Airy-like hyperbolic shear polariton in high symmetry van der Waals crystals

Abstract: 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 interfacial techniques (e.g., refractive optics, meta-optics and moire engineering) to manipulate polariton wavefront. Here, we propose that orbital angular momentum (OAM) of incident light could offer a new degree of freedom to structure vdW polaritons. With vortex excitations, we observed a new class of accelerating polariton waves - 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 vortex beam leads to asymmetry excitation of polaritons, as a result, the Airy-like PhPs possess asymmetric propagation feature even with a symmetric mode, analogous to the asymmetry hyperbolic shear polaritons in low-symmetry crystals. Our finding highlights the potential of OAM to manipulate polaritons in vdW materials, which could be further extended into a variety of applications such as active structured polaritonic devices.