Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction

Abstract

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.

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

“…Since the transient strain curves nearly coincide, our data evidece a linear fluence dependence for this strain oscillation feature in Nb up to 13.3 mJ/cm². The interpretation of the UXRD data from the thick TbFe 2 layer is based on the insights on ultrafast lattice response of photoexcited thin films studied by UXRD that were previously discussed by Schick et al 43 The laser illumination leads to the excitation of coherent and incoherent phonons, which superimpose in the strain response of the absorbing layer. 2 The strain pulse composed by the coherent excitation of phonons subsequently propagates at the longitudinal acoustic phonon velocity, whereas the thermal energy leaves the excited layer by a slower diffusion process.…”

Optical spectroscopy on the photo-response in multiferroic BiFeO3 at high pressure

“…However, ultra-fast timeresolved x-ray diffraction (XRD) studies on BFO disagree with this classical explanation and claimed in the case of BFO the creation of excitons during light illumination. 41 Two recent optical spectroscopy studies 42,43 observed three absorption features on BFO single crystals during laser illumination which are energetically close to the crystal field excitations and the absorption onset. These features were interpreted in terms of exci-tons.…”

Nanosecond laser pulse heating of a platinum surface studied by pump-probe X-ray diffraction

“…The intrinsic Bragg peak profile depends on multiple factors as crystal mosaicity, static strain due to dislocations, and dynamic strain due to the temperature gradients. 39,40 The X-ray beam divergence of the set-up is much larger than any of the other effects mentioned to give rise to intrinsic Bragg peak broadening. That is, in the region À100 ns < Dt < 100 ns, the measured Bragg peak profile consists of both the pumped and un-pumped states.…”

“…This means that one can safely neglect the formation of thermoelastic waves, which manifest themselves on different time-scales. 40 In the proposed method the near-surface temperature is determined indirectly by measuring the strain which can be detected 27 with the sensitivity better than 10 À7 corresponding to the uncertainty of the temperature change detection of 0.01 K. However, the accuracy of the temperature determination essentially depends on the accuracy of the linear expansion coefficient b used for the strain-temperature mapping. In this letter we assumed an isotropic approximation of the thermal expansion.…”