Changing light’s polarization can reverse the structure of a patterned light field, opening a new way to control geometry and information in optics.
(Nanowerk Spotlight) Light can be bent, focused, or split, yet its potential to hold structure within itself is only beginning to be understood. Researchers have learned that light can carry not just energy but geometry, forming patterns that remain stable because of their topology, a mathematical property that describes how something is connected rather than how it is shaped.
The idea comes from solid-state physics, where electrons in certain magnets can form tiny whirlpools of spin called skyrmions. These patterns behave like knots that cannot be undone without cutting through them, which makes them remarkably stable. That stability has inspired interest in using skyrmions to store or process information in magnetic materials.
Physicists have wondered whether light could support similar structures, stable configurations that would resist small disturbances while still allowing controlled switching between states. If possible, it could lead to optical systems that handle information through the geometry of their fields rather than through electrical charge.
Yet light presents unique challenges. Unlike spins in a crystal, it has no fixed background to hold its shape. Its structure must be engineered through materials that manipulate its phase, amplitude, and polarization with extreme precision.
Metasurfaces, patterned materials that can sculpt the behavior of light at scales smaller than its wavelength, have recently made that level of control possible. They can direct, delay, and rotate light with great accuracy, letting researchers build optical fields with almost any desired pattern.
At the same time, terahertz technology has advanced to the point where scientists can map electric fields in both space and time. These capabilities together allow direct observation of light’s internal twists and rotations and make the study of optical topology an experimental reality.
Schematics and microscopic image of the designed metasurface. a) Schematic image of multiple plasmonic vortex interference with topological charges of 0, 3, -3 on the metasurface. b) Schematic image of the 3D SAM vector distribution of optical spin multimeron. c) Microscopic image of metasurface which are composed by 246 slit resonators with identical geometric parameters: width = 10 μm and length = 70 μm. These slit resonators are distributed to 4 rings with radii of 1850, 1950, 2050, and 2150 μm, respectively. (Image: Reprinted with permission from Wiley-VCH Verlag)
Using a metasurface that produces surface plasmons, which are waves formed by coupled oscillations of light and electrons along a metal surface, the researchers created a cluster of optical vortices whose topological charge changes sign when the polarization flips from left-handed to right-handed. The ability to trigger a discrete topological switch is important because such changes can encode information in a form that is inherently resistant to noise and distortion.
The paper describes the creation of a multimeron, which is a group of six merons with alternating topological charge, in a surface plasmon field at terahertz frequencies. Surface plasmons combine oscillations of free electrons on a metal with an electromagnetic field that is confined to the surface. The experiment shows that by switching the polarization of the light, the sign of the topological charge reverses in a distinct jump rather than a smooth shift.
To understand what that means, it helps to recall how topology is used in magnetism. A skyrmion is a twist of spins that covers every possible orientation exactly once, giving it a charge called the skyrmion number. A meron covers half that sphere of orientations and carries a charge of plus one half or minus one half.
In optics, the concept of spin refers to the local rotation of the electromagnetic field, known as optical spin angular momentum. In the surface plasmon mode used here, called transverse magnetic, the electric field lies mostly along the surface with a smaller component pointing out of it. This gives rise to a three-dimensional spin vector at every point on the surface. By mapping how that vector twists from place to place, the researchers can calculate the skyrmion number and determine the topology of the field.
The metasurface in the experiment is made of aluminum patterned on a silicon substrate. It contains four concentric rings of narrow slits, each about 70 micrometers long. When illuminated by circularly polarized light at 0.75 terahertz, corresponding to a wavelength of about 400 micrometers, the rings generate three surface plasmon vortices with charges of zero, plus three, and minus three. The interference among these vortices produces a sixfold pattern of light intensity around a bright center.
More importantly, the interference locks in a stable arrangement of the optical spin that flips direction three times as one moves around the center, forming the pattern known as a multimeron. The researchers measured the electric field above the surface using a near-field terahertz microscope, which allowed them to record both the strength and the phase of the field. From these data they reconstructed all three components of the local spin vector.
