Airy-Like Hyperbolic Shear Polaritons in High Symmetry van der Waals Crystals

Controlling light at the nanoscale by exploiting ultra-confined polaritons-hybrid light and matter waves-in various van der Waals (vdW) materials empowers unique opportunities for many nanophotonic on-chip technologies. So far, mainstream approaches have relied on interfacial techniques (e.g., refractive optics, meta-optics, and moire engineering) to manipulate the polariton wavefront. Here, it is proposed that orbital angular momentum (OAM) of incident light can offer a new degree of freedom to structure vdW polaritons. With vortex excitations, a new class of accelerating polariton waves is observed-Airy-like hyperbolic phonon polaritons (PhPs) in high-symmetry orthorhombic vdW crystal alpha-MoO3. In analogous to the well-known Airy beams in free space, such Airy-like PhPs also exhibit self-accelerating, nonspreading, and self-healing characteristics. Interestingly, the helical phase gradient of the vortex beam leads to asymmetry excitation of polaritons, as a result, the Airy-like PhPs possess asymmetric propagation features even with a symmetric mode, analogous to the asymmetry hyperbolic shear polaritons in low-symmetry crystals. The finding highlights the potential of OAM to manipulate polaritons in vdW materials, which can be further extended into a variety of applications such as active structured polaritonic devices. This work demonstrates a new class of accelerating polariton waves-Airy-like hyperbolic phonon polaritons (PhPs) in vdW crystal alpha-MoO3 through orbital angular momentum (OAM) excitation. Such in-plane waves exhibit typical Airy characteristics and possess asymmetric propagation features, analogous to shear polaritons in low-symmetry crystals. Such OAM-assisted generation of PhPs opens up new avenues for actively controlling light at the nanoscale. image