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

Abstract

We study the selective excitation at infrared and THz frequencies of optical and acoustic plasmonic modes supported by thin topological insulators. These modes are characterized by effective net charge or net spin density, respectively, and we study their excitation by combining many-body and electromagnetic calculations. We first show that non-locality can significantly modify the plasmonic response: it changes the energy of propagating plasmons up to tens of percent. We then discuss how, by changing the distance between a dipolar source and a semi-infinite 10 nm thin film, it is possible to control the excitation of acoustic and optical propagating plasmons, which can propagate over a distance of several plasmonic wavelengths. Furthermore, we consider 10 nm thin TI nanodisks and study the excitation of acoustic and optical localized plasmon modes by a point dipole source and plane wave illumination, respectively. The resonant plasmonic modes appear at frequencies that strongly depends on the size of the disk, and that can be potentially tuned by applying electrostatic gating to modify the Fermi Energy of the conductive 2-dimensional layer that forms at the interfaces of the TI. We observe a spectral shift from ~29 $\mu$m to ~34 $\mu$m by changing the Fermi Energy from 250meV to 350meV. Last, the electromagnetic energy of these plasmonics modes can be confined to very small regions, of effective volume ~(120 nm)^3 for the smaller disk considered, much less than the free-space wavelength cubed $\lambda$^3 ~(35000 nm)^3. The strong confinement is desirable for achieving very efficient coupling with nearby systems. Our detailed study thus shows that thin topological insulators are a promising system to control both the spin and charge oscillations associated with the plasmonic resonances, with possible applications to fast, compact and electrically-controlled spintronics devices.