Double character of excitons in extremely fast time scales: atomic type or solid type?

Attosecond measurement of an exciton in a crystal of MgF2. Credit: Polimi

Excitons are quasiparticles that can transport energy through solid substances. This makes them important for the development of future materials and devices – but more research is needed to understand their fundamental behavior and how to manipulate it. Researchers from Politecnico di Milano, in collaboration with the Institute of Photonics and Nanotechnologies IFN-CNR and a theory group from the University of Tsukuba (Japan) and the Max Plank Institute for the structure and dynamics of matter (Hamburg, Germany), have discovered that an exciton can simultaneously assume two radically different characters when stimulated by light. Their work, recently published in Nature communications, provides crucial new information for current and future research on excitonics.

Excitons consist of a negatively charged electron and a positively charged hole in solids. This is a so-called multi-body effect, produced by the interaction of many particles, in particular when a strong light pulse hits the solid material. Over the past decade, researchers have observed effects on multiple bodies down to the incredibly short attosecond timescale – in other words, billionths of a billionth of a second.

However, scientists have yet to achieve a fundamental understanding of excitons and other multi-body effects due to the complexity of the dynamics of ultra-fast electrons when many particles interact. The research team from Politecnico di Milano, University of Tsukuba and the Max Planck Institute for Structure and Dynamics (MPSD) wanted to explore the light-induced dynamics of ultra-fast excitons in MgF2 single crystals. using the peak attosecond. transient reflection spectroscopy and theoretical microscopic simulations.

By combining these methods, the team discovered an entirely new property of excitons: the fact that they can simultaneously exhibit atomic and solid characteristics. In excitons with an atomic character, electrons and holes are closely linked by their Coulomb attraction – just as the electrons of atoms are linked by the nucleus. In solid-character excitons, on the other hand, electrons move more freely in solids, much like waves in the ocean.

“These are important discoveries – says lead author Matteo Lucchini of Politecnico di Milano – because understanding how excitons interact with light at these extreme timescales allows us to envision how to exploit their unique characteristics, promoting the creation of ‘a new class of optical electro-devices. “

During their attosecond experiment carried out at the Center for Research on Attosecond (ARC) as part of the ERC project AuDACE (“Attosecond Dynamics in AdvanCed MatErials”), the researchers succeeded in observing for the first time the sub-femtosecond dynamics. excitons, with fast components and. This phenomenon has been explained with advanced theoretical simulations, adds co-author Shunsuke Sato of MPSD and Tsukuba University: “Our calculations clarified that the slower component of the signal comes from the atomic character of the exciton while that the fastest component comes from. of the solid character – a revolutionary discovery, which demonstrates the coexistence of the double characters of the excitons! “

This work opens an important new avenue for the manipulation of exciton properties and materials by light. It represents a major step towards an in-depth understanding of the dynamics of out-of-equilibrium electrons in matter and provides the fundamental knowledge for the development of future ultrafast optoelectronic devices, electronics, optics, spintronics and excitonic.

Reference: “Unraveling the atomic and massive nature of interlaced excitons localized by attosecond spectroscopy” by Matteo Lucchini, Shunsuke A. Sato, Giacinto D. Lucarelli, Bruno Moio, Giacomo Inzani, Rocío Borrego-Varillas, Fabio Frassetto, Luca Poletto, Hannes Hübener , Umberto De Giovannini, Angel Rubio and Mauro Nisoli, February 15, 2021, Nature communications.
DOI: 10.1038 / s41467-021-21345-7

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