Materials for binary and/or multibit and/or volume data storage which have azo dyestuffs as antennae for the incident light already exist in the prior art. In particular, the class of side chain polymers in which, in addition to the azo dyestuffs, components having form anisotropy are also used as side chains, is distinguished by the level of the birefringence which can be induced by light. In these materials, the dyestuffs, in combination with components having form anisotropy, bring about an orientation in the actinic light field if they are irradiated with polarized light of a suitable wavelength: A light-induced birefringence builds up.
U.S. Pat. No. 5,384,221 describes an optical recording material for binary and/or multibit and/or volume data storage comprising at least one polymeric and/or oligomeric azo dyestuff which changes its spatial arrangement on irradiation with polarised electromagnetic radiation, and optionally at least one grouping having form anisotropy.
However, these materials do not meet all requirements, and in particular, because of their absorption properties (too high an optical density) at 400 nm, they cannot be used for data storage: In reflection constructions, the actinic light is already noticeably absorbed in the recording layers after a few layers of atoms, and the actinic light cannot act on the complete thickness of the recording material. This means on the one hand that during storage of optical information a significant increase in the sample temperature occurs due to the absorption of light. A significant increase in the sample temperature is understood as meaning a temperature increase of at least 20° C., more precisely of at least 50° C., in particular of more than 100° C., and especially particularly of at least 140° C. This increase in temperature can be measured, for example, with an IR thermal head (Inframetrics Thermocam PM 290).
On the other hand, the high absorption impedes reading of the light-induced information with the wavelengths which have been used for writing: The light arriving from the recording material at the detection construction is too weak still to have an adequate signal/noise ratio. An adequate signal/noise ratio exists when the intensity of the detected light stands out from the scatter background by an order of magnitude.
These known materials sometimes do not show an adequate capacity for being rewritten on: It has already been shown that in the case of irradiation with linearly polarized light the previously induced birefringence can be deleted by rotating the direction of polarization of the actinic light by, for example, 90°. Alternatively, the birefringence can be reduced, for example, by using circularly polarized light. During a renewed writing operation, however, on irradiation with linearly polarized light, in the prior art the birefringence value such as has been achieved during writing on an isotropic sample is not produced.
For the abovementioned reasons (too high an optical density), rewriting on was not possible, especially at wavelengths of about 400 nm.
The capacity for being written on at 400 nm is of great importance, however, for the use of the blue laser diodes (NICHIA) which have just become commercially obtainable, since these laser diodes allow the generation of high storage densities.