Near-field THz detection of phonon-polariton modes in thin flakes of topological insulator materials: Bi2Se3 and Bi(Te1-x Sex)3

We report on the study of phonon polaritons modes in thin crystalline flakes of Bi 2 Se 3 and Bi 2 (Te 1-x Se x ) 3 as a function of the flake thickness, by means of two near-field THz techniques: phase resolved self-detection and THz time domain spectroscopy (TDS-SNOM). Background-free near-field imaging with nanoscale spatial resolution is demonstrated.