Tailoring Phonon Polaritons with a Single-Layer Photonics-Empowered Polaritonic Crystal

Photonic crystals (PhCs) are artificial periodic structures that can compress and control light at the nanoscale. Recently, the emerging van der Waals (vdW) materials with extreme anisotropy exhibit exotic hyperbolic phonon resonances and ray-like propagation. However, localization and manipulation of these hyperbolic phonon polaritons (PhPs) in polaritonic crystals (PoCs) on a scale deeply below the polariton wavelength have remained elusive so far. Here, we experimentally demonstrate tailored PhP localization in a single layer PoC of patterned α-MoO₃. The biaxial PoCs have a rotation degree of freedom between the lattice orientation and the optical axis of α-MoO₃. By tailoring the rotation angle, the bandgap exhibits a transition from open to closed at certain wavelengths. Two-dimensional localization of PhPs in these rotated PoCs have been directly revealed in real space by infrared nanoimaging. Our work provides a robust platform to construct polaritonic cavities for ultrastrong light-matter interactions and nanoscale polariton manipulation.