Direct Generation of Terahertz Vector Beams with Symmetry-Controlled Optoelectronic Metasurface

A broad range of materials can serve as sources of terahertz radiation when exposed to femtosecond pulsed optical fields, due to effects such as coherent optical rectification, transient photocurrent generation, or ultrafast demagnetization. While various sources have become quite powerful, the ability to design and actively control polarization properties of the emitted terahertz fields remain limited by often restrictive light-matter interaction symmetries. Here, we introduce a new class of optoelectronic metasurfaces consisting of asymmetric gold nanostructures on graphene, which can be designed for arbitrary local (nanoscale) and global (millimeter-scale) transient vectorial currents and corresponding terahertz radiation distributions. In particular, we demonstrate continuous terahertz polarization control with a C_3v symmetric Kagome lattice, as well as direct generation of elusive terahertz vector beams with radial and azimuthal arrays. Field strengths comparable to conventional 1 mm thick ZnTe sources have already been observed for linear arrays, with 30,000-fold thinner metasurfaces. These initial results suggest a variety of new capabilities for integrated terahertz sources with a high degree of design flexibility and active optical control opportunities.