When a crystal is hit by an
intense ultrashort light pulse, its atomic structure is set in motion. A team
of scientists from the Max Planck Institute of Quantum Optics (MPQ), the
Technischen Universität München (TUM), the Fritz-Haber Institute in Berlin
(FHI) and the Universität Kassel can now observe how the configuration of
electrons and atoms in titanium dioxide, a semiconductor, changes under the
impact of an ultraviolet laser pulse, confirming that even subtle changes in
the electron distribution caused by the excitation can have a considerable
impact on the whole crystal structure.
The scientists illuminated a
titanium dioxide crystal with an intense ultraviolet laser pulse of less than
five femtoseconds duration. The laser pulse excites the valence electrons in
the crystal and generates a small number of hot electrons with a temperature of
several thousand Kelvin. The continuous interplay between the positions of the
atomic cores and the valence electrons determines the material characteristics
such as electric conductivity, optical properties or the crystal lattice
structure.
Following the first, intense
laser pulse, the changes in the reflectivity of the crystal on the femtosecond
timescale were observed by a second, weak light pulse. This measurement
provides the scientists with information on the changes in the crystal induced
by the first laser pulse: the intense ultraviolet laser pulse did not only heat
up the valence electrons but also changed the electron distribution within the
lattice. The electron density was reduced around the oxygen atoms and increased
around the titanium atoms. This redistribution of the electrons causes a shift
of the equilibrium position of the oxygen atoms relative to the titanium atoms,
which leads to an oscillatory motion of the oxygen atoms around the new
equilibrium position. In an intuitive picture the oxygen atoms in the crystal
potential surface can be compared to a ball in a bowl. In the ground state, the
ball is at rest at the center of the bowl. The excitation of the electrons
corresponds to a sudden shift of the bowl, and the ball oscillates around its
new minimum position.
In their experiment, the
scientists also observed a suprising effect: after the excitation with the
laser pulse, the electrons cool down to room temperature within about 20
femtoseconds, while the crystal is only heated slightly on these timescales.
The cooling of the electrons led to an additional significant change in the
valence electron distribution. In consequence, the equilibrium position of the
lattice was shifted even further from the initial position of the ground state.
Such a dependence of the crystal structure on the electron temperature has long
been predicted theoretically. Now it could be observed experimentally for the
first time. The results show that even subtle changes in the electron
distribuition can have a significant impact on the equilibrium state of a
crystal. The understanding of such phenomena can be helpful in the design of
new materials.
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