JPH08318674A - Write-once organic coloring matter optical recording medium - Google Patents

Abstract

PURPOSE: To obtain a write-once org. coloring matter optical recording medium having high laser beam transmissivity and high sensitivity and excellent in long-term preservability by forming a recording film from a compsn. prepared by compounding a specific light stabilizer with specific laser beam absorbing coloring matter. CONSTITUTION: A write-once org. coloring matter optical recording medium is equipped with a laser beam previous circular substrate having first and second main surfaces, the recording film formed on the first main surface of the substrate and containing laser beam absorbing coloring matter represented by formula I and a light stabilizer represented by formula II of which the ratio is 5-25% by wt. of the coloring matter and the metal laser beam reflecting film formed thereon. In the formula I, R<1> and R<2> are independently an alkyl group or the like, L is a methylene chain group consisting of 1-3 units -(CH=CQ)- (wherein Q is hydrogen or an alkyl group), A is a carbon atom or a sulfur atom, Y is a 2-l8C substituent having an unsaturated bond, Z is a hydrogen atom or a carbon atom and m is an integer of 1-4. In the formula II, W is a nitro group or the like and R<21> is a l-4C alkyl group.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a write-once type organic dye-based optical recording medium capable of recording and / or reproducing information by a laser beam and having a portion capable of being written (recorded) at least once. In particular, the present invention relates to a write-once type organic dye-based optical recording medium having an organic dye recording film with excellent stability and durability.

[0002]

2. Description of the Related Art A so-called write-once type optical recording medium is an optical recording medium of a so-called shape change type recording system in which pits are mainly formed, and a user can freely record and reproduce signals. In this write-once type optical recording medium, other data cannot be written in a portion where data is once recorded, but data can be freely recorded in another portion.

As a structure of such a write-once type optical recording medium, there is known a structure in which a recording film is formed on a circular transparent substrate, a laser light reflecting film is provided on the recording film, and an ultraviolet curable resin is formed as a protective layer. Has been. As a recording film, a metal film such as a chalcogenide-based amorphous material typified by tellurium (Te) has been put into practical use, but recently, in addition to recording properties, productivity is good and toxicity is low, so an organic dye is used. Thin films (especially cyanine dyes) have been studied.

In the optical recording medium having this structure, the recording film is irradiated with a laser beam for recording / reproducing from the back surface of the substrate and reflected by the reflecting film. At the time of recording, the organic dye recording film heats the organic dye in the portion irradiated with the laser beam and decomposes it to form pits. The pits apparently have a lower reflectance than the other parts, and the laser light transmittance is almost zero. Reproduction is performed by utilizing the change in transmittance due to the formation of the pits.

In order to make the write-once type organic dye-based optical recording medium conform to a recordable compact disc (CD-R), the organic dye recording film has a certain degree of absorption for laser light for recording. However, it is necessary that the laser light transmittance be large to some extent and the laser light reflectance be small.

The cyanine dyes conventionally used can satisfy the above requirements to some extent as a recording film, but it is necessary to solve the problems of reproduction deterioration and long-term storage stability.

The causes of reproduction deterioration are the phenomenon that the dye itself deteriorates and discolors due to heat accumulation in the recording film portion due to the exposure of the reading light for a long time, and the transmittance of the reading light causes the pit portion and the non-recording portion. The phenomenon that the recording film surface to be distinguished gradually melts or thermally deforms, and the phenomenon that the contour of the pit collapses due to the same heat accumulation also occurs, which causes a decrease in the SN ratio and jitter. Cause rise in. In addition, the deterioration phenomenon during long-term storage is also caused by changes in the transmittance of the dye due to the oxidation of cyanine dyes and natural light (such as ultraviolet rays), and changes in the transmittance and noise due to the association and aggregation of the dyes due to oxygen and water. Cause to cause.

Various proposals have been made in the past for solving these problems for a recording medium using a cyanine dye (for example, JP-A-62-201288 and JP-A-6-2012).
2-2012289, JP-A-55-22961, JP-A-57-66541, JP-A-59-124894,
JP-A-59-198193, JP-A-59-20324
7, JP-A-62-133173, JP-A-63-13.
783, JP-A-63-198096, JP-A-59-
213039, JP 57-11090 A, JP 5 A
9-124895, JP-A-60-44389, JP-A-60-71296, JP-A-60-257290,
JP-A-63-1594, JP-A-57-11090 and the like) have not yet been fully resolved.

