JP2002074744A - Optical information recording medium and optical information recording method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical information recording medium for enabling high density recording and reproduction using near-field light in a practical-level optical information recording medium and to provide an optical information recording method using the medium. SOLUTION: A refraction adjusting layer 2 having a refractive index nr which satisfies the relation of NA<nr to the numerical aperture NA of an optical system which projects laser light is formed on a light transmissive substrate 1 and a dielectric layer 3 for converting near-field light to propagative light, a recording layer 4 and a protective layer 5 are further formed in succession to obtain the objective optical information recording medium. The refraction adjusting layer 2 is irradiated with light from the substrate 1 to emit evanescent light and this evanescent light is converted to propagative light through the dielectric layer 3 to form a pit in the recording layer 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical information recording / reproducing method, and more particularly to an optical information recording method capable of performing high-density recording using near-field light.

[0002]

2. Description of the Related Art In recent years, an optical information recording medium has been required to have a further increased storage capacity, and accordingly, an increase in the density of the optical information recording medium has been studied.

In order to improve the recording density of an optical information recording medium, it is essential to reduce the size of a recording pit. The size of the recording pit basically depends on the wavelength of a laser beam to be used and the numerical aperture of an objective lens. NA).

[0004] Therefore, conventionally, the two directions of increasing the density have been focused on (1) shortening the wavelength of the laser and (2) increasing the numerical aperture (NA) of the objective lens. .

[0005] However, there is a limit in increasing the density of an optical information recording medium by these methods, and a new method is required to further increase the capacity.

Therefore, an optical recording technique using near-field light has recently attracted attention.

In general, near-field light does not propagate like ordinary light, but is localized so as to cling to the vicinity of the medium surface. Therefore, the spatial distribution follows the shape and dimensions of the medium. Therefore, by using a medium having a fine shape, it becomes possible to form a near-field light spot having a fine spatial distribution that exceeds the restrictions imposed by the wavelength.

Based on this principle, a scanning near-field optical microscope (SNO) which uses a fine needle probe as a near-field light generating element and aims at obtaining a resolution lower than the diffraction limit of light.
M: Scanning Near-Field Optical Microscope) has been developed for several years.

Recently, a principle experiment of optical recording using the SNOM technique has been performed, and it is possible to record a very small pit of nanometer order corresponding to the geometric size of the tip opening of the probe. It has been shown that ultra-high-density recording, which greatly exceeds the limits of current optical recording technology, is possible.

[0010]

However, since the distribution of the near-field light is as short as about 50 nm, the distance between the optical information recording medium rotating at a high speed and the tip of the probe is kept constant (up to several tens nm). This technique is difficult to control and causes problems such as the contact between the probe and the optical information recording medium and the destruction of the recorded data, which makes it difficult to apply this technology to practical optical information recording media such as optical disks. It was difficult.

In addition, the process of processing the tip of the probe on the order of several tens of nanometers is complicated and unsuitable for mass production.

It is an object of the present invention to provide an optical information recording medium and an optical information recording method for enabling high-density recording and reproduction using near-field light on a practical level optical information recording medium. .

[0013]

To achieve the above object, the present invention provides a near-field light generating layer for generating near-field light by irradiating a laser beam, and a near-field light generating layer generated by the near-field light generating layer. A recording layer on which recording is performed by field light,
The near-field light generating layer is formed of a material having a refractive index n satisfying n <NA with respect to a numerical aperture NA of the optical system that emits the laser light.

Further, it is preferable that a dielectric layer for converting near-field light generated in the near-field light generating layer into propagation light is provided between the near-field light generating layer and the recording layer.

The dielectric layer for propagating the near-field light is not always necessary, for example, when the near-field light generating layer is formed of an organic dye. May be determined according to the material forming the near-field light generating layer.

The thickness of the near-field light generating layer is controlled to at least the wavelength of the laser light.

The recording layer records information corresponding to the laser light by changing its optical characteristics reversibly or irreversibly by the near-field light.

Further, when a buffer layer is formed between the near-field light generating layer and the recording layer or between the dielectric layer and the recording layer, the recording layer has improved overwrite resistance.

