U.S. patent number RE28,971 [Application Number 05/218,567] was granted by the patent office on 1976-09-21 for light modulator element.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Keiichiro Aizu, Yoshio Furuhata, Akio Kumada.
United States Patent |
RE28,971 |
Kumada , et al. |
September 21, 1976 |
Light modulator element
Abstract
A device for modulating and a nondestructive readout storage
device employing modulation of light transmitted through an
irregular ferroelectric crystal before and after the rotation of
the vibration plane thereof caused by an applied electric field
equal to or larger than the coercive field thereof.
Inventors: |
Kumada; Akio (Kodaira,
JA), Aizu; Keiichiro (Kodaira, JA),
Furuhata; Yoshio (Kodaira, JA) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JA)
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Family
ID: |
27457458 |
Appl.
No.: |
05/218,567 |
Filed: |
January 17, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
810161 |
Mar 25, 1969 |
03586415 |
Jun 22, 1971 |
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Foreign Application Priority Data
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Mar 30, 1968 [JA] |
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43-20815 |
Mar 30, 1968 [JA] |
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43-20816 |
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Current U.S.
Class: |
359/251; 365/121;
359/322 |
Current CPC
Class: |
G02F
1/05 (20130101); H01G 7/025 (20130101) |
Current International
Class: |
H01G
7/02 (20060101); G02F 1/05 (20060101); H01G
7/00 (20060101); G02F 1/01 (20060101); G02F
001/03 () |
Field of
Search: |
;350/150,149,157,16R
;340/173.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Corbin; John K.
Assistant Examiner: Koren; Matthew W.
Attorney, Agent or Firm: Craig & Antonelli
Claims
We claim: .[.1. A device for modulating a beam of light comprising
a pair of light polarizer plates disposed substantially in parallel
with each other and having their surfaces substantially
perpendicular to the direction of incident light thereon an
irreguar ferroelectric element having a pair of Z-cut planes, said
irregular ferroelectric element being arranged between said pair of
light polarizer plates in such a manner that said Z-cut planes of
said element are substantially parallel to said light polarizer
plates, a pair of transparent electrodes each provided on each of
said pair of Z-cut planes, and means for applying an electric field
not lower than the coercive field of said element across said
element through said pair of transparent electrodes..]. .[.2. A
device according to claim 1, further comprising a quarter
wavelength plate for the central wavelength of white light..].
.[.3. A device according to claim 1, wherein said irregular
ferroelectric material is a single crystal having molybdate
gadolinium oxide structure represented by (R.sub.x
R'.sub.11x).sub.2 O.sub.3.3Mo.sub.1-e W.sub.c O.sub.3, where R and
R' are at least one element of the rare earths, x is a number of
from 0 to 1.0, and e is a number of from 0 to 0.2..]. .[.4. A
device according to claim 1, wherein each of said pairs of
transparent electrodes comprises a plurality of parallel strips at
equal intervals, said strips comprising oppositely biased
electrodes crossing over substantially perpendicularly to each
other..]. .[.5. A device according to claim 4, wherein said
electrodes are made of one of SnO.sub.3 and InO.sub.2..]..Iadd. 6.
A device for modulating a beam of light comprising:
a pair of light polarizer plates disposed substantially in parallel
with each other;
an irregular ferroelectric element having a pair of Z-cut
planes;
a pair of transparent electrodes each provided on each of said pair
of Z-cut planes; and
means for applying an electric field not lower than the coercive
field of said element across said element through said pair of
transparent electrodes, said irregular ferroelectric element being
made of molybdate gadolinium oxide crystal structure given by the
formula
where e is a number of from 0 to 0.2, and R is an element selected
from the group consisting of Sm, Tb, Dy and Eu. .Iaddend. .Iadd. 7.
A device for modulating a beam of light comprising:
a pair of light polarizer plates disposed substantially in parallel
with each other;
an irregular ferroelectric element having a pair of Z-cut
planes;
a pair of transparent electrodes each provided on each of said pair
of Z-cut planes; and
means for applying an electric field not lower than the coercive
field of said element across said element through said pair of
transparent electrodes, said irregular ferroelectric element being
made of molybdate gadolinium oxide crystal structure given by the
formula
where R is constituted by at least one element selected from the
group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Eu, Tb, Tm, Yb,
and Lu, R' is constituted by at least one element selected from the
group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Gd, Eu, Tb, Tm,
Yb, and Lu, x is a number of from 0 to 0.1 and e is a number of
from 0 to 0.2. .Iaddend. .Iadd. 8. A birefringent device
comprising:
a crystal element made of molybdate gadolinium oxide crystal
structure given by the formula
where e is a number of from 0 to 0.2, and R is an element selected
from the group of Sm, Tb, Dy and Eu; and
means, connected to said crystal, for placing said crystal in one
of the reversibly birefringent states thereof. .Iaddend..Iadd. 9. A
device according to claim 8, wherein said means for placing said
crystal in one of the reversibly birefringent states thereof
comprises means for imparting a mechanical stress to the crystal at
least equal to the coercive stress thereof. .Iaddend. .Iadd. 10. A
device according to claim 8, wherein said means for placing said
crystal in one of the reversible birefringent states thereof
comprises means for applying an electric field across said crystal
at least equal to the coercive field of said crystal.
.Iaddend..Iadd. 11. A device according to claim 10, wherein said
means for applying an electric field comprises a voltage source and
a pair of electrodes connected to said crystal for receiving the
voltage from said voltage source and applying said electric field
across said crystal. .Iaddend..Iadd. 12. A device according to
claim 10, wherein said means for placing said crystal in one of the
reversibly birefringent states thereof comprises means for applying
a voltage pulse to said crystal. .Iaddend..Iadd. 13. A birefringent
device comprising:
a crystal element made of molybdate gadolinium oxide crystal
structure given by the formula
where R is constituted by at least one element selected from the
group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Eu, Tb, Tm, Yb,
and Lu, R' is constituted by at least one element selected from the
group consisting of Y, La, Ce, Pr, Nd, Pm, Sm, Dy, Gd, Eu, Tb, Tm,
Yb, and Lu, x is a number of from 0 to 0.1 and e is a number of
from 0 to 0.2; and
means connected to said crystal, for placing said crystal in one of
the reversibly birefringent states thereof. .Iaddend..Iadd. 14. A
device according to claim 13, wherein said means for placing said
crystal in one of the reversibly birefringent states thereof
comprises means for imparting a mechanical stress to the crystal at
least equal to the coercive stress thereof. .Iaddend..Iadd. 15. A
device according to claim 13, wherein said means for placing said
crystal in one of the reversible birefringent states thereof
comprises means for applying an electric field across said crystal
at least equal to the coercive electric field of said crystal.
