The Efficient Excitation of High-Order Hyperbolic Phonon Polaritons in Boron Nitride Nanoshells.

High-order hyperbolic phonon polaritons provide superior light confinement and multimodal information pathways, offering a new degree for manipulating light at deep-subwavelength scales. However, their efficient excitation remains challenging due to the stringent requirements for both large wavevector composition and spatial symmetry matching. In this work, we demonstrate that zero-dimensional hexagonal boron nitride nanoshells (BNNSs), featuring atomically smooth nanocavities with high light field confinement, enable effective excitation of high-order hyperbolic phonon polariton whispering gallery modes (HHPP-WGMs). Using electron energy loss spectroscopy, we directly observed the HHPP-WGMs in BNNSs. Moreover, we show that BNNS-supported HHPP-WGMs can function as multimodal optical sources for coupling with hBN waveguides. By tailoring the sidewall thickness of BNNSs, the excitation amplitude ratio of the first high-order hyperbolic phonon polariton to the fundamental hyperbolic polariton can be tuned from 0.05 to 2 in hBN waveguides. These findings establish BNNSs as a promising platform for developing advanced multimodal nanophotonic devices.