Steering Electrons with Light

An illustration of the setup and the interaction of a short laser pulse (red oscillating line) with the lattice of titanium atoms. The red and blue structures represent the redistribution of the electron density in the vicinity of a titanium atom. (Source: Ultrafast Laser Physics group, ETHZ)

The distribution of electrons in transition metals is responsible for many of their interesting properties used in appli­cations. The magnetic properties of some of the members of this group of materials are, for example, exploited for data storage, whereas others exhibit excellent electrical conduc­tivity. Transition metals also have a decisive role for novel materials with more exotic behaviour that results from strong inter­actions between the electrons. Such materials are promising candi­dates for a wide range of future appli­cations. In their experiment, Mikhail Volkov and colleagues in the Ultrafast Laser Physics group of Ursula Keller exposed thin foils of the transition metals titanium and zirconium to short laser pulses. They observed the redistri­bution of the electrons by recording the resulting changes in optical proper­ties of the metals in the extreme ultraviolet (XUV) domain.

In order to be able to follow the induced changes with sufficient temporal resolution, XUV pulses with a duration of only few hundred attoseconds were employed in the measurement. By comparing the experi­mental results with theoretical models, developed by the group of Angel Rubio at the Max Planck Institute for the Structure and Dynamics of Matter in Hamburg, the researchers established that the change unfolding in less than a femtosecond is due to a modi­fication of the electron locali­zation in the vicinity of the metal atoms. The theory also predicts that in transition metals with more strongly filled outer electron shells an opposite motion is to be expected.

The electron distri­bution defines the microscopic electric fields inside a material, which do not only hold a solid together but also to a large extent determine its macro­scopic properties. By changing the distr­ibution of electrons, one can thus steer the charac­teristics of a material as well. The experiment of Volkov et al. demonstrates that this is possible on time scales that are considerably shorter than the oscillation cycle of visible light. Even more important is the finding that the time scales are much shorter than the thermali­zation time, which is the time within which the electrons would wash out the effects of an external control of the electron distri­bution through collisions between themselves and with the crystal lattice.

Initially, it came as a surprise that the laser pulse would lead to an increased electron locali­zation in titanium and zirconium. A general trend in nature is that if bound electrons are provided with more energy, they will become less localized. The theoretical analysis, which supports the experi­mental obser­vations, showed that the increased locali­zation of the electron density is a net effect resulting from the stronger filling of the characteristic partially filled d-orbitals of the transition-metal atoms. For transition metals that have d-orbitals which are already more than half filled, the net effect is to the opposite and corresponds to a delocali­zation of the electronic density.

While the result now reported is of funda­mental nature, the experiments demonstrate the possi­bility of a very fast modi­fication of material properties. Such modulations are used in electronics and opto­electronics for the processing of electronic signals or the trans­mission of data. While present components process signal streams with frequencies in the gigahertz range, the results of Volkov and co-workers indicate the possi­bility of signal processing at petahertz frequencies. These rather funda­mental findings might therefore inform the development of the next generations of ever-faster components, and through this indi­rectly find their way into our daily life. (Source: ETHZ)

Reference: M. Volkov et al.: Attosecond screening dynamics mediated by electron localization in transition metals, Nat. Phys., online 5 July 2019; DOI: 10.1038/s41567-019-0602-9

Link: Ultrafast Laser Physics group, ETH Zurich, Zurich, Switzerland

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