Probing Dirac plasmon polaritons in bismuth selenide coupled nano-antennas by terahertz near-field microscopy

The study of Dirac plasmon polaritons (DPPs) in two-dimensional materials has raised considerable interest in the last years for the development of tunable optical devices, plasmonic sensors, ultrafast absorbers, modulators, and switches. In particular, topological insulators (TIs) represent an ideal material platform by virtue of the plasmon polaritons sustained by the Dirac carriers in their surface states. However, tracking DPP propagation at terahertz (THz) frequencies, with wavelength much smaller than that of the free-space photons, represents a challenging task. Herein, we trace the propagation of DPPs in TI-based coupled antennas. We show how Bi2Se3 rectangular nano-antennas effectively confine DPPs propagation to one dimension, enhancing their visibility despite intrinsic attenuation. Furthermore, plasmon dispersion and loss properties of coupled antenna resonators, patterned at varying lengths and distances are experimentally determined using holographic near-field nano-imaging at different THz frequencies. Our study evidences modifications on the DPP wavelength along the single nano-antenna ascribable to the cross-talk between neighbouring elements. The results provide insights into DPPs characteristics, paving the way for the design of novel topological devices and metasurfaces by leveraging their directional propagation capabilities.