Optical bimeron: A new topological state of light

Topological quasiparticles with sophisticated spin textures are intriguing objects in particle physics and magnetic materials that exhibit exotic physics and have potential applications in information storage and processing. The most fundamental and exemplary topological spin texture is called the skyrmion, which is a nanoscale circular domain wall carrying a nonzero integer topological charge.

The skyrmion texture was recently realized in structured optical fields, as a powerful tool to open new research directions of topological photonics. Since the first observation of optical skyrmions, the researchers have focused on the exploration of more general topological spin textures evolved from the skyrmion.

In a recent theoretical work carried out by a research fellow in University of Southampton, Dr. Yijie Shen, a new type of topological quasiparticle state in optical field beyond the limit of skyrmion, which is called the optical bimeron. Bimeron is a topological counterpart of skyrmion and can be seen as a combination of two half-skyrmions (merons) with opposite polarities.

The importance of bimerons has been well endorsed in fundamental particles and magnetic materials, because it possesses more general topological states than conventional skyrmion, but it was never studied in optical fields. Dr. Shen’s paper fills the gap, which has been published online on 29 July in the journal Optics Letters (https://doi.org/10.1364/OL.431122). This article has been highlighted as an Editor’s Pick.

The method to realize optical bimeron is to tailor a special family of structured vector beam in which the polarization Stokes vectors can construct devise textures akin to bimerons. Tailoring light is much like tailoring cloth, cutting and snipping to turn a bland fabric into one with some desired pattern.  In the case of light, the tailoring is usually done in the spatial degrees of freedom, such as its amplitude and phase (the “pattern” of light), and its polarization, while the cutting and snipping might be control with spatial light modulators and the like. For optical bimeron, we just need to tailor a bimeronic cloth for structured light.

“The meaning of bimeronic beams is towards the general control of topological light, which can be transformed to diverse generalized topological textures, including all the intermediate skyrmionic states among Néel-, Bloch-, and anti-skyrmion types as simple members.” says Dr. Shen. Moreover, in the paper, a graphical model is proposed to universally represent such general topological evolution of tunable bimeronic beams onto a 3D Poincaré-like sphere, as a vivid graphical toolkit to guide future applications.

As a remarkable merit of optical bimeron, it can propagate in free space with salient topology-dependent dynamics. A bimerionic beam, constructed by a fundamental and vortex spatial modes with different polarizations, will undergo a propagation-dependent dynamic phase difference between the two polarized components, which induces the longitudinal-variant vector patterns. Therefore, you can theoretically see multiple topological states in different transverse planes along the light propagation direction. Also, such 3D vector patterns of bimerionic beams with elegant topological characterization meet the urgently demanded techniques for higher-dimensional structured light control, which promises the applications of modern optical communications and encryption with increasing demands of large capacity and high speed in information encoding.

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