The resulting maps reveal six wedge-shaped regions surrounding the center. Each region contains a meron with a skyrmion number close to plus or minus one half. The boundaries between the regions correspond to lines where the vertical component of the spin becomes zero. These are known as L lines.
Each region also contains a point where the spin points exactly upward or downward, called a C point. The distribution of these features shows an alternating pattern of positive and negative topological charge. When the researchers integrated the skyrmion density, also called the Chern density, over each wedge, they obtained an average magnitude of about 0.42, close to the theoretical 0.5.
Small deviations arise from measurement noise and the limits of reconstructing continuous fields from discrete samples. When they integrated over the whole area, the positive and negative contributions nearly canceled, giving a global value near zero, as expected for a structure containing equal numbers of positive and negative merons.
To test control over the topology, the researchers built a second metasurface that responds differently to left- and right-handed light. For left-handed circular polarization, it produces the same three-vortex interference pattern as before. For right-handed light, the order of the vortices reverses, flipping the signs of the local topological charges while leaving the overall pattern unchanged.
They gradually changed the polarization by rotating a quarter-wave plate relative to a polarizer so that the state moved smoothly from left-handed through linear to right-handed on the Poincaré sphere, which represents all possible polarization states. At each position they recorded new field maps and calculated the skyrmion number within two representative meron regions.
The skyrmion number remained nearly constant while the polarization stayed within one hemisphere of the sphere. When the polarization crossed the equator, corresponding to linear polarization, the skyrmion number switched abruptly from positive to negative. The transition matched numerical simulations and confirmed that the handedness of light determines the sign of the topological charge.
This behavior reflects a direct topological relationship. Within a single hemisphere of the Poincaré sphere, the polarization keeps the same handedness and the field remains topologically identical. Crossing the equator reverses handedness, forcing a discrete change in topology. The field cannot deform continuously between these two states. This makes polarization a simple and effective control for topological switching in optical systems.
Transition of optical spin topological texture. a) Topological textures with various incident polarization states characterized by points on the Poincaré sphere. The points on the yellow line from the north pole to the south pole of Poincaré sphere represent the angle 𝜃 between the fast axis of the quarter-wave plate and the linear polarizer is -45°, -35°, -25°, -15°, 0° (green star, linear polarization), 15°, 25°, 35°, and 45°, respectively. All insets consist of four small panels, describing the characteristics of the SP field excited by the incident polarization states on the Poincare sphere. The top-left panel shows the calculated electric field intensity distribution, while the bottom-left panel represents the calculated z-component SAM distribution. The topright and bottom-right panels show the corresponding experimental results. All data are normalized. b) The theoretical and experimental Chern density distributions of topological textures on red and blue star, respectively. c) The evolution of skyrmion numbers as the variation of incident polarizations, demonstrating the topological transition. (Image: Reprinted with permission from Wiley-VCH Verlag) (click on image to enlarge)
The experiment included several checks for accuracy. The researchers reconstructed all three components of the spin vector rather than inferring them from partial data. They identified the singular points and lines that define the field boundaries and tested how the total topological charge changes with area and measurement noise. The uniformity of results across all six merons supports the reliability of the reconstruction.
The approach can be extended. Because the multimeron field results from interference among several vortices, adding more vortices or changing their charges could generate more complex textures such as lattices of merons or skyrmions. The same design principles could be applied at visible or infrared wavelengths using other materials.
Fields with well-defined regions and stable boundaries may be useful for trapping and manipulating microscopic particles. Polarization control would allow rapid, reversible changes to those trapping patterns without altering the underlying structure.
This study demonstrates that topological invariants in light can be both measured and switched using polarization alone. By showing that the handedness of light directly determines the sign of the skyrmion number in an optical multimeron, it provides a clear experimental link between optical chirality and topological charge. This link establishes a framework for developing photonic systems where information or function depends on the geometry of light itself rather than on the materials that guide it.
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