[0009]

Therefore, the present invention provides an organic dye-based recording which has a high laser light transmittance and a high sensitivity and is particularly excellent in long-term stability (improvement of recording / reproduction and long-term storage stability). It is an object of the present invention to provide a CD-compatible write-once organic dye-based optical recording medium having a film.

[0010]

As a result of intensive studies to solve the above problems, the present inventor has mixed a specific cyanine dye (laser light absorbing dye) with a specific amount of a specific light stabilizer to form a recording film. The present invention has been completed by finding that a write-once type organic dye-based optical recording medium for CD, which has less reproduction deterioration and is excellent in long-term stability, can be obtained by forming.

That is, according to the present invention, the first and second
A laser-transmissive circular substrate having a main surface of
Of the laser light absorbing dye represented by the formula (I) represented by the following chemical formula 4 and a ratio of 5 to 25% of the weight of the laser light absorbing dye represented by the following chemical formula (II) And a metal-based laser light reflection film formed on the recording film, and a write-once type optical recording medium.

[0012]

[Chemical 4] (In the formula (I), R ¹ and R ² are each independently an alkyl group, an aryl group (eg, phenyl) or an aralkyl group (eg, phenyl C ₁ -C ₆ alkyl),
L is a methylene chain group consisting of 1 to 3 units-(CH = CQ)-(wherein Q is hydrogen, an alkyl group, a halogen, an alkoxy group or a diphenylamino group) or a formula shown below. A methylene chain group in which any one of the groups represented by (1) to (5) or any one of the groups represented by formulas (1) to (5) represented by the following chemical formula 6 intervenes, A is a carbon atom , Sulfur atom or selenium atom, X
Is an anion (for example, perchlorate, fluoroborate, iodide, chloride, bromide, etc., but absent when forming an internal salt), Y
Are each independently a substituent having 2 to 18 carbon atoms having an unsaturated bond, Z is a hydrogen atom, a carbon atom, or a carbon number 1
To 18 alkyl groups, nitro groups, hydroxyl groups,
Carboxyl ^{_{group, -OCF 3, -R 12 OH,}} -R 12 CO
^{OH, -R 12 R 13, R} 12 COOR 13, -CH = CHC
N, -COOR ¹³ or -COR ¹³ (wherein, R ¹² is an alkylene group having 1 to 20 carbon atoms, R ¹³
Is a C 1-18 alkyl group), or Z is a benzene ring which is condensed with a benzene ring to which it is bonded to form an optionally substituted naphthalene ring, and m is 1 to 4
Integer)

[0013]

Embedded image (In the formula (II), W is a nitro group, a halogen atom,
A hydroxyl group, a carboxyl group, -R ²² COR ²³ , or -R ²² COOR ²³ ; R ²¹ and R ²³ are alkyl groups having 1 to 4 carbon atoms; R ²² is an alkylene group having 1 to 4 carbon atoms)

[0014]

[Chemical 6] (In formulas (1) to (5), R ¹¹ is a hydrogen atom,
Halogen atom, alkyl group having 1 to 6 carbon atoms, or aryl group (for example, phenyl) or diphenylamino group) In a preferred embodiment of the present invention, the recording film is a nickel metal complex compound having absorption in the near infrared. Contains.

It is preferable that two or more kinds of laser light absorbing dyes represented by the formula (I) are present. Further, it is preferable that the recording film contains a dye that suppresses the association of the laser light absorbing dye represented by the formula (I) in a proportion of 20 to 70% of the weight of the laser light absorbing dye.

The substrate usually has pre-formed pre-addresses and pre-grooves, and can be formed of laser-transmissive polycarbonate, polyacryl, polyester, polyolefin, polyepoxy, polyimide or glass.

The reflective film can be formed of a thin film of a metal material selected from the group consisting of aluminum, gold, copper, silver and alloys thereof. Hereinafter, the present invention will be described in more detail.

