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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. Speedy surface structure shifts 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 techniques 1 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 femtoseconds 2 , 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 longer 7 , 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 detectors 10 , 11 .

Using ultrafast, time-resolved, 1.54 angstrom x-ray diffraction, thermal and ultrafast nonthermal melting of germanium, involving passage through non-equilibrium extreme states of matter, was observed. Such ultrafast, optical-pump, x-ray diffraction probe measurements provide a way to study many other transient processes in physics, chemistry, and biology, including direct observation of the atomic motion by which many solid-state processes and chemical and biochemical reactions take place.

The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye–Waller analysis at large momentum transfer of 9.3 Å−1 ≤ Δ k ≤ 21.8 Å−1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τint of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τint is caused by the nonlinearity of the exponential function in the Debye–Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye–Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.

Intense femtosecond laser excitation can produce transient states of matter that would otherwise be inaccessible to laboratory investigation. At high excitation densities, the interatomic forces that bind solids and determine many of their properties can be substantially altered. Here, we present the detailed mapping of the carrier density-dependent interatomic potential of bismuth approaching a solid-solid phase transition. Our experiments combine stroboscopic techniques that use a high-brightness linear electron accelerator-based x-ray source with pulse-by-pulse timing reconstruction for femtosecond resolution, allowing quantitative characterization of the interatomic potential energy surface of the highly excited solid.

Ultrafast reflection high-energy electron diffraction is employed to follow the lattice excitation of a Bi(111) surface upon irradiation with a femtosecond laser pulse. The thermal motion of the atoms is analyzed through the Debye–Waller effect. While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps. This transient nonequilibrium situation is attributed to the weak coupling between bulk and surface phonon modes which hampers the energy flow between the two subsystems. From the absence of a fast component in the transient diffraction intensity, it is in addition concluded that truncated bulk phonon modes are absent at the surface.

In the last few years, bent crystal x-ray mirrors have played an important role in time-resolved x-ray diffraction experiments when x-ray pulses from femtosecond laserproduced plasmas were used. Improvements in manufacturing techniques have significantly increased the quality of this type of mirror.

The study of phase-transition dynamics in solids beyond a time-averaged kinetic description requires direct measurement of the changes in the atomic configuration along the physical pathways leading to the new phase. The timescale of interest is in the range 10 -14 to 10 -12  s. Until recently, only optical techniques were capable of providing adequate time resolution 1 , albeit with indirect sensitivity to structural arrangement. Ultrafast laser-induced changes of long-range order have recently been directly established for some materials using time-resolved X-ray diffraction 2 , 3 , 4 , 5 , 6 , 7 , 8 . However, the measurement of the atomic displacements within the unit cell, as well as their relationship with the stability limit of a structural phase 9 , 10 , 11 , has to date remained obscure. Here we report time-resolved X-ray diffraction measurements of the coherent atomic displacement of the lattice atoms in photoexcited bismuth close to a phase transition. Excitation of large-amplitude coherent optical phonons gives rise to a periodic modulation of the X-ray diffraction efficiency. Stronger excitation corresponding to atomic displacements exceeding 10 per cent of the nearest-neighbour distance—near the Lindemann limit—leads to a subsequent loss of long-range order, which is most probably due to melting of the material.

Most information on atomic structure at everyone's disposal today comes from diffraction experiments. One of these is time-resolved electron diffraction. Von der Linde discusses this experiment, which has made a landmark in time-resolved structural investigations.

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

The influence of absorption and ionization on high-order harmonic generation in a gas-filled hollow fiber is analyzed within the framework of slowly varying envelope approximation. Harmonic spectra and pressure dependencies are calculated for high-order harmonic generation in hollow fibers filled with different rare gases. Ionization of the gas filling the fiber gives rise to self-phase modulation of the pump pulse, changing the phase mismatch within the pump pulse and shifting the maxima in pressure dependencies of high-order harmonics.