Photonics Breakthroughs 2024: Nearfield observation of spin-orbit interactions at nanoscale

The spin-orbit interaction (SOI) of light describes the interaction between spin angular momentum, related to circular polarization, and orbital angular momentum, associated with spatial wavefront structures. While typically trivial in conventional farfield optics, the SOI can become significant at subwavelength scales in the nearfield, where complex vectorial light fields arise. This phenomenon offers exciting prospects for applications in micro-/nano-photonic devices, optical sensing and quantum information technology. In this review, we highlight recent advances in the investigation of the SOI, with particular emphasis on the demonstration of accurate nearfield SOI characterization using photoinduced force microscopy. This technique enables direct, high-resolution mapping of longitudinal and transverse nearfield components of sharply focused optical vortex beams, providing quantitative insight into the spin-to-orbit conversion process. We also discuss existing challenges and outline future directions for exploratory research on the SOI of light in advanced optical systems.