Abstract:
A method of fabricating a color laser, comprising growing a first thin layer of ionic crystal on a substrate. The crystal can comprise many types of ionic crystals, such as sodium chloride or potassium chloride. A second thin layer of a different type of ionic crystal can be deposited above the first ionic crystal layer, such as lithium fluoride or sodium fluoride. An inert metal layer can be deposited between the first and second layers of ionic crystal and above the second layer of ionic crystal. When the first and second ionic crystal layers are radiated with gamma rays, they form color centers at the spots radiated. Because of the difference in crystalline properties of the two different ionic crystal centers, their color centers have different wavelengths. Each of the ionic crystal layers emit light at different characteristic wavelengths when illuminated at their unique absorption frequencies, and can be made to lase separately.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/827,170, filed Jul. 11, 2007 now abandoned, which is a divisional of U.S. application Ser. No. 10/934,251, filed Sep. 3, 2004, now U.S. Pat. No. 7,260,127, which issued on Aug. 21, 2007, which is a continuation of U.S. application Ser. No. 10/120,698 filed Apr. 11, 2002, now U.S. Pat. No. 6,795,465, which issued on Sep. 21, 2004, the entire disclosures of which are hereby incorporated by reference as if being set forth in their entireties herein. 
    
    
     FIELD OF INVENTION 
     The present invention relates to forming and utilizing defects formed in ionic crystal. 
     BACKGROUND OF THE INVENTION 
     The use of color centers in ionic crystals has been known for some time. A color center laser, for example, is a known light source that operates on a basis of random defects formed in an ionic crystal. See U.S. Pat. No. 5,889,804 entitled, “Artificial Color Center Light Source” which issued on Mar. 3, 1999 to Y. Takiguchi. In that patent there is described a color center light source where a color center is formed artificially. A predetermined single atom is removed from the surface of a defect-free ionic crystal so as to form a lattice defect. Optical transition of the defect is utilized so that it functions as a light source. In the past, these color centers have been formed by methods such as exposing the crystals to gamma radiation or heating in the presence of excess cations or other impurities. These methods cause anions to be displaced from the crystal lattice. The hole left by the cation can then be filled by an excess electron that is attracted to the void due to the positive ions surrounding it. The electron can then be treated as if in a potential well whose size is smaller than the wavelength of the electron; such a well has discrete energy levels which can be predicted quite easily. When an incident photon hits the trapped electron it will be absorbed if the energy of the photon is the same as the difference between the two energy levels of the electron in the well; this will also cause the electron to be excited into the higher energy state. In this way the electron can be used to absorb only select wavelengths of light that correspond to the energy levels in the well. Once the electron is in an excited state, the surrounding crystal will relax, thereby changing the energy gap between the excited and ground states of the potential well. When the electron decays back into the ground state it will emit a photon with different energy and therefore a different wavelength than the incident photon. This is commonly referred to as an F-center type color center, so called because the absorption of discrete wavelengths gives a unique color to the ionic crystal. In general, for most ionic crystals, an F-Center has an absorption peak within the visible light spectrum, however when an excited electron decays back to the ground state it does so over a smaller energy gap and emits light of a longer wavelength. There are other types of color centers such as F A , F B , F 2 +, and others which can be created through various types of annealing and bombardment by radiation. The other color centers are caused by various other impurities and dislocations present in the crystal and they will each absorb and emit at different areas of the spectrum. 
     Use of color centers has been employed in the prior art. See for example, the above-noted patent, U.S. Pat. No. 5,889,804. See also U.S. Pat. No. 4,990,322 entitled, “NACL:OH Color Center Laser” which issued on Feb. 5, 1991 to C. R. Pollock et al. and is assigned to Cornell. See also U.S. Pat. No. 4,839,009 entitled, “NACL:ON Color Center Laser” which issued on Jun. 13, 1989 to C. R. Pollock et al. See U.S. Pat. No. 4,638,485 entitled, “Solid State Vibrational Lasers Using F-Center/Molecular-Defect Pairs in Alkali Halides” which issued on Jan. 20, 1987 to W. Gellermann et al. See also U.S. Pat. No. 5,267,254 entitled, “Color Center Laser with Transverse Auxiliary Illumination” which issued on Nov. 30, 1993 to I. Schneider et al. 
     In most of these patents the color centers are created throughout the ionic crystal so that the whole crystal can be used to lase light. A notable exception to this is the point light source patent where a scanning electron microscope is used to create a single F-center dislocation to be used as a point light source. In all cases only a single type of ionic crystal is used so that there is only one absorption and emission peak. 
     It is an object of the present invention to provide an apparatus and a method of forming and utilizing defects formed in the ionic crystals. 
