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BACKGROUND OF THE INVENTION
The present invention relates to an optical memory which is capable of
storing and retrieving information by light exposure. More specifically,
the invention relates to an optical recording disc comprising a recording
layer that has optical property which changes when the layer is irradiated
with a laser.
For storing and retrieving information, an optical disc comprising a layer
or film of semiconductor materials is widely used in industrial and
consumer recording systems such as video recorders, digital audio
recorders and document digital recorders. A signal is recorded on the
optical disc by applying a laser beam so that bits of information are
formed in a layer of semiconductor materials. The optical disc permits
direct read after write and random access to the recorded information.
Generally, a layer of semiconductor material, such as Se, Ge, Te and InSb,
is well-known for its ability to assume two stable states, amorphous and
crystalline. The concept of an optical memory based on the
amorphous-to-crystalline transition of the semiconductor materials
mentioned above is disclosed by S. R. Ovshinsky et al, "Reversible
Structural Transformations in Amorphous Semiconductors for Memory and
Logic," METALLURGICAL TRANS. 2:641-45 (1971). These semiconductor
materials, however, are chemically unstable and are gradually corroded in
the air when they are made into a thin film. Therefore, they are not used
in practice as the recording film of an optical disc memory.
Since 1971, substantial effort has been expended to produce a durable
recording film of semiconductor materials. For example, in "Reversible
Optical Recording in Trilayer Structures" APPL. PHYS. LETT. 38:920-921
(1981), A. E. Bell et al disclose an erasable optical recording medium
based on the amorphous-to-crystalline transition of pure tellurium. The
optical recording disc of Bell et al includes silicon dioxide capping
layers formed on the both sides of the tellurium film to protect the pure
tellurium film from corrosion and to prevent the tellurium from
evaporating. In the resulting trilayer structures, the thickness of each
layer must be precisely controlled, making the film forming-process unduly
complicated.
Another example of an optical disc is reported by M. Takenaga in
Proceedings of the 116th Study Meeting of No. 131 Film Conference of the
Japan Society for the Promotion of Science, May 20, 1983, at pp. 21-26.
This optical disc is based on the reversible transition of a tellurium
sub-oxide thin film which is formed as a deposition layer by simultaneous
evaporation of TeO.sub.2 and Te that contains Ge or Sn as an impurity.
Therefore, it is difficult to control the quality of the film produced via
the disclosed method, because the method includes decomposing unstable
TeO.sub.2 at a high temperature. Moreover, the tellurium sub-oxide thin
film had a low reflectivity (about 15%) and a low rate of reflectivity
change (about 12%), resulting in a low signal-to-noise (SN) ratio.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an
improved optical memory wherein data can be optically stored and
retrieved.
It is a further object of the present invention to provide an optical
memory which is excellent in durability and has a high SN ratio.
It is still a further object of the present invention to provide a simple
film-forming process for producing an optical memory as described above.
In accordance with the present invention, the foregoing objects, among
others, are achieved by providing an optical memory comprising a substrate
and a recording layer, supported on the substrate, which comprises a
chemically stable dielectric material and a semiconductor material capable
of undergoing a change in optical reflectivity when exposed to light.
In accordance with another aspect of the present invention, the
above-stated objects are achieved by providing a method of storing and
retrieving information comprising the steps of (a) providing a recording
layer comprising a chemically stable dielectric material and a
semiconductor material capable of undergoing a change in optical
reflectivity when exposed to light, and (b) applying a light beam to
selected portions of the layer to cause a structural change in the
semiconductor material in the selected layer portions, whereby optical
reflectivity in those portions is altered. In a preferred embodiment, the
aforesaid method further comprises after step (b) the step of optically
detecting the selected structural change in the selected portions of the
recording layer.
There has also been provided, in accordance with yet another aspect of the
present invention, a method for producing an optical memory element,
comprising the step of co-evaporating a semiconductor material and a
dielectric material onto a transparent substrate in a high vacuum to form
on the substrate a recording layer comprising the semiconductor material
and the dielectric material, the semiconductor material being capable of
undergoing a change in optical reflectivity when exposed to light.
