So, how does recording affect the dye?

The recording mechanism in a CD-R such as that described in the introduction is complex. As the dye layer heats up, there is a distortion at the interface between the dye and the polycarbonate substrate, causing a bump. There is also decomposition at the dye metal interface, giving a bubble. This bubble causes a pit in the dye layer as shown in the diagram.

 

 

It is this pit which records the data and reduces the reflectivity of the dye in the infra red. All three effects lead to the pit formation by changing the discs optical properties.

It has been shown that the laser pulse causes most heating near the polycarbonate as the gold on the other side is an efficient heat conductor. The bump caused here is due to two combining effects:

1) Firstly, the dye melts at a laser power of around 3 mW (depending on the dye used).

2) Secondly, the Polycarbonate base layer reaches it's glass transition temperature (ca. 140 degrees) in a 460ns pulse at 4mW power. This secondary effect may help to cause the bumping seen in the dye PC interface.

The bubble in the gold-dye interface is due to a higher temperature phenomenon. At 250 degrees (a 460ns pulse of a 5mW laser) the dye begins to decompose and leaves a deformation in the interface which drastically changes the optics of the reflector. Due to the poor thermal conductivity of the dye, the heating effects in the beam are very localised. At 5mW power, the temperature has fallen below the melting point of the dyes within a radius of aroung 500nm. This gives the definition needed for very high data densities.

The following table contains thumbnail graphs and these can be enlarged to show some features of the recording process with a brief explanation. Simply click on the graphs below

The variation of temperature with distance from the laser centre The variation of temperature with time at several different points in the disc. The reflectance of the dye after heating to various temperatures.
Graph (Described above) Graph (Described above) Graph (Described above)