On-Chip Asymmetric Steering of Phonon Polaritons via Grating-Driven Twisted α-Phase Molybdenum Trioxide Bilayers.

Asymmetric light transmission at the nanoscale constitutes a fundamental challenge and a key goal in integrated photonic circuits. Polaritons in van der Waals (vdWs) materials offer a promising platform for such nanoscale light confinement and control. However, achieving complex and functional vdWs polaritons control across interfaces remains a challenge. Here, we demonstrate grating-driven on-chip asymmetric steering of phonon polaritons (PhPs) in hyperbolic vdWs crystal α-MoO3 bilayers that incorporate twisted stacks and engineered interfaces. By combining twist-induced dispersion engineering and polaritonic grating diffraction, we achieve tunable deflection and the asymmetric transmission of PhPs. PhP transmission can be switched between bidirectional and unidirectional modes by varying either the excitation frequency or the grating period. We further designed an asymmetric polaritonic lens that focuses forward-propagating PhPs while allowing normal backward transmission. These results provide a novel strategy for polaritonic steering and lay a solid foundation for designing on-chip optical isolators, diodes, and nonreciprocal routers.