The present invention also relates to a near-field light generating layer of an optical information recording medium formed of a material having a refractive index n satisfying n <NA with respect to a numerical aperture NA of an optical system for emitting laser light. By irradiating light, near-field light is generated from the near-field light generating layer, and information is recorded by changing optical characteristics of the recording layer with the near-field light.

Further, the present invention provides a near-field light generating layer of an optical information recording medium formed of a material having a refractive index n satisfying n <NA with respect to a numerical aperture NA of an optical system for emitting laser light. By irradiating light, a near-field light is generated from the near-field light generating layer, and the near-field light generated from the near-field light generating layer is converted into a propagating light that propagates to the recording layer side by the dielectric layer. Information is recorded by changing the optical characteristics of the recording layer with the propagating light converted by the dielectric layer.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of an optical information recording medium and an optical information recording method according to the present invention will be described below in detail with reference to the accompanying drawings.

Normally, when light is incident from a high refractive index medium to a low refractive index medium at a critical angle or more, the incident light is totally reflected at the boundary surface of the medium. At this time, the evanescent light is applied to the low refractive index medium side of the boundary surface. Near-field light called light is generated.

Here, assuming that the refractive index of the high refractive index medium is nH, the refractive index of the low refractive index medium is nL, the incident angle of light is θi, and the refractive angle is θr, the total reflection is sin θr = (nH / nL) sin θi> 1 (1) It occurs when:

Therefore, when the high refractive index medium and the low refractive index medium are determined and the refractive indices nH and nL are determined, whether or not total reflection occurs at the interface of the medium is determined by the incident angle θi of the light.
Only depends on.

The incident angle θc satisfying (nH / nL) sin θc = 1 is called a critical angle.

By the way, the numerical aperture NA of the lens is as follows: θ is the angle at which the outermost periphery of the lens looks at the object point, and n is the refractive index of the medium filling the space between the object point and the lens.・ It is expressed as (2).

Therefore, when the refractive index n of the medium that fills the space between the lens and the object point is nH in the equation (1), if the angle θ <θc at which the outermost peripheral portion of the lens looks at the object point,
The laser light condensed on the low-refractive-index medium by this lens is not totally reflected at the interface of the medium but is entirely absorbed by the low-refractive-index medium (shown in FIG. 4A).

However, when the laser beam is condensed by a lens in which θ> θc, the incident angle of the laser beam on the low refractive index medium becomes larger than θc (the optical path Cr and the optical path in FIG. 4B). b): hereinafter referred to as total reflection area
Is totally reflected at the medium interface.

Therefore, if the optical system is designed such that the outermost portion of the lens has an angle θ> θc at which the object point can be seen, a part of the laser light emitted from this optical system will be at the interface of the medium as shown in FIG. As shown in (1), evanescent light can be generated on the low refractive index medium side of the medium interface.

The larger the value of θ, that is, from equation (2), the larger the value of the numerical aperture NA of the optical system, the larger the total reflection area of the laser light focused on the interface of the medium, Evanescent light is easily generated.

However, as the value of NA increases, the focal length of the lens becomes shorter (L1> L2 in FIG. 4), so that it is difficult to control to keep the distance between the lens and the medium constant. Further, NA is an important factor that determines the stability of the optical system such as the depth of focus, and generally, the smaller the numerical aperture NA, the more stable the optical system, and therefore, it is not preferable to set the NA too large. .

Therefore, an optical information recording medium capable of causing total reflection sufficient to generate evanescent light even with a low NA lens will be described below.

FIG. 1 is a sectional view of an optical information recording medium according to the present invention.

In FIG. 1, the optical information recording medium comprises a light-transmitting substrate 1, a refraction adjusting layer 2, a dielectric layer 3, a recording layer 4, and a protective layer 5.

The translucent substrate 1 has high transparency, heat resistance,
It is formed using a material having excellent moisture resistance and impact resistance. Specifically, polycarbonate, acrylic, or the like can be used, but is not limited thereto.

A pre-groove for controlling the position of the laser head is formed on the translucent substrate 1 at an appropriate depth and width as needed.

The refraction adjusting layer 2 is formed on the surface of the light transmitting substrate 1.

The refraction adjusting layer 2 is formed of a material having a lower refractive index than the translucent substrate 1, and generates evanescent light at the interface with the translucent substrate 1.

