Electrically Switchable Ultraslow Dispersionless Polaritons via Twist Engineering in van der Waals Heterostructures.

Polaritons in van der Waals materials provide a powerful platform for controlling slow light at nanoscales. However, realizing ultraslow polaritons in the technologically critical low-momentum regime (<10 μm-1) has so far remained elusive, which severely limits their practical applications. Here, we report a mechanism for realizing dispersionless phonon polaritons in hyperbolic hetero-bicrystals via twist engineering. We demonstrate ultraslow polaritons with group velocity down to 10-5c in the low-momentum regime, representing a 100 times improvement over previous reports. This approach is broadly applicable across diverse material systems, enabling highly tunable polaritons with wide spectral coverage. Furthermore, integration of a gated graphene layer enables dynamic electrical switching between ultraslow and fast polariton modes with 2 orders of magnitude tunability. Our findings open up a new avenue for developing nanoscale programmable slow light devices.