An international team with participation of the Paul Scherrer Institute PSI
shows how light can fundamentally change the properties of solids and how
these effects can be used for future applications. The researchers summarize
their progress in this field, which is based among other things on
experiments that can also be carried out at the Swiss X-ray free-electron
laser SwissFEL, in the scientific journal Reviews of Modern Physics.
The researchers 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. In addition to PSI researcher Simon
Gerber, it was written by MPSD group leaders James McIver and Michael Sentef
as well as Dante Kennes from RWTH Aachen University, Alberto de la Torre
(Brown University, U.S.) and Martin Claasen (University of Pennsylvania,
U.S.). The team 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 (such as temperature and pressure) 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.
Like a second compared to the age of the universe
In recent years, the field of 'pump-probe experiments' has seen tremendous
progress. In these experiments, which can are carried out at the Swiss X-ray
free-electron laser SwissFEL, 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."
To review this rapidly growing field, the scientists formed a team involving
both experimentalists—de la Torre, McIver and Gerber—and theorists—Claassen,
Kennes and Sentef. "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."
Inspiration through the combination of theory and practice
"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 that "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."
Reference:
Alberto de la Torre et al, Colloquium: Nonthermal pathways to ultrafast
control in quantum materials, Reviews of Modern Physics (2021).
DOI: 10.1103/RevModPhys.93.041002
Tags:
Physics