Edge-oriented and steerable hyperbolic polaritons in anisotropic van der Waals nanocavities

Abstract

Highly confined and low-loss polaritons are known to propagate isotropically over graphene and hexagonal boron nitride in the plane, leaving limited degrees of freedom in manipulating light at the nanoscale. The emerging family of biaxial van der Waals materials, such as α-MoO3 and V2O5, support exotic polariton propagation, as their auxiliary optical axis is in the plane. Here, exploiting this strong in-plane anisotropy, we report edge-tailored hyperbolic polaritons in patterned α-MoO3 nanocavities via real-space nanoimaging. We find that the angle between the edge orientation and the crystallographic direction significantly affects the optical response, and can serve as a key tuning parameter in tailoring the polaritonic patterns. By shaping α-MoO3 nanocavities with different geometries, we observe edge-oriented and steerable hyperbolic polaritons as well as forbidden zones where the polaritons detour. The lifetime and figure of merit of the hyperbolic polaritons can be regulated by the edge aspect ratio of nanocavity.

Efficient and Tunable Reflection of Phonon Polaritons at Built‐In Intercalation Interfaces

“…[ 45 ] In the meantime, particular attention should be paid to the directional reflection interface and in‐plane anisotropic (hyperbolic and elliptical) propagation of PhPs in α‐MoO 3 . [ 26,46 ] We also note that all the thicknesses of α‐MoO 3 flakes used in our experiment and simulation range from 60 to 160 nm. In flakes down to the few‐layer or monolayer limit, the reflection of PhPs may follow a different mechanism due to the extreme confinement in this regime.…”

“…Efficient reflections at oriented interfaces only happen when the polariton wave vector intersects with the dispersion lines. [ 26,39 ] Clearly, the <302> orientations of our embedded interfaces fulfill this requirement and hence allow for the effective reflections of PhPs in both RBs 2 and 3.…”

“…Beyond in‐plane isotropic hBN, more recent findings have discovered the strong dependence on the edge orientation of the reflection of PhPs in in‐plane hyperbolic alpha‐phase molybdenum trioxide (α‐MoO 3 ). [ 26 ] Nevertheless, the characteristics and tunability of PhP reflections remain largely unexplored, particularly at non‐topographic interfaces.…”

Tailoring Light with Layered and Moiré Metasurfaces

“…Particularly dramatic effects arise when each layer is extremely anisotropic, such as in the case of a hyperbolic response, which corresponds to an optical response that is metallic along one characteristic direction but dielectric in the orthogonal one [31,32]. Such extreme anisotropy can guide the flow of near-field electromagnetic waves in the form of polaritons, which are coupled excitations and oscillations of photons and matter [100][101][102][103]. Polaritons can emerge in various forms, for example, plasmon polaritons (photons interacting with free or nearly free electrons), phonon polaritons (photons coupled with ion lattice vibrations), exciton polaritons (photons coupled with electron-hole pairs), and many others [103].…”

Enhanced broadband absorption with a twisted multilayer metal–dielectric stacking metamaterial

“…[28][29][30] As the twisted stacking structures enable the exotic dispersion engineering and enhanced light-matter interactions. 31,32 Inspired by the idea of twisted magic angle and the popularity of three-dimensional structures, 33,34 a new method is proposed to enhance the absorption bandwidth of multilayer metaldielectric stacking metamaterial in this paper. By individually rotating each stacking layer along the vertical symmetry axis at a certain angle, the absorption bandwidth of the proposed structure at higher frequency region can be effectively enlarged.…”