Optical recording medium

A recordable optical recording medium having a sufficiently high reflectance and modulation factor to comply with the CD specifications. The light-absorbing layer in the optical recording medium has a first cyanine dye having a light absorption band in the wavelength region of recording or reproducing light, and a second cyanine dye having a light absorption band in the wavelength range shorter than the first cyanine dye and having a smaller light absorption in the wavelength region of recording or reproducing light. The second cyanine dye is contained in the light-absorbing layer in an amount larger than the first cyanine dye.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a recordable optical recording medium and, 
more particularly, to an optical recording medium having a light-absorbing 
and a light-reflecting layer formed in superposition on a 
light-transparent substrate. 2. Description of the Related Art 
The use organic dyes, such as cyanine and phthalocyanine dyes, in the 
recording membrane of "recordable" optical recording media is generally 
known. In order to write information on such optical recording media, a 
laser beam is focused in a small area of the recording membrane and 
converted to thermal energy which changes the characteristics of the 
recording layer (i.e., forming a pit). The common practice adopted to 
insure a smooth change in the properties of the recording membrane is to 
prepare two substrates each bearing a recording membrane and these 
substrates are combined together with the two recording membranes facing 
each other, thereby providing a so-called "air sandwich" structure. 
The laser beam used to write data on these type of optical recording media 
is aimed into the outer face of each transparent substrate so as to form 
an optically readable pit in either or both of the recording membranes. In 
order to reproduce the recorded data, a reading laser beam having a lower 
power than the writing beam is focused onto the pit surface and the 
contrast between the area where the pit is formed and the area where no 
pits are formed is read as an electric signal. 
Media of the read-only memory (ROM) type having prerecorded data are also 
available and have been commercially used in the audio recording and 
information processing fields. The ROM media have no recording membrane 
into which data can be recorded. Stated more specifically, prepits which 
correspond to the data to be reproduced are preformed in the surface of a 
plastic substrate by press forming with a master. Then, a reflecting layer 
made of a metal such as Au, Ag, Cu or Al is formed over the prepits and 
then a protective layer typically is formed on the reflecting layer. 
A typical class of ROM media are compact disks which are commonly referred 
to as CDs. Information is written into and read from CDs with signals of 
standardized specifications. In accordance with such specifications, CD 
reproducing apparatus are widely used as compact disk players (CD 
players). 
The so-called recordable optical recording media are the same as CDs 
insofar as a laser beam is used for reading/writing and that both media 
are in a disk form. It has been strongly desired to develop a recordable 
medium that complies with the ROM or CD specifications and, as a result, 
is adapted for use in CD players. However, a problem is encountered in 
attempts to achieve this object. 
In particular, if the air sandwich structure, which is widely adopted in 
conventional structures of recordable media, is merely replaced by the 
provision of a light-reflecting layer on the pitted surface of the 
conventional recordable recording membrane, as needed in a ROM or CD 
structure, the reflectance and modulation factor of a laser beam cannot be 
made sufficiently high to satisfy the CD specifications. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a recordable optical 
recording medium which has a sufficiently high reflectance and modulation 
factor to comply with the CD specifications. 
It has been discovered that this object and other objects of the present 
invention can be attained by providing an optical recording medium 
comprising a light-transparent substrate having a light-absorbing layer 
which, in turn, is overlaid with a light-reflecting layer, wherein the 
light-absorbing layer contains a first cyanine dye and a second cyanine 
dye. Moreover, the first cyanine dye is selected having a light absorption 
band in a wavelength region of recording or reproducing light, and the 
second cyanine dye is selected having a light absorption band in a 
wavelength range shorter than the first cyanine dye and which has smaller 
light absorption in the wavelength region of recording or reproducing 
light. Additionally, the second cyanine dye is present in the 
light-absorbing layer in an amount larger than the amount of first cyanine 
dye in the light-absorbing layer. 
In the present invention, it is desired to record or reproduce information 
with light having a wavelength range of about 780-830 nm. Therefore, the 
first cyanine dye to be incorporated in the light-absorbing layer 
preferably has the following general formula (I): 
##STR1## 
wherein R.sub.1 and R.sub.2 are each an alkyl group having 1-8, preferably 
3-5, carbon atoms. X.sup.- represents a counterion as exemplified by 
ClO.sub.4, I.sup.- or Br.sup.-. 
The second cyanine dye which is also preferably incorporated into the 
light-absorbing layer preferably has the following general formula (II): 
##STR2## 
wherein R.sub.3 and R.sub.4 are each an alkyl group having 1-8, preferably 
3-5, alkyl groups. X.sup.- represents a counterion and may be exemplified 
by the same ions as already mentioned above for this coefficient. In 
keeping with the broader concepts of the invention, the second cyanine dye 
of the general formula (II) is present in the light absorbing layer in a 
greater amount than the first cyanine dye of the general formula (I). 
