JPH0441916B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to optical recording media, particularly erasable optical recording media.

(Prior Art) The optical recording method, in which a laser beam is focused on a minute spot by a condensing lens and irradiated onto the surface of an optical recording medium to cause an optical change on the medium and record information, is a high-density information recording method. This method is attracting attention as a possible method. A wide variety of media have been proposed for use in the optical recording. Particularly erasable optical recording media according to the present invention include so-called magneto-optical recording media using magneto-optical materials exhibiting magneto-optical effects such as Faraday effect and Kerr effect, and magneto-optical recording media using phase change of materials such as chalcogenide compounds.
An optical recording medium using a so-called phase change medium that produces differences in optical properties is known as a typical example.
In addition to this, various other media have been proposed, and information recording media using polymeric liquid crystals as well as the present invention have also been proposed. (JP-A-59-10930, JP-A-59-
35989).

(Problems to be solved by the invention) Magneto-optical recording media are currently the most powerful recording media and are close to being put into practical use, but their S/N is generally as low as ~40 dB, and the external voltage applied to the media is low. The response speed of the magnetic field is slow, and it is a unit of recording information. Since it is not possible to partially erase each bit, one track must be erased all at once. This has the disadvantage that rewriting information becomes complicated. Furthermore, the media currently being considered as phase change media are Te-based oxides or Te-based alloys, but these have the problem of material toxicity and are feared to have an adverse effect on the human body. Although these recording media can be made into thin films by techniques such as vapor deposition and sputtering, they have the drawback of high costs in terms of materials and manufacturing. There is a strong desire for an optical recording medium that is simple to manufacture and inexpensive. As mentioned above, a recording medium using a polymer liquid crystal has been proposed as a way to solve the above-mentioned drawbacks. Since it is used in this state, the incident light is scattered and the detection mechanism is limited, so it has the disadvantage that the S/N ratio is not necessarily sufficient.

(Means for Solving the Problems) The object of the present invention is to improve the above-mentioned drawbacks, that is, to provide a method that is simple and inexpensive to manufacture, and has a high S/S/
The purpose of the present invention is to provide an erasable recording medium that can achieve a high N ratio. The structure of the optical recording medium according to the present invention is that a light absorption layer is provided on a substrate, and a cholesteric polymer liquid crystal whose helical pitch p and refractive index n are approximately λ=np with respect to the reading light source wavelength λ is disposed on the light absorption layer. The structure is such that the optical recording layer is the optical recording layer.

(effect) The basic operating principle of the present invention will be explained.

A cholesteric polymer liquid crystal is used for the recording layer. It is known that liquid crystal groups in cholesteric polymer liquid crystals have a helical arrangement, and because of the helical arrangement, they exhibit optical properties unique to cholesteric. A typical optical property is a wavelength-selective reflection phenomenon of light corresponding to the periodic pitch of the helical structure. This is a phenomenon in which, when the pitch of the cholesteric polymer liquid crystal is P and the refractive index is n, only circularly polarized light whose wavelength input is np and rotates in the same direction as the rotation direction of the helix is selectively reflected. The present invention is based on this selective reflection phenomenon. The light selective reflection wavelength is
It can be changed by changing the refractive index pitch of the polymer liquid crystal. Conventionally, it has been known that low molecular weight cholesteric liquid crystals change depending on pitch temperature, etc.

As a result of intensive studies by the present inventors, it has been found that when a cholesteric polymer liquid crystal in a cholesteric phase is rapidly heated by laser beam irradiation, a color change occurs due to a change in the light selection wavelength, which seems to correspond to a change in the helical pitch length or rotation of the helical axis. (Transmittance change) occurs, and it has been found that the color change state is preserved when rapidly cooled by removal of laser beam irradiation. This is thought to be because the changed pitch state was preserved as it was due to the rapid cooling effect.
On the other hand, when slowly cooling, the color change state disappears,
It turned out that it returned to its original state. When the irradiation energy of the laser beam is further increased, unlike the color change corresponding to the pitch length change, pits are formed and are preserved by rapid cooling, and furthermore, the formed pits disappear by reheating and slow cooling, returning to the original state. confirmed to return.

The present invention will be explained in more detail using figures.

FIG. 1 is a schematic cross-sectional view of an embodiment of the optical recording medium of the present invention. In FIG. 1, a light absorbing layer 2 is formed on a plastic or glass substrate 1 to efficiently convert the writing light beam into heat. The light absorption layer is preferably one that has large absorption in the wavelength range of the light beam, has a relatively high melting point, and can be made into a thin film. A light absorption layer is essential in the present invention.
The optical change of the cholesteric polymer liquid crystal is due to heat, but the cholesteric polymer liquid crystal is almost transparent in the visible to near infrared region and has a low ability to absorb laser beams and convert them into heat. Therefore, in order to achieve high sensitivity, high speed, and high S/N, it is necessary to have the above-mentioned absorption layer.

