Few-Femtosecond XUV Pulse Pairs with Independently Tunable Topological Properties

By leveraging a recently developed slippage-compensation scheme in externally seeded free-electron lasers (FELs), we propose a method for generating few-femtosecond extreme-ultraviolet (XUV) pulse pairs with tunable topological properties and adjustable temporal delay. Simulations show that the topological charge l of each pulse can be independently set to l=-1, 0, 1, while the delay between the pulses can be tuned from a few to several tens of femtoseconds. Compared with superradiance-the only other currently viable approach for producing XUV pulse pairs with these characteristics-the proposed scheme offers greater flexibility, as it does not require driving the FEL into saturation, which typically necessitates a cascade configuration. Furthermore, when combined with pulse-to-pulse polarization switching, this approach has the potential to extend XUV studies of circular and helical dichroism in chiral systems and magnetic materials, as well as high-resolution imaging, into the few-femtosecond regime.