhauser shiftwhich is attributed to dark-bright exciton mixing originating fromelectron-hole exchange interaction in dots with reduced symmetry. Nuclear spinpolarization degrees reported in this work under ultra-low power opticalpumping are comparable to those achieved by techniques such as resonant opticalpumping 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, wheremanipulation 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