United States Patent (19) 11, 3,909,270 Kishi et al. (45) Sept. 30, 1975

(54) DICHROIC REVERSIBLE 56 References Cited PHOTOSENSITIVE MATERALS UNITED STATES PATENTS 75) Inventors: Yasuo Kishi, Hirakata; Toshiaki 3,580,688 5/1971 Schneider...... , 356/256 Yokoo, Osaka; Terutoyo Imai, 3,720,926 3/1973 Schneider...... 340/173 CC Neyagawa; Keiichi Kanatani, 3,761,267 9/1973 Gilman et al...... 96/ 108 Hirakata; Takehito Yazaki, Katano, 3,814,601 6/1974 Schneider...... 96/88 all of Japan 73 Assignee: Director of Agency of Industrial Primary Evanniner-Won H. Louie, Jr. Science & Technology, Japan 57) ABSTRACT 22 Filed: Aug. 28, 1973 New materials are the dichroic reversible photosensi (21) Appl. No.: 392,302 tive materials which are the crystals of an halide in which are contained an alkali metal ion hav 30) Foreign Application Priority Data ing smaller ion radius than that of the alkali metal in said alkali metal ion and one or more kinds of metal Aug. 30, 1972 Japan...... 47-86608 ions other than alkali metal ion which possess an abil 52 U.S. C...... 96/88; 96/90 PC; 252/300 ity to act as an electron trap; which have M-center. (5) Int. Cl...... G03C 1/00 Said materials are useful as a reversible ultramicro 58 Field of Search...... 96/88, 90 PC, 108; fish, an optical storage element for optical memory 340/73 CC: 356/256; 252/3OO system and the like. 10 Claims, No Drawings

3,909,270 1 2 DICHROIC REVERSIBLE PHOTOSENSTEVE rial which consists of the crystals of an alkali metal hal MATERIALS ide intermixed with one kind of a specific metallic cat This invention relates to dichroic reversible photo ion in which M-center is formed, and as a result of fur sensitive materials and a method for the preparation ther studies on using a new theory developed by them, thereof. More particularly, it concerns dichroic revers have succeeded to obtain new dichroic reversible pho ible photosensitive materials which consist of the crys tosensitive materials having a remarkably improved tals of an alkali metal halide which contain an alkali photosensitivity which consist of the crystals of an al metal ion having smaller ion radius than that of the al kali metal halide intermixed with one kind of alkali kali metal in said alkali metal halide and also one or metal ion having smaller ion radius than the alkali more kinds of other metal ions than alkali metal ion O metal in said alkali metal halide used as the base mate which possess an ability to act as an electron trap; rial and one or more kinds of metal ions other than the which have M-center, and a method for the preparation alkali metal ion which possess an ability to act as an therof. electron trap, which have M-center. The objects of this invention will become clear in the The alkali metal halide crystal to be used as the base following explanation. 15 material of this invention means the crystals of of It has been known that when crystals of an alkali an alkali metal such as , , , ru metal halide such as , potassium chlo bidium or cesium, with a such as , bro ride or the like are irradiated with an electron beam, mine, or . The most preferred example proton, X-ray or y-ray, or treated in an atomosphere of of the alkali metal halide may be . Pre alkali metal steam, negative ion vacancy is generally 20 ferred examples may be sodium chloride, sodium bro produced in the crystals. And the negative ion vacancy mide, , , potas is one of color centers, because it can catch an electron sium bromide, , fluoride, ru and absorb light at a definite wave length by which bidium bromide and the like. Besides, cesium fluoride, crystals are colored. An M-center is one of such color cesium bromide, and the like may be centers wherein two negative ion vacancies (F-center) 25 mentioned as other examples to be used in this inven stand adjacently to each other to the direction of (110) tion. or (110) and have two absorption bands and together The alkali metal ion in the compound producing an catch an electron. That is, the M-center oriented to the alkali metal ion having smaller ion radius than that of direction of (110), e.g., that of sodium fluoride shows the alkali metal in the alkali metal halide means, for in a maximum absorption band at 505 mpu to the light po 30 stance, lithium ion to sodium ion, or sodium or lithium larized to the direction of (110) and an absorption ion to potassium ion or the like. The compound pro band at 340 Mpu to the light polarized to the direction ducing an alkali metal ion having smaller ion radius of (110). The band at long wave length side between than that of the alkali metal in the alkali metal halide two absorption bands is called as the first absorption is one producing an alkali metal ion as mentioned band and one at short wave length side as the second 35 above, preferably a halide compound of the same kind absorption band. M-center possesses a property of of halogen as that of the alkali metal halide used. being not reoriented by the light of first absorption The metal ion in the compound producing metal ion band but reoriented by the light of second absorption other than the alkali metal ion which possesses an abil band to the direction of