Optical disc


The coated material in a DVD can be heated by the laser light. The material in the amorphous form crystallizes above the glass transition temperature, Tg by slow cooling. On the other hand, when fast cooling is done an amorphous bit is formed from the crystallized material. The amorphous and crystalline regions acts as 0 or 1 's. The amorphous state has a lower reflectance than the crystalline state.

The material used in a DVD : Ge2Sb2Te5 (GST)

or Ag-In-Sb-Te (AIST) alloys

GST is used in DVD -ROM

AIST is used in DVD -RW

The DVD is made from polycarbonate substrate with the recording layer of the above material sandwitched between ZnS-SiO2 which protects from scratches. The laser beam is focussed on the recording layer for RW.

* Slow cooling of GST is achieved in 30 ns to crystallize the material

* Fast cooling to make it amorphous is achieved within femtoseconds

* The bond lengths are in the range of 2.83, 3.2 A for Te-Ge and 2.91, 3.2 for Te-Sb.


Media type



Data capacity

4.7 GB

4.7 GB


18-30 %

45-85 %


650 nm

650 nm

Track pitch

0.74 microm

0.74 microm

Scan velocity

1 – 2.4 x

1 x (3.5 m/s)