Numerical Temporal-Spectral Analysis of Non-scattering THz nanoscopy with Resonant Cantilevered Tips

Scattering-type scanning near-field optical microscopy (<i>s</i>-SNOM) under the excitation of single cycle picosecond (ps) pulse provides access to terahertz (THz) time-resolved nanoscopy. However, the development of THz nanoscopy has been greatly limited due to the inherently low efficiency of the scattered field and the convolution of the intrinsic material response with the extrinsic response of the cantilevered tip. In this work, we quantitatively study the near-field time-delayed pulse transients of resonant cantilevered tips, observing localized tip-enhanced coupling as well as delocalized collective charge oscillations propagating as resonant surface waves along cantilevered tips. By numerical temporal-spectral analysis, the phonon resonance of the topological insulator Bi₂Se₃ can be effectively extracted from the resonant surface waves at the end of the cantilever. We demonstrate that after propagating 600 <i>µ</i>m, the intensity of near-field signals extracted from the resonant surface waves is about three orders higher than that from the traditional scattering field. Our research reveals the delocalized tip-enhanced light-matter interaction in propagating surface waves and proposes a promising route to non-scattering THz nanoscopy.