Controllable Refraction of Highly Confined Hyperbolic Phonon Polaritons Using van der Waals Dielectric Superstrates

In stark contrast with traditional refraction phenomenon in isotropic media, refraction of hyperbolic polaritons enables extreme confinement and nanofocusing of mid-infrared light. However, the tuning of hyperbolic refraction typically relies on plasmonic systems (e.g., gating graphene and phase-change materials), and therefore, the electron–electron scattering introduces inevitable propagation losses of polaritons. Herein, we demonstrate that refraction of highly confined hyperbolic polaritons in α-phase molybdenum trioxide (α-MoO3) crystal strongly depends on the dielectric superstrate. We theoretically and experimentally indicate that both the wavevector (its direction and magnitude) and the propagation direction of refracted hyperbolic polaritons can be controlled by varying the twist angle between the vdW dielectric layers of tungsten diselenide (WSe2) and α-MoO3 crystals. Moreover, we observe the total internal reflection and zero-phase propagation of nanoscale-confined polaritons in hyperbolic media. Our study of the propagation of hyperbolic polaritons not only deepens the understanding of polariton refraction at the nanoscale and in strongly anisotropic media but also promises the development of functional nanophotonic elements and devices.