NICE2026 Nonlinear Imaging and Coherent Effects

May 20, 2026, 8:00 AM → May 21, 2026, 6:00 PM Europe/Rome

Latisana, Italy

The workshop aims to bring together the linear and non-linear spectroscopy communities with the rapidly advancing field of coherent imaging. While second harmonic generation and two-photon excitation fluorescence microscopy at optical wavelengths are already established as powerful tools for visualizing cellular and tissue structures, as well as functionalized surfaces, non-linear imaging approaches at sub-100 nm wavelengths are still in their infancy.

This workshop will open a dynamic forum to explore the scientific opportunities unlocked by non-linear effects in advanced imaging, leveraging the unique coherence properties of free-electron lasers, high-brilliance fourth generation synchrotrons, and table-top lasers sources. The anticipated outcome of this meeting is to take concrete steps toward fostering collaborations between experimental and theoretical groups, and to inspire the design of next-generation instrumentation for probing intensity-dependent light–matter interactions.

A key feature of the meeting will be the active involvement of early-career researchers. Young scientists will have the chance to engage directly with world leaders in the field, exchange ideas, and gain exposure to cutting-edge methodologies and perspectives. 

This workshop is co-organised with COST Action NEXT, CA22148, supported by COST (European Cooperation in Science and Technology) and Elettra Sincrotrone Trieste.

 

COST (European Cooperation in Science and Technology) is a funding agency for research and innovation networks. Our Actions help connect research initiatives across Europe and enable scientists to grow their ideas by sharing them with their peers. This boosts their research, career and innovation.

 

Attendance at the workshop is free of charge and by invitation only, following registration with COST Action CA22148.

!!Please note: 

During the week from 19 to 24 May, railway services on the Trieste – Portogruaro – Venice line will be significantly reduced due to maintenance works scheduled by the Infrastructure Manager, RFI.

Trenitalia’s sales and information systems will be updated with full details of all connections starting from 25 April.

All trains will terminate at either Portogruaro or Monfalcone. A replacement bus shuttle service will operate between these two locations at hourly intervals.

Although the replacement bus service will be reinforced during peak commuting hours, it will have limited capacity and longer travel times compared to the train.

Assistance services will be available at both bus terminal locations to manage passenger transfers and address any potential issues.

Further information and updated timetables will be available soon on the Trenitalia website.!!

 

 

Wed, May 20

8:30 AM Registration

9:00 AM Welcome

Flavio Capotondi, Elettra Sincrotrone Trieste, Trieste, Italy and Giulia Fulvia Mancini, University of Pavia, Italy

Coherent-based Diffractive Imaging

Chair Maurizio Sacchi, Johannes Kelper University Linz

1 Exploring the Limits of Coherent Diffractive Imaging

If the amplitude and phase of a coherently scattered monochromatic wave can be measured or determined, then that wave can be numerically propagated backwards in time to form a map of the interaction of the wave with an object (i.e. an image). The phase of the wave can often be inferred from a measurement of the intensity pattern alone. This is particularly useful for imaging with X-rays, where atomic resolution can be reached. When imaging with intense pulses from X-ray free-electron lasers, with the goal of capturing processes occurring at ultrafast time scales, assumptions of stationarity of the scattering may break down. Here, I will discuss some methodologies of imaging with XFEL pulses, attempts to take advantage of the dynamics of the sample for imaging, and discuss ways that we might perform imaging in the attosecond domain.

Speaker: Henry N. Chapman (CFEL DESY)

2 Broadband ptychography for soft X-ray imaging with a high-order harmonic source

Recent advances in ultrafast laser technology have facilitated the development of high-brightness, high-order harmonic generation (HHG) sources. These compact tabletop systems now enable high-resolution coherent diffractive imaging (CDI) experiments previously restricted to large-scale facilities [1, 2]. To date, HHG-based imaging has been largely confined to the EUV spectral range (<100 eV), as these sources often lack sufficient brightness and stability in the soft X-ray regime (>100 eV). However, numerous applications would benefit from soft X-ray radiation, including the imaging of biological samples in their natural aqueous environments (the "water window", 280–530 eV) and the investigation of magnetic materials at L-edges (∼700 eV).
In this contribution, we demonstrate what is, to our knowledge, the first soft X-ray ptychography measurements acquired using a high-harmonic generation (HHG) source at 185 eV photon energy. These results are enabled with a custom-engineered, ultrahigh-stability infrared laser system [3]. Operating at a central wavelength of 1.58 μm, this driver facilitates uninterrupted, 24/7 generation of soft X-ray harmonics. When driven in helium, the source produces a broadband X-ray continuum spanning from 150 eV to beyond 350 eV. Multilayer mirrors are typically used to select a narrow bandwidth (~1%) from the broadband continuum to meet the temporal coherence requirements of coherent diffractive imaging. However, this approach leaves the majority of generated soft X-ray photons unused and, therefore, leads to long exposure times for soft X-ray imaging. To address this, we have developed strategies to utilize a significantly broader spectral bandwidth, thereby maximizing usable photon flux. In a proof-of-concept experiment at 92 eV, we demonstrate broadband ptychography with 10% bandwidth without significant degradation of spatial resolution. Applying this approach to the soft X-ray regime offers a substantial increase in photon flux, promising sub-100 nm spatial resolution for biological specimens within the water window.