In another embodiment of the present invention, a quencher compound is 
preferably added to the light absorbing layer for the purpose of 
preventing deterioration of the cyanine dyes upon exposure to light during 
handling. 
The invention itself, both as to its construction and its method of 
fabrication, together with additional objects and advantages thereof, will 
be better understood from the following description of preferred 
embodiments of the present invention when considered in conjunction with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, the optical recording medium 1 of the present invention 
generally comprises a light-transparent substrate 11 that is overlaid with 
a light-absorbing layer 12, a light-reflecting layer 13 and a protective 
layer 14, in that sequence. 
From the viewpoint of high production rate, the light-transparent substrate 
11 is preferably made by injection molding transparent resins such as 
polycarbonate resins (PC) and polymethyl methacrylate resins (PMMA). The 
substrate 11 is typically formed in a thickness of about 1.0-1.5 mm. 
The substrate 11 is overlaid with the light-absorbing layer 12 which 
contains a first cyanine dye selected having a light absorption in the 
wavelength region of recording or reproducing light, and a second cyanine 
dye is selected having a light absorption band in the wavelength range 
shorter than said first cyanine dye and having smaller light absorption in 
the wavelength region of recording or reproducing light. The amount of 
second cyanine dye in the light-absorbing layer is provided to be larger 
than the amount of first cyanine dye in the light-absorbing layer. 
In the present invention, information is generally recorded or reproduced 
with light having a wavelength range of 780-830 nm. Therefore, the first 
cyanine dye to be incorporated in the light-absorbing layer 12 preferably 
has the following general formula (I): 
##STR3## 
wherein R.sub.1 and R.sub.2 are each an alkyl group having 1-8, preferably 
3-5, carbon atoms. If the number of carbon atoms in R.sub.1 or R.sub.2 
exceeds 8, accelerated deterioration will occur in a test under hot and 
humid conditions. Furthermore, the dye will become waxy, causing 
inconvenience in handling. 
In the general formula (I), X.sup.- represents a counterion, as 
exemplified by ClO.sub.4, I.sup.- or Br.sup.-. 
The second cyanine dye which is also to be incorporated in the 
light-absorbing layer 12 preferably has the following general formula 
(II): 
##STR4## 
wherein R.sub.3 and R.sub.4 are each an alkyl group having 1-8, preferably 
3-5, carbon atoms. If the number of carbon atoms in R.sub.3 or R.sub.4 
exceeds 8, the same disadvantages as described in connection with the 
general formula (I) will occur. 
In the general formula (II), X.sup.- represents a counterion and may be 
exemplified by the same counterions as already mentioned above. 
The second cyanine dye of the general formula (II) is contained in a 
greater amount than the first cyanine dye of the general formula (I) and 
the preferred weight ratio of the first to the second cyanine dye is in 
the range of from 1:1.5 to 1:3. If the second cyanine dye is contained in 
an amount that is equal to or smaller than the first cyanine dye, it is 
impossible to achieve the necessary reflectance or, as another possible 
disadvantage which might occur, the thickness of the dye film cannot be 
selected at such a value as to achieve a good output between the push-pull 
tracking error and the reflectance. 
The light-absorbing layer 12 containing the first and second cyanine dyes 
is coated by a conventional means such as spin coating. To coat the light 
absorbing layer, the concentration of a mixture of the first and second 
cyanine dyes in solution is in the range of 0.01-0.20 mol/l, with the 
range of 0.04-0.12 mol/l being more preferred. If the concentration of the 
dye mixture is less than 0.01 mol/l, the absorption sensitivity that can 
be achieved is too low to perform signal recording with a semiconductor 
laser. If the concentration of the dye mixture exceeds 0.20 mol/l, the 
dyes will not readily dissolve in solvents. 
The light-absorbing layer 12 is coated in a thickness that preferably 
ranges from about 30 to 900 nm, with the range of 100-300 nm being more 
preferred. If the thickness of the layer 12 is less than 30 nm, the light 
absorption will decrease to the extent that the sensitivity to light in 
the operating wavelength of a semiconductor laser is too low to achieve 
satisfactory signal recording. If the thickness of the layer 12 exceeds 
900 nm, the dye layer becomes so thick as to cause increased absorption 
and hence lower reflectance. 
Various known solvents can be used for coating the absorption layer 12, as 
exemplified by diacetone alcohol, ethyl cellosolve, methanol, 
tetrafluoropropanol, and the like. 
In a still another preferred embodiment of the present invention, a 
quencher is preferably contained in the light-absorbing layer 12 for the 
purpose of preventing deterioration of the cyanine dyes upon exposure to 
light. 
Representative examples of preferred quenchers are shown below as compounds 
(Q-1) to (Q-4): 
##STR5## 
Among these quenchers, the one represented by the structural formula (Q-1) 
is particularly preferred for the purpose of preventing the deterioration 
of the cyanine dyes upon exposure to light. 
