JP2001291279A - Optical recording medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical recording medium which shows high superresolution characteristics and in which the recording density can be improved. SOLUTION: In the optical recording medium 1 having a superresolution film 3 on a recording layer, the superresolution film 3 contains at least a substance having <=0.4 eV halfwidth of the absorption peak or a substance having <=30 cm-1 lowest vibrational level calculated by using the ground function STO-3G for a structure optimized by using the Hartree-Fock approximation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical recording medium.

[0002]

2. Description of the Related Art In recent years, optical recording media, especially digital video disks (DVDs), have become indispensable recording media for the information industry.
Is gaining attention and the market is growing. At present, several means have been proposed to achieve further higher recording density of such an optical recording medium.

One of the means for achieving high recording density is to shorten the wavelength of laser light used. Currently, a GaAlInP-based semiconductor laser uses a wavelength of 650 nm,
In recent years, GaN-based semiconductor lasers have been
It seems to shift to a wavelength of ~ 450 nm. When the wavelength of the laser beam is shortened, the laser spot diameter can be reduced by the square thereof, and the recording density can be increased accordingly.

Another means for achieving a higher recording density is a super-resolution method. FIG. 3 is a schematic diagram showing the concept of the super-resolution method. In the super-resolution method, a super-resolution film 14 is formed on a recording layer 12 of an optical recording medium 13 having a recording layer 12 in which information is recorded in recording pits 11. The super-resolution film 14 has a property of transmitting only a high intensity portion of the laser beam 15 focused on the recording layer 12, and an aperture smaller than the diffraction limit of the reproduction light is formed. The laser light beam 15 is applied to the recording layer 1 through such a super-resolution film 14.
By irradiating No. 2 with a laser beam 15 having a spot diameter smaller than the diffraction limit, the recording density is improved.

As one of such super-resolution methods, a method utilizing a saturation absorption phenomenon of a material constituting a super-resolution film is disclosed in, for example, JP-A-06-243508 and JP-A-07-296.
No. 419. The super-resolution film used in this method has a structure in which a saturable absorbing dye is dispersed in a transparent matrix. The saturable absorbing dye molecule has a property that, when irradiated with light, electrons are excited from a ground state to an excited state, but when the intensity of the exciting light is significantly strong, absorption is reduced. Utilizing this property, the super-resolution film exhibits a super-resolution operation in which only a central portion of a laser beam having a high light intensity is transmitted.

In the above publication, a phthalocyanine dye is mentioned as a specific example of such a dye molecule, and a super-resolution film showing a super-resolution operation when reproducing by irradiating a 780 nm laser beam. An optical recording medium comprising:

[0007]

As described above, a method of increasing the recording density of an optical recording medium by a super-resolution method has been conventionally proposed. However, in order to further improve the recording density, it is necessary to use a super-resolution film having higher super-resolution characteristics.

The present invention has been made in view of the above problems, and has as its object to provide an optical recording medium which exhibits high super-resolution characteristics and can improve the recording density.

[0009]

According to a first aspect of the present invention, there is provided a recording layer capable of forming a recording pattern corresponding to information to be recorded, and the recording is performed by irradiating the recording layer with light. In the optical recording medium from which the information is reproduced, the optical recording medium includes a super-resolution film for narrowing a beam diameter of the light on the recording layer, and the super-resolution film has at least a full width at half maximum of an absorption peak of 0. An optical recording medium comprising a substance that is 4 eV or less.

A second invention has a recording layer capable of forming a recording pattern corresponding to information to be recorded, and the recorded information is reproduced by irradiating the recording layer with light. In the optical recording medium, the optical recording medium includes a super-resolution film on the recording layer to narrow the beam diameter of the light,
The super-resolution film has a minimum vibration level of at least 30 cm calculated in a molecular structure optimized using the Hartree-Fock approximation using the basis function STO-3G.
An optical recording medium comprising a substance of ^-1 or less.

In the optical recording medium according to the first and second aspects of the present invention, as in the prior art, a film exhibiting super-resolution characteristics is formed on a recording layer on which information is recorded as irregularities or changes in optical characteristics on a substrate. (Super-resolution film). The super-resolution film has a property of transmitting only a portion having a high light intensity when irradiated with light, whereby an aperture smaller than the diffraction limit of light is formed, and a spot smaller than the diffraction limit is formed. Light having a diameter is applied.

In order to realize a super-resolution film having high super-resolution characteristics, in the first invention, the full width at half maximum of the absorption peak is 0.4 e.
A super-resolution film including a substance having a value of V or less is used.