The cyanine dye used as the laser light absorbing dye in the present invention is represented by the above formula (I). In formula (I), each substituent is as described above, but R ¹ and R ² are present when A is carbon, in which case they are C ₁ -C ₆ alkyl groups. It is preferable. In formula (I), L is more specifically
-(CH = CH) _p -CH =, -CH = C (Q1) -C
H =, -CH = CH-C (Q1) = CH-CH =, -C
H = CH-CH = C (Q1) -CH = CH-CH =, or-(CH = CH) _n- M = (CH-CH) _n = (In these formulas, p is an integer of 1 to 4, Q1 is Q other than hydrogen, M is a group of any of formulas (1) to (5), n is 0, 1
Or 2). Further, as Y, allyl, vinyl, arylidene, allyloxy, crotonoyl, styryl, vinylidene, vinylene, methylidyne, acryloxy, metaallyloxy, isopropenyl,
Each group such as ethynyl and butenyl can be exemplified.

The cyanine dye represented by the formula (I) has appropriate absorption and transmission properties required for laser light. This cyanine dye has good compatibility with the surface of the substrate because the characteristic Y substituent has unsaturation. This cyanine dye is known per se. Preferred specific examples of the laser light absorbing dye represented by the formula (I) are shown in the following chemical formulas 7 to 18 (compounds I-1 to I-
36).

[0020]

[Chemical 7]

[0021]

Embedded image

[0022]

[Chemical 9]

[0023]

[Chemical 10]

[0024]

[Chemical 11]

[0025]

[Chemical 12]

[0026]

[Chemical 13]

[0027]

Embedded image

[0028]

[Chemical 15]

[0029]

Embedded image

[0030]

[Chemical 17]

[0031]

Embedded image

These compounds should be construed not only as specific examples of the compound of the formula (I) but also as examples of each substituent in the formula (I). The laser light absorbing cyanine dyes may be used alone, but by mixing two or more thereof, their spectral characteristics (laser light transmittance,
The absorptance) can be adjusted, and for example, the amount of return light from the reflective film can be adjusted to R _top > 65% as the CD standard. That is, the cyanine dye represented by the formula (I) may agglomerate due to water or the like during long-term storage alone, but by mixing a plurality of these, aggregation and the like can be prevented, and the stability of the recording film can be improved. Is improved and noise is prevented from occurring.

In some cases, the dyes may associate with each other by mixing only the cyanine dye of formula (I). Therefore, in order to prevent such association of the cyanine dye of formula (I) and control the film state and spectral characteristics, the addition of a cyanine dye of a different structure that suppresses the association of the laser light absorbing dye of formula (I). Is preferred. More specifically, such an association-inhibiting cyanine dye has a basic skeleton similar to that of the laser light absorbing dye of formula (I). When using this formula (I), Y is a hydroxyl group or a carboxyl group. A group or an alkyl group which may be substituted with a sulfone group, having the same structure as the definition of the formula (I) (provided that M is the following exemplary compound (a cycloalkyl group represented by III-3). Butenyloxy ring may be used.) A cyanine dye of the type in which an alkyl group or an aromatic group is directly condensed is preferable, and a specific example of such an association-inhibiting cyanine dye is shown in Chemical Formula 19 below (compound III- 1 to III-4).

[0034]

[Chemical 19]

The association-inhibiting cyanine dye is 20 to 70% by weight, preferably 10% by weight, based on the laser light absorbing dye.
It can be added in a proportion of up to 50% by weight. By the way, the cyanine dye (the cyanine dye of the formula (I),
Or a combination of a cyanine dye of formula (I) and an association-inhibiting cyanine dye), long-term storage stability even when a recording film is formed,
It has been found that a sufficient effect cannot be obtained with respect to deterioration in light resistance reproduction.

According to the present invention, the above-mentioned cyanine dye (a cyanine dye of formula (I) or a combination of a cyanine dye of formula (I) and an aggregation-inhibiting cyanine dye) has an absorption wavelength shorter than that of the cyanine dye film. It was found that long-term storage stability and resistance to deterioration by reproduction can be significantly improved by combining with the light stabilizer (ultraviolet absorbing dye) of the above formula (II) at (300 to 500 nm). This formula (I
The light stabilizer represented by I) traps radicals generated by deterioration of the cyanine dye due to ultraviolet rays, laser light, etc., and terminates the chain reaction. As a result, it exerts a remarkable effect in stabilizing the retention of the recording film and preventing the occurrence of cracking. Thus, the recording film of the present invention in which the cyanine dye is blended with the light stabilizer of formula (II) can be stored for a long period of time, and the film flow due to heat generated by light can be suppressed,
It is possible to suppress the generation of noise and the increase in jitter. In the formula (II), the compound in which the substituent W is NO or a halogen group and R is an alkyl group having 1 to 3 carbon atoms has particularly excellent compatibility with the cyanine dye, and the above effects are large. Preferred specific examples of the light stabilizer represented by the formula (II) are shown in the following chemical formula 20 (compounds II-1 to II-6).

