Topological insulator nanoparticles for strong light-matter interaction in the terahertz regime

We study the spontaneous emission (SPEM) for a quantum emitter (QUEM) near a topological insulator Bi₂Se₃ nanosphere. We calculate numerically the QUEM Purcell factor near nanospheres of radii between 40 nm and 100 nm, with and without taking into account the topologically protected delocalized states at the surface of the nanosphere. We find exceptionally large Purcell factors up to 10¹⁰ at distances between the QUEM and the nanosphere as large as half its radius in the terahertz regime. By computing the SPEM dynamics for a QUEM with transition frequencies in the terahertz and free-space decay rates in the nanosecond to millisecond range, we observe intense reversible dynamics, as well as population trapping effects. This work demonstrates that a Bi₂Se₃ nanosphere provides the conditions for strong light-matter interaction at the nanoscale in the terahertz regime.