Abstract

The feasibility of ultrashort laser pulse driven phase change optical recording has been recently demonstrated in GeSb thin films in which reversible write/erase (crystallization/amorphization) cycles have been performed with picosecond laser pulses [1]. Furthermore, it has been shown that the fluence threshold required for crystallizing a spot is considerably reduced for write pulses shorter than 1 ps, which has been interpreted in terms of electronic excitation effects [2]. However, the optimization of the material performances, including the determination and subsequent minimization of the transformation time, makes necessary to investigate the ps time scale transformation dynamics during the write/erase (w/e) events. This can be done by means of optical reflectivity measurements with ps-time resolution in a pump-and-probe scheme. The drawback of this approach is that the measurement would require the exposure of a fresh region of the recording media for each pair of pump and probe laser pulses since the pump pulse is aimed to modify the structure of the surface. Therefore several hundreds of write (erase) operations in different regions have to be performed in order to reconstruct the reflectivity evolution for a given fluence over a time window of just a few nanoseconds. Additionally, a laser system with an excellent pulse-to-pulse energy stability would be required since the w/e processes are critically dependent on changes of the irradiation pulse fluence. Streak cameras combine a high (ps-) time resolution with the feasibility of performing single pulse measurements, but their use has been reported very rarely for transient reflectivity measurements, and never in a real single-pulse configuration. This work reports on streak camera based, single-shot, optical reflectivity measurements with picosecond time resolution during ps laser w/e in Sb-rich films.

© 1998 IEEE