     SUMMARY OF INVENTION 
     In the present invention a thin layer of ionic crystal is grown on a substrate. The crystal could be of any type of ionic crystal such as NaCl or KCl. The crystal could be a pure form of the chosen compound or could contain contaminates which would shift the wavelength of the created color centers. On top of the thin layer, a second thin layer of a different type of ionic crystal is deposited. The second layer, for example, can be LiF or NaF. When these two layers are irradiated with gamma rays, they will each form color centers at the spots which are irradiated. Because of the differences in crystal properties of the two different ionic crystal layers, their color centers will be at different wavelengths. For instance, NaCl absorbs light at a wavelength of 459.6 nm while LiF absorbs light at 248.2 nm. Once the F-center has absorbed light of a certain wavelength, it will eventually decay and emit light at a different higher wavelength. Accordingly the two separate ionic crystals also emit light at different characteristic wavelengths when illuminated at their unique absorption frequencies. Each layer can be made to lase separately. It is important to make sure that the top layer has absorption energy greater than that of the bottom layer. This way the lower light energy of the bottom layers absorption peak will pass through the top layer and be absorbed only by the bottom layer. By selectively exposing different areas of each layer of the crystals to gamma radiation, it is possible to create unique areas in each layer that contain color centers. If the crystal layers are exposed to light at the wavelength characteristic of the absorption of one layer of crystal, that layer&#39;s pattern will be apparent and emit light at its emission wavelength. In any event, if the device is exposed to light of the second layer&#39;s absorption wavelength, then the second pattern will be exposed and light will be emitted at its characteristic emission wavelength. Thus, as one can ascertain, by the utilization of the above-noted invention, two different wavelengths of light can be emitted in a single device. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows several different types of color centers (F) which can be produced in an ionic crystals. 
         FIG. 2  is a graph depicting the energy gap between the ground state and the excited state, as it varies with the distance between surrounding anions. 
         FIG. 3  is a graph depicting the energy of the absorption and emission in an F-center. 
         FIG. 4  is a table showing the principal bulk characteristics of alkali halide crystals. 
         FIG. 5  is a cross sectional view depicting an embodiment of a device according to this invention. 
         FIG. 6  depicts the device of  FIG. 5  after being exposed to gamma rays. 
         FIG. 7  is a cross-sectional view showing the device of  FIGS. 5 and 6  exposed to light of the absorption wavelength of the top crystal layer. 
         FIG. 8  shows another embodiment of a device according to this invention. 
         FIG. 9  shows the results of gamma irradiation on a device depicted in  FIG. 8 . 
         FIG. 10  shows the device of  FIG. 8  operative with light of the characteristic absorption wavelength of the top layer incident on the device. 
         FIG. 11  shows still another embodiment of the device according to this invention. 
         FIG. 12  shows the device of  FIG. 11  after both the first and second layers have been treated with the anion layer and heated. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , there is shown several different types of color centers which can be produced in ionic crystals, including the F-center depicted by reference numeral  10 . The name F-center comes from the German word for color, Farbe. F-centers can be produced by heating a crystal in excess alkali vapor or by x-irradiation. The central absorption band (the F-band) associated with F-centers in several alkali halides is well known. The F-center has been identified by electron spin resonance as an electron bound at a negative ion vacancy. The F-center  10  of  FIG. 1  is a negative ion vacancy with one excess electron bound at the vacancy. Reference numeral  11  refers to a self-trapped hole, which essentially shows two vacancies where two holes are indicated by reference numeral  11 . There is also shown another F-center  12  in  FIG. 1 . 
     As one can understand, the lattice vacancies are well known and reference is made to the above-noted patents. One can also understand that the simplest imperfection is a lattice vacancy, which is a missing atom or ion, which is also known as a Schottky defect. A Schottky defect in a perfect crystal can be created by transferring an atom from a lattice site in the interior to a lattice site on the surface of the crystal. In thermal equilibrium a certain number of lattice vacancies are always present in an otherwise perfect crystal, because the entropy is increased by the presence of disorder in the structure. 
     Referring to  FIG. 2 , there is shown a graph of energy versus displacement and depicts the energy gap between the ground state depicted by reference numeral  14  and the excited state  15 . This is depicted as it varies with the distance between surrounding anions. It can be seen that the energy of absorption  16  is greater than that of emission  13 . This is due to the shift in energy levels with the relaxation of the crystal after absorption. 
     Referring to  FIG. 3 , there is shown the energy absorption  17  and emission  18  in a KBr F-center. It can be seen that the energy of emission  18  is significantly lower than the energy of absorption. This means that the wavelength of the emitted light will be longer than that of the absorbed light. The peaks are spread out due to fluctuations in bond lengths at temperature. 
