Uncovering hidden light-matter interactions at the nanoscale
Progress in nanoscience increasingly depends on our ability to control light at spatial and temporal scales matching those characteristic of nanostructures. In particular, controlling the polarization of light is essential for investigating materials whose properties depend not only on how much light they absorb, but also on the specific orientation of the electric field. Such polarization-sensitive effects are central to the behavior of magnetic and chiral materials, which are of great interest across condensed matter physics, chemistry, and materials science.
At visible wavelengths, artificial structures with dimensions comparable or smaller than the wavelength of the light can be used to shape its intensity or polarization with nanoscale precision. However, in the extreme ultraviolet (EUV), where the wavelengths are much shorter, no comparable tools exist. To overcome this limitation, an international team led by researchers at the TIMER beamline of the FERMI free-electron laser (FEL) has developed a breakthrough method. By combining their unique instrument with a tailored configuration of the FEL, they created a novel type of transient grating in which it is not the intensity, but the polarization of the light that varies periodically. Imagine this as a series of stripes where the electric field rotates in opposite directions from one stripe to the next, with inter-stripe spacing as small as 43 nanometers. Unlike artificial structures, this grating is not static, but exists for the same time duration of the FEL pulses, enabling the ultrafast dynamics of the material illuminated by the light pulse to be probed.
Figure 1: (a) in an intensity grating excitation the polarization is constant in space and the intensity is sinusoidally modulated. The resulting signal is dominated by the thermoelastic response, in this case associated with surface acoustic waves; (b) in a polarization grating excitation the intensity is constant across the volume while the polarization is periodically rotating from circular left to circular right. In the resulting signal the thermoelastic response is strongly suppressed (dashed line) and a previously hidden ultrafast dynamics revealed. This dynamic is due to the direct coupling of the light helicity with the magnetic response of the sample.
The researchers then tested how a thin film of a magnetic alloy (CoGd) responds to this polarization-modulated grating, comparing it with the response induced by an intensity-modulated grating. In the case of intensity-modulated gratings, the signal was dominated by thermal effects, such as heat-induced vibrations (Fig. 1a). In contrast, the polarization grating suppressed this thermo-elastic background, revealing an otherwise hidden signal (Fig. 1b). Numerical simulations showed that this signal can be attributed to helicity-dependent magnetic effects, that is, to changes in the magnetization directly induced by the circular polarization of light. This suggests that polarization-modulated gratings can be used to selectively trigger and monitor non-thermal, ultra-fast magnetic dynamics that were previously inaccessible.
By enabling precise control of light polarization on nanometer length and femtosecond time scales, this new methodology opens exciting opportunities for studying a wide range of materials. These include systems with complex magnetic structures, topological phases, or valley-dependent properties — fields of growing interest in materials science. This approach adds a powerful new tool to the expanding field of ultrafast EUV and X-ray spectroscopy and paves the way for investigating fundamental processes in condensed matter physics with unprecedented precision.
This research was conducted by the following research team:Laura Foglia1, Björn Wehinger1,2,*, Giovanni Perosa1,3, Riccardo Mincigrucci1, Enrico Allaria1, Francesco Armillotta3, Alexander Brynes1, Matthew Copus4, Riccardo Cucini5, Dario De Angelis1, Giovanni De Ninno1,6, W. Dieter Engel7, Danny Fainozzi1,#, Luca Giannessi1,8, Ezio Iacocca4, Nupur N. Khatu1,2,9, Simone Laterza1,3, Ettore Paltanin1,3, Jacopo Stefano Pelli-Cresi1, Giuseppe Penco1, Denny Puntel3, Primož Rebernik Ribič1, Filippo Sottocorona1,3, Mauro Trovò1, Clemens von Korff Schmising7, Kelvin Yao7, Claudio Masciovecchio1, Stefano Bonetti2 and Filippo Bencivenga1
1 Elettra - Sincrotrone Trieste S.C.p.A., Basovizza, Trieste, Italy.
2 Department of Molecular Sciences and Nanosystems, Ca’Foscari University of Venice, Venezia, Italy.
3 Department of Physics, Universitá degli Studi di Trieste, Trieste, Italy.
4 University of Colorado Colorado Springs, Center for Magnetism and Magnetic Nanostructures, Colorado Springs, USA.
5 CNR- Istituto Officina dei Materiali (IOM), Unità di Trieste, Basovizza, Trieste, Italy.
6 Laboratory of Quantum Optics, University of Nova Gorica, Ajdovščina, Slovenia.
7 Max-Born-Institute for Nonlinear Optics and Short Pulse Spectroscopy, Berlin, Germany.
8 INFN Laboratori Nazionali di Frascati, Frascati, Roma, Italy.
9 European XFEL, Schenefeld, Germany.
* Present address: European Synchrotron Radiation Facility, Grenoble, France.
# Present address: Institute of Applied Physics, University of Bern, Bern, Switzerland.
Reference
L. Foglia, B. Wehinger, G. Perosa et al.; “Nanoscale polarization transient gratings”, Nat. Commun. 15, 10742 (2024). DOI: 10.1038/s41467-024-54799-6.
