Relativistic spin-polarized (SP) lepton beams are important in fields such as nuclear physics, high-energy physics, materials physics, and atomic physics. In practice, transversely SP lepton beams are typically generated first, then rotated into longitudinal polarization through spin rotators for subsequent interactions and experimental studies. However, current spin manipulation systems mainly rely on dipole magnets or solenoids, resulting in significant challenges in terms of facility design, cost, limited tunability, and operational flexibility. Terahertz (THz) acceleration technology, particularly when based on a dielectric-lined waveguide (DLW), is one of the research hotspots for new desktop accelerators. It features moderate structural dimensions, flexible and controllable electromagnetic modes, high electric field breakdown thresholds, and a large charge capacity for particle beams. We explore the possibility of utilizing a THz wave in a DLW to attain spin rotation capability. We find that this method can rotate transversely SP lepton beams into longitudinal polarization within picoseconds, achieving a rotation efficiency of over 98%. Moreover, using THz waveguides not only maintains the high quality of the particle beam but also significantly reduces its energy spread. This novel lepton beam spin rotation scheme not only provides a fresh perspective for spin manipulation of particle beams but is also expected to promote the widespread application of SP particle beams in fields such as nuclear physics, high-energy physics, materials physics, and atomic physics, thereby opening up more potential application prospects.

relativistic electron beam,waveguide,spin polarization