In general, an organic material has a large number of absorption peaks, and the absorption peak referred to here is a peak containing the wavelength of light (reproduction laser light) used. In general, excited electrons tend to be more stably present at lower energy (longer wavelength) peaks, and therefore it is better to use light (reproduced laser light) having a wavelength substantially coincident with the peak wavelength on the lower energy side.

When the energy of electrons at the wavelength of X nm is Y eV, Y = 1239.5 / X is established as an energy conversion approximation.

Further, the full width at half maximum referred to here means that the peak
Of the peak value on each side of the peak
Shows the interval between half-valued independent variable values. (Iwanami
 Physical and Chemical Dictionary 4th Edition Iwanami Shoten (1987) p. 10
08 "half width") In the second invention, the basis function
Hartree-Fock approximation using STO-3G
Calculated lowest vibration levels in molecular structures optimized using
Position is 30cm ^-1Use a super-resolution film with the following substances
I have.

The reason why the full width at half maximum of the absorption peak is set in the above range in the first invention will be described below.

The present inventors believe that the super-resolution characteristic of the super-resolution film is related to the phase relaxation time of the material constituting the super-resolution film.

The relaxation time when a certain molecule is irradiated with light of a certain wavelength is a value specific to each substance, and is generally described in JP-A-6-243508 and JP-A-7-296419. There is an energy relaxation time and a phase relaxation time according to the present invention.

The phase relaxation time refers to the degree of time during which the coherence of light that excites a molecule when the molecule is irradiated with light is disturbed. For example, the plane of light can be represented by a sine wave, but coherence is a state in which the phase of this sine wave is continuously connected in time and space, and agrees with coherence. It is.

On the other hand, the energy relaxation time is the time when an electron changes from a high energy state to a low energy state, and has a different definition from the phase relaxation time. The general energy relaxation time of a dye is from about sub-nanosecond to nanosecond, but the phase relaxation time of the dye is from about sub-femtosecond to about femtosecond, and the time regions are greatly different.

Hereinafter, the relationship between the super-resolution characteristics of the super-resolution film and the phase relaxation time will be described.

In the present invention, it is considered that the absorption saturation phenomenon of the super-resolution film is caused mainly by the third-order nonlinear optical characteristics of the material constituting the super-resolution film, for example, the dye.

First, when the electrons excited by irradiation with light in the dye molecules vibrate harmonically, anharmonic vibration is induced and the absorption changes. Harmonic vibration is a vibration, such as a spring, in which the distance from the center position and the force to return to the center are proportional. Anharmonic oscillation is when it is not proportional.

The electron polarization P (that is, proportional to the magnitude of absorption) of a dye molecule excited by an electric field E by light (electromagnetic wave) is generally P = P ₀ + χ ⁽¹⁾ · E + χ ⁽²⁾ · E
E + χ ⁽³⁾ · E · E · E +... (However, P ₀ is spontaneous polarization, χ ⁽¹⁾ is linear susceptibility, χ ⁽²⁾ ,
χ ^{( 3)} ... are second-order, third-order,. ) The electric field (light) is not strong, χ ⁽²⁾ , χ ⁽³⁾ …
Is small, the term below χ ^{(2 )} becomes small, and P = P
₀ + χ ⁽¹⁾ · E This is the general situation.

However, χ ⁽²⁾ , χ ⁽³⁾ ... are large,
When the electric field (light) becomes very large like a laser,
^{(2) The} following terms cannot be ignored and a non-linear characteristic appears.

The wavelengths at which absorption occurs are χ ⁽¹⁾ , χ ⁽²⁾ ...
異 な り⁽¹⁾ is called linear absorption, and χ ⁽²⁾ is called two-photon absorption. Here, the wavelength of the laser used is related to a term related to linear absorption, that is, χ ⁽¹⁾ of the second term.

Here, since the dye molecules in the super-resolution film exhibiting absorption saturation have a macroscopic inversion symmetry, χ ⁽²⁾ , χ ⁽⁴⁾ ,
χ ⁽⁶⁾ . . . . . . Is zero. That is, the absorption of the dye molecules in the super-resolution film exhibiting absorption saturation is mainly caused by χ ⁽¹⁾ , χ ⁽³⁾ , χ ⁽⁵⁾ ,
χ ⁽⁷⁾ . . . . . ⁽³⁾ has the greatest effect on nonlinear susceptibility.