The light-absorbing layer 12 is then overlaid with the light-reflecting 
layer 13, which is composed of a metal film such as Au, Ag, Cu or Al. 
These metal films can be formed by deposition via a suitable technique 
such as vacuum evaporation, sputtering or ion plating. The 
light-reflecting layer 13 preferably has a thickness of about 0.02-2.0 
.mu.m. 
The light-reflecting layer 13 is usually overlaid with the protective layer 
14 for protecting the light-absorbing layer 12 and the light-absorbing 
layer 13. The protective layer 14 is typically formed by a process that 
consists of spin coating a uv curable resin and then curing it under 
exposure to uv radiation. Other materials that can be used to form the 
protective layer 14 include epoxy resins, acrylic resins, silicone resins 
and urethane resins. The protective layer 14 typically has a thickness of 
around 0.1-100 .mu.m. 
An intermediate layer may be provided between the substrate 11 and the 
light-absorbing layer 12 in order to protect the substrate 11 from a 
coating solvent used in the light-absorbing layer. If necessary, an 
intermediate layer may be provided between the light-absorbing layer 12 
and the light-reflecting layer 13 in order to enhance the efficiency of 
light absorption. 
Recording light is applied to the optical recording medium of the present 
invention as the medium rotates, whereupon a part of the light-absorbing 
layer 12 which is struck by the light is melted away to form a pit. To 
reproduce the recorded data, reading light is applied to the medium as it 
rotates and a difference is detected between the intensity of the light 
reflected from the area where the pit is formed and that of the light 
reflected from the area where no pits are formed and the difference is 
read as an electric signal. 
The present invention is described below in greater detail by reference to 
the following nonlimiting examples. 
EXAMPLES 
Dyes (D-1) and (D-2) identified below were used as the first and second 
cyanine dyes, respectively, to be contained in the light-absorbing layer. 
These dyes were dissolved in ethyl cellosolve and the solution was coated 
in a thickness of 250 nm onto a polycarbonate substrate having a diameter 
120 mm and a thickness of 1.2 mm. A spiral groove was preliminarily formed 
on the substrate by injection molding. 
A gold (Au) light-reflecting layer was deposited in a thickness of 0.1 
.mu.m on the light-absorbing layer by vacuum evaporation. A photopolymer 
protective film was then formed on the light-reflecting layer. 
By this basic procedure, various samples of optical recording medium were 
fabricated, with the mixing weight ratio of the first cyanine dye to the 
second cyanine dye being varied at 1:1, 2:3 and 1:3 and with the 
concentration of the mixture of the cyanine dyes in solution being varied 
over the range of 0.01-0.16 mol/l. 
First cyanine dye (D-1) 
Dye of the general formula (I) wherein R.sub.1, and R.sub.2 are each 
n-C.sub.4 H.sub.9 and X.sup.- is ClO.sub.4.sup.-. The absorbance 
characteristic of the dye (D-1) is depicted in FIG. 6 showing the dye has 
a light absorption band at 580-720 nm. 
Second cyanine dye (D-2) 
Dye of the general formula (II) wherein R.sub.3 and R.sub.4 are each 
n-C.sub.3 H.sub.7 and X.sup.- is ClO.sub.4.sup.-. The absorbance 
characteristic of the dye (D-2) is depicted in FIG. 7 showing the dye has 
a light absorption band at 540-680 nm. 
EFM signals were recorded and reproduced from the respective media samples 
under the following conditions. 
EFM signal record/reproduce conditions 
Wavelength: 778 nm 
Linear speed: 1.4 m/s 
Write power: 6.0 mW 
Read power: 0.5 mW 
For each of the media with which recording was conducted under the 
conditions specified above, the following eight potentials were measured: 
potential in the specular portion, I.sub.O ; potential in the land, 
I.sub.l ; potential in the groove, I.sub.g ; potential in the brightest 
portion of recorded signals that had an amplitude of 11 T (196 kHz) , 
I.sub.top ; potential of recorded signals that had an amplitude of 11 T 
(196 kHz), I.sub.11T ; potential of recorded signals that had an amplitude 
of 3 T (720 kHz), I.sub.3T ; push-pull tracking error potential, TE.sub.pp 
; and three-beam tracking error potential, TE.sub.3b. 
The results of potential measurements are shown in FIGS. 3(a)-3(c), FIGS. 
4(a)-4(d) and FIGS. 5(a)-5(d). 
