Photonic Crystal Slab Generates Meron Spin Textures in Momentum Space, Advancing Compact Light-Field Control

A thin film of dielectric material, pierced by a regular pattern of tiny etched holes and hanging free with nothing beneath it, sits at the center of a new paper in Physical Review Letters. When researchers shine a circularly polarized laser of a single color onto it, the light that comes through carries a structured pattern: a meron spin texture, written into the light's momentum space.

The work comes from teams at Fudan University and Nanyang Technological University. Lei Shi is a co-senior author and Jiajun Wang a co-corresponding author. Both spoke with Phys.org's Ingrid Fadelli about it. The paper is credited to Rao and colleagues and carries the DOI 10.1103/3g3j-mnh9.

Some background helps. Topological spin textures are spatial patterns tied to the intrinsic angular momentum of particles, and they have proved useful for spintronics and quantum technologies. Skyrmionic textures, which are stable two-dimensional patterns of spin orientation, are among the most studied. Interest has recently spread into optics and photonics. Physicists there have mostly examined these textures in real space. Momentum space, where light is described by its in-plane momentum, has received less attention, and that is where this team worked.

The starting point was Shi's group's long interest in bound states in the continuum, or BICs. These are unusual states that do not radiate and stay bound even in open systems, at energies where waves would normally leak or spread out. Shi describes them as important topological singularities that allow extremely high quality factors and carry polarization vortex configurations in momentum space. His group had already studied their topology in vectorial lasing, the generation of optical vortices, and the spin Hall effect of light.

At the same time, the researchers were drawn to progress on skyrmionic light fields, structured patterns in the physical vectors of light that mirror the topology of magnetic skyrmions. Shi says the field is still early and that generating such light fields is the critical step on which further study and applications depend. Earlier research had linked their generation to optical vortices, and the team built on that link. They wanted to understand how the polarization topology of BICs relates to skyrmionic light fields and what mechanisms drive the connection. They also wanted a compact, simple way to make them. Most proposed generation methods are complex and need bulky systems, and Wang calls compact, miniaturized generators a cutting-edge research focus.

Their answer was the photonic crystal slab, made by etching periodic holes into a free-standing dielectric film. Focusing the circularly polarized laser onto it produces optical merons, one type of skyrmionic light field. To check the result, the team used a home-made measurement system based on Fourier optics. It recorded the far-field polarization distribution of the transmitted light and yielded the corresponding Stokes parameters. The normalized Stokes vectors serve as the pseudo-spins of the structured light. Analysis of those measurements revealed momentum-space spin textures in the transmitted light, validating the approach.

Shi says the method also reveals a connection among BICs, momentum-space polarization vortices, phase vortices and skyrmionic light fields. Wang points to a practical advantage over traditional metasurfaces: because BICs exist in photonic crystal slabs, which are periodic structures with no real-space center, the method needs no alignment.

Other groups could soon adopt the approach to study topologically structured light. Longer term, the authors see uses in optical information processing, optical measurements and quantum optical devices. Their next steps are to keep exploring the hidden effects of momentum-space topology enabled by BICs, and to examine how the generated light fields propagate and how stable they are under disturbance.