FERMI | TIMER Shaping light to uncover hidden light– matter interactions at the nanoscale Progress in nanoscience increasingly depends on our ability to manipulate light at scales that match the world of atoms and molecules: distances measured in nanometers and timeframes in femtoseconds. At these extremes, the understanding of how light behaves can uncover new physics. One of the key properties of light is its polarization, that is the direction in which the electric field oscillates. Controlling polarization is vital for exploring materials whose responses vary depending not only on how much light they absorb, but on the specific “twist” of that light. These so-called polarization-sensitive effects lie at the heart of many phenomena in magnetic materials and chiral systems, foundational in condensed matter physics, materials science, and even biology. At visible wavelengths, we already have tools to sculpt light with nanoscale precision. Advanced structures like metasurfaces and photonic crystals can shape the intensity and the polarization of visible light, opening doors to applications from flat lenses to nextgeneration displays. But when we shift into the extreme ultraviolet (EUV) range —where the wavelengths shrink to just a few tens of nanometers— those tools stop working. Standard optics become ineffective, and crafting nano-structured elements to manipulate EUV light is extremely challenging. Yet this is the regime where many of the most intriguing light–matter interactions unfold, especially when probing materials at their electronic and magnetic resonances. Now, a team of researchers led by scientists at the EIS-TIMER instrument of the FERMI Free-Electron Laser (FEL) has overcome this longstanding roadblock. They’ve developed a method to generate polarization transient gratings in the EUV range, which is a new kind of light patterning, where polarization, not intensity, varies across space and time. Instead of alternating bright and dark bands of light (as in traditional intensity gratings), this method creates stripes where the light’s electric field rotates differently from one stripe to the next, from left-circular to right-circular, while keeping the intensity constant. These stripes are spaced just 43 nanometers apart and because the pattern is formed by intersecting ultrafast FEL pulses, it exists only for tens of femtoseconds. That means the excitation is not only nanoscale in space, but also ultrafast in time. This elegant light pattern is generated by customizing the FEL itself. Using a specialized “split-undulator” configuration, the FERMI team steered the FEL’s electron beam along two slightly different paths through its undulators, the magnetic structures that generate the laser’s EUV light. The two paths produce two synchronized EUV pulses with orthogonal linear polarizations. These beams are then recombined on a sample in time coincidence, so that they interference creates the transient polarization grating. A third EUV pulse, delayed in time and with a fixed polarization, probes how the material responds by measuring the diffraction of light off the induced pattern. To demonstrate the power of this new method, the team investigated a thin layer — just 15 nanometers thick— of a magnetic cobalt-gadolinium (CoGd) alloy, chosen for its known ultrafast magnetic properties and strong dichroic response to circularly polarized light. When the material was excited using a conventional intensity grating, the researchers observed strong thermal effects: a spatially modulated temperature increase, mechanical expansion and surface acoustic vibrations, all of which created a long-lived, sinusoidal signal. This thermal “background” is common in such experiments and often obscures more subtle material responses. But with the new polarization grating, the behavior changed dramatically. The thermal signals were almost entirely suppressed. Instead, the team saw a fast, non-oscillatory signal that rose within 2 picoseconds and decayed within 10 picoseconds, which is too rapid and too sharp to be thermal in origin. With the help of sophisticated simulations, the researchers attributed this new signal to helicity-dependent magnetic dynamics. In other words, changes in magnetization that directly follow the spatial twist of the light’s polarization. The light’s handedness was communicating with the material’s magnetic order in a way that had previously been hidden. By selectively exciting these non-thermal processes and filtering out heat-induced response, polarization transient gratings allow researchers to directly observe a dichroic form of light–matter interaction. This not only provides a new lens on the magnetic dynamics of CoGd, but also demonstrates a general method that could be applied to many other systems. Any material that responds differently to different polarizations —whether due to chirality, magnetic texture, topological properties, or valley polarization— can now be investigated with unmatched resolution in both space and time. In a broader sense, this work brings us closer to the dream of ultrafast, nanoscale light control. It extends the toolkit for EUV and x-ray spectroscopy and creates new possibilities for probing fundamental physics. Researchers could explore emergent phenomena in quantum materials, track information flow across spintronic devices, or even investigate chiral molecular dynamics central to biochemistry and drug design. All with light that’s twisted, timed, and selectively sensitive to given atomic species. This breakthrough shows how rethinking our tools — from how we generate light to how we let it interfere— can unlock entirely new windows into the hidden behavior of matter. And in doing so, it gives scientists the power to not only observe but steer ultrafast processes at the smallest scales. Elettra Highlights 2024–2025 156
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