Gouy Phase Induced Optical Skyrmion Transformation in Diffraction Limited Scale

Optical skyrmions are topologically stable quasiparticles that can be constructed with electric field, spin angular momentum, polarization Stokes vector, pseudospin, etc. In this letter, both theoretical and experimental studies are carried out to reveal the role of Gouy phase in the topology transformation during the tight focusing of Stokes skyrmions. The Stokes skyrmionic beam can be constructed by superposing two orthogonally polarized components with orthogonal spatial modes. The Gouy phase produced in the focused field depends on the orbital angular momentum carried by the high order mode component of the incident Stokes skyrmionic beam. While the beam size of the focused field is diffraction limited, the variation of the Stokes vectors in the skyrmion topology is in the sub-diffraction limited scale. The presented results shed light on the understanding of the topology transformation between the incident and the tightly focused fields, paving the way for engineering the optical skyrmions in micro-nano scale and their applications in information processing, quantum technology, metrology, etc.