Spatial stress profiling by picosecond ultrasonics with X-rays using buried detection layers.

Understanding the dynamics of laser-excited nanolayers thicker than their optical absorption length requires access to the inhomogeneous energy distribution along the depth. Here, we demonstrate the quantitative extraction of the excitation profile from the launched picosecond strain pulse detected in a buried crystalline layer using ultrafast hard-X-ray diffraction. We apply this approach to Ni and Cu thin films, revealing stress profiles that differ from the optical absorption profile because of rapid electron-mediated energy redistribution. We further examine the influence of the detection layer thickness and an insulating interlayer on the profiling sensitivity. Resolving the subtle differences in the excitation profile in Ni for 400 and 800 nm pump pulses highlights the power of our approach and benchmarks the experimental sensitivity. With a crystalline buried detection layer as the only requirement, our approach is broadly applicable to quantitatively study the spatial energy distribution even in amorphous layers of functional materials.