Circular dichroism of the dipole transition in nanosized topological insulators

The vitality of topological insulators (TIs) arises in large part from the conducting edge states, which support the dissipationless transport along the edge. Whereas chiral edge states have been identified by electrical transport measurements, observations that capture the topological order of the insulating bulk states are rather scarce. Instead, contributions from bulk states are assumed to behave like those of ordinary insulators, and are thus undesirable. Here, we theoretically demonstrate that in nanosized TIs, the topological order of bulk states can be manifested by the distinct circular dichroism of the dipole transition in high harmonic generation (HHG), even though the energy band structure changes negligibly compared to the trivial one in bulk TIs. The circular dichroism is mapped onto the large ellipticity of HHG, which keeps the same sign as the Chern number of TIs despite fluctuations of driving laser ellipticity, defects, and doping of TIs. It also enables a micromagnetic trap that realizes magnetic confinement more than three orders of magnitude stronger than that generated by the conventional method. Our results reveal a counterintuitive feature of dynamic responses in nanosized TIs and provide additional degrees of freedom for the design of novel TI-based devices.