Topologically protected optical polarization singularities in four-dimensional space

Optical singularities play a major role in modern optics and are frequently deployed in structured light, superresolution microscopy, and holography. While phase singularities are uniquely defined as locations of undefined phase, polarization singularities studied thus far are either partial, i.e., bright points of well-defined polarization, or are unstable for small field perturbations. We demonstrate a complete, topologically protected polarization singularity; it is located in the four-dimensional space spanned by the three spatial dimensions and the wavelength and is created in the focus of a cascaded metasurface-lens system. The field Jacobian plays a key role in the design of such higher-dimensional singularities, which can be extended to multidimensional wave phenomena, and pave the way for unconventional applications in topological photonics and precision sensing.

Automated summary

Optical singularities are widely used in modern optics for applications such as structured light, superresolution microscopy, and holography. In this study, we demonstrate a complete and topologically protected polarization singularity that is created in the focus of a cascaded metasurface-lens system. This singularity, located in a four-dimensional space, opens up possibilities for unconventional applications in topological photonics and precision sensing.