Note: Descriptions are shown in the official language in which they were submitted.
~13~774
RCA 72,387
OPTICAL RECORDING ~DIUM
This invention relates to a novel optical record-
ing medium. More particularly, this invention relates to
an optical recording medium for use with an AlGaAs solid
state injection laser.
BACKGROUND OF THE INVENTION
Spong in U. S. Patent 4,097,895, issued June ~7,
1978, has described a recording medium which comprises a
light reflecting material, such as aluminum or gold, coat-
ed with a light absorbing layer, such as fluorescein
which is operative with an argon laser light source. The
thickness of the light absorbing layer is chosen so that
the structure has minimum reflectivity.
An incident light beam ablates, vaporizes or
melts the light absorbing layer, leaving a hole and ex-
posing the light reflecting layer. After recording, at the
wavelength of the recording light there will be maximun
contrast between the minimum reflectance of the light
absorbing layer and the reflectance of the light reflect-
ing layer. Further, when the light reflecting material is
itself a thin layer on a non-conductive substrate, little
energy is lost either by reflection from the thin absorb-
ing layer or by transmission through the reflecting layer.
Thus, the energy absorbed from the light beam is con-
centrated into a very thin film and recording sensitivity
is surprisingly high.
Because of its low input power requirement, small
size and its capability for direct modulation of its opti-
cal output power by modulation of the electrical drivecurrent, a solid state injection laser, in particular the
AlGaAs laser which operates in the wavelength range from
about 750 to 850 nanometers (nm), is a preferred light
source for an optical recording system. Thus, materials
36 which ablate, vaporize or melt at low temperatures upon
absorption of optical energy in this wavelength range
would be most useful in an optical recording system.
In order to be useful as a light absorbing layer
for the above-described recording medium, materials
must be able to be applied to a substrate in the form of a
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thin, smooth layer of high optical quality and a predeter-
mined thickness; they must be absorbing at the frequency
of the optical source employed; and they must ablate,
vaporize or melt to form uniform holes.
Bloom et al, U. S. Serial No. 837,853, filed
September 29, 1977, have described an ablative optical
recording medium operative with an AlGaAs laser which
comprises a substrate coated with a light reflecting layer,
which in turn is coated with a light absorbing layer
selected from the group consisting of lead phthalocyanine,
chloroaluminum phthalocyanine, vanadyl phthalocyanine,
stannic phthalocyanine and chloroaluminum chlorophthalo-
cyanine.
These materials have the disadvantage, however,
that their ablation temperature is in the range from 300C.
to 400C. Materials with a lower ablation temperature
would require less energy to reach this temperature and
would thus be more sensitive. Thus, materials which
absorb light between 750 nm and 850 nm, form specular,
amorphous films and have a low melting temperature would be
a significant improvement in the art.
SUMMARY OF THE INVENTION
An optical recording medium comprises a light
reflecting layer and a layer which absorbs light having
a wavelength of about 750 nm to about 850 nm comprising
a platinum complex of bis-(dithio-a-diketones) where
the substituents on the ethylenic group are phenyl or
substituted phenyl groups.
BRIEF DESCRIPTION OF THE DRAWINGS
. .
- FIG. 1 is a cross-sectional view of a recording
medium embodying the invention prior to recording.
FIG. 2 is a cross-sectional view of a recording
medium embodying the invention after recording.
FIG. 3 is a schematic view of a recording and
playback system in which the present recording medium can
be employed.
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DETAILED DESCRIPTION OF THE INVENTION
The light reflecting layer should reflect the
light used for recording. Suitable light reflecting
materials include aluminum, rhodium, gold and the like.
The light reflecting material has a thickness such that it
reflects substantially all the recording light.
Generally, the light reflecting layer is applied
to a substrate. The substrate should have an optically
smooth, flat surface to which the subsequently applied
light reflecting layer is adherent. A glass plate or disc
or a plastic disc is suitable. If the light reflecting
material can be formed so it is a self-sustaining layer and
optically smooth, the need for a substrate may be
eliminated.
Materials which we have found useful as a light
absorbing layer in this recording medium are Pt complexes
of bis-(dithio-~-diketones) which have the formula:
C / \
R 5 Pt
where at each occurrence R is a phenyl group or a phenyl
group substituted with alkyl or alkoxy groups such as p-
isopropylphenyl or p-methoxyphenyl. The preparation of
these compounds has been described by Schranzer and ~ayweg
in J. Amer. Chem. Soc., Vol. 87, pp. 1483 - 89 (1965).
These materials absorb at the solid state
injection laser wavelengths between 750 nm and 850 nm and
all can be evaporated onto a light reflecting layer to
give smooth, optical quality light absorbing layers in
which information can be recorded at high signal to
noise ratios.
The materials of the invention can be applied to
the light reflecting layer by conventional vacuum evapor-
ation. The material is charged to a suitable vessel
fitted with a resistance heater and placed in a vacuum
chamber. The heater is then connected to a source of
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electrical current. A substrate is positioned above the
dye on a rotating holder. The substrate is then spun
at about 50 rpm.
The vacuum chamber is evacuated to about 10 6
torr and current is applied to the heater to raise the
temperature of the material to its evaporation temperature.
Evaporation is continued until an absorbing layer of the
required thickness is deposited onto the light reflecting
layer, at which time the electrical current is shut off
and the chamber vented.
The thickness of the evaporated layer is monitored
using an optical system which measures the reflectivity
of the reflecting surface coated with the material. The
evaporation is stopped when the reflectivity reaches its
minimum value.