That is, when the light intensity is small, the excited electron polarization (absorption magnitude) is expressed linearly. However, when the light is strong, the proportion of the non-absorbed polarization increases in proportion to the cube of the electric field. Thus, as the light becomes stronger, the absorption appears to saturate.

Here, the third-order nonlinear optical constant χ ^{(3) is the} (third-order nonlinear susceptibility). The magnitude of the transition dipole moment (absorption) per unit molecule is μ, and the angular frequency of the energy level is omega _0, the angular frequency of the laser beam omega, the phase relaxation (transverse relaxation) constant (inverse of the phase relaxation time) and gamma _p,

(Equation 1) It is expressed as

A larger super-resolution effect can be expected when χ ⁽³⁾ is larger. However, ω and ω ₀ coincide, and (when the laser beam is resonated to the energy level), χ ⁽³⁾ becomes the maximum. further more the phase relaxation constant gamma _p is small, that is (the more the phase relaxation time is large) super-resolution effect increases.
This means that the so-called resonance effect increases.

On the other hand, it is generally considered that the full width at half maximum of the absorption peak is proportional to the phase relaxation constant Γ _p (that is, the reciprocal of the phase relaxation time). Therefore, a material in which the full width at half maximum of the absorption peak is equal to or less than a specific value exhibits a high super-resolution effect.

The present inventors have found that a substance having an absorption peak full width at half maximum of 0.4 eV or less is used for a super-resolution film to exhibit a large absorption saturation effect and, consequently, a large super-resolution characteristic.

Also, in the second invention, the basis function STO-3
The reason why the lowest vibration level calculated in the structure optimized by using G and the Harttree-Fock approximation is set in the above range will be described.

As described above, the longer the phase relaxation time of the substance constituting the super-resolution film (the smaller the phase relaxation constant).
Shows high super-resolution characteristics. As one of the causes of the phase relaxation, it is considered that the collision between the excited electrons and the molecular (or crystal lattice) vibration.

Therefore, it is considered that there is a correlation between the vibration level of the material constituting the super-resolution film and the phase relaxation. The present inventors have found that the lower the lowest vibration level is, the smaller the measured phase relaxation constant is, and in particular, using the basis function STO-3G,
It has been found that a super-resolution film using a substance whose lowest vibration level calculated in a molecular structure optimized using the tree-Fock approximation is 30 cm ⁻¹ or less exhibits high super-resolution characteristics.

The basis function STO-3G used in calculating the vibration level is described in “WJ Hehre, R., et al.
F. Stewart and J.M. A. Pople,
J. Chem. Phys. , 51, 2657 (196)
9)). Also Hartree-
The Fock approximation is described in "(CCJ Roothaan,
Rev .. Mod. Phys. , 23, 69 (195
1); 32, 179 (1960)) ".

[0037]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic diagram showing an example of an optical recording medium according to the first to third aspects of the present invention.

In the optical recording medium 1, a super-resolution film 3 is formed on a transparent substrate 2 made of glass or plastic. Further, a reflective film 4 made of Al or the like is formed on the super-resolution film 3.

Further, the thermal conductivity of the reflective film 4 is 1 W /
A heat dissipating film 5 having a thermal conductivity of 1 W / m · K or more is provided so as to be in contact with the super-resolution film 3, and the film in contact with both surfaces of the super-resolution film is 1 W / m · K It is desirable that the heat dissipation film having the above-described thermal conductivity be configured.

When the super-resolution film 3 is composed of a matrix polymer and a low molecular weight organic compound and has a thermal conductivity of about 1 or less, the thermal conductivity is preferably 1 W / m on both sides of the super-resolution film 3. A heat radiation effect occurs by contacting a heat radiation film of K or more, and deterioration of the super-resolution film can be reduced. The higher the thermal conductivity of the heat radiation film, the better.
A heat dissipation film of 0 W / m · K or more has a high effect. Especially 1W / m
As the heat dissipation film of K or more, specifically, an aluminum thin film, a gold thin film, a copper thin film, aluminum nitride, germanium nitride and the like are particularly desirable.

It is desirable that the thickness of the heat radiating film be in the range of 1 nm to 100 nm because of the heat radiating efficiency and the necessity that the entire film of the optical recording medium be within the depth of focus.

An uneven pattern corresponding to information to be recorded is recorded on the transparent substrate 2 of the recording medium 1 to form a recording layer. Reproduction of recorded information is performed by irradiating the reproduction light 6 from the transparent substrate 2 side and reading a change in reflected light. The optical recording medium 1 shown in FIG. 1 reproduces information by reading a change in reflected light. For example, the information is reproduced by irradiating a reproduction light 6 and reading a change in transmitted light. Is also good.

