Frequency-dependent topological polaritons in carbon nanotube array/hBN heterostructures.

Plasmons in carbon nanotubes (CNTs) have attracted significant attention due to their strong spatial confinement and high-quality factors. However, constrained by CNTs' intrinsic electronic characteristics, modulation of their plasmon dispersion relations remains a significant challenge. Here, we report frequency-dependent topological polaritons arising from the coupling between hyperbolic plasmons and phonon polaritons in CNT-array/hexagonal boron nitride (hBN) heterostructures. In particular, we achieved a controllable topological transition of the polariton wavefront from hyperbolic to elliptical. Moreover, we demonstrated a whispering-gallery polaritonic mode confined in closed-loop CNT array on hBN. Our findings provide fundamental insights into optical topological transitions in low-dimensional heterostructures, and a promising route to manipulate light propagation and energy transfer at the nanoscale.