Tailored spectral tuning of polaritons in isotope-enriched hyperbolic hetero-bicrystals via twist engineering

Phonon polaritons, which are hybrid light–matter quasiparticles, enable the confinement and manipulation of optical fields at the nanoscale. On-demand spectral tuning of polaritons is crucial for future integrated nano-optics yet remains a formidable challenge. Here, we report an unexplored family of isotopically enriched van der Waals hetero-bicrystals that provide a powerful platform for flexibly tailoring phonon polaritons via twist-engineered hybridization. We show that the polariton eigenmodes in such bicrystals feature dual spectral gaps accessible at distinct frequencies, thereby allowing polaritonic closed-loop circulation and configurable in-plane directional propagation within the “forbidden” regime. We demonstrate that the polariton dispersion can be selectively switched and controlled among three states (0, 1, or 2 gaps) by adjusting the thickness ratio and twist angle (over the entire 0°–90° range). Moreover, this dual-gap feature is broadly realizable across diverse material systems, enabling tailored polaritons with wide spectral coverage. These findings open up a new avenue for engineering polaritons and offer on-demand nano-light for various nanophotonic functionalities.