Twisted Rainbow Light and Nature-Inspired Generation of Vector Vortex Beams

Twisted vector light beams (optical vortices) arise from a spiral modulation of the geometric (Pancharatnam-Berry) phase converting the light spin to the orbital angular momentum. The preferred geometric-phase elements using liquid crystals and plasmonic metasurfaces realize this conversion by structuring their building blocks, i.e., precisely orienting individual crystal molecules or plasmonic nanoantennas. Here, an analogous mechanism is discovered in the spiral phase modulation of light reflected by dielectric spheres and first demonstrated in natural phenomena, namely in the rainbow formation. The spiral geometric phase is documented by holographic imaging of full circle primary and secondary rainbows created in the laboratory. The measurement uses a wide-angle holographic camera (field of view approximate to 120 degrees) taking time-resolved self-correlation holograms (300 ms). The holograms allow a quantitative restoration of the spiral geometric phase of light reflected from thousands of randomly falling water drops. The capability of individual drops to generate vector vortex beams under circularly polarized illumination is proven theoretically and demonstrated in experiments using glass microspheres. The spherical reflectors are discovered as simple generators of vector vortex beams and vortex arrays, inspiring novel geometric-phase elements