Enhancing FEL imaging resolution with OAM and ptychography

The broad research fields of microscopy and computational imaging actively seek higher resolution, better quality, faster acquisition, versatile and easy-to-operate instrumentation to accommodate as many scientific applications as possible. Ptychography is one of the computational microscopy techniques based on diffraction imaging principles, where algorithms are used to reconstruct a high-fidelity image of the sample without the use of magnifying lenses. While powerful, ptychography’s effectiveness can, however, be limited by the characteristics of the probing light, particularly in the case of samples exhibiting a high degree of symmetry.

In this study, researchers at the DiProi beamline explored the use of Extreme Ultraviolet light carrying Orbital Angular Momentum (OAM), often referred to as "twisted light" to enhance ptychographic imaging resolution. Unlike conventional (Gaussian) beams, OAM light has a helical phase structure, which combined with the structured illumination of the beam profile, allows for a selective amplification of high-frequency spatial component in the diffraction process. The net effect is an increase in the amount of information (spatial resolution) that can be captured about the sample.

The experiments were conducted using the FERMI seeded free-electron laser (FEL) source, which provides a highly coherent and intense beam of light. By pairing state-of-the-art ptychography reconstruction algorithms designed by the Scientific Computing Team with the high-quality characteristics of the source, the researchers were able to reliably use the OAM beam to improve the spatial resolution of the reconstructed images.

The results showed a significant improvement in imaging resolution - up to 30% when compared to conventional Gaussian beams. The enhancement allows for more detailed visualization of nanostructures, revealing fine details that are otherwise difficult to be resolved (see the central part of the nanostructured star in Figure 1, panels c and d, and the quantitative resolution analysis in Figure 1, panel e).

Figure 1: (a) The ptychography setup built at the DiProI beamline employs different spiral zone plates to structure the FEL beams exploited in the experiment, from Gaussian (ℓ=0) to OAM (ℓ=±1, ±2, ±3). A sample stage is used to scan the sample in the xy-plane (nanostructured star). Diffraction patterns emerging from the beam-sample interaction are directly acquired with a CCD camera, without using a magnifying lens. (b) By using OAM (ℓ=±1, ±2, ±3) instead of a simple Gaussian beam (ℓ=0), the amount of information acquired extends to the borders of the frame (see the radial lines extending up to the edges) and provides a stronger signal. (c) By applying a modified reconstruction algorithm on the acquired diffraction patterns (SciComPty), it is possible to reconstruct the complete image of the nanostructured star, as well as the wave-field of light at the sample plane (shown in magnitude and phase; note the twirl in panel d). By comparing panels (c) and (d), it can be seen that higher image quality and resolution are associated with the use of an OAM beam (ℓ=-3). An increase of about 30% in resolution (120 nm vs 80 nm) can be observed by quantitatively assessing for the resolution enhancement.

Furthermore, the approach uses single-shot measurements, where only one light pulse is acquired per position. This method not only speeds up the imaging process, but also reduces the potential for errors due to beam instability that can occur with multiple exposures. The resulting efficiency paves the way for fast and reliable measurements, crucial for real-time applications including in-situ experiments for monitoring dynamic processes.

The ability to achieve higher resolution with OAM beams opens up new possibilities for various applications. For instance, in materials science, it can help in studying the intricate patterns in nanofunctional materials, as well as their collective excitations under optical stimuli. Furthermore, the reported resolution enhancement can find applications in biology, improving imaging quality and providing reliable phase contrast information of cellular structures, aiding in the understanding of complex biological processes.

By integrating ptychography with twisted light, researchers have developed a more effective method for high-resolution microscopy based on the state-of-the-art transverse and longitudinal coherence of FEL radiation. This technique holds promise for advancing research in multiple fields, offering clearer, faster, and more detailed views of the microscopic world.

Matteo Pancaldi^1,2,†, Francesco Guzzi^1,†, Charles S. Bevis³, Michele Manfredda¹, Jonathan Barolak⁴, Stefano Bonetti^2,5, Iuliia Bykova⁶, Dario De Angelis¹, Giovanni De Ninno^1,7, Mauro Fanciulli^8,9, Luka Novinec¹, Emanuele Pedersoli¹, Arun Ravindran⁷, Benedikt Rösner⁶, Christian David⁶, Thierry Ruchon⁹, Alberto Simoncig¹, Marco Zangrando^1,10, Daniel E. Adams⁴, Paolo Vavassori^11,12, Maurizio Sacchi^13,14, George Kourousias¹, Giulia F. Mancini3 and Flavio Capotondi¹

¹ Elettra-Sincrotrone Trieste S.C.p.A., Trieste, Italy
² Department of Molecular Sciences and Nanosystems, Ca’ Foscari University of Venice, Venezia, Italy.
³ Laboratory for Ultrafast X-ray and Electron Microscopy (LUXEM), Department of Physics, University of Pavia, Pavia, Italy.
 ⁴ Department of Physics, Colorado School of Mines, Golden, Colorado, USA.
⁵ Department of Physics, Stockholm University, Stockholm, Sweden.
⁶ Paul Scherrer Institut, Villigen, Switzerland.
⁷ Laboratory of Quantum Optics, University of Nova Gorica, Nova Gorica, Slovenia.
⁸ CY Cergy Paris Université, CEA, LIDYL, Gif-sur-Yvette, France.
⁹ Université Paris-Saclay, CEA, LIDYL, Gif-sur-Yvette, France.
¹⁰ Istituto Officina dei Materiali, CNR, Trieste, Italy.
¹¹ CIC nanoGUNE BRTA, Donostia-San Sebastián, Spain.
¹² IKERBASQUE, Basque Foundation for Science, Bilbao, Spain.
¹³ Sorbonne Université, CNRS, Institut des NanoSciences de Paris, INSP, Paris, France.
¹⁴ Synchrotron SOLEIL, L'Orme des Merisiers, Saint-Aubin, Gif-sur-Yvette, France.
^† These authors contributed equally to this work

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