Controlling surface charge and spin density oscillations by Dirac plasmon interaction in thin topological insulators

Thin topological insulator (TI) films support optical and acoustic plasmonic modes characterized by effective net charge or net spin density, respectively. We combine many-body and electromagnetic calculations to study how these modes can be selectively excited at films and nanodisks at infrared and THz frequencies. We first discuss the excitation of propagating plasmons in a thin film by a point dipolar source. We emphasize how changing the distance between the dipolar source and the film allows us to control the relative strength of the acoustic and optical plasmons and thus to excite net-spin or net-charge waves on demand. The acoustic and optical modes in a nanodisk structure can be efficiently tuned by changing the size of the disk or by applying electrostatic gating. Furthermore, these modes can be confined to regions of dimensions much smaller than the wavelength. The control of the excitation of acoustic and optical modes indicates that thin topological insulators are a promising system to manipulate the spin and charge properties of the plasmonic response, with potential applications in fast, compact, and electrically-controlled spintronic devices.M.A.P, J.A., and R.E. acknowledge funding from project FIS2016-80174-P of the Ministry of Economy, Industry and Competitiveness MINEICO and from NIST Grant No. 70NANB15H32 of the Department of Commerce of the US. M.H. acknowledges financial help from Grant Agency of the Czech Republic (Grant No. 15-21581S), Technology Agency of the Czech Republic (Grant No. TE01020233) and MEYS CR (project No. LQ1601 CEITEC 2020). I.A.N. and V.M.S. acknowledge support from the Spanish Ministry of Economy, Industry and Competitiveness MINEICO (Project No. FIS2016-76617-P) and from Saint Petersburg State University (Grant No. 15.61.202.2015). A.Y.N acknowledges the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (national projects MAT2014-53432-C5-4-R and MAT2017-88358-C3-3-R). R.E. and V.M.S. acknowledge project PI2017-30 of the Departamento de Educación, Política Lingüística y Cultura of the Basque government.Peer Reviewe