Here, in order to simplify the discussion, the refractive index of the translucent substrate 1 is set to the same na as that of air, and
(1) and (2) are as follows, where nr is the refractive index of

(Na / nr) sin θ> 1 (1) ′ NA = (na) sin θ (2) ′ Then, when the expression (2) ′ is substituted into the expression (1) ′, the following expression is obtained. / Nr) NA> 1 NA> nr (3) is derived.

Therefore, the numerical aperture NA of the optical system used
In contrast, if the refraction adjusting layer 2 is formed of a material having a refractive index nr that satisfies the expression (3), even a part of the laser light condensed by the low NA lens is transmitted to the refraction adjusting layer 2. Total reflection at the interface with the conductive substrate 1.

For example, Au shows nr = 0.16 for a laser beam having a wavelength of 650 nm, and Al shows nr = 0.49 for a laser beam having a wavelength of 400 nm. By forming the layer 2, NA = 0.
Evanescent light can also be generated by the 60 optical systems.

In order to convert the evanescent light generated at the interface between the refraction adjustment layer 2 and the translucent substrate 1 into propagation light at the dielectric layer 3, the thickness of the refraction adjustment layer 2 is set to be equal to the propagation distance of the evanescent light. It is necessary to control the following.

The normal distribution of near-field light is very short, several tens of nanometers. However, since the evanescent light propagates to the wavelength of the used laser light, the thickness of the refraction adjusting layer 2 should be at least the wavelength of the used laser light. What is necessary is just to control to below (to several hundred nm).

The dielectric layer 3 is made of, for example, ZnS-SiO
2, formed of Si3N4 or the like, and has a function of converting near-field light generated in the refraction adjusting layer 2 into propagation light.

The recording layer 4 is made of a material whose optical characteristics change in response to light or heat. Specifically, for example, a GeSbTe alloy, an AgInSbTe alloy, or the like, which is a phase change material whose optical characteristics change reversibly, is used. A film is formed using a target obtained by alloying these, or several units are formed. It can be manufactured by forming a film separately.

Further, in order to use these alloys as the recording layer, a so-called as-deposited state immediately after the film formation may be used as it is, but before the recording, the film is formed on the translucent substrate 1. After that, once the phase is stabilized by changing the as-deposited state from a large random state to a crystalline state by light or heat, the temperature is raised again to the melting point or higher of the film by laser light or overheating, and this is performed at an ultra-high speed. It is effective to use a material that has been brought into an amorphous state by cooling at a low temperature.

A protective layer 5 is formed on the surface of the recording layer 4, and the protective layer 5 is made of, for example, the same material as the light-transmitting substrate 1.

FIG. 2 is a cross-sectional view of the optical information recording medium of the optical information recording medium of FIG. 1 when an intermediate layer 6 is provided between the translucent substrate 1 and the refraction adjusting layer 2.

Here, similarly to FIG. 1, the incident angle of light is θ
i, the refractive index of the translucent substrate 1 is na, which is the same as that of air, the refractive index of the refraction adjusting layer 2 is nr, and the refraction angle is θr.
If the refractive index is nm and the refraction angle is θm, the relationship of (na) sin θi = (nm) sin θm = (nr) sin θr is established from the law of refraction angle, and therefore (na) sin θi = (nr) sin θr sin θi = (na / nr) sin θr> 1 (1) ′ Since the relationship of (1) ′ is derived, the numerical aperture that satisfies the condition of expression (3) is the same as in the optical information recording medium of FIG. By using the NA optical system, evanescent light can be generated at the interface between the intermediate layer 6 and the refraction adjusting layer 2.

That is, even in an optical information recording medium in which some other layers are formed between the light-transmitting substrate 1 and the refraction adjusting layer 2, an NA optical system satisfying the condition of the expression (3) is used. Thereby, evanescent light can be generated in the refraction adjusting layer 2, and the same function as the optical information recording medium in FIG. 1 can be realized.

