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

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

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.

Data: CORDIS, © European Union