     Referring to  FIG. 4 , there is shown a table of energies of absorption and emission in many common ionic crystals. As one can see from  FIG. 4 , the crystal is depicted in the left-hand column with the cation and anion distance, the exciton energy and various other characteristics depicted. Such tables are well known and many examples exist in the prior art. In any event, one can understand from  FIGS. 1 through 4  how color centers are formed and basically they describe an indication of the nature of such centers. 
     As will be explained subsequently, in the present invention there are two ways in which to create patterns of F-centers using ionic crystals. In the first way, collimated gamma rays are used to expose only certain areas of each crystal to the radiation necessary to produce crystal dislocation and therefore F-centers. The gamma radiation passes through both layers of crystals creating color centers in each crystal at the same location. As will be explained, because of the difference in absorption and emission wavelengths of each layer, the two crystals would activate at different wavelengths. Such a device is used to create color center lasers capable of lasing at two unique discrete frequencies. 
     Referring to  FIG. 5 , there is shown the following structure. Reference numeral  20  depicts a substrate upon which is grown a crystal such a NaCl or KCl or another crystal as shown, for example, in  FIG. 4 . The crystal is grown on substrate  20  by known crystal growing techniques and substrate  20  may consist of silicon, a metal, plastic or another substrate material upon which a crystal could be grown. The substrate is an inorganic material. The ionic crystal thickness is on the order of a few milliinches, but the thickness is not critical. It can be deposited on the substrate by sputtering. On top of the crystal layer  21  is a metal layer  22  which is deposited either by CVD or other metal deposition techniques as sputtering and so on. The metal layer  22  has openings or holes  23  and  23 L. The metal layer is an inert metal as, for example, gold, platinum, or any other inert metal and is very thin as between 0.05 to 0.2 millimeters. Deposited on top of the metal layer  22  is a second layer  24 , which is a thin layer of a different type of ionic crystal, which for example could be LIF or NaF. The layer  21  absorbs light at a wavelength different than the layer  24 . As seen, deposited on top of layer  24  is another metal layer  25  which has openings  26 ,  26 B and  26 C. The openings  26 , as one can see, are in alignment with openings  23 , while opening  26 B is associated with layer  24  and therefore gamma rays  27 , which are directed through opening  26 B, do not in any manner impinge on layer  21 . Whereas, for example in the event of gamma rays  27 L enter aperture  26 C and pass through aperture  23 C to impinge upon both layers  24  and layer  21 . As can be seen, gamma rays  28  are absorbed by metal layer  25 L and do not further enter the substrate. 
     As indicated, and as seen in  FIG. 5 , the holes  26  and  23  allow gamma rays to pass through. Therefore, the gamma rays  27  would pass through appropriate apertures  26  and impinge upon layer  24 . Gamma rays will also pass through appropriate apertures  23  to impinge upon layer  21 . Gamma rays  27  will pass through aperture  26  and be blocked by the metal layer  23 M to prevent them from passing to layer  21 . In a similar manner, gamma rays  27 R pass through aperture  26 C, to irradiate layer  24  and also impinge upon layer  21  as they pass through aperture  23 . The use of gamma radiation as shown in  FIG. 5  will produce the F-centers depicted in  FIG. 6 . 
     In  FIG. 6  there is shown the laminate device of  FIG. 5  after being exposed to gamma rays. F-centers such as  30 ,  31 ,  32 ,  33  and so on are present. Essentially,  FIG. 6  shows F-centers located at all areas that were exposed to radiation. Also shown in  FIG. 6  are light beams such as  40 ,  41 ,  42  and  43 . These light beams, for example, are light of the absorption wavelength of the lower ionic crystal. If the light beams  40 ,  41  and  43  are at absorption wavelengths of those of the lower ionic crystal, then the lower ionic crystal based on the F-centers in that layer will emit light as shown by reference numeral  50 . Thus, light  40  which emanates and strikes layer  21  causes layer  21  to lase and emit light  50  at the emission wavelength of layer  21  due to the F-center. It is seen that light  41 , which is of the same frequency as light  40 , does not cause layer  24  to lase because of the different absorption frequency. 
       FIG. 7  depicts a same device a s shown in  FIG. 6 , except that the light  60 ,  60 M and  60 R depicted in  FIG. 7  is of the absorption wavelength of the top crystal layer  24 . Also shown in  FIG. 7 , only the top crystal layer emits light as, for example, rays  61  and  62 , when light  60  impinges thereon. Light rays such as  60 R and  60 L which is the same frequency as light  60  impinge upon the metal surfaces  25  and  25 L and thereby do not cause lasing. The lasing is caused because the structure depicted has the access holes such as  26  and  23  to enable light to be directed to either the top ionic crystal  24  or the bottom ionic crystal  21 . 