.Iaddend..Iadd. 16. A device according to claim 15, wherein said
means for applying an electric field comprises a voltage source and
a pair of electrodes connected to said crystal for receiving the
voltage from said voltage source and applying said electric field
across said crystal. .Iaddend..Iadd. 17. A device according to
claim 15, wherein said means for placing said crystal in one of the
reversibly birefringent states thereof comprises means for applying
a voltage pulse to said crystal. .Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a light modulator utilizing the
variation in the orientation of the vibration plane of an irregular
ferroelectric crystal accompanying the polarization reversal
thereof.
2. Description of the Prior Art
There are various conventional optical switching elements such as
ammonium dihydrogen phosphate (hereinafter referred to as ADP)
employing an electrooptical effect and Kerr cells employing
birefringence caused when a substance such as nitrobenzene is
placed in an electric field. All of these elements are such that
the intensity of light transmitted through these elements is
controlled by placing the elements between two polarizers the
vibration planes of which are orthogonal and applying thereto an
electric field. In such elements
1. The quantity of light transmitted therethrough is proportional
to the applied field. A high voltage is necessary for intensifying
the brightness of the transmitted light.
2. Since the quantity of transmitted light is proportional to the
applied field, light is not transmitted when the applied voltage is
reduced to zero, that is, these optical elements have no memory
function. Therefore, in order to maintain the brightness at a
constant value, it is necessary to keep the elements impressed with
a voltage corresponding thereto.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an optical
switching element having a memory function and capable of
controlling switching time.
It is another object of the present invention to provide a
ferroelectric storage device having no dependency on voltage,
frequency and time.
It is a further object of the present invention to provide a
storage device wherein information stored in a ferroelectric
storage element is nondestructively read out.
It is still another object of the present invention to provide a
large capacity storage device wherein information stored in a
ferroelectric storage element is read out with a high S/N
ratio.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1a is a hysteresis loop of polarization versus electric field
of a ferroelectric material;
FIG. 1b is a hysteresis loop of generated electric charge versus
stress of an irregular ferroelectric material;
FIG. 1c is a hysteresis loop of mechanical strain versus electric
field of an irregular ferroelectric material;
FIG. 1d is a quantity of transmitted light versus voltage
characteristic of an irregular ferroelectric material;
FIG. 2 is a diagram showing the change in the dimension of an
irregular ferroelectric crystal wherein (a) is the state of the
crystal with no stress nor applied electric field, and (b) is the
state of the crystal with an applied electric field higher than the
coercive field.
FIG. 3 is a part of the indicatrix ellipsoid of a biaxial
birefringent crystal;
FIG. 4 is a diagram schematically showing how white light is
polarized;
FIG. 5 is a diagram showing the state of interference of the light
passed through the device of FIG. 4;
FIG. 6 is a crystal element used for an optical shutter device;
FIG. 7 is an embodiment of the optical shutter device according to
the invention;
FIG. 8 is another embodiment of the invention;
FIG. 9 is an arrangement of electrodes on a storage element
according to the invention;
FIG. 10 is still another embodiment of the invention;
FIG. 11a is a wave form of a readout signal;
FIG. 11b is a current versus time characteristic of a readout
current when a storage element is in a "0" state; and
FIG. 11c is a current versus time characteristic of a readout
current when a storage element is in a "1" state. .Iadd.
FIG. 12 illustrates an embodiment of the invention employing
mechanical stress applying means. .Iaddend.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Ferroelectric material has generally a hysteresis characteristic as
shown in FIG. 1a between an applied field E and an electric
polarization P. In other words, as the electric field applied to
the ferroelectric material grows high, the polarization of the
ferroelectric material reaches the state indicated by CA in FIG.
1a. Then, as the applied field is gradually reduced, the
polarization also becomes reduced, and when the applied field
exceeds the negative coercive field -E.sub.c after having passed
through zero, the polarization is reversed. As the intensity of the
applied field is further increased in the negative direction, the
polarization reaches the state indicated by DB in FIG. 1a.
According to studies on ferroelectrics made by the inventors it was
found that some kinds of ferroelectrics such as potassium
dihydrogen phosphate (hereinafter referred to as KDP) and
gadolinium molybdenum oxide (hereinafter referred to as GMO) have
the property that when a stress of more than a certain value
(hereinafter referred to as coercive stress), to say nothing of an
electric field of more than a certain value (hereinafter referred
to as coercive field), is applied to the ferroelectrics, the
direction of the spontaneous polarization 5 thereof is reversed
and, at the same time, the crystal lattice thereof undergoes
deformation as shown in FIGS. 2a and 2b as contrasted to the known
ferroelectrics such as triglycine sulfate, lead zircon-titanate and
barium titanate whose spontaneous polarizations are reversed in
their direction by the application of the coercive field and which
undergo no crystal lattice deformation.
Generally, a crystal having an electric polarization called
spontaneous polarization in the absence of stress and electric
field and capable of being reversed in its spontaneous polarization
depending on an applied electric field as shown in FIG. 1a is
conventionally called a ferroelectric crystal. In some of the
ferroelectric crystals, the strain in the crystal lattice is
different depending on the direction of the spontaneous
polarization as shown in FIG. 2. Such a ferroelectric crystal will
hereinafter be referred to as an irregular ferroelectric crystal.
The above-mentioned KDP and GMO are examples of irregular
ferroelectric crystals. Irregular ferroelectrics belong to
ferroelastoelectrics. In contrast, a ferroelectric crystal of which
the strain in the lattice is independent of the direction of the
spontaneous polarization is called a regular ferroelectric crystal.
Referring to FIG. 2 in which h, k and l indicate the length of the
edges of the crystal along the crystallographic axes a, b and c
respectively, the crystal in the state (a) is expanded in a
direction perpendicular to the sheet of the drawing, while in the
state (b), it is elongated in the horizontal direction. That is,
the crystal in the state (a) corresponds to that in the state (b)
rotated 90.degree. around the axis c. With this change in
deformation, the tensorial properties of the crystal change
accordingly.
There are basically two methods of transforming an irregular
ferroelectric crystal from one state to the other. According to one
method, a crystal which is in the state of FIG. 2a is give a
compressive force in the direction of k to cause a strain. If the
compressive force exceeds a certain value, the crystal will be
transformed into the state of FIG. 2b, and the polarity of the
electrification on both end surfaces perpendicular to the direction
of the spontaneous polarization will be reversed. This phenomenon
corresponds to the generation of electric charge or electromotive
force due to a mechanical stress. In this case, the relation
between the stress X and the charge density Q is expressed by a
hysteresis loop as shown in FIG. 1b, and both polarized states
opposite to each other are stable in the absence of an electric
field or a mechanical stress. The other method of transforming the
state of the crystal is to impose an electric field on the crystal
in the direction opposite to that of the spontaneous polarization
to reverse the polarization as described previously. Accompanying
the reversal of the polarization there occurs a change in strain as
shown in FIG. 2. In this case, the relation between the electric
field and the mechanical strain is as shown in FIG. 1c.
