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Structured attosecond pulses for ultrafast nanoscience

Periodic Reporting for period 2 - ATTOSTRUCTURA (Structured attosecond pulses for ultrafast nanoscience)

Periodo di rendicontazione: 2021-09-01 al 2023-02-28

Since the invention of the laser in 1960, our ability to harness light for practical applications has increased exponentially. Yet for many years, the interest in the spatial profile of the laser beams was dominated by the quest of smooth and cleaner Gaussian beams, without undesired spatial inhomogeneities, as they can be focused to a minimum-size spot. However, advanced applications of light benefit from non-uniform intensity, phase or polarization distributions. Paradigmatic examples are optical tweezers and the applications of adaptive wavefront optics. The control of light in its transverse dimension has boosted the interest in the development of new techniques for structuring, sculpting, or tailoring light beams. The general term structured light refers to light beams with non-homogeneous, non-trivial, distribution of intensity, phase and/or polarization properties in the plane transverse to propagation.

Recent advances in structured laser pulses in the ultrafast regime, down to femtosecond laser pulses (1 fs=10-15 s), including the understanding of their nonlinear interaction with solid and gaseous systems, have enabled the up-conversion of structured light into the extreme-ultraviolet regime, a very attractive ground for the manipulation of magnetic and chiral systems at ultrafast timescales. Such milestone was not trivial, as most generation techniques are wavelength dependent, and very inefficient in the high-frequency/ultrafast world. Thus, the generation of structured light at these extreme regimes must be accompanied with theoretical efforts to understand their topology and the conservation rules of spin-orbit exchange between light and matter.

ATTOSTRUCTURA aims to develop novel ultrafast structured laser light tools in the, and to explore their application in ultrafast magnetism. Thanks to the process of high harmonic generation, today it is possible to generate attosecond laser pulses structured in their polarization and orbital angular momentum properties. In ATTOSTRUCTURA we aim to harvest the properties of such structured laser pulses to their limits, looking for strategies towards the generation of attosecond/x-ray tools with controlled angular momentum. Also, we aim to design new scenarios of harmonic generation in atomic and solid systems, where structured pulses would give access to novel ultrafast phenomena. Last but not least, we aim to explore scenarios of ultrafast magnetism that make use of structured laser pulses.
In ATTOSTRUCTURA we aim to develop the most advanced computational tools for the simulation of the generation of ultrafast structured laser pulses. In particular, we develop advanced theory of high harmonic generation, covering micro to macroscopic physics in a wide range of laser-matter configurations. We pay especial attention to the macroscopic description of high harmonic generation, in order to be able to describe accurately the up-conversion of topological light properties. Up to now, at the microscopic level we have developed strong-field approximation (SFA) and time-dependent Shcrödinger equation (TDSE) models in most of standard atomic gases (argon, helium, neon, etc.), and two-dimensional materials, such as graphene. We have combined such calculations with a proper macroscopic description relying on the integral expressions of the Maxwell equations. Both at the microscopic and macroscopic levels we have used the most advanced high-performance computing capabilities. Thanks to these implementations, we have proposed novel designs of ultrafast structured light beams, which have been later-on realized by our experimental collaborations. These include the generation of ultrafast vector-vortex beams, the generation of harmonic beams with ultra-high topological charges (the highest obtained in the ultrafast regime), and the generation of necklace-driven harmonics with controllable frequency and divergence properties, among others.

During the first period of the project we have performed a deep investigation of the role of intense femtosecond magnetic pulses in the magnetization dynamics, a completely unexplored field. A wide and generic benchmark of calculations have showed us that if a linearly polarized magnetic field is considered, huge magnetic fields, up to 10000 Tesla are required to drive measurable magnetization dynamics. In contrast, we have found that if circularly polarized magnetic fields are considered, a new chiral and nonlinear response appears. In such scenario, all-optical magnetization switching in fs timescales can be achieved with moderately high amplitudes of 10-100 Tesla. This result represents the first theoretical proposal for magnetization dynamics control through structured lasers.

These works have been published in high impact factor journals such as Science Advances or Optica, and presented in several conferences such as JEMS, Gordon Research Conference, Frontiers in Optics or CLEO, among others.
The field of structured ultrafast pulses (down to the attosecond timescale), and their interaction with nanostructures, are at the focus of interest in the non-linear optics community. ATTOSTRUCTURA is ideally placed to add value by tackling some very prominent current scientific challenges. The scientific and the technological activities within the Action drive a new direction in the field of light-matter interaction, a field of relevance not only for fundamental physics but also other disciplines ranging from material science, chemistry, and biology to optoelectronics and spintronics.

ATTOSTRUCTURA is producing highly specialized software products covering high harmonic generation in a broad range of situations, from the most common set-ups to structured hard x-rays scenarios. Advanced computational tools, including high performance computing and artificial intelligence, are opening exciting possibilities to tackle macroscopic strong-field scenarios that were not theoretically accessible some years ago. A multiplatform, open-source, user-friendly version of the high harmonic generation code will be developed, to be tested and validated by several laboratories. As a result, a stand-alone multiplatform application will be produced to serve as a practical tool for the attosecond community in the EU.

In collaboration with our experimental collaborators, we shall propose a new set of experiments that can benefit from the uniqueness of structured electromagnetic fields in diverse fields, in particular, in femtosecond, or even attosecond, ultrafast magnetism. Our first micromagnetic simulations results demonstrate that all-optical magnetization switching can be achieved through the use of structured laser pulses. These results imply the first theoretical proposal for ultrafast magnetic switching through the use of an isolated magnetic field. However, the generation of such ultrafast magnetic fields is challenging. Several approaches, including azimuthally polarized beams and metallic apertures, have been theoretically proposed. We aim to get closer to experiments and to propose scenarios in which such fields can be generated with state-of-the-art laser technology.