Erasable Phase-Change Optical Materials

Abstract

Almost all stones on a lane will become glassy if they are melted and quenched. They will become transparent and quite different in appearance from before vitrification. This visible change constitutes the recording of information. We might refer to the stone as “1 bit.” If the vitrified stone is subsequently kept at a high temperature under its melting point, it will lose its transparency and turn back to the appearance it had before melting and quenching. Thus the “1 bit” is erased. This is the simple mechanism of an erasable phase-change optical memory. In practical systems, a laser beam focused into a diffraction-limited spot is used for recording. This enables the spatial size of the “1 bit” to be very small (of submicron order) so that the recording density is very high.Figure 1 shows a transmission-electron-microscope (TEM) photograph of an actual optical disk. The elliptical smooth areas are recording marks in the amorphous state that were formed by high-power and short-duration laser irradiation. The shortest mark length is about 0.5 μm. The area surrounding the amorphous marks is in the crystalline state and consists of small grains. The two states differ from each other in optical properties such as refractive indices and optical absorption coefficients. Accordingly when the bits are illuminated with low-intensity laser light, the reflected light from the amorphous and crystalline regions is different and may be detected as information signals.The amorphous marks are erased by heating above the glass-transition temperature by laser irradiation, but with lower power than is used in the case of recording.