Detectorless scanning near-field microscopy in the THz range as tool for the characterization of topological insulator-based nanophotonic structures

he 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, plasmon dispersion and loss properties of Bi2Se3 coupled antenna resonators, patterned at varying lengths and distances, are experimentally determined using scattering-type scanning near-field optical microscopy (s-SNOM) at different THz frequencies in a detector-less configuration, which exploits the self-mixing (SFMX) phenomenon occurring in quantum cascade lasers (QCLs). The propagation of DPPs in TI-based coupled antennas is traced, thereby showing how the designed rectangular nano-antennas effectively confine DPPs propagation to one dimension, enhancing their visibility despite intrinsic attenuation. Furthermore, the results evidence modifications on the DPP wavelength along the single nano-antenna ascribable to the cross-talk between neighbouring elements. The outcomes provide insights into DPPs characteristics, paving the way for the design of novel topological devices and metasurfaces by leveraging their directional propagation capabilities.

Seminar realized in the framework of the funded project
-Extreme Optical Nonlinearities in 2D materials for Far Infrared Photonics (GA No. 964735).