References
[1] R. Sandberg et al., Phys. Rev. Lett. 99, 098103 (2007)
[2] M. Seaberg et al., Optica 1, (2014)
[3] D. Morrill, et al., APL Photonics 1 2025; 10 (11): 116101

Speaker: Wilhelm Focko Eschen (CU Boulder, JILA)

3 Using the longitudinal coherence of a seeded free electron laser to enable single-shot videography

Stroboscopic imaging is a popular use of free electron lasers (FELs), enabling studies following the dynamics of a variety of systems ranging from fast-moving objects [1] to ultrafast magnetic dynamics [2]. However, these methods face a fundamental limitation: that each flash of light from the FEL produces a single image. Therefore, to stitch together a full movie of a time-dependent process, the process either needs to happen slower than the FEL repetition rate, or it needs to be repeated many times, with one frame of data extracted per repetition.

This causes challenges when one wants to study a system which cannot be prepared in a repeatable state, such as a thin film hosting natural magnetic domains, on the femtoscale timescales comparable to the pulse length of a FEL. To overcome this challenge, we have recently demonstrated a new method, multi-frame randomized probe imaging (RPI). In this method, a single diffractive optic introduces a series of time delays, while simultaneously producing the structured illumination patterns that enable phase retrieval with RPI [3,4].

Here, we will show the results of a demonstration experiment studying the process of ultrafast demagnetization in a thin film of CoGd. However, one key limitation of this method is the need to work with stable and nearly transform-limited FEL pulses. For this reason, we performed the experiment at the FERMI FEL [5]. More broadly, continuing to develop multi-frame RPI will place a number of unusual requirements on the light source. In this presentation, we will finish by discussing the key opportunities of multi-frame RPI, with a focus on how the limitations of the method can potentially be alleviated by future developments in FEL technology.

References
[1] Vagovič, P. et al. Megahertz x-ray microscopy at x-ray free-electron laser and synchrotron sources. Optica 6, 1106 (2019).
[2] von Korff Schmising, C. et al. Imaging Ultrafast Demagnetization Dynamics after a Spatially Localized Optical Excitation. Phys. Rev. Lett. 112, 217203 (2014).
[3] Levitan, A. L., Keskinbora, K., Sanli, U. T., Weigand, M. & Comin, R. Single-frame far-field diffractive imaging with randomized illumination. Opt. Express 28, 37103 (2020).
[4] Levitan, A. L. et al. Single-shot imaging with randomized structured illumination at a free electron laser. Opt. Express 34, 8043 (2026).
[5] Allaria, E. et al. Highly coherent and stable pulses from the FERMI seeded free-electron laser in the extreme ultraviolet. Nature Photon 6, 699–704 (2012).

Speaker: Abraham L. Levitan (Paul Scherrer Institute)

10:50 AM Coffee Break

NonLinear Spectroscopy

Chair Nicola Jaouen,Synchrotron SOLEIL

4 Unlocking Hidden Worlds in Quantum Materials: Metastability Controlled by Textured EUV Excitation

Metastable and long lived hidden electronic orders in quantum materials are rapidly emerging as platforms for next generation technologies, ranging from ultrafast low power memory, quantum information architectures to advanced lithography and tunable X ray optics. However, materials capable of sustaining such persistent phases remain rare, and the microscopic mechanisms that stabilize them are still not fully resolved.
Here, we use textured extreme UV pulsed excitation to manipulate electronic order in the prototypical quantum material 1T TaS2, renowned for hosting multiple exceptionally long lived hidden states. By applying precise, ultrafast nanoscale strain engineering, we show that the lifetimes of the photoinduced phases can be tuned over several orders of magnitude. Moreover, we demonstrate that distinct hidden electronic orders can be selectively stabilized by tailoring the excitation pathway that drives the phase transition.
Together, these results help map the system’s trajectory through its nonequilibrium phase space, and establish design principles for discovering and controlling materials that support robust, long lived metastable electronic orders.

