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Ultrafast opto-electronic twists: Controlling the chirality of electrons and extreme-UV photons by ultrafast laser pulses (UCHIRAL).

Periodic Reporting for period 1 - UCHIRAL (Ultrafast opto-electronic twists: Controlling the chirality of electrons and extreme-UV photons byultrafast laser pulses (UCHIRAL).)

Reporting period: 2017-07-01 to 2019-06-30

The UCHIRAL project comprise two approaches to improve the way that humanity can observe extremely fast phenomena in materials: The first approach utilizes circularly polarized high-harmonic beam. These beams of extreme-UV or X-ray light are generated from femtosecond laser pulses (1 femtosecond = 1 / 1 000 000 000 000 000 seconds) were considered to be strictly linear, and the recent demonstration of bright beams with circular polarization opened the path towards magnetic imaging. Indeed, nanoscale magnetic imaging was a proto “holy grail” in the community. The importance is that when combined with the proven femtosecond and even better temporal resolution, imaging with high harmonics would allow for the “ultimate” slow motion movies of magnetic dynamics, following the magnetization as fast as a few femtosecond and as accurately as a few nanometers. The challenges, however, are significant. A preliminary objective aimed at demonstrating that a source of circularly polarized high harmonics can be stable enough for imaging, and especially in the wavelength range around 20 nm, where the magnetization of important magnets such as iron, nickel and cobalt, can be detected by polarized light. Then, there are no lenses that work for this wavelength so the camera records the light scattered off the magnetic texture, from which the image is retrieved algorithmically. A combination of bright source, high quality beam, and a robust image retrieval approach must be combined.

The second approach utilizes the interaction of strong lasers with beams of free electrons in a transmission electron microscope (TEM). TEMs are important as they are a crucial portal to the smallest length-scales, reaching systematically below 1 nanometer. The exciting development of TEM with pulsed electron beam allowed to record nano and picosecond dynamical movies. On a fundamental level, the short electron pulses allowed to experiment with extreme optical intensities on free electrons, in the form of a finite optical energy compressed to a short pulse which overlaps temporally with the electron. In a particularly important demonstration, the Ropers group in Göttingen showed that light can compress the electrons to pulses with attosecond-scale duration (1 attosecond = 1/1000 femtoseconds). Thus, when the original objective of this approach, to generate vortex-electron beams was demonstrated by another group, the team adopted another concept, with potentially a far reaching impact. The recipient set a long-term goal to proliferate the availability of attosecond pulses to standard TEMs, not only microscopes with pulsed electron beams. To push this forward, the project considered microscopic glass structures. Even in the simplest structures, spheres, light can be trapped and circumvent them many (even millions!) times and thus, requires small input power to have strong fields.
Within the “UCHIRAL” project, the recipient, Dr. Ofer Kfir moved to Göttingen, Germany, and worked exclusively on the project. The Supervisor, Prof. Dr. Claus Ropers guided the project, and was available for important discussions throughout the duration of the project. The supervisor and also supported the project by upgrading the laser system, upgraded the camera (4 Megapixel instead of 1.7 Megapixel), and allocated further budget to promote the project. The Beneficiary, the University of Göttingen, supported the recipient and his family through the international office, and through academic infrastructure and professional training.


The results of the first part of the “UCHIRAL” project, attempting at magnetic imaging, were extremely successful. Before the project commenced, the state-of-the-art magnetic imaging which can be used for ultrafast movies was found at large-scale facilities, e.g. synchrotron. There was no laboratory-scale microscope for magnetism that can reach the scale of 100 nm. Within “UCHIRAL” project, the team found that the source of high-harmonics was extremely bright and stable, leading to the recording of high-quality diffraction patterns, which result in magnetic images having resolutions below 50 nm. The diffraction, and reconstruction of the magneto-optical (MO) phase and amplitude are below. This success, seeded a collaboration that may improve the available magnetic images from many large scale facilities (synchrotrons) around the world. Such an impact is well timed with the effort to utilize magnetic features to replace electronics as the logic in our computers. Magnetic features have the prospect of low-energy consumption, which remains intact even without power input. The results are published in Science Advances (DOI: 10.1126/sciadv.aao4641) and additional information is available on the group website (https://www.uni-goettingen.de/en/603791.html)

The second part lay the foundation for strong interaction of light confined to glass structure and electrons. In the publication accepted for publication in Physical Review Letters (currently available at arXiv:1902.07209). The recipient found quantitatively that the interaction can be strong enough such that even a single photon stored in a special ring can have strong-field effect on the electron beam. In this regime, quantum-mechanics phenomena as entanglement would become available for researchers. Specifically, we find that when using light trapped in glass resonators, its velocity can match that of the traversing electron. Under this so called “phase-matching” condition the interaction length play an equivalent role to the laser field amplitude, and propagation over a few microns, instead of the few nanometers used to date, reduces the requirement of the light field intensity by 1 million (!) and eliminate the requirement for intense pulses. An experimental work to utilize light trapped in glass spheres is ongoing, and support these concepts substantially. We hope and believe that TEMs around the world would be able to convert their continuous electron beam into a stroboscopic beam of attosecond pulses, and to freeze extremely fast motion, just like a flash in a hand-held camera freezes the image of a child running in a dimly lit room.
The two work packages of the UCHIRAL project advanced the state of the art in their respective academic fields both technologically, and conceptually. The system devoted to the advance the work package 1 is the first, and currently the only that demonstrated magnetic imaging below 50 nm with HHG radiation, which is capable of temporal resolution in the femtosecond scale. The methodology developed during the project can be extended to other radiation sources, e.g. Synchrotrons, to improve their imaging capabilities, and advance magnetic technology. The work done within work package 2 opened a new conceptual field for free-electron coupling with light, suggesting that free-electrons and cavity photons can be entangled. Importantly, it predicts that such entanglement is feasible experimentally. Based on this approach, undergoing experiments are paving the way for such strong interaction and its utilization. One impactful example would be to allow the ultimate temporal resolution (attoseconds) in standard TEMs. If the interaction is strong, even a continuous-wave weak laser can drive electrons substantially. Thus, the continuous electron beam in any TEM worldwide can be converted into a stroboscopic beam of electron-attosecond-pulse train.