Skyrmions are topologically protected field configurations characterised by a topological index, the skyrmion number. Optical skyrmions are ideally suited to investigate topological structures due to the ease of generating arbitrary light fields, and the freedom from energy constraints encountered by, for example, magnetic skyrmions. Building on our previous work of a topologically defined skyrmion number,1 here we demonstrate the conservation of the skyrmion number of hedgehog skyrmions and bimerons under propagation. We furthermore generate tunable multi-skyrmions from superpositions of oppositely polarised Gaussian and split-vortex beams of different waists, and find that the skyrmion number is conserved as a function of waist scaling. For both cases, the topological definition of the skyrmion number forms an intuitive geometric approach to understanding the underlying topology and to identifying the individual skyrmion structures.
We show that Skyrmions provide a natural language and tool with which to describe and model structured light fields. These fields are characterised by an engineered spatial variation of the optical field amplitude, phase and polarisation. In this short presentation there is scope only for dealing with the simplest (and perhaps most significant) of these namely those that can be designed and propagate within the regime of paraxial optics. Paraxial Skyrmions are most readily defined in terms of the normalised Stokes parameters and as such are properties of the local polarisation at any given point in the structured light beam. They are also topological entities and as such are robust against perturbations. We outline briefly how Skyrmionic beams have been generated to order in the laboratory. Optics gives us access, also, to the Skyrmion field and we present the key properties of this field and show how it provides the natural way to describe the polarisation of structured light beams.
Polarimeters/ellipsometers are important tools in a wide range of fields, including remote sensing, material characterisation, radar, and biomedical research. We have recently developed and demonstrated a polarimetry technique based on the total internal reflection of light in a glass cone, utilising the polarisation dependence of Fresnel reflection. This technique offers true broadband one-shot polarimetry in a compact design without moving parts. We will outline the underlying scientific principles, fidelity and efficiency, associated applications in two-photon microscopy (developed as QuantIC projects) and give an outlook on our future polarimetry development.
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