The invention will be further explained by
reference to the drawings.
FIG. 1 shows a recording medium embodying the
invention prior to exposure to a recording light beam com-
prising a glass substrate 110, a light reflecting layer 112
comprising a layer of gold about 600 angstroms thick or
other metal of suitable thickness, and a light absorbing
layer 114 of one of the above mentioned materials.
FIG. 2 shows a recording medium embodying the
invention after exposure to a recording light beam wherein
the absorbing layer 114 has been ablated to leave a hole
116, exposing the light reflecting layer 112. It will be
understood that a recording medium after recording con-
tains a plurality of holes or pits 116 rather than the
single one shown in FIG. 2.
The use of the present recording medium can be
explained in greater detail by referring to FI5. 3. For
recording, the light emitted by an AlGaAs injection laser
10 is modulated directly in response to an input
electrical signal 14. The modulated light beam is enlarg-
ed by recording optics 16 to increase the diameter of the
intensity modulated laser beam so that it will fill the
aperture of an objective lens 18 in the planes parallel
and perpendicular to the plane of the laser 10. The en-
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larged modulated laser beam is totally reflected by a
polarizing beam splitter 20 and passes through a quarter-
wave plate 22 to the objective lens 18. The modulated
recording beam then impinges upon a recording medium 24,
as described in FIG. 1, and ablates or evaporates a
portion of the light absorbing layer to expose a portion
of the light reflecting layer. Recording medium 24 is
rotated by the turntable drive 26 at about 1800 rpm. A
focus servo 28 maintains a constant distance between the
objective lens 1~ and the surface of the recording medium
24.
For readout, an unmodulated and less intense
laser beam, that is, one that will not cause ablation of
the recording medium, follows the same path as the record-
ing beam to the recording medium 24. The recorded reflec-
tion-antireflection pattern modulates the light reflected
back through the objective lens 18 and the quarter-wave
plate 22. The light, now rotated by 90 in polarization
by the two passages through the quarter-wave plate 22,
passes through the polarizing beam splitter 20 and is
directed by playback optics 30 to a photodetector 32.
The photodetector 32 converts the reflected light beam to -
an electrical output signal at terminal 34 which coxresponds
to the input signal from source 14. A tracking servo 36
monitors the light reflected through the playback optics
30 and deflects the incident light beam radially to in-
sure that the incident light beam remains centered on the
track of interest.
The invention will be further illustrated by the
following Examples but the invention is not meant to be
limited to the details described therein.
EXAMPLE 1
A vinyl disk substrate was coated by evaporating a
layer of gold about 600 angstroms thick. The coated sub-
strate was placed in a vacuum chamber above an evaporation
boat containing the bis(diphenyl-dithio-~-diketone)
complex of Pt and turned at 50 rpm. The vacuum chamber
was evacuated to about 10 6 torr and a source of current
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was connected to the boat. The boat was heated to about
225 - 275C. at which temperature the shutter was opened
and the material evaporated at a rate of about 4 angstroms
per second. Evaporation was continued until an absorbing
layer about 600 angstroms thick was deposited over the gold
layer.
A smooth, amorphous, specular and continuous layer
was deposited. The real (n) and imaginary (k) parts of
the dielectric constant of the Pt bis(diphenyl-dithio-~-
diketone) layer at 800 nm are 2.08 and 0.5 res~ectively.
The resultant recording medium was exposed to a
series of 50 nanosecond light pulses having a wavelength
of about 800 nm from an AlGaAs injection laser in an
apparatus as in FIG. 3. The absorbing material was ablated
from the disk by multiple exposure with 10 milliwatts
incident on the disk.
EXAMPLE II
Following the general procedure of Example I,
a gold coated substrate as in Example I was coated with a
layer of the bis(di-p-isopropylphenyl-dithio-a-diketone)
complex of Pt 1324 angstroms thick. The real (n) and
imaginary (k) parts of the dielectric constant of the Pt
bis(di-p-isopropylphenyl-dithio-a-diketone) layer at 800
nm are 1.75 and 0.78 respectively.
A smooth, amorphous, specular and continuous layer
was deposited.
EXAMPLE III
Following the general procedure of Example I,
a gold coated substrate as in Example I was coated with a
layer of the bis(di-p-methoxyphenyl-dithio-~-diketone)
complex of Pt 1900 angstroms thick. The real (n) and
imaginary (k) parts of the dielectric constant of the Pt
bis(di-p-methoxyphenyl-dithio-~-diketone) layer at 800
nm are 1.61 and 0.76 respectively.
~ ~ smooth, amorphous, specular and continuous layer
- was deposited. This film reduced the reflectivity of the
recording medium to about 7% from about 95% reflectivity
of the gold layer before the absorbing layer was added.
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COMPARATIVE EXAMPLES
Following the general procedure of Example I,
gold coated vinyl disks were coated with bis(dithio-~-
diketone) complexes of Pt, Pd, or Ni with the same ordiffering substituted aryl or alkyl groups. The complexes
belong to the class having the formula
~ ~ C / 1`
LR / ; S~
wherein M can be Pt, Pd or Ni and Rl and R2 can be alkyl,
phenyl, or alkyl- or alkoxy-substituted phenyl. These
materials produced either non-specular films or films with
a poor antireflection condition upon evaporation. In al-
most all cases the films were initially hazy and proceeded
to crystallize in a period of days. None of these dyes
proved suitable for the present application. The data
are summarized in Table I
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