Needless to say, in an irregular ferroelectric crystal, the
relation between the mechanical stress and strain also shows a
hysteresis loop similar to those of FIGS. 1b and 1c. Such a
mechanical behavior is entirely different from elasticity or
plasticity of ordinary materials, and it is a property rather
comparable with ferroelectricity or ferromagnetism. Therefore, it
may be called "ferroelasticity," and an irregular ferroelectric
crystal may be said to be ferroelectric as well as ferroelastic
material. According to the investigation made by the inventors, it
has been found that some crystals among the crystals belonging to
the point groups mm2, 2-I and 2-II fall within the category of the
irregular ferroelectric material. The following table I enumerates
these crystals under the respective group indices imm2, i2-I and
i2-II.
TABLE I ______________________________________ Point group Material
______________________________________ imm2 KDP, GMO. i2-I Not yet
discovered. i2-II Rochelle salt, cadmium ammonium sulfate,
dodecylhydrate of aluminum methyl-ammonium sulfate.
______________________________________
According to various investigations made by the inventors it has
been found that GMO and its crystallographic isomorphs, that is,
.[.(R.sub.x R'.sub.11x).sub.2 O.sub.3 Mo.sub.11e W.sub.e O.sub.3
.]..Iadd.(R.sub.x R'.sub.1.sub.-x).sub.2 O.sub.3 . 3
Mo.sub.1.sub.-e W.sub.e O.sub.3 .Iaddend. (where, R and R' are at
least one element of the rare earths, x is a number of 0-1.0, and e
is a number of 0-0.2) are crystals of the ferroelectric and
ferroelastic phase belonging to point group mm2, have a curie
temperature approximately at 1600.degree. C. show irregular
ferroelectric properties at temperatures ranging from the curie
point to very low temperatures, including room temperature, are
insoluble in water, resistive to moisture as well as desiccation,
and have a high mechanical strength. Further, the curie point
thereof can be lowered down to around room temperature by forming
isomorphous solid solutions. A crystal of the GMO crystal structure
employed in the present invention belongs to the orthorhombic
system of crystal crystallography.
The unit cell dimensions of GMO used in this invention have been
determined by using an X-ray goniometer and by an X-ray diffraction
method, as follows:
a=10.38.+-.0.005 A.
b=10.426.+-.0.005 A.
c=10.709.+-.0.005 A.
As to Eu.sub.2 (MoO.sub.4).sub.3,Tb.sub.2 (MoO.sub.4).sub.3,
Dy.sub. 2 (MoO.sub.4).sub.3 and Sm.sub.2 (MoO.sub.4).sub.3 which
are isomorphous of GMO, it has been found from the measurement by
an X-ray diffraction method that the unit cell dimension along the
axis a is different from that along the axis b in all of these
crystals as shown in table II.
TABLE II ______________________________________ Material a (A.) b
(A.) c (A.) ______________________________________ Eu.sub.2
(MoO.sub.4).sub.3 10.377.+-. 0.005 10.472.+-.0.005 10.655.+-.0.005
Gd.sub.7 (MoO.sub.4).sub.3 10.388.+-.0.005 10.426.+-.0.005
10.709.+-.0.005 Dy.sub.2 (MoO.sub.4).sub.3 10.331.+-.0.005
10.346.+-.0.005 10.603.+-.0.005 Sm.sub.2 (MoO.sub.4).sub.3
10.478.+-.0.005 10.511.+-.0.005 10.856.+-.0.005
______________________________________
Each single crystal of GMO, Sm.sub.2 (MoO.sub.4).sub.3, Eu.sub.2
(MoO.sub.4).sub.3, Tb.sub.2 (MoO.sub.4).sub.3 and Dy.sub.2
(MoO.sub.4).sub.3 was cut in parallel with (100), (010), (001)
planes which are perpendicular to the axes a, b, c, respectively,
and was subjected to polling by being impressed with an electric
field or a mechanical stress to be made into a single domain
structure. (This was verified by observing 080) specimen through a
polarizing microscope with plane polarized light directed in the
direction of the axis c while manipulating a crossed polar.) The
intensity distribution of light reflected from the surfaces of the
crystal was measured with an X-ray three axes goniometer. The
planes the reflected light from which was measured, were (400),
(600), (800), (1000), and also (003), (004), (005). Further, after
the measurement of the reflected light, the axes a and b of the
crystal were interchanged by applying an inverse electric field in
the direction of the axis c or by applying a stress in the
direction of the axis c, and the crystal is made of a single
domain. Then again, the intensity distribution of the light
reflected from planes (040), (060), (080) and (0100) was determined
under the following measuring condition. That is, Cu-K rays from an
X-ray source energized with a voltage of 30 kv. and a current of 10
ma. were directed to the crystal through a divergence slit 10 mm.
wide, a scattering slit 10 mm. wide and an entrance slit 0.1 mm.
wide. The scanning speed of the goniometer was one-fourth
degree/min. and the radius of a geiger counter used was 185 mm.
Further, when the crystal was heated above the curie temperature
thereby to release it from the polled state and then cooled, it
became of a multidomain structure and the difference between the
cell dimensions a and b became indistinct.
Some of the irregular ferroelectric crystals used in this invention
are single crystals and solid solutions of chemical compounds of
the GMO crystal structure. Several of them have been shown in table
I.
The structure of such a crystal is greatly affected by the size of
positive ions contained therein. If the positive ions are too large
or too small, a different structure will result. The Arrhenius ion
radii of ions of rare earths are as follows: Sm.sup..sup.+3 : 1.00
A. Eu.sup..sup.+3 : 0.98 A. Gd.sup.+: 0.97 A. Tb: 0.93 A. and Dy:
0.92 A. Therefore, .[.(R.sub.x R'.sub.11x).sub.2 O.sub.3.
3Mo.sub.11e WeO.sub.3 .]. .Iadd.(R.sub.x R'.sub.1.sub.-x).sub.2
O.sub.3 . 3 Mo .sub.1.sub.-e W.sub.e O.sub.3 .Iaddend. formed with
these ion radii will have the same GMO crystal structure.
