to refractive index changes, making them more efficient sensors than their noble metal counterparts in addition to reaching previously unobtainable wavelengths. [2] Unlike graphene, TSS electrons exhibit spin-momentum locking, in which the momentum of an electron determines its spin direction. The Dirac electrons are thus protected by time reversal symmetry from backscattering into other surface states in the absence of a magnetic perturbation. [12] TI plasmonics could therefore gain an advantage over graphene since the spin-momentum locking in TIs should lead to spin-polarized plasmons with long propagation distances. [5,13,14] In addition, it is straightforward to grow TI films and multilayered heterostructures on a wafer scale using molecular beam epitaxy. [15][16][17] It is important to note that although the TSSs are 2D, standard TI materials are usually 3D with 2D sheets of topologically protected Dirac electrons on each surface. Plasmon excitations in bulk TI materials have been investigated with electron energy loss spectroscopy measurements, and complex behavior attributed to both trivial surface and topological Dirac electrons has been observed. [18][19][20] Since the TI films discussed in this paper are all in the "thin film" limit of qd < 1 (where q is the wavevector of light and d is the thickness of the film), the plasmons on the top and bottom film surfaces will couple electrostatically. This results in an acoustic (dark) and an optical (bright) plasmon mode. There has been some initial work performed in this system on both the acoustic and optical plasmon modes. [21][22][23][24][25][26][27] Previous work by Di Pietro et al. [23] investigated the dependence of plasmon frequency on stripe width in etched TI films; their observations showed the expected W dependence for 2D plasmons. Though their results were consistent with Dirac plasmons, they were unable to completely rule out the possibility that they observed a 2D massive plasmon instead of the desired 2D massless plasmon.The inherently 3D nature of Bi 2 Se 3 allows for the existence of at least two carrier pathways beyond the TSS: bulk doping and a band bending two-dimensional electron gas (2DEG) at the interface of the TI and adjacent material. [28] One way to distinguish Dirac plasmons from coexisting massive bulk plasmons and massive 2DEG plasmons is by mapping how the plasmon frequency depends on film thickness as well as stripe width. Equation (1) shows the dispersion relation for massive bulk plasmons, where ω p is the plasmon frequency, e is the electron charge, n M is the 2D sheet density, ε r is the average permittivity of the surrounding materials, and q is the wavevector of light. For massive bulk plasmons, n M will increase as the film thickness increases (assuming Topological insulators are predicted to house spin-polarized 2D Dirac plasmons. In topological insulator thin films, Dirac plasmons on the top and bottom surfaces are coupled, giving rise to an unusual dispersion relationship. These plasmons are of interest both for fundam...