Speaker: Igor Vaskivskyi (Jozef Stefan Institute)

5 Probing nonlinear effects at X-ray and EUV wavelenghts

Using modern synchrotron and free-electron laser (FEL) sources, it has become feasible to study a wide range of nonlinear processes in the x-ray regime. With it comes the possibility to transfer ideas from parametric nonlinear optics as well as quantum optics to shorter wavelengths, for which we will explore examples in this talk. Processes of interest comprise x-ray-optical wavemixing (XOWM) that can combine diffractive imaging capabilities with spectroscopic sensitivity for new material diagnostics. As such, we have recently demonstrated the applicability of nonlinear crystallography using x-ray-optical difference-frequency generation to spatially reconstruct the valence response in diamond at sub-Angstrom resolution. Processes deriving from spontaneous x-ray parametric down-conversion (XPDC), on the other hand, can provide access to quantum features of light-matter interaction. As an example, we showcase a series of experiments, in which we found that non-degenerate XPDC allows access to polariton-formation in the extreme-ultraviolet (EUV) and soft x-ray spectral ranges. Recent developments have also shown renewed interest to extend XPDC into the degenerate regime, aiming to produce energetically equal x-ray photon pairs as a resource of entanglement. We will elaborate on the challenges to study nonlinear optics in the soft x-ray regime and explore possible future experiments with the extended capabilities of FERMI.

Speaker: Christina Boemer (Deutsches Elektronen-Synchrotron DESY)

6 Laser-driven XUV and X-ray sources for coherent imaging, ultrafast dynamics, and emerging nonlinear methods

Laser-driven XUV and X-ray sources provide versatile tools for ultrafast science and open new opportunities for nonlinear methods at short wavelengths. At ELI Beamlines, we are developing a complementary suite of laser-driven sources spanning coherent XUV high-harmonic generation (HHG), characteristic Cu Kα emission, and broadband hard X-ray betatron radiation. The HHG beamline is driven by the 1 kHz L1 laser delivering 50 mJ, 15 fs pulses and provides femtosecond coherent XUV pulses for ultrafast experiments. Using this source, we will present first results in coherent diffraction imaging as well as initial experiments in transient grating spectroscopy, demonstrating the potential of the HHG beamline for time-resolved studies and coherent XUV methodologies. The same L1 laser is also used to generate an incoherent Cu Kα source, mainly employed for time-resolved diffraction methods, while a 0.5 PW laser operating at 3.3 Hz is used to produce broadband hard X-ray betatron radiation, extending our capabilities toward short-pulse hard X-ray probing. Together, these sources form a flexible platform for pump-probe studies of ultrafast dynamics. Looking ahead, we aim to exploit their femtosecond duration, high peak brightness, and coherence to explore nonlinear spectroscopy and imaging in the XUV and X-ray range.

Speaker: Jaroslav Nejdl (Extreme Light Infrastructure ERIC)

12:45 PM Lunch Break

Novel Coherent-based Diffractive Imaging

Chair Alessandra Gianoncelli, Elettra Sincrotrone Trieste, Trieste, Italy

7 X-ray Coherent Diffractive Imaging with Nonlinear Wavemixing

Nonlinear imaging has become a powerful and widespread tool for improving the spatial resolution or selectivity of microscopy tools in the visible and infrared spectral range. Only recently has non-linear X-ray science progress to the point where imaging using wavemixing processes can reasonably be contemplated. Here I will discuss how non-linear X-ray imaging can be realized using coherent imaging methods. Instead of separating linear and nonlinear signals using chromatic optics, we instead leverage the mutual incoherence of different frequencies to numerically separate out their contributions to coherent scattering patterns. This approach has several advantages over other conceivable implementations, and I will show how nonlinear X-ray imaging can be realized using existing facility scale and tabletop sources.

References
[] A Sarkar and AS Johnson, arXiv:2512.15457 (2026)

Speaker: Allan Johnson (IMDEA Nanoscience)

8 Ultrafast single-shot computational imaging

Recent advances in computational imaging are reshaping ultrafast measurement, making it possible to recover information that is difficult or impossible to access with conventional imaging or pump–probe approaches alone. In this talk, I discuss progress over the past five years at the intersection of computational microscopy, electric-field metrology, and algorithm development, with an emphasis on how ptychographic and related computational methods can turn intensity measurements into quantitative reconstructions of ultrafast fields and transient material response.

I highlight approaches for complete pulse-beam characterization, single-shot, and three-dimensional imaging, and the study of plasma and carrier dynamics on ultrafast timescales. Examples include broadband scanning ptychography, single-shot ptychography, electron–neutral deconvolution in plasmas, three-dimensional single-shot ptychography, and applications ranging from two-photon-absorption-induced carrier dynamics in ZnSe to laser-induced and electrostatic-discharge plasma imaging. Taken together, these results illustrate how computational imaging can expand the dimensionality, fidelity, and physical interpretability of ultrafast experiments.

Speaker: Daniel E. Adams (Colorado School of Mines)

9 Ultrafast subwavelength imaging of magnetic domain walls with extreme ultraviolet radiation

Investigating ultrafast dynamics and transport phenomena at the nanoscale necessitates simultaneously achieving both femtosecond temporal and nanometer spatial resolution in the probe, which has proven to be challenging up to the present day. Using a femtosecond coherent extreme ultraviolet source via high harmonic generation, we demonstrate subwavelength imaging of magnetic domains of Co-based magnetic thin films at the Co M$_{2,3}$ edge (~60 eV, 20.8 nm), and extract local properties of magnetic domain walls with precision below 2 nm. As such, we show that domain wall widths and positions remain largely invariant during ultrafast demagnetization, which is contrary to previous propositions of ultrafast domain wall broadening and movement from diffraction-based measurements in reciprocal space. Our technique can be easily adapted to facility-scale imaging experiments at synchrotrons and X-ray free electron lasers, combining the strength of high spatiotemporal resolution, brightness, and photon energy flexibility to resolve ultrafast charge and spin dynamics in complex systems.

