Reversible structural changes of a nanostructure were measured nondestructively with subpicometer spatial and subpicosecond temporal resolution via x-ray diffraction (XRD). The spatially periodic femtosecond excitation of a gallium arsenide/aluminum gallium arsenide superlattice results in coherent lattice motions with a 3.5-picosecond period, which was directly monitored by femtosecond x-ray pulses at a 1-kilohertz repetition rate. Small changes (DeltaR/R = 0.01) of weak Bragg reflexes (R = 0.005) were detected. The phase and amplitude of the oscillatory XRD signal around a new equilibrium demonstrate that displacive excitation of the zone-folded acoustic phonons is the dominant mechanism for strong excitation.

Metal nanoparticles form potent nanoreactors, driven by the optical generation of energetic electrons and nanoscale heat. The relative influence of these two factors on nanoscale chemistry is strongly debated. This article discusses the temperature dependence of the dimerization of 4-nitrothiophenol (4-NTP) into 4,4′-dimercaptoazobenzene (DMAB) adsorbed on gold nanoflowers by Surface-Enhanced Raman Scattering (SERS). Raman thermometry shows a significant optical heating of the particles. The ratio of the Stokes and the anti-Stokes Raman signal moreover demonstrates that the molecular temperature during the reaction rises beyond the average crystal lattice temperature of the plasmonic particles. The product bands have an even higher temperature than reactant bands, which suggests that the reaction proceeds preferentially at thermal hot spots. In addition, kinetic measurements of the reaction during external heating of the reaction environment yield a considerable rise of the reaction rate with temperature. Despite this significant heating effects, a comparison of SERS spectra recorded after heating the sample by an external heater to spectra recorded after prolonged illumination shows that the reaction is strictly photo-driven. While in both cases the temperature increase is comparable, the dimerization occurs only in the presence of light. Intensity dependent measurements at fixed temperatures confirm this finding.

We report the first analysis of the polarization and lattice dynamics in a metal/ferroelectric/metal nanolayer system by femtosecond x-ray diffraction. Two Bragg reflections provide information on the coupled dynamics of the two relevant phonon modes for ferroelectricity in perovskites, the tetragonal distortion and the soft mode. Optical excitation of the SrRuO(3) metal layers generates giant stress (>1 GPa) compressing the PbZr(0.2)Ti(0.8)O(3) layers by up to 2%. The resulting change of tetragonality reaches a maximum after 1.3 ps. As a result, the ferroelectric polarization P is reduced by up to 100% with a slight delay that is due to the anharmonic coupling of the two modes.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO 3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO 3 : the relevant excited charge carriers must remain localized to be consistent with the data. Multiferroics have a great potential for application due to their possible coupling of ferroelectricity and magnetism [1][2][3]. BiFeO 3 (BFO) is one of the few room temperature multiferroics today [4][5][6][7][8], and of these, the only one that is a stable phase. Its relatively small band gap of approximately 2.7 eV [9] renders BFO an ideal candidate for applications in spintronics and memory devices [5] with a perspective for ultrafast optical switching similar to purely ferroelectric [10] or magnetic materials [11]. The photovoltaic effect in this complex material and the underlying ultrafast carrier dynamics after above-band-gap femtosecond (fs) optical excitation have been studied thoroughly [12][13][14]. The photoinduced currents in BFO lead to THz emission [15,16] and to a photostrictive response [17]. Alloptical experiments showed that the rapid photoinduced mechanical stress excites coherent phonons [18,19]. The dynamics of photoinduced strains were directly and quantitatively measured in a recent synchrotron-based ultrafast x-ray diffraction (UXRD) study with a temporal resolution of 100 ps [20]. Combined optical measurements revealed a linear dependence of the transient strain and the number of excited carriers over several nanoseconds (ns). This led to the conclusion that depolarization field screening (DFS) including macroscopic transport of the carriers to the surface and interface could be the dominant stress generating process, although the effect of excited antibonding orbitals was not ruled out [20].In this Letter, we report complementing UXRD experiments at a laser-driven plasma x-ray source (PXS) in order to monitor the coherent and incoherent lattice dynamics in a BFO thin film sample with subpicosecond (ps) temporal resolution after above-band-gap excitation. We observe a sound-velocity limited evolution of the structural response within 10 ps indicating a quasi-instantaneous stress. The substantial Bragg peak broadening is a direct evidence of an inhomogeneous spatial stress profile. It appears quasiinstantaneously and decays on nanosecond time scales as reconfirmed by new synchrotron-based UXRD data recorded at the Advanced Photon Source (APS). We obtain quantitative agreement of the transient peak shift and broadening measured with both setups and can firmly conclude that the photogenerated stress driving the film expansion has a strongly inhomogeneous sp...

The udkm1Dsim toolbox is a collection of matlab (MathWorks Inc.) classes and routines to simulate the structural dynamics and the according X-ray diffraction response in one-dimensional crystalline sample structures upon an arbitrary time-dependent external stimulus, e.g. an ultrashort laser pulse. The toolbox provides the capabilities to define arbitrary layered structures on the atomic level including a rich database of corresponding element-specific physical properties. The excitation of ultrafast dynamics is represented by an N -temperature model which is commonly applied for ultrafast optical excitations. Structural dynamics due to thermal stress are calculated by a linear-chain model of masses and springs. The resulting X-ray diffraction response is computed by dynamical X-ray theory. The udkm1Dsim toolbox is highly modular and allows for introducing user-defined results at any step in the simulation procedure.

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Using ultrafast X-ray diffraction, we study the coherent picosecond lattice dynamics of photoexcited thin films in the two limiting cases, where the photoinduced stress profile decays on a length scale larger and smaller than the film thickness. We solve a unifying analytical model of the strain propagation for acoustic impedance-matched opaque films on a semi-infinite transparent substrate, showing that the lattice dynamics essentially depend on two parameters: One for the spatial profile and one for the amplitude of the strain. We illustrate the results by comparison with high-quality ultrafast X-ray diffraction data of SrRuO3 films on SrTiO3 substrates.