FIG. 3A is a diagram showing a case where a buffer layer 7a is provided between the dielectric layer 3 and the recording layer 4 in the optical information recording medium having the above-described configuration. (B)
FIG. 4 is a diagram when a buffer layer 7b is provided between a recording layer 4 and a protective layer 5. Further, FIG. 3C is a diagram in the case where the buffer layers 7a and 7b are provided on both surfaces of the recording layer 4. As shown in these figures, by providing the buffer layer on the surface of the recording layer 4, The overwrite characteristics of the optical information recording medium are improved.

The buffer layers 7a and 7b are made of GeN or the like.

As shown in FIG. 1, when the optical information recording medium having the above-described structure is irradiated with laser light from the light-transmitting substrate 1 side, the interface S forms a part of the total reflection area of the irradiated laser light. Is totally reflected, and evanescent light is generated on the refraction adjusting layer 2 side of the interface S by the total reflection.

The evanescent light generated is transmitted to the dielectric layer 3
Is converted into propagation light, and recording on the recording layer 4 is performed by the evanescent light converted into the propagation light.

As an optical system for emitting a laser beam, an optical head similar to that used in an ordinary DVD-RAM or the like can be used, and the distance between the optical head and the optical information recording medium can be increased. Since it is only necessary to control the optical system to the same level as a normal DVD-RAM or the like, it is not necessary to significantly change the structure of the optical system or optical information recording medium of the conventional optical information recording device, and it is possible to achieve high density using near-field light that is extremely practical. Recording can be realized.

[0057]

As described above, according to the present invention, since the constituent film of the optical information recording medium exhibits the same function as the probe in the SNOM in a self-forming manner, it is not necessary to use a probe. High-density recording using near-field light enables practical-level optical information recording because near-field recording is possible without the need to significantly change the optical system and optical information recording medium structure of conventional optical information recording devices. This has the effect of being made possible with a medium.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of the configuration of an optical information recording medium and an example of an optical information recording method according to the present invention.

FIG. 2 is a diagram showing an example of a configuration of an optical information recording medium according to the present invention.

FIG. 3 is a diagram showing an example of a configuration of an optical information recording medium according to the present invention.

FIG. 4 is a diagram illustrating the concept of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transparent board | substrate 2 Refraction adjustment layer 3 Dielectric layer 4 Recording layer 5 Protective layer 6 Intermediate layer 7a, 7b, 7a Buffer layer

Claims (9)

[Claims] A near-field light generating layer that generates near-field light by irradiating a laser beam; and a recording layer on which recording is performed by the near-field light generated by the near-field light generating layer. The optical information recording medium, wherein the field light generating layer is formed of a material having a refractive index n that satisfies n <NA with respect to the numerical aperture NA of the optical system that emits the laser light. 2. The method according to claim 1, further comprising a dielectric layer formed between the near-field light generating layer and the recording layer, the dielectric layer converting near-field light generated by the near-field light generating layer into propagation light. The optical information recording medium according to claim 1, wherein: 3. The optical information recording medium according to claim 1, wherein the thickness of the near-field light generating layer is controlled to at least the wavelength of the laser light. 4. The optical information recording according to claim 1, wherein the recording layer records information corresponding to the laser light by irreversibly changing its optical characteristics by the near-field light. Medium. 5. The optical information recording medium according to claim 1, wherein the recording layer records information corresponding to the laser light by reversibly changing its optical characteristics by the near-field light. . 6. The optical information recording medium according to claim 1, wherein a buffer layer is formed between the near-field light generating layer and the recording layer. 7. The optical information recording medium according to claim 2, wherein a buffer layer is formed between said dielectric layer and said recording layer. 8. A numerical aperture NA of an optical system for emitting laser light.
By irradiating the laser light to the near-field light generating layer of an optical information recording medium formed of a material having a refractive index n satisfying n <NA, near-field light is generated from the near-field light generating layer. An optical information recording method, wherein information is recorded by changing optical characteristics of the recording layer by the near-field light. 9. A numerical aperture NA of an optical system for emitting laser light.
By irradiating the laser light to the near-field light generating layer of an optical information recording medium formed of a material having a refractive index n satisfying n <NA, near-field light is generated from the near-field light generating layer. Converting near-field light generated from the near-field light generating layer into propagation light propagating to the recording layer side by the dielectric layer, and changing the optical characteristics of the recording layer by the propagation light converted by the dielectric layer. An optical information recording method, wherein information is recorded by causing the optical information to be recorded.