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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mrow> <a:mi>l</a:mi> </a:mrow> </a:math> of each pulse can be independently set to <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mrow> <c:mi>l</c:mi> <c:mo>=</c:mo> <c:mo>−</c:mo> <c:mn>1</c:mn> </c:mrow> </c:math> , 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.