[0037]

Embedded image

By adding 5 to 25% by weight, preferably 10 to 20% by weight, to the laser light-absorbing cyanine dye, the above-mentioned light stabilizer can sufficiently obtain the above-mentioned effects, and the dye film (recording Since the reflection of the laser light at the (film) interface can be significantly reduced, the light transmission efficiency of the recording film is improved.

Furthermore, by incorporating a nickel metal complex compound having absorption in the near infrared into the recording film, the excited state of the cyanine dye, which is excited by reproduction light and is about to be decomposed, is electronically attracted and taken into self. Therefore, it was found that the photolysis of the cyanine dye due to the reproduction light can be suppressed. It is said that such a nickel metal complex functions as a quencher and has been conventionally used. However, a 1,2-benzenedithiol nickel complex or the like formed a groove when a recording film was formed. It was found that it is difficult to dissolve in a solvent that does not adversely affect the transparent substrate (for example, alcohol-based, cellosolve-based, etc.), and also has poor compatibility with the laser-absorbing dye and ultraviolet-absorbing dye of the present invention and separates. . Therefore, in the present invention, bis [4- (diethylamino) -α, β-stilbenedithiolato] nickel and bis [3-methoxy-4- (2-methoxyethoxy) -2′-, which do not cause such a problem, are obtained. Chloro-α, β-stilbenedithiolato]
A dithiolato nickel complex represented by nickel is used. The dithiolato nickel complex is preferably used in a weight ratio of about 1: 2 to 2: 1 with respect to the light stabilizer of formula (II).

In order to manufacture the optical recording medium of the present invention, the recording film material of the present invention is a solvent that does not attack the substrate (for example, alcohol, cellosolve, acetonitrile, DMF).
Etc.) and is pre-addressed and pre-grooved on a laser-transmissive substrate (eg polycarbonate, polyacryl, polyester, polyester, polyolefin, polyepoxy, polyimide or glass substrate etc.) About 700 depending on the coating method
It is formed as a uniform recording film of about 3000 Å. The film thickness can be adjusted by the film forming conditions and the concentration of the complex dye solution. A as a reflection film on this recording film
A metal thin film is formed from a metal material selected from 1, Au, Cu, Ag, Pt, Ni and alloys thereof with a thickness of about 500 to 2000 angstroms by a vapor deposition method, a sputtering method or an ion plate method. Further, a resin-based protective layer (for example, an ultraviolet curable resin layer) can be provided thereon in a thickness of 1 to 7 μm.

FIG. 1 shows the basic structure of the write-once type organic dye-based optical recording medium of the present invention produced as described above. As shown in FIG. 1, the recording medium 10 includes a laser-transmissive transparent substrate 11 having a first main surface 10a and a second main surface 10b.
In the surface of the substrate 11, a pre-groove 12 (guide groove) for tracking a laser beam for recording information is formed in advance. The organic dye-based recording film 13 of the present invention is formed on the surface 11a of the substrate 11 including the pregrooves 12. Further, on the recording film 13, a reflective film 14 for amplifying the reflected light amount of the laser light incident from the back surface 10b of the substrate is provided, and a resin protective layer is provided thereon.

The write-once type optical recording medium of the present invention has a reflectance R
_{The top} is 65% or more, the LP-jitter is 35 ns or less, and the blur (bler) is 0.3 or less. And excellent long-term storability.

[0043]

EXAMPLES The present invention will be described below with reference to examples, but the present invention is not limited thereto. In the following examples, the substrate was a polycarbonate (PC) substrate and had a tracking groove with a depth of about 10 nm and a width of about 390 nm formed in advance.