Other objects, features, and advantages of the present invention will
become apparent from the following detailed description. It should be
understood, however, that the detailed description and specific examples,
while indicating preferred embodiments of the invention, are given by way
of illustration only, since various changes and modifications within the
spirit and scope of the invention will become apparent to those skilled in
the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the present invention and many of its
attendant advantages will be readily obtained by reference to the
following detailed description considered in connection with the
accompanying drawings, in which:
FIGS. 1 through 3 each provide a sectional view, respectively, of a
different optical disc memory according to the present invention.
FIG. 4 is a schematic diagram depicting a co-evaporation system for use in
preparing an optical disc memory with the present invention;
FIGS. 5, 9 and 10 are graphs showing, respectively, the relationship
between the rate of reflectivity change and the thickness of recording
films of the present invention;
FIGS. 6 and 8 are graphs both showing the relationship between the
reflectivity and the thickness of recording films of the present
invention;
FIG. 7 is a graph showing the relationship between the reflectivity of a
recording film of the present invention and exposure time; and
FIG. 11 is a schematic diagram of an optical disc memory system within the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to FIGS. 1, 2 and 3, optical disc memories are generally
designated 10, 12 and 14, respectively. These optical discs can be used as
a computer memory medium, an image recording medium for an image
information filing system, and the like. The optical disc memories
comprise a transparent substrate 16, 16' or 16", respectively. Substrates
16, 16' and 16" are formed from acrylic resins or polycarbonate resins,
and each takes the form of a disc having a diameter of about 130 mm and a
thickness of about 1.2 mm, which disc carries grooves 18 and can be
rotated. Grooves 18, as shown in FIG. 1, are formed on the surface of
substrate 16, 16' or 16"; they are about 0.07 /.mu.m deep and about 0.8
/.mu.m wide. Similar grooves, not shown, are also provided on the surface
of substrate 16' or 16". Grooves 18 are used for optical tracking, in that
portions to record or erase information are guided as a recording track by
the grooves.
The optical disc has a recording layer or film 20, 20' or 20". The
recording film comprises a chemically stable dielectric member 24, 24' or
24" wherein are dispersed discrete regions ("particles") of semiconductor
material having a complex refractive index expressed as (n-ik), where n
and k are real numbers. Generally, the refractive index of a transparent
material such as a glass plate is defined by reference to a refraction
angle. But the case of a material that absorbs light, such as a metallic
layer, the refractive index is expressed in terms of complex number
(n-nk), where n represents the difference between the speed of light in a
vacuum and in the metallic layer, respectively, and k represents the
absorption of light in the metallic layer. As disclosed by Stuke, "Review
of Optical and Electrical Properties of Amorphous Semiconductors," J.
NON-CRYSTALLINE SOLIDS 4:1-26, and described in greater detail below, the
complex refractive index, and hence the optical reflectivity, of the
semiconductor material changes as a function of a structural transition
within the semiconductor particles of the recording layer.
The semiconductor material used in the present invention preferably has a
fractional volume factor of about 40% or more, where fractional volume
factor (q) is defined as the ratio of the volume of the semiconductor
material in the recording layer to the total volume of the recording
layer. Also, in the present description "chemically stable dielectric
member" denotes a dielectric material that is effectively unreactive with
the aforesaid semiconductor material and does not interfere with a
light-modulated phase transistion within the semiconductor particles, as
further detailed below.
In accordance with the present invention, a light beam is employed to heat
semiconductor particles in selected regions of the recording layer. For
example, recording film 20, 20' or 20" can be locally irradiated by a
laser beam L for a certain time period T, the semiconductor particles 22,
22' or 22" are heated to a certain temperature t which is proportional to
the energy density of laser beam L. After exposure to the laser beam is
terminated, the heated semiconductor particles are cooled gradually, at a
cooling speed of t/2T, as heat is lost from the particles by thermal
conduction to the surrounding dielectric member. Although coherent light
is used in the preceding example, incoherent light can be employed so long
as beam intensity is sufficient to heat the semiconductor particles and
effect the structural transition described below.
