Lateral Coupling of 2D Dirac Plasmons in Thin Films of Topological Insulator Bi₂Se_3.

2D Dirac plasmon polaritons (DPPs) excited on the surfaces of the 3D topological insulator Bi₂Se₃ have resonance frequencies in the terahertz range which has applications in security, wireless communication, and so on. Patterning thin films of Bi₂Se₃ into stripes of width W provides an additional momentum of π/W to the incoming photons, which then can excite DPPs by overcoming the momentum mismatch. When the thickness of films is much smaller than the wavelength of the incident light, the DPPs are excited on the top and bottom surfaces simultaneously and couple to each other resulting in an acoustic mode and an optical mode. Vertical coupling of DPPs in Bi₂Se₃ thin films is generally well-understood but there is no clear understanding of how the plasmons couple to each other in the same plane. We explored the effect of in-plane coupling on the resonance frequency of the DPPs by changing the gap size between the stripes while keeping stripe width and film thickness constant. We show that DPPs exhibit dipole-dipole type coupling which becomes negligible when the lattice constant exceeds approximately 2.8 times the stripe width. This value is comparable to results found for plasmons localized on the metallic nanoparticles or graphene micro-discs. Insight into the in-plane coupling of DPPs can be useful in designing TI-based metasurfaces to exploit THz light.