     Referring to  FIG. 8 , there is shown still another embodiment of a device where essentially there is depicted a substrate  80  having deposited on a top surface a first ionic crystal layer  81 . Deposited on top of first ionic crystal layer  81  is a second ionic crystal layer  82 . Both ionic crystals have different absorption wavelengths as indicated above. In the embodiment of  FIG. 8 , collimated beams of gamma rays  83 L and  83 R are directed to both crystals at particular areas, which are selected spots. As can be seen, the difference between the structure in  FIG. 7  and  FIG. 8  is that the metal layers do not exist and therefore there are no particular holes, but a selected area of the device is now irradiated by collimated gamma rays, which essentially cause F-centers to appear. As seen in  FIG. 9 , the F-centers  85  and  86  will only appear where the collimated beams are incident. 
       FIG. 9  shows the results of the gamma irradiation of the crystal layers. When light  88  and  88 A of the absorption wavelength of the bottom layer  81  is directed on the crystal, the bottom crystal lases, producing output beam  89  and  89 A. Since the wavelength of light is not the absorption wavelength of the top crystal  87 , there is no lasing of the top crystal. However, in  FIG. 10 , light  90  and  90 A, which has the characteristic absorption wavelength of the top layer  82 , the top layer  82  produces output beams  91  and  91 A. In this case, the F-centers of the top layer emit laser light  91  and  91 A indicative of the top layer&#39;s emission wavelength. 
     Referring to  FIG. 11 , there is shown still another embodiment. In  FIG. 11 , a substrate  95  has deposited thereon a first ionic crystal layer  96 . Deposited on ionic crystal layer is an anion layer  97 . The anion layer, when heated, an ionic crystal with F-centers ( FIG. 12 ). The numerals F designate the F-centers, which are created when needed. As one can ascertain, there are F-centers as  99 A,  99 B and  99 N. These F-centers are distributed throughout the layers and therefore when light, again of a particular frequency indicative of the top layer  98  or the bottom layer  96  is incident upon the device, the device will emit or lase light based on the emission wavelength of the layers. Thus, as one can ascertain from the present invention, F-centers are created by collimated gamma rays, which expose only certain areas of each crystal to the radiation necessary to produce crystal dislocation and therefore, F-centers. The gamma radiation passes through both layers of crystals, creating color centers in each crystal at the same location. However, because of the difference in absorption and emission wavelengths of each layer, the two crystals activate at different wavelengths. The device depicted is utilized to create color center lasers capable of lasing at two unique frequencies. 
     The second way of producing patterns of F-centers is by masking each layer with a thin layer of metal. This is depicted, for example, in  FIGS. 5 ,  6  and  7 . A layer of metal is deposited on the first layer of crystal and then etched to form a pattern of holes before the second ionic layer is deposited. A final layer of metal is then deposited over the second layer of crystal. This second layer can also be patterned such that some of the holes coincide with the holes of the first layer and some of them are unique to the second layer. In this layer collimated gamma ray beams can be swept or scanned over various parts of the device. If the beam is aimed at a hole that is in both layers of metal, then both ionic crystals will be exposed and become colored. If the beam passes over a hole that is only in the top layer of metal, then the top crystal will be the only one with color centers at that location. In this way, the bottom crystal layer will have a pattern that is only part of the pattern contained on the top layer. This allows the device to display two different patterns, depending on the wavelength on light incident on the surface. 
     Another way of producing the F-centers in the two layers does not involve gamma rays. This is depicted, for example, in  FIGS. 11 and 12 . It is known that if an ionic crystal is heated in the present of excess anions, F-centers can be formed in the material. In the present device, after each ionic layer is deposited, a thin layer of the anion alone, such as sodium in the case of sodium chloride, is deposited in the top. This material is then heated so that F-centers are formed in the crystal. Then the second layer is added and another layer of different anion is deposited and the whole device is heated again. In this case, there is no need for a metal masking layer, as the entire ionic layer will generate F-centers. This technique will be useful in the creating of other layers in the two different lasing frequencies. 
     It is, of course, understood that such a device having two different lasing frequencies is extremely useful and many applications, such as the transmission of information along optical fibers and other uses can be employed as well. The device can be used to create two color displays as reference numerals or other display by scanning with proper light beam. While the above-noted invention was described in terms of specific embodiments, it should be understood by those skilled in the art that many alternate embodiments could be employed as well without departing from the spirit and scope of this invention.