Nearfield observation of spin-orbit interactions at nanoscale using photoinduced force microscopy.

Optical spin and orbital angular momenta are intrinsic characteristics of light determined by its polarization and spatial degrees of freedom, respectively. At the nanoscale, sharply focused structured light carries coupled spin-orbital angular momenta with complex 3D nearfield structures, crucial for manipulating multidimensional information of light in nanophotonics. However, characterizing these interactions faces challenges with conventional farfield-based methods, which typically lack the essential accuracy and resolution to interrogate the structured nearfield with high fidelity. To address this challenge, we experimentally observe spin-orbit interactions at the nanoscale using photoinduced force microscopy. Such interactions are enabled through sharply focused circularly polarized optical vortices, which are then mapped by nearfield optical forces in high resolution. Because the optical forces can reveal both longitudinal and transverse nearfield structures with high fidelity, the spin-orbit interactions are eventually evaluated quantitatively at the nearfield, as an important inspiration to use the coupled momenta in dense optical nano-device systems.