The GMO crystal used in this invention belongs to the orthorhombic
system and to the point group mm2 and has a spontaneous strain
x.sub.S as follows: ##EQU1## A crystal having such unit dimension
is remarkably affected by the polling. The GMO crystal used in this
invention has the following properties:
______________________________________ Color: Colorless and
transparent Density: 4,600 kg./m..sup.3 Point group: Orthorhombic,
mm2, ferroelectric phase at temperatures below the curie point;
Tetragonal, 42 m., paraelectric phase at temperatures above the
curie point Phase transition temp.: 162.degree..+-.3.degree.C.
Melting point: 1,170.degree.C. Cleavage plane: (110),(001) Specific
dielectric constants in the direction of axes a, b and c:
.epsilon..sub.c =10.5, .epsilon..sub.a .apprxeq..epsilon..sub.b
=9.5 (at 20.degree.C.) Spontaneous polarization:
1.86.times.10.sup.-.sup.3 (C/M.sup.2)(axis c direc- tion)
Spontaneous strain: 1.5.times.(C/m.sup.2 10.sup.13) Elastic
compliance: 25.times.10.sup.112 (m.sup.2 /Newton) Coercive field:
6.times.10.sup.5 (V/m.sup.2) Coercive stress: 1.4.times.10.sup.5
(Newton/m.sup.2) Electrical resistivity: higher than 10.sup.10
.OMEGA. a Resistivity to water and chemicals: Good Efflorescence
and diliquescene: None ______________________________________
The following table III shows some of the isomorphs of GMO crystal
used in this invention. Reactive materials and the amounts thereof
required for forming the crystals also are shown in the table.
TABLE III
__________________________________________________________________________
Reactive material (mixture ratio) Molyb- Chemical formula of date,
Rare Number single crystal parts earth Parts
__________________________________________________________________________
2 Sn.sub.2 (MoO.sub.4).sub.3 431.8 Sm.sub. 2 O.sub.3 348.7 3
Eu.sub.2 (MoO.sub.4).sub.3 431.8 Eu.sub.2 O.sub.3 352.0 4 Dy.sub.2
(MoO.sub.4).sub.3 431.8 Dy.sub.2 O.sub.3 373.0 5 Tb.sub.2
(MoO.sub.4).sub.3 833.6 Tb.sub.2 O.sub.3 748.8 Gd.sub.2 O.sub.3
180.9 6 (Gd.sub.0.5 Sm.sub.0.5).sub.2 (MoO.sub.4).sub.3 431.8
Sm.sub.2 O.sub.3 174.3 Gd.sub.2 O.sub.3 180.9 7 (Gd.sub.0.5
Eu.sub.0.5).sub.2 (MoO.sub.4).sub.3 431.8 Eu.sub.2 O.sub.3 176.0
Gd.sub.2 O.sub.3 180.9 8 (Gd.sub.0.8 Tb.sub.0.5).sub.2
(MO.sub.4).sub.3 431.8 Tb.sub.4 O.sub.3 187.2 Gd.sub.2 O.sub.3
180.9 9 (Gd.sub.0.5 Dy.sub.0.5).sub.2 (MoO.sub.4).sub.3 431.8
Dy.sub.2 O.sub.3 186.5 Gd.sub.2 O.sub.3 343.7 10 (Gd.sub.0.95
Yb.sub.0.68).sub.2 (MoO.sub.4 ).sub.3 431.8 Yb.sub.2 O.sub.3 19.7
Gd.sub.2 O.sub.3 343.7 11 (Gd.sub.0.95 Ho.sub.0.05).sub.2
(MoO.sub.4).sub.3 431.8 Ho.sub.2 O.sub.3 18.9 Gd.sub.2 O.sub.3
343.7 12 (Gd.sub. 0.93 Lu.sub.0.03).sub.2 (MoO.sub.4).sub.3 431.8
Lu.sub.2 O.sub.3 19.9 Gd.sub.2 O.sub.3 343.7 13 (Gd.sub.0.95
Tm.sub.0.03).sub.2 (MoO.sub.4).sub.3 431.8 Tm.sub.2 O.sub.3 19.3
Gd.sub.2 O.sub.3 343.7 14 (Gd.sub.0.95 Sc.sub.0.05).sub.2
(MoO.sub.4).sub.3 431.8 Sc.sub.2 O.sub.3 6.9 Gd.sub.2 O.sub.3 343.9
15 (Gd.sub.0.95 La.sub. 0.05).sub.2 (MoO.sub.4).sub.3 431.8
La.sub.2 O.sub.3 16.3 Gd.sub.2 O.sub.3 343.9 16 (Gd.sub.0.95
Pr.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8 Pr.sub.5 O.sub.6 17.0
Gd.sub.2 O.sub.3 217.0 17 (Gd.sub.0.5 Y.sub.0.4).sub.2
(MoO.sub.4).sub.3 431.8 Y.sub.2 O.sub.3 90.3 Gd.sub.2 O.sub.3 217
18 (Gd.sub.0.6 La.sub.0.4).sub. 2 (MoO.sub.4).sub.3 431.8 La.sub.2
O.sub.3 130.0 Gd.sub.2 O.sub.3 217 19 (Gd.sub.0.60 Tb.sub.0.20
Dy.sub.0.20).sub.2 (MoO.sub.4).sub.3 431.8 Dy.sub.2 O.sub.3 74.6
Tb.sub.4 O.sub.3 78.8 20 (Gd.sub.0.70 Eu.sub.0.20
Dy.sub.0.10).sub.2 (MoO.sub.4).sub.3 Gd.sub.2 O.sub.3 235.3 431.8
Eu.sub.2 O.sub.3 70.4 Dy.sub.2 O.sub.3 37.3 21 (Gd.sub.0.60 Sm.sub.
