Abstract

Most of the experimental and theoretical work on femtosecond laser induced phase transitions has concentrated on the non-thermal solid-to-liquid transformation in group IV- and III-V-semiconductors [1]. However, recent experimental results [2] gave some indications that strong electronic excitation might be also important in triggering transitions between different solid phases of a material. In particular, amorphous GeSb can be crystallized upon short pulse laser irradiation and the fluence necessary to induce crystallization decreases significantly for pulses shorter than one picosecond. In this work we have investigated the dynamics of phase transformations in a-GeSb thin films after irradiation with 100 fs laser pulses at 620nm. Motivated by the large difference in reflectivity between the amorphous and the crystalline state of this material we used time resolved microscopy [3] to monitor the evolution of the reflectivity with 100fs time- and micrometer spatial resolution. A sequence of micrographs, covering the entire period from the initial deposition of the laser energy to the appearance of the final structural modifications, is shown in the figure below. Due to the gaussian intensity distribution of the focused pump beam different spatial locations on the sample experience different levels of excitation. The snapshots provide therefore detailed information on both the time- and the fluence dependence of the reflectivity.

© 1998 IEEE