References:
[1] Chang et al., Nat. Mater., accepted. arxiv: 2504.17917.
[2] Zayko et al., Nat. Commun. 12, 6337 (2021)
[3] Kfir et al., Sci. Adv. 3, eaao4641 (2017)
[4] Pfau et al., Nat. Commun. 3, 1100 (2012)
[5] Jangid et al., Phys. Rev. Lett. 131, 256702 (2023)

Speaker: Hung-Tzu Chang (Max Planck Institute for Multidisciplinary Sciences)

10 Imaging Electric-Field-Driven Nanotexture Dynamics in V2O3

V2O3 is a prototypical Mott insulator in which the first-order insulator-to-metal transition (IMT) is accompanied by a symmetry -breaking lattice distortion. In the low-temperature insulating - antiferromagnetic - monoclinic phase, the breaking of three-fold rotational symmetry generates an intrinsic nanotexture composed of differently oriented domains. This nanoscale texture plays a central role in determining the nucleation and propagation of the metallic phase; here, we investigate its role when the IMT is triggered by a static electric field in a resistive switching geometry.
The nanotexture is imaged by PhotoEmission Electron Microscopy (PEEM), exploiting X-ray Linear Dichroism (XLD) at the V L2,3 edge (∼513–530 eV) as a contrast mechanism sensitive to domain orientation. By combining PEEM imaging with the application of voltage across micro-patterned electrodes, we directly visualize the spatial pathway of the electrically driven transition. During resistive switching, the formation of metallic filaments is not random but nucleates at topological defects emerging from the C₃-symmetry–broken insulating nanotexture, demonstrating that the pre-existing domain pattern governs the switching process. In addition, we reveal a striking non-volatile effect of the electric field on the insulating nanotexture. When applied to a pristine device, the field induces a macroscopic reorientation of monoclinic domains persisting after removal of the current, driving the system into a global energy minimum that cannot be accessed by thermal cycling alone. These findings provide direct real-space insight into switching mechanisms in Mott insulators, highlighting the key role of the intrinsic nanotexture and its manipulation for controlling Mott resistive switching, and establishing memory effects relevant for neuromorphic functionalities.

Speaker: Alessandra Milloch (Elettra Sincrotrone Trieste)

4:10 PM Coffee Break

Advanced Computational Methods

Chair George Kourousias

11 Deep learning for 3D/4D X-ray image reconstruction at high-brilliance sources

The rapid development of high-brilliance X-ray sources, including diffraction-limited storage rings and X-ray free-electron lasers, is enabling X-ray imaging at unprecedented spatial and temporal resolution. At the same time, these facilities generate increasingly large and often highly undersampled datasets, creating challenges for image reconstruction and analysis. In this talk, I will present recent advances in deep-learning-based approaches for 3D/4D X-ray image reconstruction developed for high-brilliance X-ray imaging experiments. In particular, I will discuss physics-informed self-supervised methods for ultrafast 3D/4D reconstruction for X-ray Multi-Projection Imaging (XMPI)[1-4]. In the second part of the talk, I will briefly introduce the ongoing SLS 2.0 and cSAXS 2.0 upgrade at the Paul Scherrer Institute, where commissioning starts in June 2026, and user operation starts in August 2026.

References
[1] P. Villanueva-Perez et al., Optica 5, 1521–1524 (2018).
[2] Y. Zhang et al., Commun. Eng. 4, 54 (2025).
[3] Z. Yao et al., Meas. Sci. Technol. 36, 085403 (2025).
[4] Z. Hu et al., Adv. Sci. 13, e11933 (2026). 
[5] T. Rosén et al., arXiv:2412.09368 (2024).

Speaker: Yuhe Zhang (Paul Scherrer Institute)

12 Magnetic X-ray Imaging using Single Polarization and Multimodal Ptychography

The ongoing upgrade of synchrotrons to 4th generation, offering higher brilliance and coherence, creates a valuable opportunity to enhance the penetration depth and sensitivity of coherence-based X-ray techniques. This improvement particularly benefits the detection of weak dichroic signals from magnetic materials, relevant across fields from condensed matter to biology. Visualizing the microscopic magnetic structure provides essential insight into the material properties and macroscopic behavior of magnets, highlighting the importance of advanced microscopy techniques.

To probe the microscopic structure of magnetic and other dichroic materials, polarized X-rays are used because these materials respond differently based on the incident light’s polarization. Separating magnetic contrast from electronic signals therefore typically requires multiple polarization states or a polarization analyzer.

