Optically excited structural transition in atomic wires on surfaces at the quantum limit

Abstract

A structural transition in an atomic indium wire on a silicon substrate proceeds as fast as the indium atom vibrations and is facilitated by strong In–Si interface bonds. Ultrafast diffraction techniques enable us to observe laser-induced structural changes at the atomic scale and with high temporal resolution. A decade of such experiments has indicated that structural changes on surfaces are several orders of magnitude slower than changes in bulk materials, raising the question of whether there is a fundamental limit for low-dimensional systems. Tim Frigge et al. apply laser excitation to a one-dimensional wire of indium atoms on a silicon surface and find that structural changes take place on femtosecond timescales. This short timescale is made possible by electronic coupling to the underlying surface and indicates that structural changes at the surface can, in principle, be as fast as in the bulk material. The findings point to a new method for controlling the dynamic structural responses of solids to laser excitation. Transient control over the atomic potential-energy landscapes of solids could lead to new states of matter and to quantum control of nuclear motion on the timescale of lattice vibrations. Recently developed ultrafast time-resolved diffraction techniques1 combine ultrafast temporal manipulation with atomic-scale spatial resolution and femtosecond temporal resolution. These advances have enabled investigations of photo-induced structural changes in bulk solids that often occur on timescales as short as a few hundred femtoseconds2,3,4,5,6. In contrast, experiments at surfaces and on single atomic layers such as graphene report timescales of structural changes that are orders of magnitude longer7,8,9. This raises the question of whether the structural response of low-dimensional materials to femtosecond laser excitation is, in general, limited. Here we show that a photo-induced transition from the low- to high-symmetry state of a charge density wave in atomic indium (In) wires supported by a silicon (Si) surface takes place within 350 femtoseconds. The optical excitation breaks and creates In–In bonds, leading to the non-thermal excitation of soft phonon modes, and drives the structural transition in the limit of critically damped nuclear motion through coupling of these soft phonon modes to a manifold of surface and interface phonons that arise from the symmetry breaking at the silicon surface. This finding demonstrates that carefully tuned electronic excitations can create non-equilibrium potential energy surfaces that drive structural dynamics at interfaces in the quantum limit (that is, in a regime in which the nuclear motion is directed and deterministic)8. This technique could potentially be used to tune the dynamic response of a solid to optical excitation, and has widespread potential application, for example in ultrafast detectors10,11.

Cite this article

Frigge, T., Hafke, B., Witte, T. et al. Optically excited structural transition in atomic wires on surfaces at the quantum limit. Nature 544, 207–211 (2017). https://doi.org/10.1038/nature21432

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