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Attosecond Dynamics in Advanced Materials

Periodic Reporting for period 1 - AuDACE (Attosecond Dynamics in Advanced Materials)

Reporting period: 2020-02-01 to 2021-07-31

Speed and performances of contemporary digital electronics are limited by the available device architectures and heat dissipation. Two-dimensional materials are emerging as one of the main candidates for designing new structures capable to overcome the current device limitations and foster the establishment of the electronics of the future. These materials are characterised by exotic physical, electronic and chemical properties, which are neither fully investigated nor understood. In particular, the lack of suitable tools hinders the possibility to study the ultrafast processes unfolding during light-matter interaction. Nevertheless, a clear understanding is required in order to leverage their unique properties. AuDACE aims to enter this unexplored region and investigate ultrafast electron, exciton and spin dynamics happening in advanced materials on time scales below few femtoseconds.
To reach this ambitious goal AuDACE will go beyond the state of the art and develop an innovative beamline for optical spectroscopy based on arbitrarily polarised attosecond pulses to study ultrafast exciton dynamics in a particular class of 2D materials like transition metal dichalcogenides.For the first time, a comprehensive investigation of ultrafast physical phenomena will become feasible in these materials. Due to the wide spectrum of relevant applications for 2D materials, the outcome of AuDACE is expected to have a crucial impact on the development of many key technological areas like optoelectronics, spintronics, valleytronics and photovoltaics.
During the first year we have successfully performed first attosecond spectroscopy experiments in bulk materials, investigating ultrafast electron dynamics and setting the basis for the development of the AuDACE project.
At first, considerable time has been devoted to the development of a new attosecond beamline for transient absorption/reflection spectroscopy. We have designed the beamline and purchased all the components necessary to its final assembly. While we performed the first tests to verify the functionality of the components, we developed and optimized a first prototype. This resulted in a unique two-foci beamline for attosecond transient absorption and reflection spectroscopy whose performances go beyond the state of the art whose concept has been published on Review of Scientific Instruments (Lucarelli et al, Rev. Sci. Instrum. 91, 053002 (2020)).
This beamline allowed us to performed transient reflectivity measurements on bulk insulators and semiconductors, perfectly in line with the scope of the project AuDACE. In particular, we studied the dynamical behaviour of core excitons in magnesium fluoride. Magnesium fluoride is an ionic crystal which is known to form core excitons if lightened with extreme ultraviolet (XUV) photons having energy of about 56 eV. In particular, this exciton formation is responsible for a peaked structure in the sample reflectivity, observed in static measurements around the M2,3 magnesium edge. Going beyond the state of the art, we performed pump-probe experiments by monitoring the sample differential reflectivity, simultaneously with the pump pulse electrical field temporal evolution. Our results report the first clear observation of sub-femtosecond exciton dynamics and prove that the dual atomic-solid nature of the quasiparticle manifest itself on different time scales. The results have been published on Nature Communications (M. Lucchini et al, Nature Communications 12, 1-7 (2021)).
We are currently performing experiments on semiconductors, moving towards advanced materials. In this regard, we performed two measurements campaigns at the Italian Synchrotron (ELETTRA), to characterized the static optical properties of the samples. Various samples have been measured, including some monolayer transition metal dichalcogenides. The results are currently under evaluation, but they indicate that the attosecond experiments planned for the remaining part of the project AuDACE should be feasible.
In parallel to the experimental activity, we conducted theoretical investigations aiming at widening the limits of the current attosecond techniques. In particular, we investigated the role of atomic resonances and field coupling in attosecond photoelectron experiments, commonly used for the temporal calibration of the attosecond pulses. The results have been published in Optics Express (R. Borrego-Varillas Optics Express 29, 9711-9722 (2021)). Moreover, we studied the origin of the time-frequency mapping in two-colour photoionization which involves attosecond radiation. Our work has been published in Journal of Physics B (B. Moio et al, J. Phys. B: At. Mol. Opt. Phys. 54, 154003 (2021)). Finally, we pioneered the possibility to use iterative algorithms to reconstruct the few-femtosecond exciton dynamics observed in attosecond transient absorption/reflection measurements. Our calculations show that ptychograpich algorithms can retrieve the physical constants describing the exciton dynamics with high precision. A first version of the work has been deposited on ArXiv (B. Moio et al, arXiv:2107.08202).
The progress beyond the state of the art currently made within AuDACE can be summarized in the following points:
- Realization of a unique beamline for attosecond transient absorption/reflection spectroscopy in a sequential two-foci geometry, capable to access the absolute timing between a few-femtosecond pump field and the dynamical signal coming from the solid sample. This unique feature opens new investigation possibilities.
- First observation of sub-fs exciton dynamics in bulk crystals. We could observe, for the first time, clear sub-fs excitons dynamics and disentangle the atomic and solid nature of the excitonic response. In particular, we found that while the atomic nature of the exciton dominates the few-fs dynamics through optical Stark effect, its solid nature is responsible for the sub-fs dynamics via dynamical Franz-Keldysh effect. This opens the possibility to control the attosecond and nanometric motion of the exciton with high temporal and spatial resolution.
- Even if rich physical information is encoded in the few-fs exciton dynamics, how to extract it from an experimental trace remains a formidable task. A typical approach is based on a multidimensional fitting procedure, which requires reasonable initial guess in order to properly converge. Going beyond this approach, we proposed to use an iterative reconstruction algorithm in order to retrieve the exciton physical properties also in the case where no or little initial information is given.

The above results set the basis for the following expected outcome before the end of the project:
- Development, assembly and test of a new attosecond beamline for arbitrarily polarized pulses. This beamline will be realized in the following year, setting the basis for a new standard in attosecond physics.
- Investigation of ultrafast exciton dynamics in advanced materials with linearly polarized pulses. This class of experiment will widen our knowledge of advanced materials, fostering the exploitation of their unique exotic properties and paving the way for they exploitation in technological fields.
- Realization of attosecond spectroscopy of solids with circularly polarized light. This opportunity will allow us to access new unexplored physics and opening the way for unforeseen technological development.
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