Plasmon-phonon-intersubband transition interactions at THz frequency in Bi₂Se₃-GaAs heterostructures

In this work we probe theoretically strong coupling at THz frequency in a system consisting of Bi₂Se₃ and AlGaAs/GaAs quantum wells, focusing on the creation of new hybrid excitation modes, namely Dirac plasmon phonon intersubband transition (ISBT) polaritons. Rabi splitting arising from the interactions among the THz excitations in the system shows features of strong coupling which can be experimentally observable. Our calculations based on scattering matrix method reveal that the Rabi splitting depends strongly on doping level and scattering rate in the quantum wells, as well as on the thickness of the GaAs spacer layer which separates the quantum well structures from the topological insulator (TI) layer. Our model also addresses contributions from an extra two-dimensional electron gas (2DEG) occurring at the Bi₂Se₃/GaAs interface as predicted by density functional theory calculations. This massive two-dimensional electron gas gives rise to a shift in the dispersion of the Dirac plasmon-ISBT polaritons to higher frequencies. This work lays the foundation and serves as a guide for setting up future experimental explorations into the coupling between a TI and a III-V heterostructure.