Optical meron–antimeron pairs experimentally discovered for the first time, enabling λ/133 deep-subwavelength sensing

In a study published in PhotoniX, an international research team led by scientists from Air Force Engineering University, Nanjing University of Aeronautics and Astronautics, Southeast University, and the National University of Singapore has reported the first experimental observation of optical meron–antimeron pairs—a class of topological quasiparticles that were originally studied in magnetic systems.

Meron–antimeron pairs are topological defects that emerge from the conservation of topological charge. While they have been extensively studied in ferromagnetic materials, their optical counterparts remained largely unexplored. The team successfully generated these pairs in a plasmonic ring resonator with eightfold rotational symmetry, utilizing a dual-point-source excitation scheme that selectively addresses degenerate orbital states.

The researchers developed a group-theoretical framework to classify and control these topological states. They observed not only fundamental meron–antimeron pairs, but also higher-order pairs, target-type pairs, and even isolated merons and antimerons in plasmonic spin textures. The absolute skyrmion number of each pair was found to be strictly dictated by the orbital angular momentum index of the degenerate state. This dictation relation naturally gives a complete symmetry classification to the optical topological quasiparticles as the irreducible representations of symmetry groups. This framework further reveals a fundamental chirality-parity locking effect and enables the creation of isolated merons.

A key finding is the exceptional robustness of these topological configurations. When symmetry-breaking perturbations were introduced, the resonant frequency split while the vectorial topology remained intact. Leveraging this effect, the team demonstrated a sensing mechanism capable of detecting length variations as small as 1 mm, corresponding to an ultra-deep-subwavelength resolution of λ/133.

This work not only enriches the spectrum of optical topological quasiparticles but also establishes a symmetry-guided paradigm for designing structured light fields. The principles can be extended to other wave systems and frequency regimes, offering promising prospects for next-generation optical sensors, topological imaging devices, and high-security optical communication platforms.