The researchers, including Simon Gerber Head of the Quantum Techno­logies Group at the Paul Scherrer Institute PSI, explore how light can fundamentally alter the properties of solids – and how these effects can be harnessed in future appli­cations. The review on the latest developments in ultrafast materials science is both meant as a guide for graduate students entering the field as well as a standard reference for the community.

The team – including MPSD group leaders James McIver and Michael Sentef as well as Dante Kennes from RWTH Aachen University, Alberto de la Torre (Brown University) and Martin Claasen (University of Penn­sylvania) – discusses experiments and theoretical ideas for how solids react to excitations with short laser pulses or the coupling of light and matter during irra­diation with light. A piece of material, when left alone, is usually in thermal equilibrium and governed by the laws of thermo­dynamics, in which a few known external conditions fully determine its behavior. However, many practical applications require the knowledge not only of the equili­brium state of a given material, but also of its exci­tations. “If we could design excited states at will, this would allow us to create new applications, for instance in high-speed information processing and storage, lossless energy transfer, and quantum techno­logies,“ explains Simon Gerber.

In recent years, the field of pump-probe experiments has seen tremendous progress. In these experi­ments a short pump laser pulse drives a material into an excited state. Stroboscopic probe measurements then create stop-action movies of the ensuing dynamics. “Thanks to technical developments, scientists can now exert control over electrons, their spin and orbital degrees of freedom, and the crystal lattice of ions,“, says Michael Sentef. “Importantly, we are able to track these controlled states of matter with a time resolution of femto­seconds.“ “We believe that it is vitally important to identify unifying themes of how we can control materials with light, and to push towards appli­cations,“ says Dante Kennes.

Simon Gerber highlights the novel aspect that different probing techniques can be combined to learn about different parts of a dynamical system at the same time. “When you hit a material with a laser, the electrons get pushed around and the ions that form the crystal lattice start to move at the same time,” he explains. “Unlike in thermal equilibrium, where there is always a balance between these different constituents of a system, the laser can disrupt this balance, leading to nonequili­brium states where energy flows within the material in sometimes unexpected ways. It is invaluable to learn about how the different parts react to the external driving force but also to each other. For instance, we have learned about the mutual forces between electrons and ions by moni­toring both of their dynamics simultaneously.” Such new insights pave the way for future work, adds Sentef: “The knowledge gleaned for instance allows us to better understand which forces pair electrons to create better super­conductors, materials that conduct elec­tricity without heat losses and make for fantastic magnets.“

“New experimental capa­bilities also stimulate theoretical ideas, which in turn motivate experi­mentalists to look for ways to realize those ideas,“ says Martin Claassen. „For instance, around ten years ago theorists proposed changing a material’s topology – a quantum-mechanical property which can lead to dissi­pationless transport along its edges while being insulating in the bulk – by shining light on it. This is called Floquet engineering after the French mathematician who invented a formalism to describe dynamical systems which are driven by forces that oscillate in real time.“ The resulting Floquet topo­logical states were only recently measured in an experiment led by James McIver. “We had to invent and build a whole new experiment to achieve that,“ he says.

“In our review, we stress the synergies that are created when theory and experiment go hand in hand. We believe that the field is now ripe to move from discoveries of new effects in laser-driven materials towards harnessing these effects for potential techno­logies.“ De la Torre adds: “One way to achieve this is to make use of material growth techniques in order to design samples with desired equilibrium and excited states. These can then be controlled by short laser pulses. This is clearly a team effort, driven both by experimental progress and theo­retical under­standing, and we hope that our review can help form an even stronger community and attract particularly young researchers to join this scientific journey.“ (Source: PSI / MPSD)

Reference: A. de la Torre et al.: Nonthermal pathways to ultrafast control in quantum materials, Rev. Mod. Phys., 93, 041002 (2021); DOI: 10.1103/RevModPhys.93.041002

Link: Laboratory for Micro and Nanotechnology, Paul Scherrer Institut, Villigen PSI, Switzerland • Theoretical Description of Pump-Probe Spectroscopies in Solids, Max Planck Institute for the Structure and Dynamics of Matter, Center for Free-Electron Laser Science (CFEL), Hamburg, Germany