# Concepts and use cases for picosecond ultrasonics with x-rays

## TL;DR

This review explores how picosecond ultrasonics with x-ray probes can measure transient strain in nanostructures, providing layer-specific, quantitative insights into ultrafast energy and temperature dynamics at the nanoscale.

## Contribution

It introduces a method combining ultrashort x-ray pulses with ultrasonics to analyze nanoscale strain responses, modeling the process with elastic wave equations and Grüneisen parameters.

## Key findings

- Layer-specific strain measurement in nanostructures
- Quantitative analysis of ultrafast energy-density changes
- Application to diverse nanomaterials including metals and thermal expansion materials

## Abstract

This review discusses picosecond ultrasonics experiments using ultrashort hard x-ray probe pulses to extract the transient strain response of laser-excited nanoscopic structures from Bragg-peak shifts. This method provides direct, layer-specific, and quantitative information on the picosecond strain response for structures down to few-nm thickness. We model the transient strain using the elastic wave equation and express the driving stress using Gr\"uneisen parameters stating that the laser-induced stress is proportional to energy density changes in the microscopic subsystems of the solid, i.e., electrons, phonons and spins. The laser-driven strain response can thus serve as an ultrafast proxy for local energy-density and temperature changes, but we emphasize the importance of the nanoscale morphology for an accurate interpretation due to the Poisson effect. The presented experimental use cases encompass ultrathin and opaque metal heterostructures, continuous and granular nanolayers as well as negative thermal expansion materials, that each pose a challenge to established all-optical techniques.