Here, we leverage the redundancy in ptychographic datasets—obtained via overlapping illumination—to recover more information than a single image of the sample. Indeed, ptychography can also uniquely distinguish multiple incoherent light components in a single scan. Therefore, we demonstrate that multimodal ptychography can disentangle magnetic and electronic contributions from a single polarization measurement without the need for varying incident polarization or a polarization analyzer. This approach simplifies measurements and reduces artifacts while offering an efficient way to study complex magnetic materials. Our results open the door to imaging with high sensitivity and spatial resolution magnetic materials such as antiferromagnets and altermagnets, as well as other non-magnetic dichroic systems.

Speaker: Marisel Di Pietro Martínez (LCPMR - CNRS Paris)

Poster Session

10 min Poster Presentation from Young Scientists

8:30 PM Social Dinner

Thu, May 21

Multidimensional Spectroscopic

Chair Giulia Fulvia Mancini, University Pavia

13 Linear and non-linear X-ray studies of chirality in liquid solutions

Molecular chirality is central to biology and chemistry. Identification of enantiomers of chiral molecules is mostly based on optical circular dichroism (CD). It measures the absorption difference of left and right circularly polarised light by the sample. CD signals are intrinsically weak, usually 0.1% of the linear absorption, which makes them challenging to measure. Pushing CD spectroscopy into the X-ray domain promises several advantages: a) an enhanced signal (since the terms of light-matter Hamiltonian for CD are inversely proportional to the wavelength), b) element-selectivity; c) on top of the chemical shifts, an ability to identify identical but inequivalent atoms in a molecule. Although several theoretical studies have underlined these advantages of X-ray natural CD (XNCD), [1–5] its measurement on disordered media (powders) has remained a challenge. There are only a few studies on amino-acid residues and organic molecules in powder form. [6–8]
With the advent of the liquid microjet technique, [9,10] it is now possible to inject liquid samples into vacuum and investigate their soft X-ray absorption both in steady-state and time-resolved studies. Here, we will present the first XNCD spectra of chiral fenchone in ethanol. The measurements were carried out at the O K-edge of the molecule, which is not a chiral centre. The results show a good agreement with our theoretical results.[5]
Beyond linear XNCD, we have explored the use of Chi(2) non-linear methods such as sum-/difference-frequency generation (S/DFG) to study chiral solutions.[5] Indeed, it was shown [11] that these methods are sensitive both to the interface and the bulk of a chiral sample. We will present our efforts at implementing at an X-ray Free Electron Laser, S/DFG methods combining an optical pulse tuned to a valence transition of the chiral molecule and an X-ray probe tuned to one of its core transitions.

References
[1] A. Jiemchooroj et al, Near-edge x-ray absorption and natural circular dichroism spectra of L-alanine: A theoretical study based on the complex polarization propagator approach, J. Chem. Phys. 127, 16 (2007).
[2] A. Jiemchooroj and P. Norman, X-ray absorption and natural circular dichroism spectra of C84: A theoretical study using the complex polarization propagator approach, J. Chem. Phys. 128, 234304 (2008).
[3] C. Brouder et al, Theory of X-Ray Natural Circular Dichroism,
https://doi.org/10.1107/S090904959801680X.
[4] V. M. Freixas et al, X-ray and Optical Circular Dichroism as Local and Global Ultrafast Chiral Probes of [12]Helicene Racemization, J. Am. Chem. Soc. 145, 21012 (2023).
[5] Y. Nam et al, Linear and Nonlinear X-ray Spectra of Chiral Molecules: X-ray Circular Dichroism, Sum- and Difference-Frequency Generation of Fenchone and Cysteine, J. Phys. Chem. Lett. 16, 4652 (2025).
[6] M. Tanaka et al, First observation of natural circular dichroism for biomolecules in soft x-ray region studied with a polarizing undulator, Phys. Scr. 2005, T115 (2005).
[7] A. Agui et al., First operation of circular dichroism measurements with periodic photon-helicity switching by a variably polarizing undulator at BL23SU at SPring-8, Rev. Sci. Instrum. 72, 8 (2001).
[8] Y. Izumi et al, Characteristic oxygen K-edge circular dichroism spectra of amino acid films by improved measurement technique, J. Chem. Phys. 138, 7 (2013).
[9] M. Ekimova et al, A liquid flatjet system for solution phase soft-x-ray spectroscopy, Struct. Dyn. 2, 5 (2015).
[10] M. Fondell et al, Time-resolved soft X-ray absorption spectroscopy in transmission mode on liquids at MHz repetition rates, Struct. Dyn. 4, 5 (2017).
[11] J. A. Giordmaine, Phys. Rev. Lett., 1962, 8, 19–20.