0.20 Tb.sub.0.10).sub.2 (MoO.sub.4).sub.3 Gd.sub.2 O.sub.3 217.0
431.8 Sm.sub.2 O.sub.3 69.7 Tb.sub.4 O.sub.7 39.4 Gd.sub.2 O.sub.3
253.3 22 (Gd.sub.0.70 Eu.sub.0.20 Tb.sub.0.10).sub.2
(MoO.sub.4).sub.3 431.8 Eu.sub.2 O.sub.3 70.4 Tb.sub.4 O.sub.7 39.4
Gd.sub.2 O.sub.3 253.3 23 (Gd.sub.0.7 Y.sub. 0.2 La.sub.0.4).sub.2
(MoO.sub.4).sub.3 431.8 La.sub.2 O.sub.3 32.6 Y.sub.2 O.sub.3 45.2
Gd.sub.2 O.sub.3 253.3 24 (Gd.sub.0.7 Eu.sub.0.20
Ho.sub.0.10).sub.2 (MoO.sub.4).sub.3 431.8 Eu.sub.2 O.sub.3 70.4
Ho.sub.2 O.sub.3 37.8 Gd.sub.2 O.sub.3 253.3 Sm.sub.2 O.sub.3 34.9
25 (Gd.sub.0.7 Sm.sub.0.4 Eu.sub.0.4 Y.sub.0.4).sub.2
(MoO.sub.4).sub.3 Eu.sub.2 O.sub.3 35.3 Y.sub.2 O.sub.3 22.6
Gd.sub.2 O.sub.3 343.7 26 (Gd.sub.0.95 Nd.sub.0.05).sub.2
(MoO.sub.4).sub.3 431.8 Nd.sub.2 O.sub.3 16.8 Gd.sub.2 O.sub.3
217.0 Tb.sub.4 O.sub.7 78.8 27 (Gd.sub.0.5 Tb.sub.0.2 Y.sub.0.1
La.sub.0.4).sub.2 (MoO.sub.4).sub.3 431.8 Y.sub.2 O.sub.3 22.6
La.sub.2 O.sub.3 32.6 28 Gd.sub.2 (Mo.sub.0.95 W.sub.0.1
O.sub.4).sub.3 (MoO.sub.4).sub.3 Wo.sub.3 70.0 Sm.sub.2 O.sub.3
174.1 29 (Sm.sub.0.5 Eu.sub. 0.5).sub.2 (MoO.sub.4 ).sub.3 431.8
Eu.sub.2 O.sub.3 176.0 Sm.sub.2 O.sub.3 174.1 30 (Sm.sub.0.5
Dy.sub.0.5).sub.2 (MoO.sub.4).sub.3 431.8 Dy.sub.2 O.sub.3 186.5
Sm.sub.2 O.sub.3 174.1 31 (Sm.sub.0.5 Tb.sub.0.5).sub.2
(MoO.sub.4).sub.3 431.8 Tb.sub.4 O.sub.7 187.5 Sm.sub.2 O.sub.3
331.3 32 (Sm.sub.0.95 Yb.sub.0.05).sub.2 (MoO.sub.4).sub.4 431.8
Yb.sub.2 O.sub.3 18.7 Sm.sub.2 O.sub.3 331.3 33 (Sm.sub.0.95
Ho.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8 Ho.sub.2 O.sub.3 18.9
Sm.sub.2 O.sub.3 331.3 34 (Sm.sub.0.95 Lu.sub.0.05).sub.2
(MoO.sub.4).sub.3 431.8 Lu.sub.2 O.sub.3 19.9 Sm.sub.2 O.sub.3
331.3 35 (Sm.sub.0.95 Tm.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8
Tm.sub.2 O.sub.3 19.3 Sm.sub.2 O.sub.3 331.3 36 (Sm.sub.0.95
Sc.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8 Sc.sub.2 O.sub.3 6.9
Sm.sub.2 O.sub.3 331.3 37 (Sm.sub.0.95 Y.sub.0.03).sub.2 (MoO.sub.4
).sub.3 431.8 Y.sub.2 O.sub.3 11.3 Sm.sub.2 O.sub.3 313.4 38
(Sm.sub.0.90 Er.sub.0.4).sub.2 (MoO.sub.4).sub.3 431.8 Er.sub.2
O.sub.3 19.1 Sm.sub.2 O.sub.3 209.4 39 (Sm.sub.0.5 Eu.sub.0.3
Er.sub.0.4).sub.2 (MoO.sub.4).sub.3 431.8 Er.sub.2 O.sub.3 105.4
Er.sub.2 O.sub.3 19.1 Sm.sub.2 O.sub.3
244.0 40 (SM.sub.0.7 Tb.sub.0.2 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3
431.8 Tb.sub.4 O.sub.7 78.8 Y.sub.2 O.sub.3 22.6 Sm.sub.2 O.sub.3
278.9 41 (Sm.sub.0.8 Er.sub.0.4 Y.sub.0.4).sub.2 (MoO.sub.4).sub.3
431.8 Y.sub.2 O.sub.3 22.6 Er.sub.2 O.sub.3 19.1 Sm.sub.2 O.sub.3
278.9 Dy.sub.2 O.sub.3 37.3 42 (Sm.sub.0.8 Dy.sub.0.1 Y.sub.0.05
Er.sub.0.05).sub.2 (MoO.sub.4).sub.3 5 431.8 Y.sub.2 O.sub.3 11.3
Er.sub.2 O.sub.3 9.5 Wo.sub.3 70.0 43 (Sm.sub.0.5 Tb.sub.
0.5).sub.2 (Mo.sub.0.90 W.sub.0.1).sub.3 388.6 Sm.sub.2 O.sub.3
174.1 Tb.sub.4 O.sub.7 187.2 Dy.sub.2 O.sub.3 369.3 44 (Dy.sub.0.95
La.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8 La.sub.2 O.sub.3 16.3
Dy.sub.2 O.sub.2 369.3 45 (Dy.sub.0.95 Pr.sub.0.05).sub.2
(MoO.sub.4).sub.3 431.8 Pr.sub.5 O.sub.11 17.0 Nd.sub.2 O.sub.3
16.8 46 (Dy.sub..95 Nd.sub.0.05).sub.2 (MoO.sub.4).sub.3 431.8
Dy.sub.2 O.sub.3 369.3 Dy.sub.2 O.sub.3 298.4 47 (Dy.sub.0.3
Nd.sub.0.10 Ho.sub.0.40).sub.2 (MoO.sub.4).sub.3 431.8 Ho.sub.2
O.sub.3 37.8 Nd.sub.2 O.sub.3 33.7 Eu.sub.2 O.sub.3 211.2 48
(Eu.sub.0.5 Tb.sub.0.04 Dy.sub.0.2).sub.2 (MoO.sub.4).sub.3 431.8
Dy.sub.2 O.sub.1 74.6 Tb.sub.4 O.sub.8 102.4 Gd.sub.2 O.sub.3 217.0
Sm.sub.2 O.sub.3 34.9 49 (Gd.sub.0.5 Eu.sub.9.7 Sm.sub.0.1
Tb.sub.0.4 Dy.sub.0.1).sub.1 431.8 Eu.sub.2 O.sub.3 70.4
(MoO.sub.4).sub.3 Dy.sub.2 O.sub.3 37.3 Tb.sub.4 O.sub.7 39.4
__________________________________________________________________________
The irregular ferroelectric crystals as listed above are positively
biaxial and birefringent in the ferroelectric phase. FIG. 3 shows a
part of the indicatric ellipsoid (or, in other words, light
velocity ellipsoid) of such a crystal. In FIG. 3, axes X, Y, Z
indicate optoelastic principal axes, and n.sub..alpha.,
n.sub..beta., n.sub..gamma. indicate refractive indices of light
vibrating parallel to the axes, X, Y, Z, respectively.
