Optical meron-antimeron pairs in doubly degenerate plasmonic orbitals

Abstract Meron-antimeron pairs are topological defects that are created in the context of topological charge conservation. Although meron-antimeron pairs have been intensively studied in ferromagnetic materials where they nucleate from pairing of merons and their antiparticles, antimerons, their equivalents in optics have not been fully explored so far. Here, we experimentally demonstrate the existence of a universal class of optical meron-antimeron pairs in doubly degenerate plasmonic orbitals, including fundamental and higher-order meron-antimeron pairs, as well as their target-type counterparts. We develop a group-theory-based framework revealing the symmetry-dictated topological invariants, excitation conditions and symmetry classifications of meron-antimeron pairs. In particular, we demonstrate how the topological configurations of meron-antimeron pairs in degenerate orbitals give rise to highly-localized isolated (anti)merons in plasmonic spin textures beyond the conventionally observed lattice or clustered forms. A chirality-parity locking relation is further explored between the (anti)meron handedness and the orbital parity. The revealed meron-antimeron pairs exhibit excellent robustness to perturbation, whose responses to the perturbation introduce a splitting of orbitals. The effect numerically demonstrates an extremely deep-subwavelength sensing resolution down to $$\lambda /133$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>λ</mml:mi> <mml:mo>/</mml:mo> <mml:mn>133</mml:mn> </mml:mrow> </mml:math> . Generalizing the meron-antimeron pairs to optical systems provides various possibilities for the applications in optical vectorial imaging, deep-subwavelength sensing and metrology.