Speaker: Majed Chergui (Elettra Sincrotrone Trieste)

14 Amplification of weak spontaneous emission from low-lying doubly excited states in helium gas

We have detected self-amplified spontaneous emission (ASE) from 3a $^1P^o$ doubly excited state (DES) in He. In an isolated atom, this autoionizing resonance features a small $5\times 10^{-4}$ branching for fluorescence decay, preferably populating the singlet 1s3s singly excited state. To locate FEL wavelengths resonant with DES, the microfluidic gas cell was modified by adding the two parallel 100 nm diameter Pt wires to measure the charges generated in the gas by single FEL pulses. The optimum voltage difference between the two wires 400 $\mu$m apart was 30-40 V, which secured reasonably high signal and still avoiding discharges. The collected charge signal followed Fano profiles of autoionizing DES states and enabled a direct measurement of FEL spectral shift between the ASE emission maximum and Fano maximum and verification of a weak ASE amplification of fluorescence emitted by the 4a $^1P^o$ DES. We believe that the lack of ASE in this case comes because of the emission wavelength, which is already quite close to the 1s-2p energy difference in He$^+$. Since these ions are abundantly created in the gas, the ASE signal from the $n\geq4$ resonances is strongly absorbed in a given length of the gas column (9 mm). On the other hand, we saw a significant ASE signal from the 3b $^1P^o$ resonance. This means that regarding the ASE, 100 times smaller oscillator strength of the $3b$ versus $3a$ $^1P^o$ series can compensated by longer DES lifetime and 100-times larger fluorescence branching ratio of the $3b$ state. We found also unexpectedly large ASE signal emitted from the $2a$ $^1P^o$ state. This is surprising because the $2a$ state features 4-times smaller fluorescence branching ratio than $3a$ and has a significantly shorter lifetime (10 fs) than FEL pulse duration (50 fs). For $2a$ resonance, we found a strong excimer effect on ASE meaning that the wavelength of ASE emission from 2a drifts with the target pressure. The closeness of He atoms in the target not only allows for the amplification of emitted light but also affects emission wavelength by shifting energies of the upper and lower state via potential energy dependence on interatomic distance. At a given gas pressure we have detected a typical atomic linear Raman dispersion of the XUV emission signal meaning that non-linear ASE process just amplifies emission of an isolated atom while preserving its properties such as chemical shift.

Speaker: Matjaz Zitnik (Jozef Stefan Institute)

15 Attosecond momentum-resolved resonant x-ray scattering of photo-excited molecules: observing charge in motion.

Improving our understanding of electron dynamics is essential for advancing energy transfer, optoelectronics, light-harvesting systems, and quantum computing. Recent developments in attosecond x-ray sources provide the fundamental possibility of observing these dynamics with atomic-scale spatial resolution. However, connecting a time-resolved signal to the dynamics remains challenging due to the broad bandwidth of an attosecond probe pulse. This makes exploring the capabilities of different attosecond imaging techniques crucial. Here, we propose attosecond momentum-resolved resonant inelastic x-ray scattering (RIXS) as a prominent technique for tracking ultrafast dynamics. We demonstrate that the scattering signal contains information about the instantaneous distribution of charge density across the scattering atoms. To illustrate this, we consider scattering from an $\alpha$-sexithiophene molecule in which coupled electron-hole dynamics are excited.
The prominent advantage of this technique is the resonant enhancement of the signal from moving particles. We introduce a method to extract information about the time-dependent charge density from momentum-resolved RIXS signals. Through an in-depth analysis of the signal from a non-stationary electronic system and the transitions involved, and by illustrating the approach with accurate many-body calculations, we identify favorable experimental conditions for linking signal features to charge-density properties. By analyzing the momentum distribution at different scattered photon energies, it becomes possible to identify the sites with a significant hole population and those with substantial populations of both holes and electrons. This technique is particularly useful for studying exciton dynamics and charge separation, as it provides a way to distinguish coupled and decoupled electron-hole dynamics.

Speaker: Maksim Radionov (Brandenburg University of Technology)

16 Sum-Frequency Generation Spectro-Microscopy using the FHI-FEL

In this talk infrared-visible sum-frequency generation (SFG) spectro-microscopy using the FHI-FEL is introduced. This method combines benefits from second harmonic generation imaging such as local information on structural symmetry and benefits from sum-frequency generation spectroscopy such as details of the atomic structure and bonding via vibrational resonances.

We demonstrate an enhanced spatial resolution of a nineth of the IR-wavelength by imaging surface phonon polaritons in 4H-SiC nanostructures as a proof-of-concept [1]. In an extended work on a metasurface of SiC micropillars, spectro-microscopy is shown to measure the polariton dispersion simultaneously in momentum space by angle-dependent resonance imaging, and in real space by polariton interferometry [2]. At last, we demonstrate the feasibility of our method for spectroscopy using in-plane anisotropic wurtzite-type aluminum nitride as a model system [3] and show first data of our method’s application to ferroelectrics.

