Broadband twisted light beams generation using flat optics

In recent years, metasurfaces, a flat version of three-dimensional metamaterials, have become a versatile nanophotonics platform for unprecedented light manipulation. Amongst the numerous realizations of exotic optical phenomena through metasurfaces such as meta-lensing, meta-holography, and structured beams, Bessel beam generation caught significant attention due to its non-diffracting and self-healing nature. Bessel beams can be produced using various conventional methods, including spatial light modulators, composite holograms, and diffractive optical elements. These methods, however, are unfit to integrate with cutting-edge on-chip devices due to low throughput, polarization dependence, excessive bulkiness, and limited numerical aperture. In the method of generating Bessel beams through flat optics, the selection of suitable material is a crucial factor. Thus, due to constraints of inherent material properties, broadband operation with high efficiency remains challenging. Here, we demonstrated highly efficient broadband Bessel beams generating meta-devices in the visible domain using single-layer all-dielectric transmissive metasurfaces. The constituent nanoresonators of zinc sulfide (ZnS) are optimized for high-resolution phase modulation. ZnS offers the best-suited optical properties that ensure high transmission efficiency throughout the visible spectrum. To verify the proposed design technique, we realized single-element-driven meta-devices to generate Bessel beams with higher numerical apertures and different topological charges, illustrating their exceptional non-diffraction properties. For real-life applications like optical communication, the proposed design strategy could speed up consumer-level device implementation.