Time-Resolved Laser-Induced Phase Transformation in Aluminum

Abstract

Phase transformation in condensed matter is an important area of study in solid state physics since it relates to the genesis and evolution of new microstructure. The mechanisms responsible in such critical phenomena are still not fully understood. Previous experimental information has left unmeasured such important parameters as the minimum number of nuclei and their common critical radius for a transformation to occur as well as the interphase velocity. Several probe techniques have now been developed to time-resolve phase transformations in semiconductors during laser annealing. However, most of these probes (eg. electrical conductivity,^1,2 optical reflection,^3,4 optical transmission,⁵ Raman scattering,⁶ and time of flight mass spectrometry⁷) supply no direct information about the atomic structure of the material. Probing the structure can reveal when and to what degree a system melts as it is defined by degradation in the long range order of the lattice. True structural probes based on x-ray⁸ and low energy electron⁹ diffraction and EXAFS¹⁰ with nanosecond time resolution have been developed offering fresh insight into both the bulk and surface dynamics of material structure. Also, a subpicosecond probe based on structural dependent second harmonic generation¹¹ has been demonstrated. But at present, only the technique of picosecond electron diffraction¹² can produce an unambiguous picture of the structure on the picosecond timescale. In this letter we report on the results of using this probe to directly observe the laser-induced melting of aluminum.