  The surface states of the 3D topological insulator (TI), Bi$_2$Se$_3$, are
known to host two-dimensional Dirac plasmon polaritons (DPPs) in the terahertz
spectral range. In TI thin films, the DPPs excited on the top and bottom
surfaces couple, leading to an acoustic and an optical plasmon mode. Vertical
coupling in these materials is therefore reasonable well-understood, but
in-plane coupling among localized TI DPPs has yet to be investigated. In this
paper, we demonstrate in-plane DPP coupling in TI stripe arrays and show that
they exhibit dipole-dipole type coupling. The coupling becomes negligible with
the lattice constant is greater than approximately 2.8 times the stripe width,
which is comparable to results for in-plane coupling of localized plasmons
excited on metallic nanoparticles or graphene plasmon polaritons. This
understanding could be leveraged for the creation of TI-based metasurfaces.
