Hyperbolic plasmon–phonon dispersion on group velocity reversal and tunable spontaneous emission in graphene–ferroelectric substrate

Abstract This work presents a new substrate platform, which provides tunability of the group velocity and spontaneous emission of a dipolar scatterer graphene–ferroelectric slab hybrid system in the terahertz ranges. We use analytical models to determine the hybridization of graphene surface plasmon and ferroelectric LiNbO₃ type I and type II reststrahlen hyperbolic phonon–polariton. The variation of the chemical potential of graphene and the thickness of the ferroelectric layer results in several distinct features. Flipping the group velocity, strongly coupled hybrid hyperbolic surface plasmon–polaritons, and surface plasmon–polariton mode exists for the same momentum at different frequencies. The group velocity sign reversal for both a single-graphene- and double-graphene-integrated system depends on the thickness of the hyperbolic layer and the chemical potential of graphene. Comparative analysis of Purcell radiation is presented for a quantum emitter positioned at different locations between ferroelectric and graphene-integrated ferroelectric layers, revealing that this system can support strong spontaneous emission that can be modulated with the graphene chemical potential. Changing the chemical potential through selective voltage biasing demonstrates a substantial increase or decrease in the decay rate for spontaneous emission. Further analysis of the emission phenomenon shows a dependence on factors, such as the relative radiating source position and the thickness of the ferroelectric film. These characteristics make graphene–ferroelectric materials promising candidates to modify the light–matter interaction at the low terahertz ranges.