polarization, e.g., the M-center ity to act as an electron trap, means the ion derived oriented to the direction of (110) is reoriented to the 40 from metals as mentioned below. direction of (110) by irradiation of the light at second i. an alkali earth metal such as beryllium, magnesium, absorption band polarized to the direction of (110). calcium, strontium or barium. Accordingly, it will be possible to write and read an in ii. a metal belonging to the group I B of the periodic formation in using such property of M-center. When an table such as copper or silver. information O is applied for the M-center oriented to 45 iii. a metal belonging to the group II B of the periodic the direction of (1 1 O) and an information 1 for the M table such as zinc, cadmium or mercury. center oriented to the direction of (110), writing of the iv. a metal belonging to the group III B of the periodic information O in crystals by the light of second absorp table such as gallium, indium or thallium. tion band polarized to the direction of (110) will be v. a metal belonging to the group IV B of the periodic possible. And it will be also possible to erase an infor SO table such as lead or tin. mation 0 in M-center by the light polarized to the direc vi. a transition metal such as iron, cobalt, nickel, tita tion of (l 10) and stimutaneously to write a new infor nium, Vanadium, chronium or magnesium. mation 1. vii. a rare earth metal such as samarium or europium. Also, as numbers of M-center to be reoriented are The compound producing metal ion other than alkali proportional to exposing energy of writing light, the 55 metal ion which possesses an ability to act as an elec crystals having M-center will possess a capacity of an tron trap means the compound producing the metal analogue record for voluntary pattern such as charac ions as mentioned above. Preferable one is a halide ters, figures and the like as well as of a digital record compound of the same kind of halogen as that of the as mentioned above. They will, therefore, be usable for alkali metal halide to be used. a photosensitive material as equivalent to photographic 60 Speaking of the method for preparing the object ma films. Furthermore, a great advantage to use M-center terials in this invention, an appropriate amount of alkali as above is that crystals having M-center become a re metal halide crystal is firstly melted, e.g., in a melting versible photosensitive material which is capable of re pot or bath, to which are added one kind of compound cording and erasing an information repeatedly. There producing alkali metal ion and one or more kinds of fore the crystals are promising as a memory element. 65 compounds producing metal ions and melted. Crystals In utilization of M-center, the inventors of this inven are grown and isolated in accordance with a conven tion have discovered a reversible photosensitive mate tional method, e.g., Kyropoulos method. See, e.g., 3,909,270 3 4. Color Centers In Solids, J. H. Schulman and W. D. plicatively increased, as compared with that of the crys Compton, Pergamon Press, New York (1963), pages tals of alkali metal halide intermixed with either alkali 32-34. metal ion or other metal ion. The crystals of this inven The addition amount of each of the compound pro tion may be cleaved to make a thin plate which can be ducing alkali metal ion and the compounds producing used as a reversible ultramicrofiche, to which a graphic metal ion may be about 0.00001 - 1 O.O molar percent description is copied through a polarizer. Besides, they age to the crystals of alkali metal halide as the base ma may be used as an optical storage element for optical terial, preferably about 0.1 ~ 2.0 molar percentage. memory system, the material for pattern information However, even though the addition amount of each of filing system and the like. the compounds is increased over the above figure, it The following examples serve to illustrate the inven will be meaningless, because an amount to be practi tion without being limited thereto. cally intermixed with the alkali metal halide crystal de pends upon a coefficient of segregation of the crystals EXAMPLES used, applying ?cnr-'" Sodium fluoride (80 - 100 g) of a purity of 99.999 Next, M-center is formed in the resulting crystals by 15 % was melted in a platinum melting pot, to which a pre a conventional method. As its examples may be men scribed amount of each of an alkali metal fluoride and tioned a method for applying, an electron beam, X-ray metal fluoride(s) were added and melted. Then crystals or y-ray, an electrolytic coloring method and the like. were grown and isolated by Kyropoulos method. The It is desired to make M-center at a concentration as resulting crystals were irradiated with electron beam (2 high as possible, preferably a concentration of about MeV, 5 puA) for 10 mins. to form M-center, by which 107 cm 18 10/cm. For example, when a compara concentration of M-center became about 10/cm. A tively weak electron beam is applied to the crystals for photosensitivity was measured by applying a light of a long time, it can produce the crystals having a high super pressure mercury lamp to each of crystals ob concentration of M-center. tained. The results are shown in the following table. There are various combinations of the alkali metal 25 TABLE halide crystal, another alkali metal ion and other metal ions as the components of the materials in this inven Addition Content of Compound tion. Taking an example of sodium fluoride as the alkali Compound Amount Added in Crystals Sensitivity metal halide crystal, there may be mentioned as fol Added no.2, no.2 Ocm/m lows. 30 Pure NaF 3.O