Example 1 Weight ratio of laser light absorbing dye (I-4), laser light absorbing dye (I-2) and light stabilizer (II-3) 3: 2:
A mixture of 0.7 was mixed with solvent A (a mixture of methyl cellosolve: ethanol: diacetoalcohol in a weight ratio of 10:30:20) and solvent B (ethyl cellosolve) in a weight ratio of 6: 4.
Was dissolved in a mixed solvent of 4 at a concentration of 4 g / 100 cc using ultrasonic waves and filtered through a 0.2 μm filter. This solution is dropped on a PC substrate and spin-coated to a thickness of about 300.
A recording film of nm thickness was formed. Gold was formed as a reflection film on the recording film by sputtering to a thickness of 1000 angstroms. A usual UV-curable resin was applied and cured on the gold reflection film to form a protective film having a thickness of 5 μm to obtain a desired optical recording medium.

Example 2 As a dye, a laser light absorbing dye (I-5), a laser light absorbing dye (I-2), an aggregation-inhibiting cyanine dye (III-1)
And and the weight ratio of the light stabilizer (II-2) is 3: 1: 1: 0.
An optical recording medium was obtained in the same manner as in Example 1 except that the mixture of 7 was used.

Example 3 The dye mixture of Example 2 was added to Solvent A of Example 1 at 3.3 g /
It was dissolved by ultrasonication at a concentration of 60 cc. On the other hand, bis [3-methoxy-4- (2-methoxyethoxy) -2 '
-Chloro- [alpha], [beta] -stilbenedithiolato] nickel was similarly dissolved in Solvent B of Example 1. Both solutions obtained were mixed using ultrasonic waves, and an optical recording medium was obtained in the same manner as in Example 1 below.

Example 4 Laser light absorbing dye (I-1), laser light absorbing dye (I-
5), a laser light absorbing dye (I-2), an aggregation-inhibiting cyanine dye (III-2), a light stabilizer (II-2) and bis [3-
Methoxy-4- (2-methoxyethoxy) -2'-chloro-α, β-stilbenedithiolato] nickel in a weight ratio of 1: 0.5: 0.5: 1.5: 1: 0.7. An optical recording medium was obtained in the same manner as in Example 3 except that the recording film was formed.

Example 5 An optical recording medium was obtained in the same manner as in Example 4, except that an aluminum reflecting film was formed instead of the gold reflecting film.

Example 6 Laser light absorbing dye (I-8), laser light absorbing dye (I-
7), a laser light absorbing dye (I-28), an aggregation-inhibiting cyanine dye (III-2), a light stabilizer (II-1) and bis [3
-Methoxy-4- (2-methoxyethoxy) -2'-chloro-α, β-stilbenedithiolato] nickel in a weight ratio of 1: 1.0: 0.5: 0.5: 0.7: 0.7. An optical recording medium was obtained in the same manner as in Example 3, except that the recording film was formed by using the above method.

Example 7 Laser Light Absorption Dye (I-26), Laser Light Absorption Dye (I
-5), a laser light absorbing dye (I-2), an aggregation-inhibiting cyanine dye (III-2), a light stabilizer (II-2) and bis [3
-Methoxy-4- (2-methoxyethoxy) -2'-chloro-α, β-stilbenedithiolato] nickel in a weight ratio of 1: 2: 1: 1: 0.7: 0.7 to form a recording film. An optical recording medium was obtained in the same manner as in Example 3 except that the optical recording medium was formed.

Comparative Example 1 As the dyes, laser light absorbing dyes (I-4) and (I-
Using the mixture of 2) in the weight ratio of 3: 2, except for the recording film,
A comparative optical recording medium was obtained in the same manner as in Example 1.

Comparative Example 2 Cyanine dye (III-1), Cyanine dye (III-
4), light stabilizer (II-2) and bis [3-methoxy-4]
-(2-Methoxyethoxy) -2'-chloro-α, β-
Stilbene dithiolato] nickel in a weight ratio of 3: 2: 0.
Example 3 except that a recording film was formed using 7: 0.7.
An optical recording medium was obtained by a method similar to.

The characteristics of the optical recording media obtained in Examples 1 to 7 and Comparative Examples 1 and 2, that is, the reflectance (R
_top ), jitter (-L, -P), and blur using an optical recording / reproducing evaluation device (recording power: 6 mW, laser wavelength 784 nm, NA = 0.5) for an initial period of time (2 × 10 6 times). ) After regeneration, evaluation was carried out after storage at high temperature and high humidity (80 ° C., relative humidity 80% for 50 hours), and under accelerated light resistance test conditions according to the Orange Book standard. The results are shown in Table 1 below.

