Dr. Benedikt Rösner

Benedikt Rösner
Profilbild Benedikt Rösner in Anzug mit Krawatte

Scientist for X-ray Optics

Paul Scherrer Institute
Forschungsstrasse 111
5232 Villigen PSI
Switzerland

Benedikt Rösner is scientist in the Beamline Optics group at the Paul Scherrer Institute. He studied chemistry at the FAU Erlangen-Nürnberg, where he also obtained his PhD. His research focused on spectroscopic and microscopic characterization of electrically switchable metal-organic and organic materials. After his PhD thesis, he started a position as post-doctorial researcher in the X-ray Optics and Applications group, where he focused on transmissive diffractive optics. His main research goals where the fabrication of lenses for high-resolution X-ray microscopy, where 7 nm spatial resolution were achieved, and the design and application of dedicated optical elements for experiments at X-ray free electron lasers. Since August 2019, he works as tenure-track scientist at the Swiss Light Source, where he plans the upgrade of beamline optics for SLS 2.0.  

Benedikt Rösner's responsibilities are the planning, specification, procurement and commissioning of X-ray optics with an emphasis of the upgrade of the synchrotron to SLS 2.0. His main tasks are the evaluation of the changes on the accelerator side and their impact on beamline performance, the assessment of different beamline upgrade scenarios, and the procurement of optical elements according to the input from the managment team of the upgrade project.

Soft X-ray microscopy with 7 nm spatial resolution

A Fresnel zone plate for high resolution imaging- D = 240 µm, dr = 8.8 nm

Fresnel zone plates are diffractive lenses widely used in X-ray microscopy. As an intrinsic limit from diffraction, their resolution is approximately in the order of their outermost zone width. This induces a major challenge in nanofabrication to produce smaller and smaller nanostructures striving for better resolution. In recent years, zone dimensions and resolution in X-ray micrscopy have been approaching the 10 nanometer level.
We surpassed a spatial resolution of 10 nm, a long-standing limit for almost a decade. While zone plates have been reported on with focal spots well below 10 nm (cp. Mohacsi et al., Sci. Rep. 2017, Döring et al., Opt. Express 2013) reconstructed from scattering patterns in the far field, directly recorded X-ray micrographs have not been obtained so far. We thus fabricated zone plates with iridium zones with 9 nm (see above), and tested their resolution at the Hermes beamline at Soleil and the Pollux beamline at the Swiss Light Source. Indeed, these zone plates prove to be capable of resolving test structures with typical sizes down to 7 nm. The following image shows a well-resolved periodic iridium structure with 9 nm line width.

A test structure consisting of periodic iridium lines with 9 nm line width. a) Scanning electron micrograph of a 200 x 200 nm wide field. b) X-ray transmission micrograph of a similar area with 1 nm step size. c) Fourier shell correlation, showing a frequency cut-off value of 7.1 nm.

X-ray transient gratings

Scheme of an X-ray Transient grating experiment. a) Experimental geometry at the Alvra endstation of SwissFEL. b) Fresnel simulation of the Talbot carpet: intensity modulation for a 1D diamond phase grating with a 200 nm pitch and 2.985 keV photon energy, resulting in a grating period of 190 nm at the sample (c).

The extension of transient grating spectroscopy to the X-ray regime is very appealing, opening possibilities ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. In an experiment at SwissFEL, X-ray transient gratings were prepared using a simple method based on the Talbot effect for converging beams. By analyzing printed interference patterns on polymethyl methacrylate and gold samples using ∼3 keV X-ray pulses, a the experimental feasibility transient gratings in the hard X-ray regime was demonstrated.
Towards X-ray transient grating spectroscopy - Optics Letters


Tackling the timing problem in ultrafast spectroscopy: Simultaneous single shot, time-resolved demagnetization dynamics at two energies

Schematic illustration of the time-streaking principle. In this way, a time window of 3.3 ps can be covered at the iron M-edge (52.7 eV).

Streaking the time information in one dimension on a detector
X-ray free electron lasers exhibit unique capabilities to conduct resonant x-ray spectroscopy techniques on ultrafast time scales. Utilizing diffractive optical elements, the pathway difference inherent to diffraction can be exploited to streak the arrival time of an x-ray probe along a geometric dimension. This concept has been applied in a pioneering experiment in reflection geometry at FLASH with a time resolution of 120 fs: Single-shot Monitoring of Ultrafast Processes via X-ray Streaking at a Free Electron Laser.
Adapting the experiment to a transmission geometry, we investigated demagnetization dynamics of a CoDy film together with collaborators from CNRS, the University Pierre and Marie Curie, and at the FERMI free electron laser, see figure above. The time window reachable with this setup is 1.5 ps, theoratically limited by the wavelenght divided through the speed of light. In practice, the duration of the pump puls of approx. 100 fs is the limiting factor.

Detector image of a time-streaking experiment using two energies. In this particular example, the sample is removed, and a shadow mask is placed in front of the off-axis zone plate to keep track of the experimental geometry.

