Dynamic Control of Light Chirality with Nanostructured Monolayer Black Phosphorus for Broadband Terahertz Applications

Abstract Dynamically tunable polarization conversion at the nanoscale is essential for the realization of novel on‐chip photonic devices. To achieve the desired polarization control, typically, nanopatterning of resonant metallic or dielectric structures in anisotropic or chiral geometries is required, in many cases limiting the performance to narrowband, nontunable operation. In this work, by taking advantage of the strong anisotropic surface conductivity of black phosphorus (BP), polarization conversion is demonstrated via the coherent excitation of localized surface plasmons (LSPs) in symmetrically patterned monolayer BP nanosquare arrays. Using finite‐difference time‐domain simulations, linear‐to‐circular and circular‐to‐linear polarization conversion, dynamic tunability and switching, and broadband operation in the terahertz (THz) regime, all enabled by means of electrostatic control of the carrier concentration of BP, are demonstrated. Last, it is demonstrated how near‐field coupling between the LSPs and dipole emitters positioned at chosen hotspots can mediate the emission of circularly polarized light (CPL), also providing enhanced emission rates. The proposed design offers a flexible, ultrathin, platform for wave manipulation in the THz regime, enabling active control of light polarization by nanoscopic on‐chip optical sources.