[0054]

[Table 1] Further, the spectral curves of the recording media of Examples 2, 3 and 4 and Comparative Example 2 are shown in FIG.

[0055]

As described above, the write-once type optical recording medium of the present invention provided with the recording film in which the laser light absorbing dye and the ultraviolet absorbing dye are composited is used for preventing the optical deterioration of the recording film due to light and heat and the operating environment. The recording film is stabilized by preventing the aggregation and association of the dye molecules underneath, and recording of noise etc.
The environmental resistance stability of the recording film in the reproduction area is improved. The write-once type optical recording medium of the present invention is compatible with CD-R,
It can also be used to make OM.

[Brief description of drawings]

FIG. 1 is a sectional view schematically showing an example of a basic structure of a write-once type optical recording medium of the present invention.

FIG. 2 is a graph showing a spectral curve of the write-once type optical recording medium of the present invention together with that of a comparative example.

[Explanation of symbols]

11 ... Laser light transmitting substrate, 12 ... Pre-groove, 13
... organic dye-based recording film, 14 ... metal-based reflective film, 15 ... protective layer.

Claims (6)

[Claims] 1. A laser light transmitting circular substrate having first and second main surfaces, and laser light represented by the formula (I) shown in the following chemical formula 1 formed on the first main surface of the substrate. A recording film containing an absorbing dye and a light stabilizer represented by the formula (II) represented by the following chemical formula 2 in a proportion of 5 to 25% by weight of the laser light absorbing dye, and a metal formed on the recording film. A write-once type organic dye-based optical recording medium comprising a system laser light reflecting film. Embedded image (In the formula (I), R ¹ and R ² are each independently an alkyl group, an aryl group or an aralkyl group, and L is
1 to 3 units- (CH = CQ)-(wherein Q
Is a methylene chain group consisting of hydrogen, an alkyl group, a halogen, an alkoxy group or a diphenylamino group), or any one group represented by the formulas (1) to (5) shown in the following chemical formula 3 or the following chemical formula 3. Formulas (1) to (5)
A methylene chain group intervening with any one group represented by, A is a carbon atom, a sulfur atom or a selenium atom, and X is
Anions, Y are each independently a substituent having an unsaturated bond and having 2 to 18 carbon atoms, Z is a hydrogen atom, a carbon atom, an alkyl group having 1 to 18 carbon atoms, a nitro group, a hydroxyl group, a carboxyl group, -OCF _3, -R ¹² O
^{H, -R 12 COOH, -R 12} R 13, R 12 COOR 13, -
CH = CHCN, -COOR ¹³ or -COR ¹³ (wherein R ¹² is an alkylene group having 1 to 20 carbon atoms, R ¹³ is an alkyl group having 1 to 18 carbon atoms), or Z is benzene to which it is bonded. A benzene ring which is condensed with a ring to form an optionally substituted naphthalene ring, m is an integer of 1 to 4) (In the formula (II), W is a nitro group, a halogen atom,
A hydroxyl group, a carboxyl group, -R ²² COR ²³ , or -R ²² COOR ²³ ; R ²¹ and R ²³ are alkyl groups having 1 to 4 carbon atoms; R ²² is an alkylene group having 1 to 4 carbon atoms) 3] (In formulas (1) to (5), R ¹¹ is a hydrogen atom,
A halogen atom, an alkyl group having 1 to 6 carbon atoms, or an aryl group). 2. The optical recording medium according to claim 1, wherein the recording film contains a nickel metal complex compound having absorption in the near infrared. 3. The optical recording medium according to claim 1, wherein the recording film contains two or more kinds of laser light absorbing dyes represented by formula (I). 4. The recording film according to claim 1, 2 or 3, further comprising a dye that suppresses association of the laser light absorbing dye represented by the formula (I) in an amount of 20 to 70% of the weight of the laser absorbing dye. Optical recording medium. 5. The substrate has pre-formed pre-addresses and pre-grooves and is made of laser-transmissive polycarbonate, polyacryl, polyester, polyolefin, polyepoxy, polyimide or glass. 5. The optical recording medium according to any one of items 1 to 4. 6. The optical recording medium according to claim 1, wherein the reflective film is a thin film of a metal material selected from the group consisting of aluminum, gold, copper, silver and alloys thereof.