References
[1] R. Niemann, S. Wasserroth, G. Lu, S. Gewinner, M. de Pas, W. Schöllkopf, J.D.Caldwell, M.Wolf and A.Paarmann, “Long-wave infrared super-resolution wide-field microscopy using sum-frequency generation” Appl. Phys. Lett. 120(13),131102, (2022).
[2] R. Niemann, N. S. Mueller, S. Wasserroth, G. Lu, M. Wolf, J. D. Caldwell and A. Paarmann,“Spectroscopic and Interferometric Sum-Frequency Imaging of Strongly Coupled Phonon Polaritons in SiC Metasurfaces” Adv. Mat. 36(33), 2312507, (2024).
[3] D. S. Mader, R. Niemann, M. Wolf, S. F. Maehrlein and A.Paarmann,” Sum-frequency generation spectro-microscopy in
the reststrahlen band of wurtzite-type aluminum nitride” J. Chem. Phys. 161, 094706 (2024)

Speaker: Dorothee Mader (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

10:40 AM Coffee Break

Structured illumination – Multidimensional Imaging

Chair Simone Finizio, Paul Scherrer Institut

17 Structured Light in Nonlinear Spectroscopy: Breaking Selection Rules and Engineering Chiral Response

Structured light allows us to move far beyond the simple plane waves that underpin most of optics, introducing twist, structure, and topology into the electromagnetic field. This added complexity is not just aesthetic – it opens new ways of controlling how light interacts with matter.

In this talk, I will give a broad overview of what structured light brings to the table, and then focus on its role in nonlinear spectroscopy. I will highlight how structured fields can modify, relax, or even bypass familiar selection rules, enabling optical responses that are weak or inaccessible under conventional illumination.

The aim is to show how thinking of light itself as a design tool – rather than just a probe – can create new opportunities for nonlinear spectroscopy, and chiral light–matter interactions.

References:

[] K. A. Forbes “Vortex Light at the Nanoscale: Twists, Spins, and Surprises" Reports on Progress in Physics 89, 016401 (2026): https://doi.org/10.1088/1361-6633/ae3971
[] K. A. Forbes “Twisted Light and Twisted Matter: The Photonic Frontier of Chirality" Photonics Research 14, 1, B193-B208 (2026): https://doi.org/10.1364/PRJ.574843

Speaker: Kayn Forbes (University of East Anglia)

18 Magnetization Dynamics probed with Helicoidal Dichroism

Optical probing of magnetization dynamics has traditionally relied on analyzing the polarization of plain Gaussian beams. Recently, we introduced a novel approach – Magnetic Helicoidal Dichroism (MHD) – based on analyzing the reflectivity of beams with twisted wavefronts, i.e., beams carrying orbital angular momentum.
While the spin angular momentum of light is widely used in dichroic studies — such as X-ray magnetic circular dichroism or Magneto-Optical Kerr Effect — the orbital angular momentum (OAM) degree of freedom has been far less explored. Recently, some of us demonstrated that OAM can enhance ptychographic XUV imaging resolution. Besides, by controlling the sign of the incoming OAM, we also showed that it can reveal the orientation of magnetic textures, such as the curling sense of a magnetic vortex, an effect termed Magnetic Helicoidal Dichroism.
Recently, we leveraged this technique to track the ultrafast dynamics of an oriented inhomogeneous magnetic texture. In particular, we identified a transient inversion of the curling sense of a magnetic vortex during its recovery following partial demagnetization initiated by a femtosecond laser pulse. A key advantage of Time-Resolved MHD is its ability to operate without a real space imaging of the spin texture, enabling rapid measurements of a meaningful parameter with just a few intensity patterns. These results hold great promise not only for monitoring the dynamics of complex magnetic textures but also for uncovering new forms of magnetic optical spin-orbit interactions.

Speaker: Thierry Ruchon (LIDYL, CEA-Saclay)

19 From Focusing to Structured Light: The Evolution of KAOS at the FERMI FEL

KAOS (Kirkpatrick–Baez Active Optical System) has been a long-standing companion of the FERMI Free Electron Laser and, more broadly, of the XEUV community, evolving together with the scientific questions it was meant to address. Conceived, developed and continuously refined within PADReS, KAOS was initially designed as an adaptive optical system aimed at achieving—and preserving—the best possible focus at the sample.
Over time, however, its role expanded beyond focal optimization. KAOS progressively became a versatile platform for beam shaping, enabling controlled illumination conditions tailored to different experiments. These include deliberately defocused configurations with decoupled focal dimensions, integration with diffractive optics such as Fresnel zone plates, and the extension toward non-conventional illumination schemes including beams carrying orbital angular momentum (OAM). Such capabilities have supported a broad range of techniques, from pump–probe spectroscopy and coherent diffraction imaging to nano-spectroscopy, nonlinear FEL experiments, interferometry, and structured illumination with vortex beams.
Throughout this evolution, wavefront sensing has played a central role. If KAOS were a sports car, wavefront sensing would be its pilot. Initially introduced to optimize mirror alignment and optical performance, it progressively became a key element for advanced beam shaping, evolving from a tool for tuning the optics to a direct diagnostic of the wavefront itself. A recent example is the characterization of OAM beams used by FERMI scientists and collaborators to explore new opto-magnetic interactions.
This contribution briefly revisits the experience accumulated with KAOS—not only to summarize what has been achieved, but also to stimulate new ideas. If the past showed what active wavefront control can do, the next question is simple: what could the next generation do with it?