LiF 2O O.28 5.7 AgF 0.5 0.0004 5.4 NaF : Li - Ag, NaF : Li - Pod, NaF : Li - Mg, PbF, 0.3 0.00018 1.2 NaF: Li - Be, NaF : Li - Sr, NaF : Mg - Cr, ZnF 0.5 0.014 30. NaF : Li - Mg. - Cr, NaF : Li - Ag - Be - Cr, 35 BeF 0.5 0.014 27.9 NaF : Li - T - Mg - Ni, etc. MgF2 0.5 O.OO7 37.7 CaF 0.5 0.04 26. SrF, 10 0.026 9.8 MnF, O. O.OOO5 0.8 Also, in case of potassium chloride as the base material, NiF 0.5 O.0003 9.9 there may be mentioned as follows. CoF 0.5 0.03 9.8 40 CrF 0.2 OOO2 12.7 In F 0.5 O.0003 4.7 LiF 0.5 0.077 KC : Na - Ag, KCl : Li - Ag, KCI : Na - Mg, AgF O 0.00009 36.4 KCl : Li - Be, KC: i - Sr., KCI : Na - Mg Lif 0.5 O.O85 -Cr, PbF, O.S O.OO25 29.6 KC : Li - Mg - Cr, KCI : Na - Fe - Mg, 45 LiF 2.0 O.O. KCI : Li - Cr - Be - Ag, KCl : Na - Ag - Be - Cr, MgF, O 0.5 213 KCl : Na - T - Mg - Ni etc. F 2.0 0.26 BeF 1.0 0.053 197 LiF O SrF, 1.0 48O Combinations in case of other alkali metal halides will LF O.2 m 50 MgF. O.2 125.0 be easily understood from the above concrete exam CrF O.2 ples, without further illustrations. Furthermore, num bers of the kinds of the metal ions other than alkali metal ion to be added will be suitably one to five, What is claimed is: though they are particularly limited. 1. A dichroic reversible photosensitive material, Thus, when writing and erasure of an information are 55 which consists of an alkali metal halide crystal having carried out in the use of crystals having M-center which M-centers, said crystal containing an alkali metal ion have been previously known, M-center will migrate one having a smaller ion radius than that of the alkali metal latice length and combine with another M-center or F in Said alkali metal halide and one or more metal ions center to form other color centers during repeated which possess the ability to act as an electron trap se writings and eliminations, by which the M-center disap 60 lected from the group consisting of alkali earth metal peares. Contrary to this, the crystals to be obtained by ions and transition metal ions. this invention have few such possibility, because at 2. The material in accordance with claim 1, in which least two kinds of impure materials which have a differ. the alkali metal halide crystal is crystal of sodium fluo ent action to each other are fixed in the crystals. There ride, sodium chloride, , sodium iodide, fore, they will be usable as a photosensitive material 65 potassium fluoride, potassium chloride, potassium bro having a long life, i.e., a reversible photosensitive mate mide, lithium fluoride, , cesium fluo rial. In addition, their photosensitivity itself has multi ride or cesium bromide. 3,909,270 S 6 3. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali metal halide crystal is sodium fluoride or so the alkali metal ion having smaller ion radius than that dium chloride crystal. of the alkali metal of said alkali metal halide is lithium 4. The material in accordance with claim 1, in which ion and the metal ion other than alkali metal ion which crystal contains said alkali metal ion and said alkali possesses an ability to act as as electron trap is beryl earth metal ions or a mixture of said alkali earth metal lium ion. ions and transition metal ions. 9. The material in accordance with claim 1, in which 5. The material in accordance with claim 1, in which the alkali metal halide crystal is sodium fluoride crystal, the alkali earth metal ion is an ion of beryllium, magne the alkali metal ion having smaller ion radius than that sium, calcium, strontium or barium. 10 of the alkali metal of said alkali netal halide is lithium 6. The material in accordance with claim 1, in which ion and the metal ion other than alkali metal ion which the transition metal ion is an ion of iron, cobalt, nickel, possesses an ability to act as an electron trap is stron titanium, Vanadium, chromium or magnesium. tium ion. 7. The material in accordance with claim , in which 10. The material in accordance with claim 1, in the alkali metal halide crystal is sodium fluoride crystal, 15 which the alkali metal halide crystal is sodium fluoride the alkali metal ion having smaller ion radius than that crystal, the alkali metal ion having smaller ion radius of tha alkali metal of said alkali metal halide is lithium than that of the alkali metal of said alkali metal halide ion and the metal ion other than alkali metal ion which is lithium ion and the metal ions other than alkali metal possesses an ability to act as as electron trap is magne ion which possess an ability to act as an electron trap sium ion. are magnesium ion and chromium ion. 8. The material in accordance with claim 1, in which ck k k

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