JPH0619851B2 - Optical recording medium - Google Patents

Description

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 Faraday effect or Kerr effect, particularly an optical recording medium in which a thin film using an alloy containing a rare earth terbium is formed on a substrate or a chalcogenal compound is used. An optical recording medium using a material that causes a change in light reflectance due to a crystal-amorphous phase change of the material is known.

In addition, an optical recording medium that utilizes optical anisotropy due to alignment of liquid crystal groups of polymer liquid crystals has been proposed.

[Problems to be Solved by the Invention] Conventional magneto-optical recording media and phase change media have a special protective layer because they use harmful materials, are manufactured by sputtering, etc., and have low stability. There are drawbacks such as being necessary. Therefore, there is a strong demand for an optical recording medium that is simple in manufacturing method, harmless, and inexpensive. A medium in which a polymer liquid crystal is enclosed between a pair of electrode substrates has been proposed as a medium for solving the above drawbacks (Japanese Patent Laid-Open No. 59-10930,
JP-A-59-35989 and JP-A-60-114823), which have an S / N ratio of about 50 dB, which is still insufficient.

It is another object of the present invention to provide an optical recording medium having an improved S / N ratio.

[Means for solving problems]

The optical recording medium according to the present invention has a transparent electrode layer on a transparent substrate,
In an optical recording medium in which an optical recording layer including a dielectric layer and a polymer liquid crystal layer, a dielectric layer, a light absorbing layer, and a counter electrode layer are sequentially laminated, a polymer liquid crystal used for the optical recording layer The respective refractive indices n ₁₁ and n ₁ for ordinary and extraordinary rays of (However, n and m are odd numbers), the film thickness d of the optical recording layer is (Where λ is the wavelength of the semiconductor laser).

Light incident on an optical recording layer made of a polymer liquid crystal layer having uniform birefringence and birefringence is divided into ordinary rays and extraordinary rays by birefringence. Those that follow Snell's method are ordinary rays, and those that do not follow are extraordinary rays.

[Action]

The optical recording medium 1 of the present invention will be described with reference to FIG.

As the transparent substrate 2, it is possible to use a substrate used for a conventional optical disc such as a glass substrate or a polycarbonate substrate.

The optical recording layer 5 in which ITO is vapor-deposited on the substrate 2 and used as the transparent electrode layer 3 is capable of holding a recording state at room temperature, and has a binary state with different refractive indexes due to a phase transition. is necessary.

As a material satisfying the above requirements, a polymer having a glass transition temperature higher than room temperature, for example, a mixture of polystyrene and polyethylene having a degree of polymerization of about 100 with a liquid crystal can be used. The mixing ratio is such that the liquid crystal does not lose its liquid crystallinity and the isotropic phase transition temperature is 50 ℃
It must be chosen to be between 150 ° C and have a glass transition temperature above room temperature.

In addition, as a material satisfying the above requirements, it is possible to use a so-called side chain type polymer liquid crystal layer obtained by bonding a conventionally known liquid crystal to an acrylate, a methacrylate, or a polysiloxane using a flexible alkylene or the like as a spacer. it can.

The optical recording material is formed into a thin film on the transparent electrode layer 3 formed on the transparent substrate 2 by a method such as coating or spin coating, which is used as the optical recording layer 5. As the first dielectric layer 4,
Transparent dielectrics such as MgF ₂ can be used. The light absorption layer 7 is obtained by depositing a material that efficiently absorbs laser light, for example, vanadium oxide phthalocyanine. As the second dielectric layer 6, a SiO ₂ layer or the like formed by a sputtering method can be used. The reflective electrode layer can be formed by vapor deposition of ITO, NESA, aluminum or the like.

A conventional optical recording medium is shown in FIG. 2 for comparison. The optical recording medium according to the present invention is the same as the optical recording medium except that the thickness of the optical recording layer is not adjusted by the method described later and no dielectric layer is provided. The same components are assigned the same reference numerals and explanations thereof are omitted.

Both the medium of the present invention and the conventional medium have the same operation in writing. That is, first, the entire optical recording layer is heated to the glass transition temperature or higher, and cooled to room temperature to form a homogeneous alignment state in which liquid crystal groups are aligned in a certain direction.
This is the erased state. Homogeneous orientation is realized by rubbing the dielectric material, and is maintained at room temperature due to the polymer nature of the polymer liquid crystal. By irradiating the medium 1 with a pulse linearly polarized in the homogeneous alignment direction of high-density energy light such as a semiconductor laser light, the optical recording layer 5 is heated to an isotropic transition temperature or higher, and an electric field is applied at the same time, It is dot-shaped and shows a uniform homeotropic orientation in it, and exhibits a refractive index of n ₁ with respect to the incident laser.

Next, the reflectance characteristics of the medium of the present invention will be described.

The refractive index of the recording layer 5 is n ₁ at the time of writing and n ₁₁ at the time of non-writing. Let's calculate the reflectance with these refractive indices as n first.

Dielectric layer with refractive index n ₀ , recording layer with refractive index n and refractive index ng
The reflectance when using the dielectric layer is given by the following equation. Ie In equation (1), when n> ng, n ₀ or ng, n ₀ > n is satisfied, the film becomes a reflection-increasing film, and when δ = (2m + 1) π, the maximum value is obtained. Take. As shown in FIG. 3, it is at the position A, while δ =
Minimum value for 2 mπ Take and come to the position of B.

On the other hand, in equation (1), if n ₀ >n> ng or ng>n> n ₀ , then the minimum value when δ = (2m-1) π To take the position of C. In the case of δ = 2mπ Take and get to position B.

