JPS62162254A - Optical recording and reading method - Google Patents

Abstract

PURPOSE:To produce a recording medium conveniently and at a low cost and to realize high S/N by using a cholesteric liq. crystal as a recording medium by equalizing the rotating direction of the circularly polarized light beam for reading with the spirally rotating direction of the cholesteric high molecular liq. crystal. CONSTITUTION:A light absorbing layer 2 and a cholesteric high molecular liq. crystal thin film 3 are successively formed on a substrate 1. Light is selectively reflected by the high molecular liq. crystal with the wavelength shown by lambda=np of the counterclockwise circularly polarized light as the center. When the pitch p is previously controlled to conform almost to lambda-np against the wavelength of the reading light source, extremely high reflectance is exhibited at a region wherein information is not written, meanwhile, the reflectance is rapidly decreased at the region wherein information is written in the mode to form a pit, and consequently high S/N can be realized. Besides, when the pitch p is previously controlled to lambda>>np against the wavelength of the reading light source and information is written in the light quantity region to increase the pitch length, extremely low reflectance is obtained in the unwritten part of information, high reflectance is obtained in the written part, and high S/N is similarly realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical recording and reading method, and particularly to a reading method for erasable optical recording media.

(Prior art) An 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 to 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 and considered for use in the optical recording. Particularly erasable optical recording methods according to the present invention include a so-called magneto-optical recording method using a magneto-optical material exhibiting magneto-optic effects such as Faraday effect and Kerr effect, and optical recording method using a phase change of a material such as a chalcogenide compound. A typical recording method is known to use a so-called phase change medium that produces differences in characteristics. In addition to this, various media systems have been proposed, and an information recording system using polymer liquid crystals, similar to the present invention, has also been proposed (Japanese Patent Application Laid-Open No. 10930-1989,
Unexamined Japanese Patent Publication No. 59-35989>.

(Problems to be solved by the invention) The magneto-optical recording method is currently a popular method and is close to being put into practical use, but the diK is generally as low as ~40 dB, and the external magnetic field applied to the medium is Since the response speed is slow, partial erasure of each bit, which is the recording unit of information, is not possible, and one track is erased all at once, which has the disadvantage that rewriting information is complicated. In addition, although the medium currently being considered as a phase change medium is 1tTe-based oxide or Te-based alloy,
These materials have a problem of toxicity, and there are concerns that they may have an adverse effect on the human body. Furthermore, although these recording media can be made into thin films using 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 or optical recording system that is simple to manufacture and inexpensive. As mentioned above, a recording method using a polymer liquid crystal has been proposed as a method to solve the above-mentioned drawbacks. Since the incident light is scattered and the detection mechanism is limited, the shear ratio is not necessarily sufficient.

On the other hand, the method of reading information is determined by the optical characteristics of the medium used and the optical system. In a magneto-optical recording system that uses the Faraday effect or the Kerr effect, the highest SA is expected when the light beam incident on the medium for reading information is linearly polarized. On the other hand, in the conventional phase change recording system using materials such as chalcogenide compounds, the characteristics of the light beam incident on the medium are independent of the properties of the medium itself, and are determined mainly by the constraints of the optical head optical system.

That is, the optical properties of the medium itself are almost independent of the polarization state of the light beam. The reading light beam is formed to be the most suitable for the optical head optical system, but this is not restricted in any way by the characteristics of the medium.

The conventional method for reading information from an optical recording medium using the above-mentioned polymeric liquid crystal is a method of detecting scattering of a light beam, and there is a problem in that the scattering characteristics are not high.

(Means for Solving the Problems) The purpose of the present invention is to improve the above-mentioned drawbacks, that is, to optically record erasable polymer liquid crystals that are easy to manufacture, low-cost, and capable of achieving high ratios. The present invention provides a reading method for optical recording that can be used as a medium and realize Takamaki.

The optical recording/reading method of the present invention uses a cholesteric polymer liquid crystal as a recording layer of an optical recording medium, and the helical pitch of the cholesteric polymer liquid crystal is such that it is selectively reflected from visible light to near-infrared wavelength range. It is adjusted to
The rotation direction of the circularly polarized light of the readout light beam is the same as the helical rotation direction of the cholesteric polymer liquid crystal.

(Operation) The basic operating principle of the present invention will be explained. A cholesteric polymer liquid crystal is used for the recording layer. Cholesteric polymer liquid crystals are known to have liquid crystal groups in 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 means that the pitch of the cholesteric polymer liquid crystal is p, and the refractive index is n.
Then, this is a phenomenon in which only circularly polarized light having a wavelength λ=np and rotating in the same direction as the helical rotation direction is selectively reflected. The present invention is based on this selective reflection phenomenon. The light selective reflection wavelength can be changed by changing the refractive index and pitch of the polymer liquid crystal. It has been known that the pitch of low-molecular-weight cholesteric liquid crystals changes depending on temperature and other factors.

As a result of intensive studies by the present inventors, it has been found that when a cholesteric polymer liquid crystal in the cholesteric phase is rapidly heated by laser beam irradiation, a change in the light selection wavelength that seems to correspond to a change in the helix pitch length or rotation of the helical axis occurs. It has been found that a color change (transmittance change) occurs and that the color change state is preserved when rapidly cooled by removal of laser beam irradiation. This is considered to be because the changed pitch state was preserved as it was due to the quenching effect.

