Loss-engineered reconfigurable polaritons.

Phonon polaritons, with their strong field confinement, enable extreme miniaturization of photonic devices. While optical loss is conventionally perceived as a fundamental limit to be compensated, here we demonstrate a novel scheme by harnessing leakage-induced loss as a degree of freedom for mode manipulation. By coupling 2D hyperbolic polaritons (e.g., α-MoO3) with 3D bulk leaky channels (e.g., β-Ga2O3), we achieve a topological reconstruction into quasi-one-dimensional (quasi-1D) polaritons over a broad frequency range. In the frequency domain, out-of-plane phonon-driven leakage in the substrate potently controls the propagation direction of quasi-1D polaritons, enabling continuous frequency-dependent rotation and culminating in an abrupt transition at the phonon resonance. In real space, the quasi-1D isofrequency contours enable on-chip reflectionless propagation across wide angles (>170°) due to their extreme momentum-space collimation. This approach is further generalized to other 2D/3D heterostructures (e.g., α-MoO3/CdWO4), demonstrating operability across various discrete infrared spectral windows (total Δλ ~ 7 μm). Our findings not only introduce a loss-engineering framework for precise polariton control but also open new possibilities for zero-reflection on-chip optical routing and advanced optical information processing.