Meron Spin Textures in Momentum Space Spawning from Bound States in the Continuum.

Topological spin textures, such as merons and skyrmions, have shown significance in both fundamental science and practical applications across diverse physical systems. The optical skyrmionic textures in real space have been extensively explored, but those in momentum space are still rarely studied. Here, we report the experimental generation of momentum-space meron spin textures via bound states in the continuum (BICs) in photonic crystal slabs. We show that under circularly polarized illumination, the momentum-space vortex topology of BICs can transform light with meron spin textures in momentum space. These merons exhibit polarity-switchable configurations controlled by incident light polarization. We theoretically and experimentally verify the generation of momentum-space merons and demonstrate their operational flexibility across a broad spectral range. Our results establish a connection between different momentum-space topologies and provide a robust and compact platform for generating topological spin textures.