Ultrafast XUV pulse pairs with tunable topological properties

In recent years, the FERMI free-electron laser (FEL) has progressively expanded the control of the transverse structure of intense extreme-ultraviolet (XUV) pulses, from the first demonstration of coherent XUV optical vortices to the recent synthesis of fully structured Poincaré beams with spatially varying polarization and phase profiles. These advances have established seeded FELs as a unique platform for engineering complex light fields in the XUV and exploring their interaction with magnetic, chiral, and topologically non-trivial systems.

Building on this progress, researchers from the FERMI FEL and RIKEN SPring-8 Center have now proposed a new method for generating pairs of ultrashort XUV laser pulses with independently controllable orbital angular momentum (OAM), polarization, and temporal separation. The work, recently published in Physical Review Letters, opens new opportunities for investigating ultrafast topological light–matter interactions in the XUV regime.

Over the past decade, FELs have become powerful tools for tailoring intense XUV radiation in time, frequency, polarization, and spatial structure. In particular, structured XUV beams carrying OAM — often referred to as optical vortices or twisted light — have attracted growing interest because of their sensitivity to magnetic textures, chirality, and nanoscale spatial symmetries. However, combining these structured beams with few-femtosecond temporal control has remained an outstanding challenge.

The newly proposed FEL scheme addresses this limitation by exploiting a recently developed slippage-compensation technique in externally seeded FELs. The approach uses two tapered radiators separated by a small magnetic chicane. A chirped ultraviolet seed laser first imprints a tailored energy modulation onto a relativistic electron beam in a short undulator (modulator), which is then converted into a corresponding density modulation using a bunching chicane, before the beam enters the tapered radiators. As the beam propagates through the radiators, two ultrashort XUV pulses are generated sequentially, with adjustable temporal separation; see Fig. 1.

Figure 1: Scheme to generate two delayed few-femtosecond XUV pulses with tunable OAM.

Importantly, the OAM — or equivalently the topological charge — of each pulse can be independently controlled. Simulations demonstrate pulse pairs with topological charges l = -1, 0, and 1, including configurations where the two pulses carry opposite OAM and opposite circular polarization. The generated pulses are only about three femtoseconds long while maintaining megawatt-level peak powers; see Fig. 2.

Figure 2: (a) Temporal, (b) spectral, and (c), (d) transverse profiles of an XUV pulse pair generated by the proposed scheme for a cross-polarized radiator configuration when the delay chicane is turned off – in this case, two partially overlapping pulses with opposite OAM and opposite circular polarization are generated. This shows up as a double-arm spiral in the transverse distribution of the S₁ Stokes parameter (describing linear polarization) as seen in (d). (e) Demonstration of tunability of the temporal delay between the pulses by increasing the strength of the delay chicane.

Unlike previously proposed approaches such as superradiance, the method does not require operation deep into FEL saturation, making the setup more flexible and easier to implement at externally seeded FEL facilities while also reducing spurious spontaneous emission from the electron beam.

The scheme naturally supports pulse-to-pulse polarization switching on ultrafast timescales. In a cross-polarized configuration, the radiation switches from right- to left-circular polarization between the two pulses — see the dependence of the S₃ Stokes parameter (describing circular polarization) in Fig. 2a — while the OAM simultaneously changes sign. By adjusting the strength of the magnetic (delay) chicane between the undulators (Fig. 1), the researchers demonstrate controllable pulse separations ranging from partially overlapping pulses to delays of several tens of femtoseconds, while preserving balanced pulse intensities, see Fig. 2e.

The ability to generate delayed XUV pulse pairs with independently tunable OAM extends structured-light studies into the ultrafast domain. By comparing the response of materials and molecules to different OAM states at controlled time delays, the scheme could enable time-resolved measurements based on topological contrast, including studies of magnetic vortices, skyrmionic textures, chiral systems, and ultrafast symmetry-breaking dynamics. The combination of tunable OAM, polarization switching, and few-femtosecond temporal resolution also opens new opportunities for circular–helical dichroism experiments and investigations of coupled spin–orbital light–matter interactions that remain difficult to access with other XUV sources.

More broadly, the work establishes a versatile platform for engineering complex space–time structures of intense XUV light and exploring their interaction with matter on femtosecond timescales.

Primož Rebernik Ribič¹ and Takashi Tanaka^2
1 Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
² RIKEN SPring-8 Center, Koto, Sayo, Hyogo, Japan

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