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.
Highlights
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
In van der Waals nanomaterials and their layers, the interaction of light with different carriers leads to half-light-half-matter quasiparticles, such as plasmon polaritons in graphene[8,9,10,11,12], exciton polaritons in semiconductor monolayers[13,14], and phonon polaritons (PhPs) in polar materials[6,15,16,17,18,19,20,21,22], which all enable diffraction-less confinement and guiding of light at the nanoscale
PhPs in polar van der Waals (vdW) materials, such as hexagonal boron nitride endowed with natural hyperbolic response, offer a low-loss, highly confined and ray-like light propagation, enabling high-quality resonances, hyper-lensing, and nanoimaging[23,24,25,26]. This extreme form of optical anisotropy is inherently out of plane, while only recently we have discovered inplane hyperbolic PhPs in vdW α-MoO327–29 and α-V2O530 layers, which provides an unusual material platform for nanoscale light manipulation
Summary
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. PhPs in polar vdW materials, such as hexagonal boron nitride (hBN) endowed with natural hyperbolic response, offer a low-loss, highly confined and ray-like light propagation, enabling high-quality resonances, hyper-lensing, and nanoimaging[23,24,25,26]. This extreme form of optical anisotropy is inherently out of plane, while only recently we have discovered inplane hyperbolic PhPs in vdW α-MoO327–29 and α-V2O530 layers, which provides an unusual material platform for nanoscale light manipulation. Our work offers rational control of the optical field at the subdiffraction scale and opens an avenue for engineering PhPs in biaxial polar vdW materials
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