In this field, it may be advantageous to provide recording media which can be neutralized irreversibly, for example, for limiting the number of read accesses in the case where it is desired to prevent an unauthorized use of the recorded data. In particular, in the optical disk (CDROM, audio CD, DVD, etc.) memory, irreversible erasing of the data or of some of the data may serve to protect against unauthorized copying of the information contained in the memory.
Optical data are, in principle, stored on the medium in the form of physical marks which are irregularities of control dimensions which present an optical contrast allowing them to be read by a laser beam detection system.
The physical marks may be impressions formed by molding a polycarbonate substrate (for example, DVD-ROM); they are then recorded once and for all; they may also be formed by recorded zones in layers that are sensitive to the action of a writing light beam; the recording may then be reversible (erasure is possible, even re-recording) or irreversible (no erasure possible nor overwriting).
Typically, in the case of an irreversible optical recording, the recording is carried out by irradiating, by means of a laser diode, a colored layer which is locally degraded when the power of the writing laser exceeds a threshold. This local degradation defines marks whose length is defined by the time during which the laser acts on the rotating disk, taking into account the rotational speed of this disk.
For rewritable disks, the writing is usually carried out by heating a material known as a “phase change material” using a writing laser diode. The material is, for example, initially in a crystalline phase; it locally changes into an amorphous state where the writing laser acts. The optical contrast (for example, in reflectivity) between the amorphous zones and the zones that remain crystalline is sufficient to enable the reading of information thus recorded. Erasing is carried out by again exposing these zones, via the laser diode, to a power greater than the power of the read laser but lower than the power of the laser for writing information. The zones that had become amorphous recrystallize, those that were crystalline remain crystalline, and the disk is ready for a new writing operation.
When it is sought to increase the density of information recorded on an optical disk, this objective is generally limited by the performance of the information read device. The basic principle is that the physical information written to the disk can only be read with great difficulty when its size is smaller than the resolution limit of the optical system which will be used to read this information. Typically, when reading with a red laser having a wavelength of 650 nm and a numerical aperture of 0.6, there is normally no hope of correctly reading information having a resolution below 0.4 microns, or at the limit 0.3 microns.
However, methods known as super-resolution methods have been devised for reading information whose physical size is smaller than, or even much smaller than, the wavelength. These methods are based on the non-linear optical properties of certain materials. The expression “non-linear properties” is understood to mean the fact that certain optical properties of the material change depending on the intensity of the light which they receive. The read laser itself will locally modify the optical properties of the material by thermal, optical, thermooptical and/or optoelectronic effects on dimensions smaller than the dimension of the read laser spot; due to the change in properties, a piece of information present in this very small volume becomes detectable whereas it would not have been detectable without this change.
The phenomenon which is exploited is mainly based on two properties of the read laser that will be used:                on the one hand, the laser is very highly focused so as to have an extremely small cross section (of the order of the wavelength) but whose power distribution is Gaussian, very strong at its center, very attenuated at the periphery; and        on the other hand, a read laser power is chosen such that the power density over a small part of the cross section, at the center of the beam, significantly modifies an optical property of the layer, whereas the power density outside of this small cross section portion does not significantly modify this optical property; the optical property is modified in a direction that tends to allow the reading of information which would not be readable without this modification.        
Everything then takes place as if a beam had been used that was focused on a diameter much smaller than that which its wavelength allows.
In a previous patent application, filed under the number FR 0700938 on 9 Feb. 2007, an optical storage structure was proposed operating in super-resolution. This structure comprises a substrate (preferably made of polycarbonate) equipped with physical marks whose geometrical configuration defines the information recorded, a superposition of three layers on top of the substrate marks, and a transparent protective layer on top of this superposition, the superposition comprising a layer of indium or gallium antimonide inserted between two dielectric layers of a zinc sulfide and silicon oxide (ZnS/SiO2) compound.
This structure is favorable because it requires a relatively low read laser power to read the super-resolution information with a satisfactory signal/noise ratio. However, the question of the reading power is critical as, on the one hand, a sufficiently high power is necessary to obtain a super-resolution effect via a localized change of the optical properties, but, on the other hand, too high a power tends to gradually destroy the information recorded, limiting the number of read cycles possible whereas a number of read cycles that is as high as possible is desired.
By carrying out tests on these structures based on InSb or GaSb between two ZnS/SiO2 layers, it was surprisingly observed that it was possible, at the same time:                to read correctly, without them degrading, the information recorded in super-resolution, by using a read laser with a first power P1; and        to irreversibly degrade the information recorded in super-resolution by reading them with a power P2 less than P1.        
This observation was made from repeated measurements on samples comprising regularly distributed marks, recorded in super-resolution.
Although this phenomenon has not been able, to date, to be adequately explained scientifically, the repetition of the observations has led to the conclusion that it would be possible to use this phenomenon industrially to neutralize, at will and irreversibly, the working contents of an optical disk recorded in super-resolution. The neutralization consists of a degradation of certain zones (determined or randomly distributed) that renders the disk unusable.