In FIG. 1 showing the structure of the present invention, a first dielectric layer 4 having a refractive index n _{0 and} a second dielectric layer 6 having a refractive index ng are used. For the optical recording layer 5, a polymer liquid crystal using a liquid crystal group having a refractive index of n ₁₁ in homogenous orientation and n _{1 in} homeo orientation is used. The light used for reading and writing is linearly polarized light in the homogenized orientation direction, that is, the orientation direction of the liquid crystal group.

When the refractive index is n ₁₁ , that is, in the case of homogeneous orientation, for light polarized in that direction, And when n ₁₁ > ng, it is given as a reflection increasing film.
Adjust d so that δ = (2m + 1) π, the reflection becomes maximum and its value is Given in.

The refractive index of the spot is n ₁
And When n ₁ <ng, it is given as an antireflection film, and n ₁ becomes δ = (2m ′ + 1) π minimum, and its value is Given in.

n ₁ is one When the above condition is satisfied, the reflected light becomes 0 and the reflectance becomes extremely large, and as a result, a high contrast is obtained in the recording portion. However, in practice, δ = (2m ′ + 1) π where n ₁ is the phase condition, In the case of satisfying the above condition, conventionally, it could be done by adjusting d, but in the present invention, d has already been used for adjusting the reflection maximum at the time of homogeneous alignment. Therefore, equation (3) is Refractive index given by odd ratio such as (m '<m)
It is effective to select a liquid crystal having n ₁ and n ₁₁ .

From this point of view, if the liquid crystal is selected and the liquid crystal thickness d is adjusted, it is possible to record with a higher writing contrast than the conventional one. When a method aiming to improve the contrast by matching the change in the refractive index with the interference condition as described above using another medium, for example, a phase change medium, the change in the refractive index in this case is 0.01
In this case, the film thickness becomes 10 μm or more, and it is difficult to make the film thickness uniformity and the recording points of 1 μmφ uniformly over the film thickness 10 μm, and this method cannot be used.

[Embodiment] An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing the configuration of an optical recording medium 1 showing an embodiment of the present invention.

In the figure, a glass plate is used as the substrate 2, and ITO is vapor-deposited on the transparent electrode layer 1400Å to form a transparent electrode layer 3, and then,
The optical recording layer 5 is formed by depositing MgF ₂ as the first dielectric layer 4.
A polymer liquid crystal having the structural formula shown in FIG. SiO ₂ is provided as a second dielectric layer 4 by sputtering on this, vanadium oxide phthalocyanine is provided as 1000 Å as a light absorption layer 7, and aluminum is further vapor-deposited as a counter electrode layer 8 on this to form an optical recording medium 1. To do.

The optical recording layer is prepared by heating the polymer liquid crystal having the structural formula shown in FIG. 5 to a molten state, dropping it on the first dielectric layer 4, and then hot pressing it while rotating the roll. It can be formed and has uniaxial orientation (the liquid crystal group used for the side chain is oriented).

The thickness of the optical recording layer 5 is considered as follows in consideration of the interference effect. That is, first, n _{11 of the} polymer liquid crystal is 1.688, and n ₁
Is 1.508 and the ratio of n ₁ and n ₁₁ is 0.89336, so the closest odd ratio 17/19 = 0.89473 is adopted, If d is set so that is equal to 19π, d = 2.336μ
m.

The refractive index of the dielectric material sandwiching the optical recording layer 5 is 1.
38 and the second dielectric layer 6 is 1.547, the refractive index n ₁₁ when the optical recording layer 5 has a homogeneous orientation.
= 1.688, which is larger than the refractive index of both dielectric layers, acts as a reflection-increasing film, and has the maximum value in the case of the previous thickness, and the reflectance R ² is And get about 2%. On the other hand, when an electric field is applied at the same time as the semiconductor laser irradiation, the refractive index is changed when homeotropic alignment is performed. However, in this case the reflectance R ² is Therefore, R ² is 0.09%, which is almost a non-reflection condition, and the reflectance change is as large as 22.

As described above, a large refractive index ratio was obtained by adjusting the thickness of the optical recording layer and the refractive index.

FIG. 4 is a diagram showing changes in reflectance of laser light between the optical recording medium of the present invention and the conventional optical recording medium. According to the present invention, the change in reflectance was found to be 1.5 times or more that of the conventional one.

〔The invention's effect〕

As described above, according to the present invention, the S / N of the read signal is
It has an effect that the ratio can be further improved to improve the performance.

[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 the structure of a conventional optical recording medium used for comparison, and FIG. FIG. 4 is a diagram showing the change in reflectance of laser light, and FIG. 5 is a diagram showing the structural formula of the polymer liquid crystal used in the optical recording layer. 1 ... Optical recording medium, 2 ... Substrate, 3 ... Transparent electrode layer, 4
...... First dielectric layer, 5 ...... Optical recording layer, 6 ...... Second dielectric layer, 7 ...... Light absorbing layer, 8 ...... Counter electrode layer

Claims (1)

[Claims] 1. An optical recording comprising a transparent substrate, an optical recording layer comprising a transparent electrode layer, a dielectric layer, and a polymer liquid crystal layer, a dielectric layer, a light absorbing layer, and a counter electrode layer, which are sequentially laminated. In the medium, the respective refractive index n ₁₁ , n ₁ for ordinary and extraordinary rays of the polymer liquid crystal used in the optical recording layer, (However, n and m are odd numbers), the film thickness of the optical recording layer d is (Where λ is the wavelength of the semiconductor laser), which is an optical recording medium.