Harmonic spin–orbit angular momentum cascade in nonlinear optical crystals

• Published in: Nature photonics Vol. 14; no. 11; pp. 658 - 662
• Main Authors: Tang, Yutao, Li, Kingfai, Zhang, Xuecai, Deng, Junhong, Li, Guixin, Brasselet, Etienne
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.11.2020; Nature Publishing Group
• Subjects: 639/624/400/385; 639/766/400/385; Angular momentum; Applied and Technical Physics; Barium; BBO crystals; Circular polarization; Crystals; Letter; Nonlinear control; Nonlinear optics; Optics; Photonic crystals; Physics; Physics and Astronomy; Quantum Physics; Second harmonic generation; Spin-orbit interactions
• ISSN: 1749-4885; 1749-4893
• DOI: 10.1038/s41566-020-0691-0

Optical angular momentum-based photonic technologies demonstrate the key role of the optical spin–orbit interaction that usually refers to linear optical processes in spatially engineered optical materials 1 . Re-examining the basics of nonlinear optics of homogeneous crystals under circularly polarized light 2 , 3 , we report experiments on the enrichment of the spin–orbit angular momentum spectrum of paraxial light. The demonstration is made within the framework of second-harmonic generation using a crystal with three-fold rotational symmetry. Four spin–orbit optical states for the second harmonic field are predicted from a single fundamental state owing to the interplay between linear spin–orbit coupling and nonlinear wave mixing; three of these states are experimentally verified. Besides representing a spin-controlled nonlinear route to orbital angular multiplexing 4 , modal vortex light sources 5 , 6 , high-dimensional parametric processes 7 and multi-state optical magnetization 8 , our findings suggest that the fundamentals of nonlinear optics are worth revisiting through the prism of the spin–orbit interaction of light. An experimental study of the second-harmonic-generation process in a beta barium borate crystal shows that homogeneous optical crystals can exhibit the rich physics of the spin–orbit angular momentum cascade in the nonlinear optical regime.