How do rewritable CDs work?

All CDs and DVDs work by virtue of marks on the disc that appear darker than the background. These are detected by shining a laser on them and measuring the reflected light. In the case of molded CDs or DVDs, such as those bought in music or video stores, these marks are physical “pits” imprinted into the surface of the disc. In CD-Recordable (CD-R) discs, a computer’s writing laser creates permanent marks in a layer of dye polymer in the disc. CD-Rewritable (CD-RW) discs are produced in a similar fashion, except that the change to the recording surface is reversible.

The key is a layer of phase-change material, an alloy composed of silver, indium, antimony and tellurium. Unlike most solids, this alloy can exist in either of two solid states: crystalline (with atoms closely packed in a rigid and organized array) or amorphous (with atoms in random positions). The amorphous state reflects less light than the crystalline one does. When heated with a laser to about 700 degrees Celsius, the alloy switches from the original crystalline phase to the amorphous state, which then appears as a dark spot when the disc is played back. These spots can be erased using the same laser (at a lower power) to heat the material to a temperature of 200 degrees C or so; this process returns the alloy to its crystalline state. Most CD-RW makers suggest that one disc can be overwritten up to 1,000 times and will last about 30 years.

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