Near-Field Imaging of Surface Plasmons from the Bulk and Surface State of Topological Insulator Bi₂Te₂Se

Surface plasmons are collective oscillations of electrons that can enable confinement of electromagnetic energy to subwavelength scales. Recent progress in plasmonics has largely relied on noble metals which are not CMOS compatible. Hence there is a need to search for new plasmonic materials. Here we show that the topological insulator Bi2Te2Se is plasmonic, and we study the distinct surface plasmons arising from its bulk and surface state. We launch propagating plasmon via a nanoscale slit and detect it using near-field scanning optical microscopy. We observe that the wavelength of plasmon that originated from the surface state is very short, more than 100 times smaller than the incident light wavelength, in sharp contrast to the plasmon wavelength of the bulk which is on the order of the incident light wavelength. This short plasmon wavelength is due to the two-dimensional nature of the surface state, similar to that in graphene. The strong optical confinement of the surface state plasmons can be exploited for various applications including integrated optical circuits and subwavelength optical devices.