All-optical magnetic recording has been demonstrated by scientists at the Radboud University Nijmegen in the Netherlands. Instead of using the customary magnetic read head to flip the magnetic orientation of a tiny domain they use the fields present in a short burst of circularly polarized light.
Why use light instead of a magnet? Because the magnet is relatively slow and because the magnetic field in the light pulse is intrinsically strong-up to 5 Tesla. The pulses are perpendicularly incident on the storage medium and the helicity of the light pulse (whether the polarization is rotating left-handedly or right-handedly relative to the pulseís forward direction) establishes whether the orientation set in the domain will be up or down, or digital terms, a 1 or a 0.
The orientation of the domain (writing a bit) is accomplished partly through the lightís magnetism and partly through the localized heating by the pulse, which enhances the domainís magnetic susceptibility. The bit can be reversed with light of the opposite polarization.
The light pulse is so carefully focused that it addresses only one domain at a time (see figure at http://www.aip.org/png/2007/281.htm). The speed of the writing process is set by the duration of the laser pulse, 40 fsec, upsetting certain suggestions, made not so many years ago, that the speed of recording in optical medium could not shrink below a picosecond.
True, the size of the domain is 5 microns, which is rather large. However, one of the researchers, Daniel Stanciu (firstname.lastname@example.org, 31-24-365-3094), says he expects the domain size to get down to about 100 nm. He believes that the all-optical approach will eventually be the way of achieving the fastest writing of data in a magnetic medium. (Stanciu et al., Physical Review Letters, upcoming article)