Measurement of transient atomic displacements in thin films with picosecond and femtometer resolution

Abstract

We report measurements of the transient structural response of weakly photo-excited thin films of BiFeO3, Pb(Zr,Ti)O3, and Bi and time-scales for interfacial thermal transport. Utilizing picosecond x-ray diffraction at a 1.28 MHz repetition rate with time resolution extending down to 15 ps, transient changes in the diffraction angle are recorded. These changes are associated with photo-induced lattice strains within nanolayer thin films, resolved at the part-per-million level, corresponding to a shift in the scattering angle three orders of magnitude smaller than the rocking curve width and changes in the interlayer lattice spacing of fractions of a femtometer. The combination of high brightness, repetition rate, and stability of the synchrotron, in conjunction with high time resolution, represents a novel means to probe atomic-scale, near-equilibrium dynamics.

Cavity-Induced Modifications of Molecular Structure in the Strong-Coupling Regime

“…For a coupling strength of g = 0.016 a.u., corresponding to a Rabi splitting of Ω R ≈ 1.2 eV in absorption, we obtain a shift in the ground state bond length of ∆R 0 ≈ 0.84 mÅ= 84 fm. While small, such a change in bond length could be detectable using X-ray absorption fine structure spectroscopy or X-ray crystallography, which can obtain few-or even sub-femtometer precision for measuring bond length shifts [44,45].…”

Measurement of transient strain induced by two-photon excitation

“…From this, we calculate the mean transient out-of-plane strain (i.e., the relative change of the layer thickness) in the Bi:YIG layer, η(t ), via Bragg's law modified by a geometric scaling factor f . According to our diffraction geometry, with the sample and area detector at fixed diffraction angles, and for a convergent x-ray beam we use f ≈ 2 [43,44],…”

Atomic-scale imaging of ultrafast materials dynamics

“…This approach is sensitive to strains at the parts-per-million level. 17 Figure 1c-d shows corresponding MeV electron diffraction measurements that probe the same material but for an isolated single monolayer. Two different reflections are shown indexed by their Miller indices (Figure 1d), showing large-amplitude structure-factor modulations associated with the initial energy transfer from hot electrons to lattice degrees of freedom.…”