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Topological photonics inspired by the discovery of photonic topological insulators opens a path towards the discovery of fundamentally new states of light and potentially revolutionary applications, such as slow light, photonic circuitry, and topologically protected lasing. Recently, optical skyrmions, emerging as fire-new objects of study in topological photonics, were first proposed by Shai Tsesses and Luping Du et al. and may provide a new idea and approach to endow structured light and spatial-temporal light with topological properties. Resorting to vector field manipulations, different types of optical skyrmions have been generated in free space or by controlling the evanescent waves propagating along the surface of metal films. One of the most significant potential applications of optical skyrmions is the realization of the novel phenomena excited by the skyrmion-matter interactions at micro and nano scales, which could be applied to optical storage, high-resolution imaging, and precision metrology. However, there always exists a π/2 phase difference between the longitudinal and the transversal components of the focused light field under traditional tightly focusing systems in free space, and hence the generation of micro- or nano-scale optical skyrmions formed by electromagnetic field vectors in free space is still a challenge in this flourishing field.
Schematic of the generation of optical skyrmions formed by electromagnetic field vectors in the focal region of two pairs of counter-propagating cylindrical vector beams under 4π focal condition. PP1, PP2: homogeneous phase plate with a phase delay of π/2; BS1, BS2: beam splitter; L1, L2: objectives
Normalized distributions of the components of the Néel-type optical skyrmion formed by electric field vectors in the focal plane. (a)~(b) Radial and longitudinal components of the focused electric field in the focal plane; (c) Angular component of the focused magnetic field in the focal plane; (d) The cross sections of the individual components of the focused electromagnetic field
Vectorial distributions of the Néel-type optical skyrmion formed by electric field vectors in the focal plane. (a)~(b) Normalized energy density distributions of the focused electric field and magnetic field in the focal plane. The black arrows represent the projections of the electric or magnetic field unit vectors on the focal plane; (c) Three-dimensional vectorial structure of the optical skyrmion formed by electric field vectors within the red circle in (a); (d) Two-dimensional vectorial structure of the focal magnetic field in the central region of (b); (e) Orientations of the electric field unit vectors along the radial direction of the constructed Néel-type optical skyrmion; (f) Variation of θxy versus r; (g) Skyrmion numberdensity distribution of the constructed optical skyrmion in the focal plane
Normalized distributions of the components of the Bloch-type optical skyrmions formed by electromagnetic field vectors in the focal plane. (a)~(b) Angular and the longitudinal components of the focused electric field in the focal plane; (c)~(d) Angular and longitudinal components of the focused magnetic field in the focal plane; (e) Cross sections of the individual components of the focused electromagnetic field
Vectorial distributions of the Bloch-type optical skyrmions formed by electromagnetic field vectors in the focal plane. (a)~(b) Normalized energy density distributions of the focused electric field and magnetic field in the focal plane. The black arrows represent the projections of the electric or magnetic field unit vectors on the focal plane; (c)~(d) Three-dimensional vectorial structures of the optical skyrmions formed by electromagnetic field vectors within the red circles in (a) and (b); (e)~(f) Orientations of the electric field unit vectors and the magnetic field unit vectors along the radial directions of the constructed Bloch-type optical skyrmions; (g)~(h) Variations of θxy versus r; (i)~(j) Skyrmion number density distributions of the constructed optical skyrmions in the focal plane