Reverse Stark effect, anomalous optical transitions, and control of spin in topological insulator quantum dots

Formed through a closed domain magnetic heterostructure on the surface of a three-dimensional topological insulator, a quantum dot permits a class of quantized interfacial states of topological origin. We find that these states exhibit a remarkable reverse Stark effect in response to an applied electric field. In particular, those topological states whose energies are within the gap exhibit peculiar electrical alignments that are opposite to those associated with the conventional quantum-confined Stark effect in that the positive- (negative-) energy states tend to align with (against) the direction of the field. The phenomenon has unusual implications for the associated optical transitions. Furthermore, the exotic topological states exhibit polarized spin textures that can be effectively controlled electrically or optically, opening an avenue for potential applications in Dirac-material-based spintronics.