A Focusing Circular-Polarization THz Beam Splitter Based on a Self-Complementary Metasurface

Quasi-optical polarization beam splitters are important components of terahertz instrumentation widely used in interferometric and polarimetric measurements. Recently metasurfaces, i.e., two-dimensional periodic or quasi-periodic optically dense structures composed of unit cells with subwavelength dimensions, have been shown to operate as compact and efficient beam splitters. Typically, their design was based on careful optimization of anisotropic metal or dielectric resonant scatterers confined in each unit cell. In this work, we propose and experimentally demonstrate a simple and useful approach to designing circular-polarization beam splitters taking advantage of intrinsically frequency-independent properties of single-layer self-complementary metasurfaces (SCMSs). Theoretically, when illuminated with a circularly polarized beam, any SCMS at any frequency transmits a copolarized beam with a complex transmission coefficient of 1/2. At the same time, a cross-polarized beam of the same magnitude is produced, with a transmission phase that can be controlled at every point of a metasurface aperture. In this work, to split the copolarized and cross-polarized transmitted beams, we spatially modulate this phase by constructing a spatially nonuniform metasurface of self-complementary unit cells. With this approach, we experimentally demonstrate a focusing circular-polarization beam splitter operating near 0.345 THz.