The phase-change material can change from one phase to the other when it is heated and cooled. The material used is chosen because the two solid states reflect light differently. The amorphous state reflects less light than the crystalline state does. Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state, which will appear dark upon playback.
Heating the material with the laser beam above its melting point transforms it from crystalline to amorphous. The rapid cooling of the spot causes the material to freeze in the amorphous state. These spots can then be erased in a process known as annealing. This is accomplished by heating the material to a lower temperature, which transforms it back to its crystalline state. Existing data can be overwritten by turning the laser on continuously to the erase power, which will erase any existing marks. Switching the laser to a higher power, one sufficient to melt the material, enables the creation of a new mark.
Sony Electronics marketing manager Robert DeMoulin writes:
Compact Disc Recordable (CD-R) is a write-once technology. Once an area of the disc has been written to, it cannot be erased. The recordable layer is an organic cyanine or pthalocyanine dye. Initially, the organic dye has high reflectivity. When the laser is applied in write mode, however, a chemical reaction occurs that makes that "pit" less reflective than the "land" around it. During readout, the laser detects the difference in reflectivity between the "pits" and "lands" to read the data or music. CD-R discs are highly reflective--about 60 to 70 percent of light is reflected or bounced back to the photo detector or read laser. Because of this high reflectivity, CD-R discs can be read or played back in most CD players and CD-ROM drives.
Compact Disc Rewriteable (CD-RW) is a fully rewriteable media, meaning that any spot on a CD-RW disc can be rewritten up to 1,000 times (based on the current standard). Phase change technology enables this rewriteability. The recordable layer on a CD-RW disc is made up of a rare-earth metal alloy "sandwich," which includes silver, indium antimony and tellurium elements. This combination of elements has an important property: it allows a spot on the disc to be changed by the heat of a laser from a crystalline state (high reflectivity) to an amorphous state (low reflectivity). Heated to a lower temperature or power level, the same spot changes back to crystalline state. When overwriting data, the laser is modulated first to erase (or make crystalline) the spot to be recorded and then modulated to write power.
Phase change technology does have a limitation: it has very low reflectivity (approximately 25 percent). For this reason, a typical consumer CD player won't recognize a CD-RW disc.
Answer originally posted September 10, 2001.