Extending the concept to multiple energies
Extending this scheme to take advantage of the other spatial dimension, more advanced experiments become possible. In particular, we aim at performing ultrafast spectroscopy. Advancing towards this goal, we have designed a specific optical element, which allows to perform time-streaking experiments at two distinct energies simultaneously. This allows us to investigate multicomponent systems at two different absorption edges, and to compare dynamics of two elements with exactly the same timing (without a different time zero).
Employing this scheme, we investigated the demagnetization dynamics in iron-nickel multilayer systems and alloys. The evaluation of the data is ongoing. In principle, the number of different energies is not limited and can be extended even to a continuous energy range.
 


Optical vortices

Top row: Spiral zone plates with topological charge l=0 (Fresnel zone plate), l=1, l=2 and l=3. Bottom row: Resulting phase at the wavefront.

Creation of EUV vortices
Photons have fixed spin and unbounded orbital angular momentum (OAM). A light wave, which carries an orbital angular momentum can be imagined as an optical vortex. While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wave front. In electromagnetic radiation from free electron laser and synchrotron sources, the electric and magnetic field can rotate uniformly clockwise or counterclockwise with respect to the light propagation, resulting in circular polarization. In vortices, it is the phase of the electromagnetic field that rotates around a circularity in a helical fashion. The vortex can be characterized with an integer-numbered topologic charge, which describes how often the wavefront is shifted around 360°.
To demonstrate optical vortices at free electron lasers, we fabricated spiral zone plates, which yield a diffraction pattern with such a phase singularity. The material of choice for the extremely intense EUV radiation of the FERMI free electron laser is silicon. We thus etched spiral zone plates into ultraflat thin silicon membranes, and characterized the radiation using a Hartmann wavefront sensor in the far field.
Viewpoint in APS Physics :: Wavefront Characterization of Optical Vortices - Phys. Rev. X

Photoelectric effect with dependence on optical angular momentum
The distinctive way in which the photon spin dictates the electron motion upon light-matter interaction is the basis for numerous well-established spectroscopies that reveal the electronic, magnetic and structural properties of matter. In contrast, imprinting OAM onto a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable. Indeed, this amounts to transferring the phase of a classical electromagnetic wave, defined within several hundreds of nanometres, to a quantum particle localized within the few angstroms of an atom. In addition, the centre of symmetry of irradiated atoms does not in general coincide with the axis of the photon beam.
In an experiment at the 
FERMI free electron laser, we demonstrate for the first time that OAM-based dichroism can be observed in photoelectron spectroscopy, using an extended sample of He atomsSurprisingly, we find experimentally, and confirm theoretically, that the OAM of an optical field can be imprinted coherently onto a propagating electron wave, and that this phase information survives ensemble averaging out to macroscopic distances, where the electron is detected. We also show that electronic transitions, which are otherwise optically inaccessible due to selection rules, are essential for this process to occur. Our results reveal new aspects of light-matter interaction and point to a new kind of single-photon electron spectroscopy for accessing electronic optical transitions that are usually forbidden by symmetry.
A publication is accepted at Nature Photonics, and will be published soon.

For an extensive overview we kindly refer you to our publication repository DORA.

7 nm spatial resolution in soft X-ray microscopy and high resolution magnetic imaging
B. Rösner, S. Finizio, F. Koch, F. Döring, V. A. Guzenko, A. Kleibert, M. Langer, E. Kirk, M. Meyer, J. L. Ornelas, A. Späth, R. H. Fink, S. Stanescu, S. Swaraj, R. Belkhou, B. Watts, J. Raabe, C. David, submitted - contact me to get a draft version

The availability of intense soft X-ray beams with tunable energy and polarization has pushed the development of highly sensitive, element-specific and non-invasive microscopy techniques to investigate condensed matter with high spatial and temporal resolution. The short wavelengths of soft X‑rays promise to reach spatial resolutions in the deep single-digit nanometer regime, providing unprecedented access to magnetic phenomena at fundamental length scales. Despite considerable efforts in soft X‑ray microscopy techniques, a two-dimensional resolution of ten nanometers has not yet been surpassed in direct imaging. Here, we report on a significant step beyond this long-standing limit by combining newly developed soft X-ray Fresnel zone plate lenses with advanced precision in scanning control and careful optical design. With this approach, we achieve an image resolution of seven nanometers. By combining this highly precise microscopy technique with the X-ray magnetic circular dichroism effect, we reveal dimensionality effects in an ensemble of interacting magnetic nanoparticles. Such effects are topical in current nanomagnetism research and highlight the opportunities of high resolution soft X-ray microscopy in magnetism research and beyond.

