Plasmonic excitations in epitaxially grown ultrathin 2D Bi₂Se₃ films.

Bi₂Se₃ is a well-known 3D topological insulator whose linearly dispersive surface states host 2D Dirac plasmon polaritons in the terahertz frequency range. When the thickness of Bi₂Se₃ films decreases below 6 nm, an energy gap opens in the surface states at the Dirac point. These 2D Bi₂Se₃ films have been theoretically predicted to oscillate between a non-trivial quantum spin Hall insulator phase with conducting edge states and the trivial insulating phase depending on the film thickness. While the surface plasmons in 3D Bi₂Se₃ have been extensively investigated, there is a lack of research focusing on plasmonic excitations in 2D Bi₂Se₃. To investigate the plasmons in 2D Bi₂Se₃, the growth of wafer-scale, well-coalesced films with defined thicknesses is crucial. We have demonstrated that substrate pretreatment (growth and decomposition of a few layers of Bi₂Se₃) before the main growth results in well-integrated 2D Bi₂Se₃ films with the surface roughness below 1 nm. In this work, we present the preliminary findings on the excitation of plasmons in ultrathin Bi₂Se₃ films deposited on sapphire substrates using molecular beam epitaxy. A better understanding of plasmonic excitations and interactions in 2D Bi₂Se₃ has great interest in the development of advanced plasmonic devices.