  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.
