Femtosecond X-Ray Measurement of Ultrafast Melting and Large Acoustic Transients

Time-resolved x-ray diffraction with ultrashort $\left(\approx 300\mathrm{fs}\right)$, multi-keV x-ray pulses has been used to study the femtosecond laser-induced solid-to-liquid phase transition in a thin crystalline layer of germanium. Nonthermal melting is observed to take place within 300–500 fs. Following ultrafast melting we observe strong acoustic perturbations evolving on a picosecond time scale.