Low-Field Optical Polarization in Type-II Quantum Dots via Nuclear-Driven Dark State Mixing.

Semiconductor quantum dots (QDs) offer a rich landscape for spin control and quantum light emission. While most studies have focused on type-I band alignment, the potential of type-II systems remains underexplored. Here, we report low-field optical polarization in type-II In(Ga)As/GaAsSb QDs, enabled by hyperfine-induced mixing between bright and dark excitons via level anticrossing under magnetic fields as low as 0.17 T. The weak-field regime arises from the suppressed wave function overlap, yielding a reduced electron-hole exchange interaction. A theoretical model based on the spin Hamiltonian and the spin-split state populations accurately captures the observed mirror-symmetric luminescence helicity, reproducing the experimental polarization response. Additionally, polarization recovery measurements confirm the role of nuclear spin interactions in mediating the in-plane electron spin precession. Our work demonstrates an alternative route for light polarization control using weak magnetic fields and nonresonant linear excitation, establishing type-II QDs as promising platforms for compact sources of circularly polarized light.