  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.