The semiconductor particles in the irradiated portions of the recording
film are thus cooled quickly when heated by irradiation with a high power
density beam for a short time and, conversely, are cooled slowly when
heated by an irradiating beam of a lower power density for a longer time.
Moreover, the semiconductor particles become amorphous in structure when
they are heated by irradiation with a high power density beam for a short
time and cooled quickly. Conversely, they enter a crystalline phase when
they are heated by irradiation with a lower power density beam for a
longer time and cooled slowly. The use of semiconductor particles for
reversible recording is based on a transition in the structure of the
particles which occurs when the particles are heated and, as a
consequence, have different optical properties (i.e., reflectivity)
characterized by the complex refractive index. More specifically,
semiconductor particles 22, 22' or 22" contained in recording layer 20,
20' or 20" can undergo a temperature-dependent structural transition,
governed by the selection of the exposure time and power density of laser
beam L, between amorphous and crystalline phases which have differing
complex refractive index. The reflectivity characterized by the complex
refractive index of the irradiated portions in recording layer 20, 20' or
20" can be altered and information thereby recorded (or erased) as the
localized change in the optical reflectivity of the recording layer.
A particularly preferred optical thickness for recording film 20, 20' or
20" is less than about one-half of the wave length of the applied laser
beam, so that the intensity light reflected from the recording layer is
enhanced by constructive interference between reflected rays R and R'. By
virtue of this interference effect, recording film 20, 20' or 20"
maintains a high reflectivity after recording and erasing information, and
focusing, tracking and information signals are obtained with a high SN
ratio. In this embodiment of the present invention, recording film 20, 20'
or 20" comprises semiconductor particles 22, 22' or 22" dispersed in
chemically stable dielectric member 24, 24' or 24", so that (1)
semiconductor particles can easily change in structure from amorphous to
crystalline phase, or vice versa, and (2) disc durability, which is an
important feature for the optical disc memory of the present invention, is
extremely improved.
In another embodiment of the present invention, the surface of optical disc
memory 10, 12 and 14 is covered by a protective layer 26, 26' or 26",
respectively, and thereby stabilized. This embodiment is particularly
desirable because of its durability. The protective layer 26, 26' or 26"
is fabricated by coating the surface of the optical disc memory with
UV-curable resins and then curing the resins by ultraviolet irradiation.
With reference to FIG. 1, optical disc memory 10 comprises substrate 16, an
overlying recording layer 20, and a protective layer 26 adjacent to the
recording layer. Laser beam L is applied to recording layer 20 through
transparent substrate 16 and reflected at both boundaries of the recording
layer, so that the reflected light R is changed in its intensity according
to the reflectivity of the recording layer.
With reference to FIG. 2, optical disc memory 12 comprises substrate 16',
recording layer 20' formed on substrate 16', metallic layer 28 formed on
recording layer 20' and protective layer 26' formed on metallic layer 28.
Metallic layer 28 is comprised of Cu and has a high reflectivity.
Recording layer 20' is irradiated by laser beam L through transparent
substrate 16'. The impinging laser light is reflected at the surface of
recording layer 20' and the surface of metallic layer 28, respectively,
and the intensity of reflected light is changed as a function of the
reflectivity of the recording layer. In this embodiment, metallic layer 28
is particularly desirable because of its high reflectivity. Such a
structure assures a reading operation characterized by a high SN ratio,
since constructive interference between reflected rays R, R' and R"
enhances signal intensity.
With reference to FIG. 3, optical disc memory 14 has a semitransparent
metallic layer 30 comprised of Cu which is interposed between substrate
16" and recording layer 20". The optical disc memory also has a metallic
layer 28' which is similar to metallic layer 28 shown in FIG. 2. Thus, in
this embodiment optical disc memory 14 comprises substrate 16",
semitransparent metallic layer 30 formed on substrate 16", recording layer
20" formed on semitransparent metallic layer 30, metallic layer 28' formed
on recording layer 20" and protective layer 26" formed on metallic layer
28'. Recording layer 20" is exposed to laser beam L through substrate 16"
and semitransparent metallic layer 30, so that the light R is reflected at
the surfaces of semitransparent metallic layer 30, recording layer 20" and
metallic layer 28', respectively. As a result, the reflected light is
changed in its intensity according to the reflectivity of recording layer
20". This multilayer structure enables the intensity of the reflected
light R to be twice that of optical disc 12 shown in FIG. 2, because of an
interference effect involving rays reflected, respectively, by the
multiple layers.
