Phase-resolved terahertz near-field nanoscopy of a topological insulator phonon-polariton mode

A novel scattering-type near-field optical microscopy (s-SNOM) system based on a terahertz (THz) quantum cascade laser operating in self-detection mode is employed to probe a resonant phonon-polariton mode of a thin topological insulator flake (Bi
₂
Te
_2.2
Se
_0.8
). Background-free near-field imaging with nanoscale spatial resolution is demonstrated.