Dual layer color-center patterned light source

#The present invention relates to a method and apparatus of forming and utilizing defects formed in ionic crystal.

#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 FA, FB, F2+, 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.

#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.

#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.