Terahertz spin-selective dielectric metasurfaces from exceptional points

Abstract Exceptional points (EPs) are spectral singularities where eigenvalues and eigenvectors coalesce, typically requiring complex couplings and enabling phenomena like unidirectional propagation. Meanwhile, metasurfaces, composed of artificially engineered arrays of subwavelength structures, enable unprecedented manipulation of electromagnetic waves. Recently, by tuning the coupling within a unit cell to realize an EP in the parameter space, metasurfaces with EPs have provided insights into the manipulation of light’s spin states. However, metasurfaces with EPs based on complex couplings exhibit obvious residual zero-order diffraction in manipulating light’s spin states. Here, we experimentally demonstrate a method to construct EPs in the parameter space without complex couplings within a unit cell, effectively suppressing residual zero-order diffraction. The EPs originate from the superposition of the accumulated phases from tailoring the sizes and relative rotation angles of the two pillars in a compound unit cell, forming nodal lines consisting of EPs in the parameter space. Using terahertz imaging technology, we experimentally observe spin-selective vortex beam generators and terahertz lenses constructed by metasurfaces with EPs in the parameter space. Our finding offers alternative insights to implement spin manipulation in metasurfaces and spin-dependent wavefront engineering devices.