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Certain materials can change phase between crystalline and amorphous states. These states reflect light differently, making them a useful analogue to the pits and lands in an standard optical disc. Heating such a material with a laser to a high enough temperature switches it from a reflective crystaline state to a non-reflective amorphous state. Heating it to a lesser temperature returns it to its crystaline state. Phase-change media can be re-written thousands of times.
The ability for people to save their own content to media has always been an important selling point. It was one reason for the success of audio and video cassettes. Although CDs and DVDs took off on their strengths for pre-recorded music, video and software, the desire for writable media remained. The pits that encoded prerecorded content on those media, however, were injection molded in factories. Dyes that responded to heating by laser offered one solution (see Thermal Processes in the Chemistry section), but the chemical reaction that changes the reflectivity in that approach is irreversable. In phase-change discs, the phase of locations on the thin metallic layer can be changed repeatedly.
