Actively tuning anisotropic light–matter interaction in biaxial hyperbolic material α-MoO₃ using phase change material VO₂ and graphene

Anisotropic hyperbolic phonon polaritons (PhPs) in natural biaxial hyperbolic material α-MoO₃ has opened up new avenues for mid-infrared nanophotonics, while active tunability of α-MoO₃ PhPs is still an urgent problem necessarily to be solved. In this study, we present a theoretical demonstration of actively tuning α-MoO₃ PhPs using phase change material VO₂ and graphene. It is observed that α-MoO₃ PhPs are greatly dependent on the propagation plane angle of PhPs. The insulator-to-metal phase transition of VO₂ has a significant effect on the hybridization PhPs of the α-MoO₃/VO₂ structure and allows to obtain actively tunable α-MoO₃ PhPs, which is especially obvious when the propagation plane angle of PhPs is 90°. Moreover, when graphene surface plasmon sources are placed at the top or bottom of α-MoO₃ in α-MoO₃/VO₂ structure, tunable coupled hyperbolic plasmon–phonon polaritons inside its Reststrahlen bands (RBs) and surface plasmon–phonon polaritons outside its RBs can be achieved. In addition, the above-mentioned α-MoO₃-based structures also lead to actively tunable anisotropic spontaneous emission (SE) enhancement. This study may be beneficial for realization of active tunability of both PhPs and SE of α-MoO₃, and facilitate a deeper understanding of the mechanisms of anisotropic light–matter interaction in α-MoO₃ using functional materials.