Nonlinear effects of a visible beam interacting with a material are typically probed by a pulsed laser with high brilliance. Free-Electron-Laser facilities offer the advantage to extend such investigations in the extreme ultraviolet (XUV) but at the same time inherently limit practical applications.
Epsilon-near-zero nonlinearity enhancement in the XUV opens novel avenues for low-brilliance XUV extreme nonlinear applications in table-top systems. This finding, just published in Light: Science & Applications , emerges from a study led by Carino Ferrante from CNR-SPIN in L’Aquila and Andrea Marini from the University of L’Aquila, in collaboration with the facility FERMI of Trieste.
Third-order nonlinearity represents a key element for several optical spectroscopies and photonic devices. For example, by the simple interaction between one pulse and matter, Kerr nonlinearity can induce self-phase modulation (SPM), playing a key role in supercontinuum generation, wavelength converters, and chirped pulse amplification. The lack of stable and high brilliance XUV sources prevents the observation of SPM in such an extreme spectral regime possessing several peculiar advantages with respect to the visible:
- Interaction with core electrons;
- Enhanced spatial resolution thanks to the lower diffraction limit;
- Access to attosecond dynamics;
- Atomic specificity enabled by element-specific sharp absorption edges.
The recent advent of FEL and high-harmonic generation (HHG) sources enables the investigation of extreme nonlinear optics in the X-ray and XUV wavelength ranges and the development of innovative spectroscopic schemes taking advantage of the reduced wavelength. However, self-induced nonlinear effects in this regime currently suffer from three main drawbacks: (i) they require high peak intensity, provided only by FEL sources, (ii) the self-induced spectral modifications are small (within the pulse bandwidth), and (iii) the high peak power induces a single shot damaging of the sample.
In terms of nonlinear applications, epsilon-near-zero (ENZ) materials offer extraordinary possibilities, owing to the tremendous field enhancement that they can provide. Indeed, for low-absorbing media exhibiting low dielectric permittivity modulus, nonlinear effects become non-perturbative, thus opening novel avenues for strong-field physical applications. Currently, ENZ materials operate efficiently only in the near and mid infrared (IR) frequency part of the spectrum and, although potentially disruptive, their XUV functionality remains unexplored.
In a new paper published in Light Science & Application, a team of scientists report the first observation of ENZ-enhanced SPM in the XUV spectral range by a submicrometric free-standing foil of Aluminium. This standard material exhibits exceptional ENZ functionalities in the XUV over a broad spectral range (30-85 nm). Such results emphasize the possibility to attain self-induced spectral modulation with peak intensities three orders of magnitude smaller than the ones available at FEL facilities.
Dr. Carino Ferrante and Prof. Andrea Marini summarize: “The unconventional properties of Aluminium enable the near-zero effective-index condition for transverse magnetic radiation over a broad spectral range (30-85 nm), leading to a dramatic field enhancement that can be tuned opportunely through the incidence angle adjustment. Our measurement, performed at 44.25 nm, provides the first proof of such a nonlinearity enhancement mechanism in the XUV, which can push the applicability of extreme nonlinear optics in the XUV also for sources with lower fluence, such as table-top HHG systems.”
Dr. Emiliano Principi, coordinator of the EIS-TIMEX instrument at the FERMI FEL facility remarks:“FERMI provides unique tools for investigating unexplored phenomena in many research contexts. The high fluence and near-transform-limited pulses delivered by FERMI were crucial to the success of this experiment.”
For more details:
Carino Ferrante, Emiliano Principi, Luca Assogna, Ambaresh Sahoo, Giovanni Batignani, Giuseppe Fumero, Laura Foglia, Riccardo Mincigrucci, Luca Giannessi, Tullio Scopigno, Claudio Masciovecchio, Andrea Marini
Epsilon-near-zero nonlinearity enhancement in the extreme ultraviolet, Light: Science & Applications, 14, 374 (2025)
https://doi.org/10.1038/s41377-025-01985-w
