Near-field nanoscopy of Dirac plasmon polaritons in the 2-4.3 THz range in a topological insulator microantenna

Mr. Leonardo Viti1, Prof. Oleg Mitrofanov2,3, Dr. Valentino Pistore1, Dr. Zhengtianye Wang4, Dr. Chiara Schiattarella1, Dr. Stephanie Law5, Prof. Miriam Vitiello1
1Consiglio Nazionale delle Ricerche - Istituto di Nanoscienze, Pisa, Italy. 2Electronic and Electrical Engineering Department, University College of London, London, United Kingdom. 3Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, USA. 4Department of Materials Science and Engineering, University of Delaware, Newark, USA. 5Materials Science and Engineering, Pennsylvania State University, University Park, USA


Topological insulators (TIs) are quantum materials characterized by an insulating bulk and conductive topological surface-states (TSS). Recently, TSS have gathered significant attention thanks to the possibility to support Dirac plasmon polaritons (DPPs) at terahertz (THz) frequencies. However, their contribution to the optical response is difficult to disentangle from the contributions of massive carriers in the bulk and on the surface using traditional techniques, such as far-field THz spectroscopy. This can be partially overcome by directly imaging these hybrid plasmon polaritons with scattering-type scanning near-field optical microscopy (s-SNOM). However, the retrieval of DPP-related oscillations in the s-SNOM signal is hindered by the relatively short damping length of DPPs, comparable to their wavelength, which suppresses the propagation of DPPs within approximately one oscillation, leading to large uncertainties in determining the DPPs dispersion. 

To resolve this problem, we engineered TI micro-antennas by etching them from a uniform 80 nm thick film of Bi2Se3 grown by molecular beam epitaxy. We performed synthetic holography, via s-SNOM, employing quantum cascade lasers driven in continuous wave, operating as both THz sources and THz detectors through the self-mixing effect. In contrast to uniform bi-dimensional or irregularly-shaped flakes employed in previous studies, our rectangular (16 μm × 4 μm) antennas force DPPs to propagate along a single dimension, reducing the geometrical damping and avoiding faster dissipation due to the radial divergence of s-SNOM tip-launched waves. We measure the dispersion of Bi2Se3 DPPs in a practical device geometry in the frequency range spanning from 2.05 THz to 4.3 THz.