In a GMO crystal, the optoelastic principal axes X, Y, Z coincide
with the crystallographic axes a, b, c, respectively. The crystal
is uniaxially birefringent at temperatures above the curie point
(approximately 160.degree.C.), and its refractive indices with
respect to sodium D line, .lambda.=5,893 A, at 200.degree.C. are as
follows: ##EQU2## The crystal shows the irregular ferroelectric
characteristics at temperatures below the curie point and becomes
biaxially birefringent.
The optical axial angle 2 V (two times the angle V in FIG. 3) and
refractive indices n.sub..alpha., n.sub..beta., n.sub..gamma. of
the crystal against Na-D line at room temperature are as follows:
##EQU3## The optical axial plane of this biaxial positive crystal
is the crystallographic a plane (100), and this plane will rotate
90.degree. around the axis c if the crystal is polarized reversely.
Consequently, as is evident from FIG. 3, the retardation R.sub.a of
light transmitted through the GMO crystal in the direction of the
axis a is given by the formula, ##EQU4## where d.sub..alpha. is the
thickness of the crystal in the direction of axis a. If a
polarization reversal occurs in such a crystal and the optoaxial
plane rotates 90.degree. around the axis c, the axis a is replaced
by the axis b and the axis b by the axis a. Therefore, the
above-mentioned retardation also changes to the following value.
##EQU5## (The charge in the thickness of the crystal is due to the
deformation of the unit cell equivalent to the 90.degree. rotation
of the axes a and b of the cell.) That is, the thickness as well as
refractive index of the crystal changes with the polarization
reversal and accordingly the retardation also changes.
The retardation across a distance d of the light incident upon the
crystal in the direction at an angle .theta. to the axis c, for
example, is d(n.beta. -n.sub..alpha. '). In this case, if the
crystal is reversely polarized, the above-mentioned retardation
will become d(n.sub..beta.'-n.sub..gamma.) which is equivalent to
the retardation of light propagating in the direction oc" which is
on the plane ac and makes the angle .theta. with the axis c as
shown in FIG. 3, since it can be deemed that the optoaxial plane
(a-plane) of the crystal has been rotated 90.degree. around the
axis c.
If a crystal 3 as described above is positioned between two
parallel polarizing plates 1 and 2 as shown in FIG. 4 and white
light 4 is directed perpendicularly to the polarizer 1, the white
light 4.sub.o linearly polarized through the polarizer 1 is
refracted by the birefringence of the crystal 3 in various degrees
depending on the component wavelength thereof, becoming circularly
polarized light at a certain frequency, linearly polarized light at
another frequency and elliptically polarized light at the other
frequencies. Of the elliptically polarized light, only the light
having the same vibration plane as that of the analzyer 2 passes
through the analyzer 2 and gives an interference color. It should
be noted that if the crystal is reversely polarized, thereby
varying the retardation as described previously, the
above-mentioned interference color also varies according to the
change in the retardation.
As stated above, irregular ferroelectric crystals such as GMO are
biaxially and positively birefringent. Consequently, if
monochromatic parallel rays of light 4 are directed to an
arrangement where in a Z-cut (cut perpendicularly to the c-axis)
plate 3 of the crystal is disposed, as shown in FIG. 4, between a
ploarizer 1 and an analyzer 2 the polarization planes of which are
perpendicular to each other, an interference pattern as shown in
FIG. 5 is formed on a screen. The interference pattern of FIG. 5 is
loci of interference images formed according to whether the
difference between the optical paths of the refracted rays of the
monochromatic rays of light (wavelength: .lambda.) passed through
the crystal plate 3 is an even number times the half wavelength
1/2.lambda. or an odd number times the half wavelength 1/2.lambda..
The phase difference R between two extraordinary rays is ##EQU6##
where d is the thickness of the crystal plate 3, and n.sub.o and
n.sub.e are refractive indices of the extraordinary rays,
respectively. The spaces between the interference fringes depend on
the thickness d of the crystal plate, and become narrower as the
thickness of the crystal plate is larger.
Since the refractive index varies with the wavelength, the
positions of the interference fringes of FIG. 5 vary with the
wavelength.
If the Z-cut crystal plate 3 in FIG. 4 is provided with transparent
electrodes 6 on both its Z-planes, i.e., c-planes, and if the
crystal plate 3 is rotated around the c-axis so that the optoaxial
plane thereof coincides with the vibration plane of the polarizer,
the screen becomes dark. A diaphragm or stop may be employed in
this arrangement to improve the parallelism of rays of light and
the variation in brightness.
If the crystal plate 3 is rotated an angle .theta. from the dark
state of the screen around the c-axis, the relation between the
quantity of the transmitted light I and the rotation angle ##EQU7##
where I.sub.o is the quantity of the transmitted light when
.theta.=.pi./4. Thus, when ##EQU8##
When the spontaneous polarization of the crystal 3 is reversed by
the application of a negative voltage, the optical axis plane
thereof rotates 90.degree.. The variation in the quantity of the
transmitted light at this time is the same as that when the crystal
is rotated 90.degree. around the c-axis, and if the spontaneous
strain is neglected, the variation in the brightness of the
transmitted light can easily be detected. .alpha. changes depending
on the angles between the plane perpendicular to the optical axis
and the a- and b-axes.
Consequently, if the analyzer 2 is removed, the vibrating direction
of incident linearly polarized light can be rotated 90.degree.. If
the analyzer 2 is employed, the quantity of the transmitted light
can be varied between abundant and scanty states by a voltage at
least equal to the coercive field.
If the crystal 3 in FIG. 4 is one which is cut perpendicularly to
the optical axis, the birefringence does not take place in the
direction of the optical axis. However, if the polarization state
of the crystal is reversed, the birefringence takes place since the
optoaxial plane rotates 90.degree..
By providing transparent electrodes in the matrix form to both
surfaces of an irregular ferroelectric crystal plate element such
as a GMO crystal element, and by applying a required voltage (at
least equal to the coercive field of the crystal) to each
transparent electrode, required information can be stored at each
position of each matrix element in terms of "1" corresponding to
the +P.sub.s state of the spontaneous polarization as shown in FIG.
1b and "0" corresponding to -P.sub.s state. If light is passed
through such elements each storing information, the quantity of the
light passed through the elements differs from element to element
depending on the polarized state thereof. Thus, the stored
information can be read out nondestructively with light. Such
readout is of a high S/N ratio, and a small sized large capacity
storage device can be made of an irregular ferroelectric
crystal.
Some embodiments of the invention will now be described.
