Measurement of the thermal coefficients of erasable phase-change media

In erasable phase-change (PC) optical recording, a reversible transition between amorphous and crystalline phases of the recording medium is involved. Recording and erasure are critically dependent on the crystallization and amorphization dynamics, as well as on the time- and space-dependent temperature profiles within the media. A previous paper has described a method for measuring the thermal coefficients of non-erasable phase-change films, where a unique threshold temperature for crystallization may be assumed to exist under essentially all writing conditions. In erasable P C films, on the other hand, the crystallization process depends on the writing conditions, and a unique threshold temperature can no longer be defined. We have developed a new method for determining the thermal coefficients of erasable PC films, which we shall describe in the present paper. The method consists of monitoring the reflectivity variations of a PC sample during the writing process. By varying the laser power and pulse width, we obtain valuable information concerning the dynamics of phase transition.