Photoelectric effect with a twist
G. de Ninno, J. Wätzel, P. R. Ribi
č, E. Allaria, M. Coreno, M. B. Danailov, C. David, A. Demidovich, M. Di Fraia, L. Giannessi, K. Hansen, Š. Krušič, M. Manfredda, M. Meyer, A. Mihelič, N. Mirian, O. Plekan, B. Ressel, B. Rösner, A. Simoncig, S. Spampinati, M. Stupar, M. Žitnik, M. Zangrando, C. Callegari, J. Berakdar, Nature Photonics, accepted, 2020
Photons have fixed spin and unbounded orbital angular momentum (OAM). While the former is manifested in the polarization of light, the latter corresponds to the spatial phase distribution of its wave front. The distinctive way in which the photon spin dictates the electron motion upon light-matter interaction is the basis for numerous well-established spectroscopies that reveal the electronic, magnetic and structural properties of matter. In contrast, imprinting OAM on a matter wave, specifically on a propagating electron, is generally considered very challenging and the anticipated effect undetectable. Surprisingly, we find experimentally, and confirm theoretically, that the OAM of an optical field can be imprinted coherently onto a propagating electron wave, and that this phase information survives ensemble averaging out to macroscopic distances, where the electron is detected. We also show that electronic transitions, which are otherwise optically inaccessible due to selection rules, are essential for this process to occur. Our results reveal new aspects of light-matter interaction and point to a new kind of single-photon electron spectroscopy for accessing electronic optical transitions that are usually forbidden by symmetry.

Towards X-ray Transient Grating Spectroscopy
C. Sventina, R. Mankowsky, G. Knopp, F. Koch, G. Seniutinas, B. Rösner, A. Kubec, M. Lebugle, I. Mochi, M. Beck, C. Cirelli, J. Krempasky, C. Pradervand, J. Rouxel, G. F. Mancini, S. Zerdane, B. Pedrini, V. Esposito, G. Ingold, U. Wagner, U. Flechsig, R. Follath, M. Chergui, C. Milne, H. T. Lemke, C. David, P. Beaud, Optics Letters 44, 2019, 574-577
https://doi.org/10.1364/OL.44.000574
The extension of transient grating spectroscopy to the x-ray regime will create numerous opportunities, ranging from the study of thermal transport in the ballistic regime to charge, spin, and energy transfer processes with atomic spatial and femtosecond temporal resolution. Studies involving complicated split-and-delay lines have not yet been successful in achieving this goal. Here we propose a novel, simple method based on the Talbot effect for converging beams, which can easily be implemented at current x-ray free electron lasers. We validate our proposal by analyzing printed interference patterns on polymethyl methacrylate and gold samples using ∼3 keV X-ray pulses.


High Resolution Beam Profiling of X-ray Free Electron Laser Radiation by Polymer Imprint Development
B. Rösner, F. Döring, P. R. Ribič, D. Gauthier, E. Principi, C. Masciovecchio, M. Zangrando, J. Vila-Comamala, G. de Ninno, C. David, Optics Express 25, 2017, 30686-30695
https://doi.org/10.1364/OE.25.030686
High resolution metrology of beam profiles is presently a major challenge at X-ray free electron lasers. We demonstrate a characterization method based on beam imprints in poly (methyl methacrylate). By immersing the imprints formed at 47.8 eV into organic solvents, the regions exposed to the beam are removed similar to resist development in grayscale lithography. This allows for extending the sensitivity of the method by more than an order of magnitude compared to the established analysis of imprints created solely by ablation. Applying the Beer-Lambert law for absorption, the intensity distribution in a micron-sized focus can be reconstructed from one single shot with a high dynamic range, exceeding
. The procedure described here allows for beam characterization at free electron lasers revealing even faint beam tails, which are not accessible when using ablation imprint methods. We demonstrate the greatly extended dynamic range on developed imprints taken in focus of conventional Fresnel zone plates and spiral zone plates producing beams with a topological charge.

Extreme-Ultraviolet Vortices from a Free-Electron Laser
P. R. Ribič, B. Rösner, D. Gauthier, E. Allaria, F. Döring, L. Foglia, L. Gianessi, N. Mahne, M. Manfredda, C. Masciovecchio, R. Mincigrucci, N. Mirian, E. Principi, E. Roussel, A. Simoncig, S. Spampinati, C. David, G. de Ninno, Physical Review X 7, 2017, 031036
https://doi.org/10.1103/PhysRevX.7.031036

Extreme-ultraviolet vortices may be exploited to steer the magnetic properties of nanoparticles, increase the resolution in microscopy, gain insight into local symmetry and chirality of a material, and might even be used to increase the bandwidth in long-distance space communications. However, in contrast to generation of vortex beams in the infrared and visible spectral regions, production of intense, extreme-ultraviolet and X-ray optical vortices still remains a challenge. Here we present an in-situ and an ex-situ technique for generating intense, femtosecond, coherent optical vortices at a free-electron laser in the extreme-ultraviolet. The first method exploits nonlinear harmonic generation in a helical undulator, producing vortex beams at the second harmonic without the need for additional optical elements, while the latter one relies on the use of a spiral zone plate to generate a focused, micron-size optical vortex with a peak intensity approaching 10
14W/cm2, paving the way to nonlinear optical experiments with vortex beams at short wavelengths.