Although the recording medium 1 shown in FIG. 1 has the transparent substrate 2 also serving as a recording layer, the recording layer of the recording medium according to the present invention may be provided separately from the transparent substrate. The recording pattern corresponding to the information recorded on the recording layer of the recording medium is not limited to the concavo-convex pattern as in the recording medium 1 shown in FIG. 1, but may be any pattern due to a change in optical characteristics such as a refractive index.

The super-resolution film 3 is made of a material having at least super-resolution characteristics dispersed in a matrix.

As the matrix, polymethyl methacrylate (PMMA), polystyrene, polycarbonate, polyacetal, polyarylate and the like can be suitably used, but there is no particular limitation. However, those having a high melting point (for example, 100 ° C. or higher) and capable of containing a dye in a high concentration are better.

The super-resolution film 3 can be easily formed by spin-coating a material exhibiting super-resolution characteristics, for example, a mixture of a saturable absorbing dye, a matrix and a solvent, on an optical recording medium and drying. Yes and preferred. Therefore, the matrix material needs to be dissolved in a solvent to an extent that it can be applied with an appropriate thickness. This condition is the same for materials exhibiting super-resolution characteristics. The super-resolution film may be formed by vapor deposition of a material exhibiting a matrix and super-resolution characteristics. For example, Teflon (registered trademark) is strong against heat and good as a matrix, but Teflon cannot be spin-coated. At this time, evaporation may be performed together with a material having super-resolution characteristics.

When a dye is used as a material exhibiting super-resolution characteristics, for example, the concentration c of the dye / matrix is 0.0
It is desirable that the concentration be from 02 mol / L to 2000 mol / L. Since the film thickness is limited by the depth of focus of the optical system, if it is less than 0.002 mol / L, the steady absorption amount is small and super-resolution does not easily occur. The strength cannot be obtained. A more preferred range is 0.1 mol / L or more and 100 m.
ol / L.

It is desirable that the value of the concentration c of the dye / matrix is determined in consideration of the thickness and the absorbance of the super-resolution film. For example, 1 × 10 ⁵ L / m at the working wavelength
Assuming a dye having a molar extinction coefficient ε of ol · cm, the film thickness d exceeds the depth of focus and is at most 5 × 10 ^−5.
cm, and if the absorbance α is less than 0.01, a sufficient super-resolution effect cannot be obtained. On the other hand, in a sufficiently weak light intensity (excitation) region, that is, a linear region, α = ε · c · d, so that the concentration c needs to be 0.01 mol / L or more. The super-resolution film according to the first invention,
It includes a material having super-resolution characteristics and at least a full width at half maximum of an absorption peak of 0.4 eV or less. More preferably, it is 0.2 eV or less. Within this range, the resonance effect, that is, the super-resolution effect is increased, and a substance having a high absorption saturation effect is desirable because it is often found to be 0.2 eV or less. The super-resolution film according to the second invention is a material exhibiting super-resolution characteristics, and has a Ha
Includes substances whose lowest vibration level calculated in a structure optimized using the rtree-Fock approximation is 30 cm ⁻¹ or less. More preferably, the substance is 20 cm ⁻¹ or less. Within this range, the super-resolution effect is sufficiently large, which is desirable. The most preferred super-resolution film has a full width at half maximum of an absorption peak of 0.4 eV or less, which satisfies the conditions for the super-resolution film of the first invention and the conditions for the super-resolution film of the second invention, and Hartree-Fock using STO-3G
It is a super-resolution film including a substance whose lowest vibration level calculated in a structure optimized using approximation is 30 cm ⁻¹ or less. The dye according to the first invention and the second invention may be any dye that satisfies the conditions,
Specific examples include the dyes shown in Table 1 below.
Table 1 shows the structural formula symbols of each substance, the full width at half maximum of the absorption peak, and the basis function STO-3G.
-The lowest vibration level calculated in the structure optimized using the Fock approximation is also described.

[Table 1] The structural formulas (1) to (7) of the dyes shown in Table 1 are as follows.

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[0049]

EXAMPLES (Examples 1 to 3, Comparative Example 1) In order to evaluate the characteristics of the super-resolution film, four transparent substrates made of glass were used.
Each kind of dye film was formed by spin coating. The dye used for spin coating was coumarin 3 as in Example 1.
0, DOCI as Example 2, DASB as Example 3
TI and Coumarin 151 (Coumarin 1 as Comparative Example 1)
51, the full width at half maximum of the absorption peak was 0.61 eV, and the lowest vibration level was 41.0489 cm ⁻¹ ). In addition, the dye film contained PMMA having a molecular weight of about 70,000 as a matrix in addition to the dye.

