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

• Published in: Laser & photonics reviews 2022-07, Vol.16 (7), p.n/a
• Main Authors: Bouchal, Petr, Bouchal, Zdeněk
• Format: Article
• Language: English
• Subjects: Angular momentum; Circles (geometry); Circular polarization; Crystal structure; Electron beams; geometric phase; Holograms; Holography; Light beams; Liquid crystals; Microspheres; optical vortices; phase imaging; Phase modulation; Plasmonics; Rainbows; Reflectors; Vortices; Water drops
• ISSN: 1863-8880; 1863-8899
• DOI: 10.1002/lpor.202200080

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 ≈120°) 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. The generation of vector vortex beams needs a spiral modulation of the geometric phase provided by technologically demanding operations. Here, the spin to orbital angular momentum conversion is discovered in water drops, creating spiral phase modulation of the natural rainbow light. Inspired by the rainbow formation, a simple generation of vector vortex beams is demonstrated using light reflection in glass microspheres.