EXAMPLE 1
As shown in FIG. 7, a plate 3 of GMO single crystal in Z-cut with a
thickness of 0.65 mm. provided with transparent electrodes 6 formed
of, for example, SnO.sub.2 or InO.sub.2 on each c-plates thereof
having an area 10 mm. .times. 10 mm. is disposed between a
polarizer 1 and an analyzer 2. The b-axis of the crystal 3 forms an
angle 25.degree. with the vibration direction of the polarizer 1
and the angle between the vibration directions of the polarizer 1
and the analyzer 2 is 45.degree.. Collimated monochromatic light 4
of a wavelength .lambda.=550 m.mu. is directed to the crystal plate
3 through the polarizer 1. The light 4 changes into linearly
polarized light 4.sub.o by passing through the polarizer 1. At this
time, if the voltage (300 volts) applied to the crystal plate 3 is
adjusted by means of a controller associated with a voltage source
7, the arrangement can be used as a light modulator or an optical
shutter. The relation between the quantity of the transmitted light
and the applied voltage is as shown in FIG. 1d. Alternatively, if
the analyzer 2 is eliminated from the arrangement of FIG. 7, the
arrangement is used as a polarization plane rotating element for
rotating the polarization plane of linearly polarized light by
90.degree..
EXAMPLE 2
If, a crystal plate 3, having an appropriate thickness is used,
interference fringes as shown in FIG. 5 are observed. The crystal
plate 3 employed in this embodiment is arranged in such a manner
that the optical axis of the crystal 3 coincides with or is slight
oblique to the optical axis of the entire arrangement. Or
alternatively, a crystal 3 cut perpendicularly to the optical axis
thereof and provided with transparent electrodes on both cut
surfaces is employed and light is directed thereto perpendicularly
or slightly obliquely to the cut surfaces. Then, the quantity of
transmitted light is null, or when polarization reversal is caused
by applying an electric field thereto, the quantity of the
transmitted light increases.
Analogously to ferroelectrics ferroelastics are conceivable.
Materials having two or more states (orientations) of different
strain in the absence of any stress and capable of performing
transition between these states by the application of strain are
called ferroelastics herein. Ferroelastics generally have
rectangular strain .chi. versus stress X hysteresis loops in the
absence of applied electric field similar to the hysteresis loops
shown in FIGS. 1a to 1c.
In FIGS. 1a or 1b, curve AC corresponds to one oriented state, and
curve DB corresponds to the other oriented state. The former is
called "1 " state and the latter is called "0" state herein.
One-half of the difference between polarizations in both states or
P.sub.s or one-half of the difference between strains or
.chi..sub.x is the absence of both electric field and stress are
called spontaneous polarization and spontaneous strain,
respectively. The electric field E.sub.c and stress .chi..sub.c
necessary for the transition from "0" state to "1" state or vice
versa are called coercive field and coercive stress,
respectively.
.[.195.]. Irregular ferroelectrics such as GMO are not only
ferroelectrics, but also ferroelastics. The kind and direction of
an applied stress for the transistion of ferroelastic state are as
follows: If the z-axis is established parallel to the 4 symmetry
axis in the ordinary elastic phase (the phase above the curie
temperature), and the x- and y-axes are established perpendicularly
to two symmetry planes, a unit cell in the ferroelectric phase (the
phase below the curie temperature) orientates as shown by A and B
in FIG. 1a or 1b in "0" state and in "1" state. Therefore, in order
to make transition from "0" state to "1" state, it may well be that
a pressure is applied to the crystal plane perpendicular to the
x-axis and/or a tension is applied to the crystal plane
perpendicular to the y-axis. Or it may be good to apply shearing to
the crystal along two pairs of crystal planes forming an angle of
45.degree. with both x- and y-axes. In order to make transition
from "1" state to "0" state, it may be good to apply a pressure to
the crystal plane perpendicular to the y-axis, and/or to apply a
tension to the crystal plane perpendicular to the x-axis. Or it may
be good to apply shearing opposite to the above-mentioned shearing
to the crystal along two pairs of crystal planes forming an angle
of 45.degree. with both x- and y-axes. .Iadd.
FIG. 12 illustrates a crystal 3 to which mechanical stress applying
means 20 is coupled to apply forces as shown by the arrows f, in
conjunction with the above description. .Iaddend.
Even if the configuration of the crystal element is such that there
is no crystal face perpendicular to or forming an angle of
45.degree. with the x- or y-axis, it is possible to cause a
transition of state by a stress. The kind and direction of an
effective applied stress are determined as the case may be.
Since GMO has a spontaneous polarization the direction of which
varies with the transition of state, the spontaneous polarization
is apt to electrostatically react to the transition of state due to
stress. However, this reaction can be eliminated by applying a pair
of electrodes to appropriate crystal faces and by short-circuiting
them.
The spontaneous strain .chi..sub.s of GMO is defined by ##EQU9##
where .chi..sub.11 and .chi..sub.22 are expansion coefficients of
the crystal in the x- and y-directions, respectively.
Ferroelastics other than GMO are:
Potassium dihydrogen phosphate
Kh.sub.2 po.sub.4 (-150.degree.c. or lower)
Dideuterate of ammonium arsenate
(ND.sub.4)D.sub.2 AsO.sub.4 (27.degree.C. or lower)
Rochelle salt
KnaC.sub.4 H.sub.4 O.sub.6.4.sub.2 O (between 24.degree.C. and
-180.degree.C. inclusive)
Cadmium ammonium sulfate
(NH.sub.4).sub.2 Cd.sub.2 (SO.sub.4).sub.3 (-178.degree.C. or
lower)
Dodecylhydrate of aluminum methyl-ammonium sulfate
(-96.degree.C. or lower)
Generally, ferroelectrics vary in their refractive index by the
transition of state.
An embodiment of the invention based on the above-described
property of ferroelastics will now be described.
EXAMPLE 3
A storage unit 3 is disposed between a polarizer 1 and an analyzer
2 the polarization planes of which are perpendicular to each other
as shown in FIG. 8. The storage unit 3 is cut out from a GMO single
crystal in such a manner that its two main surfaces are
perpendicular or slightly oblique to its optical axis with a
distance of 100 microns therebetween. The storage unit 3 is then
provided on its main surfaces, after the main surfaces are
polished, with groups of transparent electrodes 8, 8', 8", --; 9,
9', 9", --made of SnO.sub.2 or InO.sub.2 each having a width of 1
mm. The groups of electrodes 8, 8', 8",--; 9, 9,', 9", --are
arranged so that they are in a row and column relation to each
other as shown in FIG. 9. A voltage source 11 for supplying a
negative voltage of one-half of the coercive field E.sub.c of the
crystal is connected to the electrodes as shown in FIG. 10. Each
group of the transparent electrodes consisted of ten electrodes in
this embodiment, thus providing a 10.times.10 bits storage device
having 10.sup.2 storage elements.
