Flatband polaritonic router in twisted bilayer van der Waals materials

Tuning polariton hybridization in hyperbolic hetero-bicrystals by twist angle engineering

Phonon polaritons are hybrid light-matter quasiparticles that enable confinement and control of electromagnetic modes at the nanoscale. Particular interest has been paid to hetero-bicrystals composed of molybdenum oxide (α-MoO3) and isotopically pure hexagonal boron nitride (h11BN), which feature polariton dispersion tailorable via the spectral gap originating from polariton hybridization. In this work, we propose unexplored hetero-crystals assembled from Ca-intercalated metal oxide α'-(Ca)V2O5 and α-MoO3, allowing the polaritons to travel along closed trajectories inside the bicrystal within the spectral gap. We systematically study the dependence of the spectral gap on the twist angle and thickness ratio of constituting layers in α'-(Ca)V2O5/α-MoO3 and the initial h11BN/α-MoO3. We show that on-off switching and strong tuning of the spectral gap in polariton dispersion can be realized by varying the twist angle in both structures. In particular, the spectral gap in α'-(Ca)V2O5/α-MoO3 can exist over a wide range of twist angles, three times as broad as that in h11BN/α-MoO3. Moreover, the spectral gap can also be significantly tuned by altering the thickness ratio. The spectral gap in α'-(Ca)V2O5/α-MoO3 emerges in a higher frequency range that is not achievable in h11BN/α-MoO3. Our results demonstrate that α'-(Ca)V2O5/α-MoO3 and h11BN/α-MoO3 provide two powerful bicrystal systems for polariton engineering via tuning the twist angle and thickness ratio.

Unidirectional hyperbolic whispering-gallery phonon-polariton excitation in boron nitride nanotubes

In two-dimensional (2D) hyperbolic materials, energy is directed into their deep subwavelength polaritonic modes through four narrow beams. Hyperbolic whispering-gallery mode nanocavity-confined phonon polaritons (PhPs) display a strongly enhanced light-matter interaction in the infrared regime. Particularly, the unidirectional phonon-polarization excitation in nanocavities has a potential application value in an on-chip integrated optical circuit design, efficient optical sensors, and enhanced spectral technology. Here, we explore the hyperbolic whispering-gallery mode PhPs on the cross section of a hexagonal BN nanotube (BNNT) and demonstrate that efficient unidirectional excitation can be achieved using a circularly polarized electric dipole, combining with optical spin-orbit coupling. Our results demonstrated that the undirectionality of the hyperbolic polariton propagation in a nanocavity can be conveniently achieved, independent of the structure symmetry of the nanocavity, providing potential applications in nanoscale light propagation, on-chip optical devices, and communication.

Guided spiraling phonon polaritons in rolled one-dimensional MoO3 nanotubes

Polaritons in reduced-dimensional materials, such as nanowire, nanoribbon and rolled nanotube, usually provide novel avenues for manipulating electromagnetic fields at the nanoscale. Here, we theoretically propose and study hyperbolic phonon polaritons (HPhPs) with rolled one-dimensional molybdenum trioxide (MoO3) nanotube structure. We find that the HPhPs in rolled MoO3 nanotubes exhibit low propagation losses and tunable electromagnetic confinement along the rolled direction. By rolling the twisted bilayer MoO3, we successfully achieve a canalized phonon polaritons mode in the rolled nanotube, enabling their propagation in a spiraling manner along the nanotube. Our findings demonstrate the considerable potential of the rolled MoO3 nanotubes as promising platforms for various applications in light manipulation and nanophotonics circuits, including negative refraction, waveguiding and routing at the ultimate scale.