Dual character of excitons in the ultrafast regime: atomic-like or solid-like?

February 15, 2021

Milan, 15 February 2021 - Excitons are quasiparticles which 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 behaviour and how to manipulate it. Researchers at Politecnico di Milano in collaboration with the Institute of Photonics and Nanotechnologies IFN-CNR and a theory group from the Tsukuba University (Japan) and the Max Plank Institute for the Structure and Dynamics of matter (Hamburg, Germany), have discovered that an exciton can simultaneously adopt two radically different characters when it isstimulated by light. Their work, now published in Nature Communications, yields crucial new insights for current and future excitonics research.

Excitons consist of a negatively charged electron and a positively charged hole in solids. They are a so-called many-body-effect, produced by the interaction of many particles, especially when a strong light pulse hits the solid material. In the past decade, researchers have observed many-body-effects down to the unimaginably short attosecond time scale, in other words billionths of a billionth of a second.

However, scientists have still not reached a fundamental understanding of excitons and other many-body effects due to the complexity of the ultrafast electron dynamics when many particles interact. The research team from Politecnico di Milano, the University of Tsukuba and the Max Planck Institute for the Structure and Dynamics (MPSD) wanted to explore the light-induced ultrafast exciton dynamics in MgF2 single crystals by employing state-of-the-art attosecond transient reflection spectroscopy and microscopic theoretical simulations.

By combining these methods, the team discovered an entirely new property of excitons: The fact that they can simultaneously show atomic-like and solid-like characteristics. In excitons displaying an atomic character, the electrons and holes are tightly bound together by their Coulomb attraction - just like the electrons in atoms are bound by the nucleus. In excitons with a solid-like character, on the other hand, the electrons move more freely in solids, not unlike waves in the ocean.

"These are significant findings - says lead author Matteo Lucchini from the Politecnico di Milano - because understanding how excitons interact with light on these extreme time scales allows us to envision how to exploit their unique characteristics, fostering the establishment of a new class of electro-optical devices."

During their attosecond experiment performed at the Attosecond Research Center (ARC, http://www.attosecond.fisi.polimi.it) within the ERC project AuDACE ("Attosecond Dynamics in AdvanCed matErials", http://www.audaceproject.it), the researchers managed to observe the sub-femtosecond dynamics of excitons for the first time, with signals consisting of slow and fast components. This phenomenon was explained with advanced theoretical simulations, adds co-author Shunsuke Sato from the MPSD and the University of Tsukuba: "Our calculations clarified that the slower component of the signal originates from the atomic-like character of the exciton while the faster component originates from the solid-like character - a ground-breaking discovery, which demonstrates the co-existence of the dual characters of excitons!"

This work opens up an important new avenue for the manipulation of excitonic as well as materials' properties by light. It represents a major step towards the deep understanding of non-equilibrium electron dynamics in matter and provides the fundamental knowledge for the development of future ultrafast optoelectronic devices, electronics, optics, spintronics, and excitonics.

Politecnico di Milano

Related Electrons Articles from Brightsurf:

One-way street for electrons
An international team of physicists, led by researchers of the Universities of Oldenburg and Bremen, Germany, has recorded an ultrafast film of the directed energy transport between neighbouring molecules in a nanomaterial.

Mystery solved: a 'New Kind of Electrons'
Why do certain materials emit electrons with a very specific energy?

Sticky electrons: When repulsion turns into attraction
Scientists in Vienna explain what happens at a strange 'border line' in materials science: Under certain conditions, materials change from well-known behaviour to different, partly unexplained phenomena.

Self-imaging of a molecule by its own electrons
Researchers at the Max Born Institute (MBI) have shown that high-resolution movies of molecular dynamics can be recorded using electrons ejected from the molecule by an intense laser field.

Electrons in the fast lane
Microscopic structures could further improve perovskite solar cells

Laser takes pictures of electrons in crystals
Microscopes of visible light allow to see tiny objects as living cells and their interior.

Plasma electrons can be used to produce metallic films
Computers, mobile phones and all other electronic devices contain thousands of transistors, linked together by thin films of metal.

Flatter graphene, faster electrons
Scientists from the Swiss Nanoscience Institute and the Department of Physics at the University of Basel developed a technique to flatten corrugations in graphene layers.

Researchers develop one-way street for electrons
The work has shown that these electron ratchets create geometric diodes that operate at room temperature and may unlock unprecedented abilities in the illusive terahertz regime.

Photons and electrons one on one
The dynamics of electrons changes ever so slightly on each interaction with a photon.

Read More: Electrons News and Electrons Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.