Controlled synthesis of THz beams spanning the higher-order Poincaré sphere
Terahertz (THz) structured fields comprising separable and nonseparable spin and orbital angular momentum states offer unique opportunities for light-matter interactions in chiral media and material systems containing topological electronic spin excitations. This requires deterministic synthesis and control of topological vector vortex states that can be conveniently mapped onto the two-dimensional higher-order Poincaré (HOP) sphere. Here, we establish a nonlinear method for synthesizing vector vortex THz beams corresponding to the HOP states using optical rectification driven by a pair of infrared pump pulses with polarization singularities. The method provides two independent control parameters that enable flexible access to arbitrary states on the HOP sphere. The resulting mapping is experimentally validated using representative states, covering linear, circular, and elliptical regimes, in excellent agreement with theoretical predictions. Our approach establishes a two-parameter control framework for structured THz field synthesis, providing a physically transparent and scalable route to topological engineering in the THz regime.