Semiconductor materials such as Ge, Te, Se, and InSb are well-known for
their ability to assume two different phases (amorphous and crystalline)
according to the manner of heating. Any semiconductor capable of a
temperature-sensitive amorphous-to-crystalline transition can be used in
the present invention. Moreover, dielectric materials that are suitable
for the present invention must have sufficient chemical stability, as
defined above, to prevent semiconductor materials from evaporating and
corroding. For example, oxides, such as B.sub.2 O.sub.3, Sb.sub.2 O.sub.3,
PbO, SiO.sub.2 and Ta.sub.2 O.sub.5, and fluorides, such as BiF.sub.3,
LiF, PbF.sub.2, MgF.sub.2, BaF.sub.2 and CaF.sub.2, are suitable for the
dielectric member of the present invention.
The fractional volume factor of semiconductor particles in the present
invention is preferably more than about 40% of the mixture of
semiconductor materials and dielectric materials, and particularly is in
the range of about 40 to 80%, to obtain sufficient reflectivity and
durability for the optical disc memory. By preparing the mixture mentioned
above, the recording layer works satisfactorily as an optical interference
film although semiconductor materials generally have a high absorbancy.
Methods based on vacuum evaporation, sputtering and the like can be used to
prepare both the recording layer and the metallic layer of the present
invention. For example, a multilayered structure as described above is
achieved using the coevaporating apparatus shown in FIG. 4. A deposition
chamber 32 contains substrate 16, 16' or 16" which is supported by a
supporting member 34 and rotated by a motor 36. Disposed below supporting
member 34 are heaters 38, 40 and 42 for heating the material to be
deposited. In the illustrated apparatus, heaters 38, 40 and 42 heat
semiconductor material, dielectric material and metal, respectively. These
heaters are charged and controlled by a power source 44. Deposition
chamber 32 is connected to a vacuum pump 46 so that the deposition chamber
can be evacuated to a pressure of about 10.sup.-3 Pa.
Cleaned substrate 16, 16' or 16" is thereafter fixed to supporting member
34, with the cleaned surface kept downward, and is rotated by motor 36.
Pump 46 is then operated so as to bring the pressure of deposition chamber
32 down to about 10.sup.-3 Pa. Then heaters 38, 40 and 42 heat the
materials to a predetermined temperature, whereby the heated materials are
evaporated and deposited. Power source 44 controls the thickness of the
deposited film and the proportion of mixed materials. Generally, the
metallic layer is deposited after the deposition of the recording layer.
Adjusting the thickness of the metallic layer renders the layer reflective
(by increasing layer thickness) or semitransparent (by making the layer
thinner). A particularly preferred example of a metallic layer within the
present invention is a high reflectivity thin film comprising Au, Ag, Cu
or Al and having a thickness of about 0.05 /.mu.m or more. A
semitransparent metallic film comprising Au, Ag, Cu or Al and having a
thickness of about 0.01 /.mu.m or less is suitable for use as a
semitransparent layer in accordance with the present invention.
After the completion of the deposition, a valve (not shown) is opened to
break the vacuum in deposition chamber 32. The prepared disc is removed
from the apparatus and coated with a UV-curable resin, which forms the
protective layer upon being irradiated with UV light.
The present invention will be described further with reference to the
following examples, which should not be considered as limitations on the
present invention.
EXAMPLE 1
In accordance with the procedure described below, an optical disc memory of
the present invention was prepared by using an apparatus as shown in FIG.
4.
A substrate comprising polycarbonate resins was provided in the form of a
disc having a diameter of about 130 mm and a thickness of about 1.2 mm and
carrying grooves thereon of 0.07 /.mu.m in depth and 0.8 /.mu.m in width.
