Skyrmions can remain its topology in nonlinear interaction

Recent advances in multicomponent condensates, quantum systems, and chiral magnetic materials highlighted nontrivial topological twists and knots in the continuous symmetries of their order parameters, which are known as skyrmions. Skyrmions are originally a special type of particles with topological spin textures, playing important roles in the realms of quantum fields, solid-state physics, magnetic materials, and recently being possible to be controlled in optical fields. Skyrmions have promised critical feature of diversified topologies, in other words, skyrmions can have many possible textures, like vortex, hedgehog, saddle, etc., being characterized by multiple topological numbers and extendable to unlimited higher dimensions, which have been touted as next-generation super-capacity information carriers. But the skymion textures are always fragile in the interaction with nonlinear media, limiting the further applications. The theoretical and experimental studies of nonlinear interaction of skyrmions are also still elusive.

In their paper recently published on Optica, a joint group of physicists, from China, UK, and Mexcio, report the world-first experimental demonstration of nonlinear conversion of optical skyrmions. Whereby, a new mechanism of nonlinear frequency conversion of arbitrary vector structured light was proposed, which was used to convert vector beams as optical counterpart of skyrmions with diversified topologies, including prior Néel-, Bloch-, and anti-skyrmion types. In contrast to conventional believe that the topological charges of light will surely be changed together with the frequency conversion, the new kind of nonlinear conversion allows the optica skyrmions to remain its topological structure in the process of nonlinear interaction, namely conformal frequency conversion. A geometric representation is also proposed to visualize the complete evolution of topologically robust skyrmions, which can vividly guide propagating skyrmions in nonlinear media for the further applications.

As a consequence of this insight, several significant advances and new perspectives are offered, “This is the first know experiment of nonlinear interaction of optical skyrmions, different topological textures have been tested in the nonlinear conversion, opening a new direction to study structured light.” says Zhihan Zhu, the corresponding author of the paper. “Skymion textures are always fragile in the interaction with nonlinear media, limiting the applications. Now this limit is broken, the optical skyrmions are pushed to be used as next-generation super-capacity optical information carriers in long-distance communications” says professor Bao-Sen Shi, the project manager.

Moreover, the researchers in this work systematically examined the conformal frequency conversion of skyrmions based on a nondegenerate sum-frequency generation with type-0 quasi-phase matchings in this study. Results show that by using the proposed methods and setup, a diversity of optical skyrmions constructed by both 2D and 3D (i.e., eigenmode and noneigenmode), in the IR region can be very efficiently upconverted into those in visible regions without changing their full spatial structure. This proof-of-principle work provides a roadmap to build a high-efficiency frequency converter for arbitrary vector-structured light/photons. Based on this solid demonstration, novel applications such as a high-dimensional quantum interface and polarization-resolved upconversion imaging will become feasible soon.