# Spatio-temporal coherent control of thermal excitations in solids

## TL;DR

This paper demonstrates the spatio-temporal coherent control of thermal surface deformations in solids using femtosecond laser-induced transient gratings and X-ray reflectivity, enabling ultrafast manipulation of surface states.

## Contribution

It introduces a method to control thermal excitations in solids via coherent superposition of acoustic and thermal gratings using X-ray diffraction.

## Key findings

- Thermal gratings can be canceled by a second delayed excitation.
- Sub-angstrom resolution measurement of surface deformations achieved.
- Limits of ultrafast X-ray shutter speed analyzed.

## Abstract

X-ray reflectivity (XRR) measurements of femtosecond laser-induced transient gratings are applied to demonstrate the spatio-temporal coherent control of thermally induced surface deformations on ultrafast timescales. Using gracing incidence X-ray diffraction we unambiguously measure the amplitude of transient surface deformations with sub-\AA{} resolution. Understanding the dynamics of femtosecond TG excitations in terms of superposition of acoustic and thermal gratings makes it possible to develop new ways of coherent control in X-ray diffraction experiments. Being the dominant source of TG signal, the long-living thermal grating with spatial period $\Lambda$ can be canceled by a second, time-delayed TG excitation shifted by $\Lambda/2$. The ultimate speed limits of such an ultrafast X-ray shutter are inferred from the detailed analysis of thermal and acoustic dynamics in TG experiments.