On the other hand, it was found that when the material was slowly cooled, the color change state disappeared and the product returned to its original state. When the irradiation energy of the laser beam is further increased, unlike the color change corresponding to the change in pitch length, bit formation occurs and is preserved by rapid cooling, and furthermore, the formed bit disappears by reheating and slow cooling and returns to its original state. It was confirmed that the condition has returned.

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 and reading method of the present invention. In FIG. 1, a light absorbing layer is formed on a plastic or glass substrate 1 to efficiently convert the writing light beam into heat. The light absorber 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. When a semiconductor laser (λ=0.78 to 0.83 μm) is used as the light beam 5, a phthalocyanine compound such as nazilphthalocyanine formed by vacuum deposition can be used.Furthermore, soluble phthalocyanine can be used in a suitable solvent. A cholesteric polymer liquid crystal thin film 3 is formed on top of the cholesteric polymer liquid crystal thin film 3. A cholesteric polymer liquid crystal 3 can be made of a solid material. For example, there is a siloxane polymer liquid crystal to which a cholesterol derivative and a nematic liquid crystal molecule are added, as shown by the following structural formula 1''l).

〇 ■ ■.

The cholesteric liquid crystal may contain a trace amount of additives such as a plasticizer 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 an appropriate solvent and coated with a spin cord, etc., transferred with gravure printing, coated with a doctor grade, etc., and the coated film can be heated and dried. Can be made into a thin film. Alternatively, it is also possible to form a thin film by molding 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 film 4 or protective layer 4 is generally formed on the polymer liquid crystal thin film. However, the protective film 4 is not an essential requirement of the present invention, and may be omitted. The rotational direction of the polymer liquid crystal is determined by the optically active substance used. A polymer liquid crystal containing a cholesterol derivative as shown in the above structural formula CI) has left-handed circularly polarized light λ=n
It is selectively reflected around the wavelength indicated by p.

Therefore, when the pitch is adjusted to adjust the selective reflection wavelength from the visible range to the near-infrared wavelength range, and when elliptical polarized light is rotated in the same direction as the cholesteric and used as the readout light (in Fig. 1, the readout light beam The polarization direction 6 in 5 indicates a counterclockwise direction.Of course, if the medium has a clockwise helical structure, the polarization direction of the readout light beam may be clockwise.), indicating an extremely large optical change. , high pasture can be achieved. 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 exhibit extremely high reflectance, while the mode that forms pits will In the area where information is written, a sharp drop in reflectance occurs, and as a result, Takamaki can be realized.

On the other hand, if the pitch p is adjusted in advance to λ > i p with respect to the wavelength λ of the readout light source, and information is written in a light amount region where the pitch length becomes long, the non-information writing area will have extremely low reflection. It is possible to achieve high reflectivity in the Sokobe area as well as in Takamaki.

Example 1) Vanadyl phthalocyanine was formed on a glass substrate by X-vacuum evaporation, and a thin film of polymeric liquid crystal represented by the structural formula (I) was formed thereon using a spin cord or the like.

The selective reflection center wavelength of the polymer liquid crystal is approximately 830 nmK.
It was adjusted. A laser diode with a wavelength of 830 nm was used as the light beam. The formation of pits was confirmed by introducing pulsed light of 8 mW and 80 μs. When the medium was reproduced with 1 mW of left-handed circularly polarized light, it was possible to reproduce with a high S/N of SA = 60 dB. On the other hand, when reproducing with clockwise circularly polarized light or non-polarized light, almost no di-N was obtained.

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) Writing was performed on a medium having the same configuration as in Example 1 except that the selective reflection wavelength of the polymer liquid crystal was adjusted to 550 nm using pulsed light of 5 mW and 80 pS, and a color change based on a change in pitch length was confirmed. When reproduced with 1 mW left-handed circularly polarized light, there was a noticeable increase in reflectance in the information recording section, and SA ~ 50 dl.
I was able to play it on the high S level of l. On the other hand, when reproduction was performed using clockwise circularly polarized light or non-polarized light, almost no di~ was obtained.

Note that when the incident power density was continuously changed from 25 to 65 mJ/-, a continuous increase in reflectance was observed corresponding to the incident energy density. This 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, according to the present invention, it was possible to provide a reading method for a polymer liquid crystal optical recording medium that can achieve a high SA ratio of 5 QdB.

[Brief explanation of drawings]

The figure is a schematic cross-sectional view of a recording medium used in an example of the present invention. In the figure,

Claims (1)

[Claims] This is an optical recording reading method in which information is recorded and erased by irradiating an optical recording medium with a light beam focused on a minute spot, and the information is read out by changing the reflectance or transmittance of the light beam. An optical recording medium having an optical recording layer made of a cholesteric polymer liquid crystal whose helical pitch has been adjusted so as to selectively reflect light in the infrared wavelength range is rotated in the same direction as the rotational direction of the helical pitch of the cholesteric polymer liquid crystal. An optical recording and reading method characterized by irradiating circularly polarized light and detecting changes in reflected or transmitted light.