Nonlinear Metasurfaces for Completed Control of Amplitude, Phase, and Polarization in Broadband Terahertz Generation.

Terahertz (THz) generation is a crucial initial step in THz applications. However, the current THz sources face challenges in fully controlling the propagation properties of generated THz waves without the use of external devices. This limitation leads to bulky systems with unavoidable insertion losses and bandwidth constraints. To overcome these challenges and facilitate compact and versatile THz applications, a novel approach using nonlinear metasurfaces is proposed to control the amplitude, phase, and polarization of broadband THz waves directly and simultaneously at the emission stage. The basic design features an elaborated coupling-controlled chiral meta-atom, providing adjustable chirality and allowing an independent amplitude and phase control strategy under a circularly polarized (CP) pump. Furthermore, the polarization state of emitted THz wave can be arbitrarily customized by designing the superposition of the generated left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) components. This control is linearly predictable, eliminating the need for complex nonlinear simulations and interleaved supercell arrangements. The effectiveness of this method is demonstrated by experimentally generating two types of unique vectorial THz fields: spatial-polarization separable and nonseparable states. The proposed approach significantly enhances the capabilities of nonlinear metasurfaces, paving the way for versatile THz generation devices.