Light Pretending to be a Ferromagnet

Scientists from the University of Warsaw have demons­trated how to structure light such that its polari­zation behaves like a collective of spins in a ferromagnet forming half-skyrmion, the merons. To achieve this the light was trapped in a thin liquid crystal layer between two nearly perfect mirrors. Skyrmions in general are found, e.g., as elementary exci­tations of magneti­zation in a two-dimensional ferromagnet but do not naturally appear in electro­magnetic fields.

Spin texture of a second-order half-skyrmion (meron) on the surface of a birefringent cavity. (Source: M. Krol, Physics UW)

In magnetism, the elementary exci­tations in a two-dimensional magneti­zation vector field have the form of skyrmions. Going clockwise around the center of such a vortex, we can observe, that the vectors attached to subsequent points on our path can rotate once or many times, clockwise or anti­clockwise. Skyrmions and half-skyrmions (merons) of various vorti­cities can be found in such different physical systems as nuclear matter, Bose-Einstein condensates or thin magnetic layers. They are also used in the description of the quantum Hall effect, cyclones, anti­cyclones and tornadoes. Especially interes­ting are experi­mental setups, in which one can create various vector fields on demand and investigate the inter­actions of their excitations.

Scientists from University of Warsaw, Military University of Techno­logy, University of Southampton, Skolkovo Institute in Moscow, and Institute of Physics PAS have demons­trated how to structure light such that its polari­zation behaves like a half-skyrmion (meron). To achieve this the light has been trapped in a thin liquid crystal layer between two nearly perfect mirrors, an optical cavity. By controlling the polari­zation of incident light and the orienta­tion of the liquid crystal molecules they were able to observe first-order and second-order merons and anti-merons.

A relatively simple optical cavity filled with a liquid crystal enables the scientists to create and investigate exotic states of polari­zation of light. The device can potentially allow to test the behavior of these exci­tations – annihilation, attraction or repulsion of skyrmions and merons – on an optical table when combined with more exotic optically responsive materials. Recog­nizing the nature of the inter­action between these objects can help understand the physics of more complex systems, requiring more sophis­ticated experimental methods. (Source: U. Warsaw)

Reference: M. Król et al.: Observation of second-order meron polarization textures in optical microcavities, Optica 8, 255 (2021); DOI: 10.1364/OPTICA.414891

Link: Polariton Laboratory, Faculty of Physics, University of Warsaw, Warsaw, Poland

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