First, PMMA: ethyl lactate =
A solution was prepared by mixing at a ratio of 4: 100, and the respective dyes were dissolved in the solution so that the absorbance at low power at the absorption peak wavelength was almost constant. This solution was spin-coated on the transparent substrate at 3000 rpm. Table 2 shows the dye names, the thickness of the dye film, and the wavelength of the incident light in Examples 1 to 3 and Comparative Example 1.

[Table 2] Laser light is incident on the dye film thus formed,
The dependence of the transmittance on the incident light intensity was investigated.

FIG. 2 is a characteristic diagram showing the incident light intensity dependency of the transmittance of the dye film of the optical recording medium according to Examples 1 to 3 and Comparative Example 1. Examples 1 to 3 and Comparative Example 1 as shown in FIG.
In, the transmittance increases as the incident light intensity increases, but the tendency is particularly strong in Examples 2 and 3.
It was confirmed that the super-resolution characteristics were excellent. Example 4 and Comparative Example 2 Next, the DOCI super-resolution film (dye film) used in Example 2 was formed on a transparent substrate in the same manner as in Example 2, and the optical recording medium shown in FIG. Produced. In this optical recording medium 1, a dye film 3 is formed on a transparent substrate 2 made of glass, and a reflective film 4 made of Al having a thickness of 50 nm is formed on the dye film 3. However, the heat radiation film 5 was not formed. In this optical recording medium 1, information was recorded by forming a concavo-convex pattern on one side, and the track pitch between adjacent tracks of the concavo-convex pattern was 0.3 μm. Here, crosstalk occurs when the track pitch or mark pitch is smaller than the laser beam spot, but generally the track pitch is shorter than the mark pitch.

Next, the information recorded on the optical recording medium 1 was reproduced by irradiating the optical recording medium with the reproducing light 6 from the transparent substrate 2 side and reading the reflected light. At this time, the N.V. A. 0.6, and the used wavelength λ was 485 nm. The condensing diameter of the laser beam on the pit without the dye film 3 is about 0.7 μm. The power of the laser beam was 3 mW, and the linear velocity was 10 m / s. As a result, information could be reproduced accurately.

On the other hand, as a comparative example 2, an optical recording medium was prepared separately under the same conditions except that the dye film 3 was not formed, and the information recorded on the optical recording medium was read by irradiating the reproducing light in the same manner. Crosstalk occurred and the signal could not be read accurately.

[0054]

As described above, according to the present invention, there is provided an optical recording medium having high super-resolution characteristics and capable of improving recording density.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of an optical recording medium according to the present invention.

FIG. 2 is a characteristic diagram showing the incident light intensity dependence of the transmittance of the dye films (super-resolution films) according to Examples 1 to 3 and Comparative Example 1.

FIG. 3 is a schematic diagram illustrating the concept of a super-resolution method.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Optical recording medium 2 ... Transparent substrate 3 ... Dye film (super-resolution film) 4 ... Reflection film 5 ... Heat dissipation film 6 ... Reproduction light (laser light beam) 11. ..Recording pits 12 ... Recording layer 13 ... Optical recording medium 14 ... Super-resolution film 15 ... Laser light beam 16 ... Reflected light

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Reiko Yoshimura 1st address, Komukai Toshiba-cho, Saiwai-ku, Kawasaki-shi, Kanagawa Pref. F-term (reference) 5D029 MA39

Claims (2)

[Claims] 1. An optical recording medium having a recording layer capable of forming a recording pattern corresponding to information to be recorded, wherein the information recorded by the irradiation of the recording layer with light is reproduced. Wherein the optical recording medium includes a super-resolution film on the recording layer for narrowing a beam diameter of the light, and the super-resolution film includes a substance having at least a full width at half maximum of an absorption peak of 0.4 eV or less. Optical recording medium. 2. An optical recording medium having a recording layer capable of forming a recording pattern corresponding to information to be recorded, wherein the recording layer is irradiated with light to reproduce the recorded information. The optical recording medium includes a super-resolution film on the recording layer for narrowing the beam diameter of the light, and the super-resolution film uses at least a basis function STO-3G;
An optical recording medium comprising a substance whose lowest vibration level calculated in a molecular structure optimized using the tree-Fock approximation is 30 cm ^-1 or less.