The substrate was cleaned and fixed to supporting member 34. Then, the
substrate was rotated and heaters 38 and 40 were operated while a high
vacuum (about 10.sup.-3 Pa) was maintained inside deposition chamber 32.
Heater 38 heated Ge (for the semiconductor particles) and heater 40 heated
Bi.sub.2 O.sub.3 (for a dielectric material member). A recording layer
comprising a mixture of Ge and Bi.sub.2 O.sub.3 was formed by
simultaneously evaporating Ge and Bi.sub.2 O.sub.3 onto the substrate,
such that the fractional volume factor of Ge in the whole recording layer
was adjusted via power source 44 to about 60%.
The disc was covered with a protective layer made of UV-cured resin, so
that it had a multi-layered structure as shown in FIG. 1. The resulting
optical disc displayed an optical reflectivity that changed upon exposure
of the disc to a laser beam having a wavelength of 0.83 /.mu.m. As shown
in FIG. 5, when the recording layer was exposed by the laser beam with
wavelength of 0.83 /.mu.m and the phase of the recording layer was thereby
irreversibly switched from amorphous to crystalline, or vice versa, the
observed rate of reflectivity change depended on the thickness of the
recording layer. Disc reflectivity was likewise a function of the
thickness of the recording layer, as shown in FIG. 6.
The results illustrated in FIGS. 5 and 6 indicate that the thickness of the
recording layer should be about 0.05 /.mu.m, i.e., in the range where disc
reflectivity and reflectivity change-of-rate underwent significant
alteration in response to exposure depending on the above-mentioned
optical interference effect, and strong read-out signals were obtainable.
EXAMPLE 2
An optical disc was prepared as described in Example 1, except that InSb
was used for the semiconductor material and PbO for the dielectric
material. In addition, the thickness of the recording layer was adjusted
to 1 /.mu.m, and no protective layer was employed.
To test durability, the optical disc thus obtained was exposed to high
temperature and humidity conditions. As shown in FIG. 7, the results
indicated that fractional volume factor q of about 0.4 to 0.8 was
associated with superior durability and reflectivity characteristics. When
the recording layer comprising InSb and PbO was scanned by a laser beam of
0.83 /.mu.m wavelength, the reflectivity of the recording layer was
observed to be a function of both the thickness of the recording layer and
the fractional volume factor q of InSb (see FIG. 8).
Accordingly, the value for fractional volume factor q is preferable
selected between about 0.4 (40%) to 0.8 (80%). If this condition is
satisfied, semiconductor material having a high absorbancy works
satisfactorily as a constituent of an optical interference film also
comprising a suitable dielectric material. Moreover, the optical
interference of reflected light at the both boundaries of the recording
layer can be increased because of the reduction of absorbancy associated
with the reduced fractional volume factor q.
EXAMPLE 3
An optical disc was prepared as in Example 2, using the apparatus
illustrated in FIG. 4. More specifically, a polycarbonate, disc-shaped
substrate having a thickness of about 1.2 mm and a diameter of about 130
mm was cleaned and fixed to supporting member 34. The substrate was then
rotated and heaters 38 and 40 were operated while the inside of deposition
chamber 32 was kept at a high vacuum. Heaters 38 and 40 heated InSb for
semiconductor particles and PbO for a dielectric member, respectively.
Thus a recording layer comprising a mixture of InSb and PbO was deposited
on the substrate by simultaneously evaporating InSb and PbO, such that the
fractional volume factor of InSb in the whole recording layer was adjusted
to about 60% via power source 44. Also, the thickness of the recording
layer was similarly adjusted to between about 0.01-0.15 /.mu.m.
Heaters 38 and 40 were then deenergized and heater 42 was operated to heat
Cu. As a result, a metallic layer having a thickness of about 0.05 /.mu.m
was formed on the recording layer. After completion of the Cu deposition,
the substrate was removed from the apparatus and was coated with
UV-curable resins; a protective layer was thereafter formed on the
metallic layer by irradiating the resin coating with UV light.
