Writing and erasing in a CD is carried out by a laser spot focused to about a micrometre in size. This spot converts the transparent light sensitive material of a `land' spot to opaque state of the `pit' spot. The sensitive data region of a CD/DVD is made up of a special material called phase-change material, which can change from one phase (crystalline) to another (amorphous) when it is heated and cooled. The material used is chosen because it reflects light differently in the two states. The amorphous state reflects less light than the crystalline state. Therefore, by starting with a disc surface in the crystalline state, heating with the laser can change small spots to the amorphous state and the rapid cooling of the spot causes the material to freeze in the amorphous state. This appears dark during `reading.' During the erasure of an amorphous spot, it is converted back to crystalline state by a process known as annealing, accomplished by heating the material to a lower temperature. The existing data in CD that has been `quick-erased,' 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, sufficient to melt the material, enables the creation of a new data.
Source:
thehindu.com