Semiconductor laser (λ = 0.78 ~
0.83 μm), a phthalocyanine compound such as vanadyl phthalocyanine formed by vacuum evaporation can be used. Furthermore, it can also be formed by coating soluble vanadyl phthalcyanine with a suitable solvent and drying it. In addition to the vanadyl phthalocyanine, commercially available near-infrared absorbers include IR-750 and IRG-
002, IRG-003 (Nippon Kayaku) NK-78, NK-123,
NK-125, NK-126, NK-2014, NK-2421,
NK-2772 (Japan Photosensitive Dye Research Institute), PA-1001,
1003, 1005, 1006 (Mitsui Toatsu Chemical) etc. can be used. A cholesteric polymer liquid crystal thin film 3 is formed on the light absorption layer. Various types of cholesteric polymer liquid crystals can be used. One example is a siloxane polymer liquid crystal with added cholesterol derivatives and nematic liquid crystal molecules as shown in the following structural formula [1]. The cholesteric liquid crystal may contain a small amount of additives such as a plasticizer in order to improve film forming properties. The polymer liquid crystal film can be made into a thin film by various methods. The polymer liquid crystal can be solubilized in a suitable solvent and applied by spin coating, etc., transferred by gravure printing, applied with a doctor blade, etc., and the applied film can be made into a thin film by heating and drying. can. Alternatively, it is also possible to form a thin film by forming it on a substrate under heat and pressure. The thickness of the polymer liquid crystal thin film is adjusted to 0.1 μm to several tens of μm. A protective layer 4 is generally formed on the thin polymer liquid crystal film. However, the protective film 4 is not an essential requirement of the present invention, and may be omitted. The helical rotation direction of the polymer liquid crystal is determined by the optically active material used. The polymer liquid crystal containing a cholesterol derivative as shown in the above structural formula [1] is selectively reflected around the wavelength of left-handed circularly polarized light, λ=np.

Therefore, when the selective reflection wavelength of the pitch is adjusted from the visible range to the near-infrared wavelength range, and circularly polarized light rotated in the same direction as the cholesteric light is used as the readout light (in Fig. 1, the readout light beam 5
The polarization direction 6 is shown counterclockwise. Of course, if the medium has a clockwise helical structure, the polarization direction of the readout light beam may be clockwise. ), it shows an extremely large optical change and can achieve a high S/N. That is, if the pitch p is adjusted in advance so that λ is approximately equal to np with respect to the reading light source wavelength λ, the area where no information is written will have an extremely high reflectance, while the area where a pit will be formed will have an extremely high reflectance. In the area where information is written in the mode, a rapid decrease in reflectance occurs, and as a result, a high S/N can be achieved. On the other hand, if the pitch P is adjusted in advance to λ≫≫np with respect to the wavelength λ of the reading light source, and information is written in a light amount region where the pitch length becomes long, the non-written portion of information is extremely low. The reflectance and writing portion have high reflectance, and high S/N can be achieved as well.

The present invention uses polymeric liquid crystal for the recording layer, and can be easily and simultaneously formed in large quantities by methods such as coating and printing. In addition, the production time is less than 1/20 that of vapor deposition, making it excellent for mass production. The material cost is less than 1/10 that of inorganic materials such as Te, resulting in low cost.

Example 1 Banardyl phthalocyanine was formed on a glass substrate by vacuum evaporation, and a thin film of polymeric liquid crystal represented by the structural formula [1] was formed thereon by spin coating or the like. The selective reflection center wavelength of the polymer liquid crystal is approximately
The wavelength was adjusted to 830nm. A laser diode with a wavelength of 830 nm was used as the light beam. Writing was performed using pulsed light of 8 mw and 80 μs, and the formation of pits was confirmed.
When the medium was reproduced with 1 mw of left-handed circularly polarized light, reproduction was possible with a high S/N of 60 dB. By heating the medium on which the information was written to 70°C or higher, the information could be completely erased. Note that no deterioration of the medium was observed.

Example 2 A 5 mW medium with the same configuration as Example 1 except that the selective reflection wavelength of the polymer liquid crystal was adjusted to 550 nm.
Writing was performed using pulsed light of 80 μs, and changes due to changes in pitch length were confirmed.

When reproducing with 1mW left-handed circularly polarized light, there was a noticeable increase in reflectance in the information recording section, and the S/N was ~
It was possible to play with a high S/N of 50dB. Incidentally, when the incident power density was continuously changed from 25 to 65 m/cm ² , an increase in reflectance was observed corresponding to the incident energy density. This also revealed the possibility of multilevel recording through multilevel control of incident energy. The recorded portion could be erased by heating at 70° C. or higher.

(Effects of the Invention) As described above, the present invention makes it possible to provide an erasable recording medium that is easy to manufacture, low cost, and can achieve a high S/N ratio of 60 dB.

[Brief explanation of drawings]

FIG. 1 is a schematic cross-sectional view of a recording medium according to the present invention. In the figure, 1...support substrate, 2...light absorption layer, 3...polymer liquid crystal, 4...protective layer, 5...light beam.

Claims (1)

[Claims] 1. It has at least a laminated structure including an optical recording layer made of cholesteric polymer liquid crystal on a light absorption layer, and the helical pitch p and refractive index n of the cholesteric polymer liquid crystal are approximately λ = λ with respect to the reading light source wavelength λ. An optical recording medium characterized by being np.