The resulting optical disc displayed an optical reflectivity that changed
upon exposure of the disc to a laser beam having a wavelength of about
0.83 /.mu.m. As shown in FIG. 9, the disc also showed a rate of
reflectivity change that depended on the thickness of the recording layer.
In light of the data shown in FIG. 9, the thickness of the recording layer
was preferably about 0.055 /.mu.m.
In FIG. 9, the dotted line represents the case when fractional volume
factor q was 1.0, that is, the recording layer consisted essentially of
InSb. The curves for q=1.0 and q=0.6, respectively, show almost the same
rate of reflectivity change, i.e., strong read-out signals were obtained
with the multi-layered disc construction even when InSb content was
reduced from 100% to about 60%.
To obtain the multilayered structure shown in FIG. 2, the recording layer
is formed by simultaneous sputtering, whereby discrete semiconductor
regions are dispersed within the chemically stable dielectric member. The
semiconductor regions dispersed in the recording layer can easily change
to amorphous or crystalline phase, and the important feature of recording
layer durability is dramatically enhanced. The deposition of the metallic
layer by sputtering also makes the fabrication operation simple.
EXAMPLE 4
An optical disc was prepared as described in the Example 3, except that the
fractional volume factor of InSb was established at about 90% and Se was
used as a dielectric material member. In addition, Al was used instead of
Cu for the metallic layer. Thus, a recording layer comprised of InSb and
Se was deposited on the substrate, and a metallic layer of Al was
deposited on the recording layer. Next, a protective layer was applied so
that the resulting optical disc had the multilayered construction shown in
FIG. 2. The thickness of the recording layer was in the range of about
0.04-0.08 /.mu.m, and the thickness of the metallic layer was about 0.05
/.mu.m. The Se dielectric material was effectively transparent to the
impinging laser beam, which had a wavelength of about 0.83 /.mu.m.
The resulting multilayer optical disc underwent an amorphous-to-crystalline
phase transition upon irradiation with the laser beam, and a large rate of
reflectivity change was observed in conjunction with the phase transition.
Furthermore, the durability of the recording layer was extremely improved.
EXAMPLE 5
An optical disc was prepared as described in Example 3, except that Te and
SiO were used for the semiconductor particles and the dielectric member,
respectively. The resulting optical disc included a recording layer
comprising a mixture of Te and SiO deposited on the substrate, with a
metallic layer of Cu deposited on the recording layer and a protective
layer coated on the metallic layer.
The thickness of the recording layer was in the range of about 0.03-0.06
/.mu.m and the metallic layer had a thickness of about 0.05 /.mu.m. The
fractional volume factor of Te was adjusted to about 80%.
The above-described optical disc showed a amorphous-to-crystalline phase
transition modulated by the intensity of the applied laser beam, and also
had a large rate of reflectivity change associated with the phase
transition.
EXAMPLE 6
In the manner of Example 1, an optical disc memory was prepared, using the
apparatus shown in FIG. 4. A polycarbonate, disc-shaped substrate having a
thickness of about 1.2 mm and a diameter of about 130 mm, and carrying
grooves for optical tracking, was cleaned and fixed to supporting member
34. The substrate was then rotated, and heater 42 operated to heat Cu,
while the inside of chamber 32 was kept at a high vacuum of 10.sup.-3 Pa.
A semitransparent metallic layer having a thickness of about 0.01 /.mu.m
was thereby deposited on the substrate. Heater 42 was then deenergized and
heaters 38 and 40 were charge to heat InSb and PbO, respectively, so that
a recording layer comprising a mixture of InSb and PbO was deposited on
the semitransparent metallic layer. The value of the fractional volume
factor for InSb was adjusted to about 60% via power source 44. The
thickness of the recording layer was in the range of about 0.01-0.15
/.mu.m. Heaters 38 and 40 were then deenergized and heater 42 was operated
to heat Cu for depositing a second metallic layer, having a thickness of
about 0.05 /.mu.m or more, on the recording layer. Finally, a protective
layer was coated on the second metallic layer.
The resulting optical disc displayed an optical reflectivity which changed
when the disc was irradiated with a laser beam having wavelength of 0.83
/.mu.m. As shown in FIG. 10, the disc also showed a large rate of
reflectivity change which was a function of recording layer thickness.
