Attosecond Dynamics in Advanced Materials

Informazioni relative al progetto

AuDACE

ID dell’accordo di sovvenzione: 848411

DOI

10.3030/848411

Progetto chiuso

Data della firma CE 10 Settembre 2019

Data di avvio 1 Febbraio 2020

Data di completamento 31 Luglio 2025

Finanziato da

• EXCELLENT SCIENCE - European Research Council (ERC)

Costo totale

€ 1 466 250,00

Contributo UE

€ 1 466 250,00

Coordinato da

POLITECNICO DI MILANO
Italy

Periodic Reporting for period 4 - AuDACE (Attosecond Dynamics in Advanced Materials)

Periodo di rendicontazione: 2024-08-01 al 2025-07-31

Sintesi del contesto e degli obiettivi generali del progetto

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. Due to their nanometric thickness, the charges are confined in two directions. As a result, 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 to leverage the unique properties of 2D materials. 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 (1 fs = 10-15 s) with unprecedented and ground-breaking possible outcome.

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 (1 as = 10-18 s). At the beginning the research will concentrate on ultrafast exciton dynamics in a particular class of 2D materials like transition metal dichalcogenides (TMDCs). In the final phase, AuDACE will focus on a new class of materials such as ferromagnetic TMDCs to investigate the elusive physical mechanism responsible for ultrafast spin and magnetic dynamics. For the first time, a comprehensive investigation of these phenomena will become feasible on these little studied time scales. 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.

Lavoro eseguito dall’inizio del progetto fino alla fine del periodo coperto dalla relazione e principali risultati finora ottenuti

During the first phase we developed a new two-foci beamline for transient absorption/reflection spectroscopy that was published in Rev. Sci. Instrum. (Lucarelli et al., 91, 053002 (2020)). This beamline, later upgraded for polarization-invariant operation (manuscript under preparation), allowed unprecedented delay calibration which was used to achieved attosecond spectroscopy experiments in bulk materials, probing ultrafast electron dynamics and laying the foundation for future studies. At first we resolved sub-femtosecond core-exciton dynamics in MgF2, discovering their dual atomic–solid nature which unfold on different, ultrafast, time scales never investigated before (Nat. Commun. 12, 1–7 (2021)).
Further experiments on insulators like diamond and LiF revealed new phenomena. In diamond, absolute timing allowed us to investigate coherent electron dynamics and show that vertical transitions of virtual charges play a key role, revising the picture of strong-field intra-band motion (Nat. Photonics 19, 999–1005 (2025)). The results suggest interpretation with Floquet theory, which has been tested against short pulses in a different experiment (Nat. Commun. 13, 7013 (2022)).
In LiF, a previously unseen excitonic resonance was detected. This transition, hidden in standard measurement, becomes detectable only under strong-field excitation, showing that light pulses can induce new functionalities on ultrafast time scales (manuscript in preparation).
The project then shifted to semiconductors. At ELETTRA, we characterized static properties of several materials, including MoS2, where contrary to expectations, no core-exciton transitions were found. Collaboration with theory attributed this to electron correlation and screening, resolving a currently debated matter (manuscript in preparation).
After synchrotron characterization, we performed attosecond spectroscopy on Ge under various conditions. The extreme time resolution allowed us to disentangle competing charge-injection mechanisms in intrinsic crystal, unravelling the process of ultrafast charge injection (Nat. Photonics 17, 1059–1065 (2023), Inzani et al., Nuovo Cim. C 46, 110 (2023), Di Palo et al., Stru. Dyn. 11, 044303 (2024)). The results obtained with doped samples, instead, proved the possibility to tune the timing of optical response and control real–virtual charge interplay (manuscript in preparation).
Parallel theoretical work advanced attosecond methodology, including two-color mapping (J. Phys. B 54, 154003 (2021); Phys. Rev. A 108, 013117 (2023)), volume-average effects (J. Phys. Photonics 4, 034006 (2022)), and iterative reconstruction (Opt. Express 29, 9711–9722 (2021), Opt. Express 30, 12248–12267 (2022), APL Photonics 8, 076101 (2023); J. Phys. Photonics 6, 025007 (2024)).
Finally, three reviews, summarizing the field, have been published (Borrego-Varillas et al., Rep. Prog. Phys. 85, 066401 (2022), Di Palo et al., APL Photonics 9, 020901 (2024); Inzani & Lucchini, J. Phys. Photonics 7, 022001 (2025)).

Progressi oltre lo stato dell’arte e potenziale impatto previsto (incluso l’impatto socioeconomico e le implicazioni sociali più ampie del progetto fino ad ora)

The progress can be summarized in the following points:
- Realization of a unique beamline for attosecond transient absorption/reflection spectroscopy in a sequential two-foci geometry which enabled unprecedented absolute delay calibration.
- 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, its solid nature is responsible for the sub-fs dynamics. This opens the possibility to control the attosecond and nanometric motion of the exciton with high temporal and spatial resolution.
- First observation of ultrafast carrier injection dynamics in bulk semiconductor like Ge demonstrating the existence of competing mechanism that unfold on different time scales. These results are of great interest for the development of electro-optical switches of new conception.
- Development and assembly of new attosecond beamline for arbitrarily polarized pulses. This beamline sets a new state of the art allowing arbitrary polarization states and the study of exotic physical phenomena.
- First experimental prove of the role of vertical virtual charge transitions in an insulator. This result set the bases for the correct interpretation of coherent electron dynamics in field-driven systems and demonstrate the existence of an intrinsic limit to the speed at which electro-optical devices can be operated.
- Discovery of a new excitonic resonance in LiF which, despite involving valence electrons has an ultrashort short lifetime. This new quasi-particle is hidden in the static data and becomes observable only with attosecond techniques, allowing insight into the intertwined coupling between exciton and solid.
- First observation of the role of doping in shaping the ultrafast response of semiconductor. These results show that doping can be used as a new tool to engineer the optical response in petahertz devices and obtain conductors whose properties can be manipulated with light on sub-femtosecond time scales.
- First attosecond study of a 2D semiconductor. Achieving a sensitivity never obtained before, these results are unique and prove that the spectroscopic technique developed during the project can give us access to unexplored regions of knowledge.

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