Fig. 1. THz s-SNOM imaging polaritons in Bi₂Se₃.

a Schematic of the THz s-SNOM. A parabolic mirror focuses a THz beam onto the apex of an AFM tip. The tip-scattered THz field is collected and recorded interferometrically as function of tip position, simultaneously with topography. b Illustration of mapping polaritons (indicated by red sine waves). $E_{\mathrm{inc}}$ and $E_{\mathrm{sca}}$ denote the electric field of the incident and tip-scattered radiation. c Topography image of a 25-nm-thick Bi₂Se₃ film on Al₂O₃ and d simultaneously recorded amplitude and phase images at a frequency of 2.52 THz. e Near-field phase line profiles of the 25 nm thick Bi₂Se₃ film at different frequencies, recorded perpendicular to the film edge.