Based on the results illustrated in FIG. 10, the thickness of the
recording layer was preferably about 0.07 /.mu.m.
The optical disc described above had a rate of reflectivity change of about
80% or more, owing to the multilayered construction shown in FIG. 3, i.e.,
to the optical interference effect of multipath reflection of light. Such
a large rate of reflectivity change permits one to regenerate information,
recorded on the recording layer as the portions of different phases, with
a high SN ratio.
The optical disc described in the foregoing examples is used for an image
recording medium for an image information filing apparatus as shown in
FIG. 11. The apparatus of FIG. 11 comprises a semiconductor laser diode 48
for reading and recording information and a semiconductor laser diode 50
for erasing information recorded on the disc. The laser beam emitted from
diode 48 or 50 is focused by a lens 52 or 54, respectively, and reflected
by a mirror 56. The light reflected from mirror 56 enters a lens 58 and
then is reflected by a mirror 60. The reflected laser beam from mirror 60
is introduced to a polarizing beam splitter 62 and then to a plate 64
having a thickness such that the laser beam is retarded by one-quarter the
wavelength of the applied laser beam. The laser beam enters then a lens 66
and is thereby focused on the optical disc. Lens 66 is movably supported
by a driving mechanism (not shown) to adjust radially and axially the
position of lens 66. Optical disc 10, 12 or 14 is rotated at a velocity of
10 m/s.
The light reflected from the optical disc enters lens 66 again and passes
plate 64. At this time, the polarization plane of the light is changed by
90.degree. because the light moves back and forth within plate 64, so that
the light is reflected by polarizing beam splitter 62 and deflected to a
converging lens 68 and a column lens 70. The reflected light from the
optical disc is detected by a detector 72. Lens 66 is moved by the driving
mechanism so that the converged spot from lens 66 focuses on the desired
groove of the optical disc, i.e., the recording track is traced in
response to the output signal of detector 72.
The laser diode emits the 0.83 /.mu.m wavelength laser beam in 0.15 /.mu.S
pulses of about 7 mW in power when a recording operation is effected. In
an erasing operation, the diode emits the beam in a 1.3 /.mu.S pulse of
lower power, i.e., about 4 mW.
When an optical disc memory of the present invention, which can have a
multilayered structure as described in Example 3, is placed in the
apparatus shown in FIG. 11 and a reading operation is carried out, laser
diode 48 continuously emits a laser beam having one-third the power of a
recording operation. The emitted light passes through the substrate and is
reflected at both boundaries of the recording layer and the metallic
layer, respectively. The intensity of the reflected light changes
according to the reflectivity of the recorded portion, where the structure
of the semiconductor particles has been switched to amorphous phase. Thus,
the recorded information is regenerated as a change of reflectivity. In an
erasing operation, laser diode 50 continuously emits an erasing beam to
which the optical disc is exposed, whereby the structure of the
semiconductor particles is switched to crystalline phase and the recorded
information is consequently erased. In a recording operation, laser diode
50 works continuously and laser diode 48 emits pulsed light which contains
information, so that the portions of the recording layer irradiated by the
erasing beam and containing semiconductor particles in the crystalline
phase are then switched to amorphous phase by the recording beam. The
converged spot of the recording beam is smaller than that of the erasing
beam and is guided on the recording track behind the spot of erasing beam,
so that the operations of recording and erasing are executed
simultaneously.
According to the present invention, the semiconductor material is
stabilized within the recording layer, even if the disc is broken in the
handling, assuring safety. Moreover, the recording layer may be easily
deposited by the above-described co-evaporating method, so that optical
disc memories of uniform quality and moderate price can be produced. As
described above, it is possible with the present invention to provide an
improved optical memory that can optically, reversibly store and retrieve
information, is excellent in durability, has a high contrast ratio, and
can be prepared using a simple film-forming process.
Numerous modifications and variations of the present invention are possible
in light of the above teachings. It is therefore to be understood that,
within the scope of the appended claims, the present invention can be
practiced in a manner other than as specifically described herein.
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