Plastic or glass disks coated with a compound (often TbFeCo) with special properties. The disk is read by bouncing a low-intensity laser off the disk. Originally the laser was infrared, but frequencies up to blue may be possible; the shorter the wavelength the higher the possible density. The polarisation of the reflected light depends on the polarity of the stored magnetic field.

To write, a higher-intensity laser is used to heat the material up to its Curie point, allowing its magnetisation to be altered and "frozen" as it cools.

Storage FAQ.