Of course, a storage device having 10.sup.2 storage elements is not
a large capacity storage device. Furthermore, the size 1 mm.
.times. 1 mm. of the element corresponding to one bit is rather
large. If a large capacity storage device of the order of 10.sup.6
bits, for example, is intended, the size of the storage device will
be large.
Since conventional phototransistors having a diameter 1 mm. were
employed as detectors in this example, the number of storage
elements was limited to 10.sup.2. If a large capacity storage
device having, for example, 10.sup.6 elements is desired, it may be
well to form a number of microminiature phototransistors in a
crystal surface by integrated circuit techniques.
The storage elements can store information by the application of a
desired signal, for example a pulse of +120 volts with a duration
of 10 microseconds to the electrodes 8, 8', 8", --; 9, 9', 9", --.
The readout of the stored information is made by directing light
through the polarizer 1 to the storage device 3 and detecting the
light passed through the element by the phototransistor 5 through
the analyzer 2. The light passed through the element is strong when
the element stores "1" and faint when it stores "0."
The above-described readout of stored information was in terms of
an analog quantity, i.e. brightness of light. However, the readout
of the stored information can be made in terms of a digital
quantity, wavelength of light.
In FIG. 4, if a GMO crystal 3 is arranged so that the z-axis
thereof is in parallel with white light, it will be lightly
colored. The GMO crystal is biaxial at room temperature and the
optical axes thereof intersect the x-axis symmetrically to each
other. The optical axis angle of the GMO crystal is about 11 at
room temperature and 0.degree. at the curie temperature and becomes
uniaxial.
By the arrangement of FIG. 4 at room temperature, interference
fringes are observed around the two optoaxial points a and b shown
in FIG. 5, and the surroundings of the interference fringes are
colored. The interference fringes are considered to be loci of the
retardation. Since the retardation R has the relation R=d(n.sub.e
.about.n.sub.o) with the thickness d of the crystal and the
refractive indices n.sub.o and n.sub.e of the two extraordinary
rays, the difference .DELTA.n=n.sub.e .about.n.sub.o between the
refractive indices is zero in the direction of the optical axis,
and the difference .DELTA.n becomes larger as the departure from
the optical axis becomes larger.
The interference color is determined by the retardation R. Bright
colors result on the interval of the retardation R of 400 m.mu. and
800 m.mu.. When the retardation R is in the vicinity of 800 m.mu.,
the color is red, and when the retardation R is near to 400 m.mu.,
the interference color is blue. Since the difference .DELTA.n
varies with the solid angle around the optical axis at a given
thickness of a crystal, the color of the light having passed
through the crystal varies accordingly. Consequently, if the
optical axis of the crystal is appropriately inclined relative to
rays of light in accordance with the thickness of the crystal, a
desired color of light can be obtained. If the crystal is fixed and
the optoaxial plane is rotated by the polarization reversal, the
color of light generally changes. It is easier to discern the two
colors when the wavelengths thereof are different as far as
possible.
The angle of the optical axis of the crystal relative to incident
light can effectively be selected by observing the interference
color which is the locus of the retardation R shown in FIG. 5. For
example, if a c-crystal plate 0.2 mm. thick is set at 11.degree. in
the direction of the axis b, and 7.degree. in the direction of the
axis a in the single domain state, the color is red in the +P.sub.s
state and blue in the -P.sub.s state.
Therefore, if the storage device 3 in FIG. 8 is replaced by a
storage device made of such a crystal, the contents of the store
can directly be identified. Further, if photodiodes having
different sensitivity to two wavelengths indicating the contents of
store are employed, or if photodiodes having sensitivity only to
either one of the wavelengths are employed, the contents of the
store can be read out with an electrical signal having a good
signal to noise ratio.
The signal to noise ratio of the readout signal can greatly be
increased by inserting a quarter wavelength plate 10 for the
central wavelength of white light between the analyzer 2 and the
phototransistors 5 in FIG. 10.
As has been described above, the storage device according to this
invention is made of an irregular ferroelectric or ferroelastic
material such as GMO, and the information stored in the storage
elements of the storage device is read out by passing polarized
light through the storage elements.
When a ferroelectric material is employed as the storage device,
there are the advantages that (1) the power consumption of the
storage element is small, and (2) a small sized large capacity
storage device can be fabricated since the storage density can be
made large.
However, since a storage device employing ferroelectric material
stores signals as polarized states of its storage matrix elements
corresponding to respective signals by being supplied with
predetermined signals, the information stored in the storage
elements is read out by being supplied with definite reverse
voltage pulses. When a pulse as shown in FIG. 11a is fed to a
storage element for such reading out, only a low current as shown
in FIG. 11b flows through the storage element if the polarity of
the pulse is the same as the polarized state of the element.
However, if the pulse is of opposite polarity with a sufficiently
large amplitude, the polarized state of the element is reversed,
accompanied by a relatively high current as shown in FIG. 11c
flowing through the storage element to read out the information
(i.e., polarized state) stored in the element.
The ferroelectric materials conventionally employed for such
storage device were barium titanate and glycine sulfate, for
example. In these ferroelectrics, there exists no coercive field
corresponding to the threshold field E.sub.c for reversing the
polarized state in the P-E hysteresis loop as shown in FIG. 1a.
This is because, since the coercive field generally has dependency
on voltage, frequency, and time, even a low voltage pulse can cause
the crystal to reverse its polarization when it is applied to the
crystal for a long time. That is, the coercive field is
substantially zero against a quasi-static change in electric field,
according to which the memory state of the crystal is apt to be
unstable.
Further, since it is necessary to apply a pulse of reverse voltage
to a storage element in order to read out the information stored
therein, the stored information is destroyed due to the
polarization reversal. Consequently, stored information cannot
repeatedly be read out.
Still further, in such a reading method, all the elements of the
i-th row and the j-th column are impressed with one-half the
negative voltage necessary for reading out an element (the
threshold voltage) in order to read the element at (i,j), for
example. Although this voltage is smaller than the threshold value
necessary for polarization reversal, i.e., the coercive field, the
polarization reversal occurs gradually to cause a noise current
since the coercive field of the conventional ferroelectric material
has not a definite threshold value. Even when the polarization
reversal does not occur but merely a charging current flows, the
current becomes a cause of noise, and hence the S/N ratio becomes
low and a large capacity storage device is difficult to
fabricate.
However, if an optical shutter element utilizing the change in
polarized state of an irregular ferroelectric or ferroelastic
material such as GMO is employed as a storage element as in the
present invention, nondestructive readout can be effected and the
S/N ratio of readout is made large, thus making it possible to
fabricate a large capacity storage device.
* * * * *