Substrate Driven Tuning of Canalized Phonon Polaritons in Twisted α-MoO₃ Stacks

Among the diverse family of van der Waals materials, molybdenum trioxide (α-MoO₃) has emerged as a powerful platform for anisotropic nanophotonics. The orthorhombic lattice provides a biaxial dielectric tensor for the material that exhibits hyperbolic dispersion in three distinct spectral regions in the mid-infrared, known as Reststrahalen bands. Interesting optical phenomena arise from hyperbolic media, such as high-momentum states, enhanced density of states, and directional propagation making it a compelling candidate for advanced photonic applications. Upon stacking two α-MoO₃​​​​​​​ flakes on top of each other, highly directional canalized PhPs are formed as a result of the hybridization of hyperbolic polaritons from the two layers. On varying the twist angle between two α-MoO₃​​​​​​​ layers, isofrequency curves (IFCs) undergo a topological phase transition from open-hyperbolic shaped curves to closed elliptical ones at a specific frequency. At the phase transition, flat IFCs form resulting in all group velocity vectors pointing in the same direction, normal to the flat segment. This leads to the propagation of energy in a unidirectional path. In this work, we show that both the canalization frequency and propagation direction can be tuned by changing the dielectric environment around the stack, specifically the dielectric function of the substrate. We observe upto 25^o change in the canalized propagation and a 25cm^-1 shift in the canalization frequency for a 70^o twisted stack of two 100nm thick α-MoO₃​​​​​​​ flakes when the substrate dielectric constant is changed from 1 to 20. The numerical results are obtained from finite element method field simulations and experimentally shown using scanning near-field optical microscopy of twisted α-MoO₃​​​​​​​ on substrates with different dielectric functions. These findings open pathways for dynamic tuning of polariton propagation, nanoscale heat transport, imaging, infrared sensing and substrate-engineered photonic devices, such as on-chip directional waveguides.