Generating Topological Nondiffracting Beams Using High-Quality-Factor Nonlocal Metasurfaces

Nondiffracting optical beams are essential tools in photonics, offering robust light transport, super-resolution imaging, and spatiotemporal control. While nonlocal metasurfaces have been proposed for structured-light generation due to their broad angular spectral dispersion and topological characteristics, experimental generation of nondiffracting beam with nonlocal metasurfaces have not been previously demonstrated. Here, we experimentally realize vortex Bessel beams using a nonlocal metasurface and uncover a link between nondiffracting beam generation and the curvature of the photonic bands. Depending on the sign of band curvature, the beams exhibit spatial asymmetry in nondiffraction, emerging either in front of or behind the metasurface. This asymmetry arises from a radial-phase gradient in momentum space, which induces an effective space compression or expansion. Furthermore, we demonstrate wavelength-dependent tunability of the beam diameter and propagation distance and show an order-of-magnitude enhancement in propagation distance compared to conventional Laguerre–Gaussian modes. These results establish nonlocal metasurfaces as a powerful platform for compact, tunable, and spatiotemporally controlled nondiffracting light.