JPH0619850B2 - Optical recording medium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an erasable optical recording medium.

[Conventional technology]

As an erasable optical recording medium, a magneto-optical material that uses a magneto-optical effect such as a Faraday effect or a Kerr effect has conventionally been used,
In particular, an optical recording medium in which a thin film using an alloy containing rare earth terbium is formed on a substrate and an optical recording medium using a material such as a chalcona gel compound that causes a change in optical reflectance due to a crystal-amorphous phase change of the material are known. There is.

In addition to the above medium, an optical recording medium utilizing optical anisotropy due to alignment of liquid crystal groups of polymer liquid crystal has been proposed.

[Problems to be solved by the invention]

By the way, conventional magneto-optical recording media and phase change media have drawbacks such as the use of harmful materials, the fact that they are manufactured by sputtering, and the need for a special protective layer due to the low stability of the media. .

For this reason, there is a strong demand for an optical recording medium that is harmless and low in cost by using a simple manufacturing method. However, a medium in which a polymer liquid crystal is enclosed between a pair of electrode substrates has been proposed as a medium for solving such a drawback (reference: Japanese Patent Laid-Open Publication No. Sho.
59-10930, JP-A-59-35989, JP-A-60-114823
No.), this has an S / N ratio of 50 dB, which is still insufficient.

An object of the present invention is to provide an optical recording medium having a further improved S / N ratio.

[Means for solving problems]

The optical recording medium of the present invention is an optical recording medium in which an optical recording layer is formed between a transparent electrode on a support and a counter reflection electrode,
Using a cholesteric polymer liquid crystal as the optical recording layer,
A circular polarizer whose center wavelength is adjusted to be equal to the selective reflection wavelength of the cholesteric polymer liquid crystal is written and placed in the optical path for reading, and the transmission circular polarization rotation direction is matched with the cholesteric spiral rotation direction. And

[Action]

 The optical recording medium of the present invention will be described with reference to FIG.

As the circular polarizer 2 of the optical recording medium 1, various ones can be used, but a left-handed circular polarizer is used here.

The central wavelength is 830 nm, which is matched to the laser light wavelength used for reading and writing.

A cholesteric polymer liquid crystal was used for the optical recording layer 5, and this was provided on the transparent substrate 3 having the transparent electrode 4 by a method such as coating or spin coating.

The reflective counter electrode 6 is obtained by depositing molybdenum and Al. It should be noted that molybdenum absorbs light, and thermal writing of the optical recording layer 5 is performed.

FIG. 2 shows a conventional optical recording medium, which is the same as the optical recording medium 11 according to the present invention except that the circular polarizer 2 is replaced with a transparent substrate 3.

The writing operation is the same in the medium of the present invention and the conventional medium. That is, by initially heating the entire optical recording layer 5 to the glass transition temperature or higher and then gradually cooling it, a planar orientation is obtained in which the direction of the helical axis is perpendicular to the substrate surface. It is known that the cholesteric polymer liquid crystal film in the planar alignment state exhibits selective reflection for circularly polarized light in the same direction as the rotation direction of the spiral axis. The rotation direction of the spiral shaft is determined by the properties of the material itself, and any material can be used in the present invention. Hereinafter, the cholesteric polymer liquid crystal film will be described as left-handed rotation. This planar orientation state is referred to as an erased state.

When cooling is performed while applying an electric field at the same time as heating, the liquid crystal group is aligned vertically to the substrate, the so-called homeotropic alignment state is fixed, and when the homeotropic state is fixed, circularly polarized light occurs. The selective reflection characteristic disappears and the normal reflection characteristic is provided. Therefore, the rotation direction of the circularly polarized light of the reflected light from the cholesteric polymer liquid crystal in the erased state is the left-handed direction, and the rotation direction of the circularly polarized light of the reflected light from the writing portion in the state where the electric field is simultaneously applied and gradually cooled is the recording state. The opposite is the right winding. Therefore, the reflected light from the writing section is cut by the circularly polarizing plate, and as a result, the reflected light amount change rate becomes about 10 times.

〔Example〕

An embodiment of the optical recording medium according to the present invention will be described below with reference to FIG.

In FIG. 1, the circularly polarizing plate 3 is a HC made by Nippon Polaroid.
N-37L was used. This is the same as the spiral rotation direction of the cholesteric polymer liquid crystal described below. For the optical recording layer 5, the cholesteric polymer liquid crystal shown in FIG. 4 was used. The reflective counter electrode 6 was formed by depositing molybdenum and Al. It should be noted that molybdenum absorbs light, and the heat generated thereby causes the thermal writing of the optical recording layer 5.

An optical recording medium 11 having the structure shown in FIG. 2 is shown as a comparative example. This medium 11 does not have the transparent electrode 4, the reflective counter electrode 7 and the circularly polarizing plate 3 in the embodiment shown in FIG.

Writing is performed by applying a semiconductor laser light (10 mw, 1 μs) to the light absorber and simultaneously applying an electric field of 10 ⁵ v / m to realize homeotropic alignment, while erasing is performed by applying a semiconductor laser light of 2 mw, 100 μs. Allowed to cool and realized as the original planar orientation. FIG. 3 shows changes in the reflectance of the laser light for the example and the comparative example. As is clear from FIG. 3, it can be seen that the embodiment shows a large reflectance change of about 20%. Therefore, it is apparent that the S / N ratio can be further improved according to the present invention.

〔The invention's effect〕

As described above, the present invention has the effect of improving the modulation factor of the recording signal and thus the S / N ratio.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of an optical recording medium according to an embodiment of the present invention, FIG. 2 is a sectional view showing a conventional optical recording medium used for comparison, and FIG. 4 and FIG. 4 are views showing the structural formula of the polymer liquid crystal used for the optical recording layer. 1 ... Optical recording medium, 2 ... Circular polarizer, 3 ... Transparent substrate,
4 ... Transparent electrode, 5 ... Optical recording layer, 6 ... Reflective counter electrode

Claims (1)

[Claims] 1. An optical recording medium in which an optical recording layer is formed between a transparent electrode on a support and a counter reflection electrode, wherein a cholesteric polymer liquid crystal is used as the optical recording layer, and a central wavelength is the cholesteric polymer liquid crystal. The optical recording medium is characterized in that a circular polarizer adjusted to have the selective reflection wavelength of is written and installed in the optical path for reading, and the rotational direction of transmitted circularly polarized light is matched with the helical rotational direction of cholesteric.