Wavelength-adaptive spin–orbit orbital angular momentum management in three-wave mixing

Abstract Here we propose the use of an adjustable liquid crystal spin–orbit device (SOD) to shape bi-colour structured light to create bimodal states. We demonstrate the proof-of-principle for two individual wavelengths in a nonlinear optics framework. The SOD has an inhomogeneous optical axis orientation and birefringence, allowing it to modulate two wavelengths of light with pre-selected transmission functions by simply tuning a voltage. We combine this bi-colour functionality in a nonlinear optical experiment by employing three-wave mixing in a periodically poled crystal to show how the combined effect of linear spin–orbit transformation rules and nonlinear selection rules gives rise to novel approaches for light to modulate light, and light to unravel light. We show that the roles of the nonlinear crystal and SOD can be switched to either characterise the device with known light, or unravel unknown light with the device. This synergy between spin–orbit and nonlinear optics offers a novel paradigm where light manipulates and reveals its own structure across spectral domains.