Speaker: Michele Manfredda (Elettra Sincrotrone Trieste)

12:45 PM Lunch Break

Sample Environment and Instrumental Developments

Chair Matteo Pancaldi, Elettra Sincrotrone Trieste, Italy

20 Diffractive X-ray optics for ultra-fast science – design, fabrication and experiments

Diffractive optics have interesting properties that make them highly attractive for use with X-ray and EUV radiation. In contrast to other kinds of x-ray optics such as mirrors and lenses, diffractive optics allow for precise control of the optical wave front and the realization of complex optical functionalities. Applications range from time dispersive elements for single-shot demagnetization experiments to beam splitting in time-resolved X-ray absorption spectroscopy and 4-wave mixing experiments. The presentation covers aspects of the design and fabrication of diffractive optical elements for FEL radiation and presents examples of resulting experiments in ultra-fast science.

Speaker: Christian David (Paul Scherrer Institut)

21 Nonlinear X-Ray Spectroscopy of Interfaces

Interfaces between different phases or materials are where all the action is. Solid and liquid interfaces are central to chemical and biological as well as technological and industrial processes, yet their molecular level structure and functioning are difficult to directly study for a variety of reasons. In this talk I will discuss the use of nonlinear X-ray spectroscopy as a direct probe of liquid and solid interfaces. X-ray second harmonic generation and sum frequency generation are interface selective probes that are just now becoming possible with the availability of Terawatt pulses from X-ray free-electron lasers. Combining these with the recent development of sub-micron liquid sheets and heterostructures, has enabled us to report the first observation of soft X-ray second harmonic generation from the surface of liquid water, revealing a surface Hydrogen bonding structure that is distinct from that of the bulk liquid. The technique is now also being applied to buried functional interfaces in Perovskite solar-cell devices.

Speaker: Jake Koralek (Stanford / SLAC)

22 Soft X-ray hybrid detectors using LGAD sensors

The customization of Low-Gain Avalanche Diode (LGAD) sensors, originally developed for high-energy physics, to detect soft X-rays promises significant performance improvements over state-of-the-art low-energy detectors in terms of sensitivity, speed, and radiation hardness. By combining the intrinsic internal gain of LGADs with a thin entrance window optimized for high quantum efficiency, these sensors enable performance in the soft X-ray and UV energy range comparable to that delivered by hard X-ray detectors.

This presentation will discuss the results obtained so far for soft X-ray ptychography and time-resolved Resonant Inelastic X-ray Scattering (RIXS) measurements. Moreover, perspectives for future LGAD technologies will be explored, along with potential detector systems achieved by pairing LGADs with readout electronics conventionally used for hard X-rays.

Speaker: Anna Bergamaschi (Paul Scherrer Institut)

23 Nanoscale coherent surface phonon XUV spectroscopy of optically-induced phase transition in FeRh

The observation of ultrafast manipulation and coherent control of spins in magnetic materials at room temperature has prompted the intense experimental and theoretical efforts to understand the underlying microscopic mechanisms and relevant interactions (exchange, spin-lattice, electron-phonon, coulomb etc.) driving such magnetic phenomena at picosecond timescales.

Stoichiometric B2 ordered epitaxial (001) FeRh undergoes a first order magnetic phase transition from antiferromagnetic (AFM) to ferromagnetic(FM) at ≈ 380K. In the AFM phase of FeRh, only Fe has net magnetic moment, whereas in the FM phase both Fe and Rh carry net moments. The phase transition is also accompanied by a ≈ 1% isotropic lattice expansion in the bulk BCC structure, and changes in electronic structure. The phase transition has been extensively studied theoretically and experimentally, in thermal equilibrium and in the time domain, but a precise understanding of the roles of the electronic, phononic and spin sub-systems remains elusive.

In this contribution, I will discuss the coherent surface phonon spectroscopy of optically induced phase transition in FeRh studied via coherent XUV diffuse scattering in reflection geometry at DiProI beamline at FERMI-FEL. The square shaped scattering pattern and dispersion curves show the biaxial anisotropic nature of surface acoustic wave (SAW) propagation, with different phase velocities along different crystallographic axes, in epitaxial FeRh thin films grown on single crystalline 3C-SiC. Further, time-frequency wavelet analysis of SAW phonon oscillations shows the nature of structural phase transition in FeRh. This study marks the demonstration of comprehensive and simple method of SAW analysis using XUV scattering in technologically important phase transition materials such as FeRh at such high wavevectors.

Speaker: Naman Agarwal (Elettra Sincrotrone Trieste)

4:20 PM Coffee Break

Round Table on Source, Science, and End Stations for the Future FERMI Seeded FEL

5:50 PM Conclusion remarks