FIGS. 3(a)-(c) are a set of graphs showing the results of potential 
measurements on the samples in which the mixing weight ratio of the first 
to the second cyanine dye was fixed at 1:1 whereas the concentration of 
the mixture of the two cyanine dyes in solution was varied over the range 
of 0.015-0.12 mol/l. Stated more specifically, FIG. 3(a) is a graph 
showing the profiles of specular potential I.sub.l and groove potential 
I.sub.g ; FIG. 3(b) is a graph showing the profile of push-pull tracking 
error potential TE.sub.pp ; and FIG. 3(c) is a graph showing the profiles 
of I.sub.top, I.sub.11T and I.sub.3T. 
FIGS. 4(a)-(d) are a set of graphs showing the results of potential 
measurements on the samples in which the mixing weight ratio of the first 
to the second cyanine dye was fixed at 2:3 whereas the concentration of 
the mixture of the two cyanine dyes in solution was varied over the range 
of 0.04-0.10 mol/l. Stated more specifically, FIG. 4(a) is a graph showing 
the profiles of specular potential I.sub.O and groove potential I.sub.g ; 
FIG. 4(b) is a graph showing the profile of push-pull tracking error 
potential TE.sub.pp ; FIG. 4(c) is a graph showing the profiles of 
I.sub.top, I.sub.11T and I.sub.3T ; and FIG. 4(d) is a graph showing the 
profile of three-beam tracking error potential TE.sub.3b. 
FIGS. 5(a)-(d) are a set of graphs showing the results of potential 
measurements on the samples in which the mixing weight ratio of the first 
to the second cyanine dye was fixed at 1:3 whereas the concentration of 
the mixture of the two cyanine dyes in solution was varied over the range 
of 0.075-0.155 mol/l. Stated more specifically, FIG. 5(a) is a graph 
showing the profiles of specular potential I.sub.O and groove potential 
I.sub.g ; FIG. 5(b) is a graph showing the profile of push-pull tracking 
error TE.sub.pp ; FIG. 5(c) is a graph showing the profiles of I.sub.top, 
I.sub.11T and I.sub.3T ; and FIG. 5(d) is a graph showing the profile of 
three-beam tracking error potential TE.sub.3b. 
Evaluating the respective samples on the basis of the results shown in the 
graphs in FIGS. 3-5, one can see that the following conditions should be 
satisfied in order to reproduce signals in accordance with the CD format: 
(1) I.sub.l &gt;I.sub.g &gt; and I.sub.O &gt;I.sub.g ; 
(2) push-pull tracking error TE.sub.pp must be at least 0.04V in order to 
perform tracking servo control in a consistent manner and to thereby 
insure reliable recording; (3) since a reflectance of at least 65% is 
necessary, I.sub.top must be at least 0.45V under the conditions for 
recording and reproduction set forth above and, at the same time, the 
ratio I.sub.11T /I.sub.top must be at least 60% whereas the ratio I.sub.3T 
/I.sub.top must be in the range of 30-70% in order to insure adequate 
reflectance and high modulation factor; and 
(4) the three-beam tracking error potential TE.sub.3b must be at least 2.5V 
which is comparable to the TE.sub.3b of commercial CDs and this is in 
order to insure that recorded signals (pits) are reproduced in a 
consistent manner on CD players. 
The graphs in FIGS. 3-5 show that the first condition to be met in order to 
satisfy all of the above-listed requirements (1)-(4) is that the second 
cyanine dye (D-2) be contained in a greater amount than the first cyanine 
dye (D-1) in the light absorbing layer, e.g. (D-1)/(D-2)=2:3 or 1:3 as 
shown in FIGS. 4 or 5. 
The second condition to be met in order to satisfy the above-listed 
requirements (1)-(4) is that the concentration of the two cyanine dyes 
(D-1) and (D-2) in solution be within the range of 0.08-0.09 mol/l for the 
combination of dyes (D-1) and (D-2). According to FIG. 5, the 
concentration range in the neighborhood of 0.15 mol/l also appears to be 
appropriate. However, but in this range, the concentration of the mixture 
of two dyes is so high that considerable difficulty is involved in forming 
a uniform solution. As a consequence, even if a solution forms, 
precipitation will ensue rather quickly thereafter on account of the high 
concentration of the dye mixture, causing a substantial practical problem. 
It was verified that optimal electric characteristics could be attained by 
the above specified combination of dyes and at the stated thickness of the 
dye film. When additional experiments were performed with the same 
protocol as above except with the thickness of the dye film being adjusted 
to various values other than 250 nm, it was found that the concentration 
of the dyes in solution was by no mean limited to a very narrow range. 
Instead, it was observed that the optimal dye concentration range will 
vary within the broader range of 0.01 to 0.20 mol/l partially as a 
function of the dye film thickness. 
As is clear from the foregoing description, the optical recording medium of 
the present invention, which can contain the exemplified cyanine dyes in 
the exemplified amounts in the light-absorbing layer, represents a 
recordable optical recording medium that achieves a sufficiently high 
reflectance and modulation factor to comply with the CD specifications. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.