Gate tunable topological transitions of biaxial hyperbolic polaritons in van der Waals heterostructures*

α-MoO3 has shown great promise for infrared nanophotonics applications due to its biaxial hyperbolicity. However, one critical shortcoming is the lack of active tunability of these biaxial phonon polaritons. In this work, we address this problem by integrating graphene with α-MoO3, yielding a mode hybridization that results in the formation of broadband biaxial hyperbolic plasmon phonon polaritons, whose dispersion can easily be tuned via electrostatic gating of graphene. We also discuss the topological transitions of the isofrequency surfaces of these polaritons as a function of gate voltage and show that this heterostructure can lead to tunable anisotropic spontaneous emission rate, which has important consequences for applications like radiative heat transfer, quantum interference, and Kerr nonlinearity enhancement.