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.
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8. A birefringent device comprising:
a crystal element made of molybdate gadolinium oxide crystal structure given by the formula
R2 O3.3 Mo1-e We O3, 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. 13. A birefringent device comprising:
a crystal element made of molybdate gadolinium oxide crystal structure given by the formula
(Rx R'1-x)2 O3.3Mo1-3 We O3 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. 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
R2 O3 . 3 Mo1-e We O3 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. 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
(Rx R'1-x)2 O3.3Mo1-e We O3 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. 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.
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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.
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.
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. (Rx R'11x)2 O3 Mo11e We O3
Even if the configuration of the crystal element is such that there is no crystal face perpendicular to or forming an angle of 45° 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 χs of GMO is defined by ##EQU9## where χ11 and χ22 are expansion coefficients of the crystal in the x- and y-directions, respectively.
Ferroelastics other than GMO are:
Potassium dihydrogen phosphate
Kh2 po4 (-150°c. or lower)
Dideuterate of ammonium arsenate
(ND4)D2 AsO4 (27°C. or lower)
Rochelle salt
KnaC4 H4 O6.42 O (between 24°C. and -180°C. inclusive)
Cadmium ammonium sulfate
(NH4)2 Cd2 (SO4)3 (-178°C. or lower)
Dodecylhydrate of aluminum methyl-ammonium sulfate
(-96°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.
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 SnO2 or InO2 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 Ec 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 10×10 bits storage device having 102 storage elements.
Of course, a storage device having 102 storage elements is not a large capacity storage device. Furthermore, the size 1 mm. × 1 mm. of the element corresponding to one bit is rather large. If a large capacity storage device of the order of 106 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 102. If a large capacity storage device having, for example, 106 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° 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(ne ∼no) with the thickness d of the crystal and the refractive indices no and ne of the two extraordinary rays, the difference Δn=ne ∼no between the refractive indices is zero in the direction of the optical axis, and the difference Δn 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 mμ and 800 mμ. When the retardation R is in the vicinity of 800 mμ, the color is red, and when the retardation R is near to 400 mμ, the interference color is blue. Since the difference Δn 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° in the direction of the axis b, and 7° in the direction of the axis a in the single domain state, the color is red in the +Ps state and blue in the -Ps 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 Ec 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.
Furuhata, Yoshio, Kumada, Akio, Aizu, Keiichiro
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