Compositions – Light transmission modifying compositions – Inorganic crystalline solid
Reexamination Certificate
2000-03-02
2003-02-04
Tucker, Philip (Department: 1712)
Compositions
Light transmission modifying compositions
Inorganic crystalline solid
C252S582000, C252S30140R, C252S30140H, C365S114000, C365S119000
Reexamination Certificate
active
06514435
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to memory storage of information, more particularly to materials, methods and apparatuses involving spectral holeburning for providing a medium for such storage.
The concept of using spectral holeburning for memory storage has been known since the discovery of the phenomenon in condensed matter.
“Spectral holes” are regions of reduced absorption (enhanced transparency) of the medium at discrete frequencies of the electromagnetic spectrum (visible light inclusive). Such holes can be created by a source of light such as a laser by selectively tuning the laser frequency to a predetermined value and impinging it on the medium of storage. This brings about changes at the atomic or molecular level altering the optical properties of a material (the medium of storage), only at the predetermined frequency. It is noted that spectral holes are not physical holes.
“Spectral hole-burning” is the process of creating or burning the spectral hole.
“Photon-gated hole-burning” is a special type of spectral hole-burning that takes place in the presence of two photons, whereas the holes can be read by one photon. It has the advantage that the process of reading the holes does not destroy the holes.
“Power-gated hole-burning” is a special type of photon-gated hole-burning where at higher powers the holes can be burned and can be read at a lower power that hardly affects them.
The potential of spectral holeburning based memory for high density storage has been demonstrated by using what is termed “transient spectral holeburning”; however, these systems would be classified as “volatile” storage systems.
“Transient” holeburning is significantly distinguishable from “persistent” holeburning. High density memory storage has been demonstrated by transient holeburning in rare earth doped materials. However, in cases of transient holeburning, if the memory is read, a constant refreshing of the memory (i.e., rewriting) is necessary. It is emphasized that “transient spectral hole burning” relates to “volatile memory,” while “persistent spectral hole burning” relates to “storage memory.”
Single photon persistent holeburning has been demonstrated but has had many disadvantages. Perhaps most notable among these disadvantages is the erasure of memory during the reading process.
The above-noted type of spectral holeburning known as “photon-gated holeburning” offers a solution to this problem. According to photon-gated holeburning, the holeburning takes place in the presence of two photons, while reading of these holes requires only one photon. Thus, according to photon-gated holeburning, the process of reading does not destroy the memory.
The multiplication factor by which the spectral holeburning can increase the storage density is determined by the maximum number of spectral holes that can be burned in an electronic transition. However, due to various material limitations, this number has not been large.
Szabo U.S. Pat. No. 3,896,420 issued Jul. 22, 1975, hereby incorporated herein by reference, discloses transient optical/spectral holeburning as a possible mechanism for memory storage.
Castro et al. U.S. Pat. No. 4,101,976 issued Jul. 18, 1978, hereby incorporated herein by reference, disclose a photon-gated holeburning method for creating persistent spectral holes which are not adversely affected by the reading process.
Other pertinent United States patents include the following, each of which is hereby incorporated herein by reference: Yagyu et al. U.S. Pat. No. 5,255,218 issued Oct. 19, 1993; Jefferson et al. U.S. Pat. No. 5,297,076 issued Mar. 22, 1994; Kodama et al. U.S. Pat. No. 5,478,498 issued Dec. 26, 1995; Gimzewski et al U.S. Pat. No. 5,588,886 issued Aug. 20, 1996; Kubota U.S. Pat. No. 5,746,991 issued May 5, 1998.
Other pertinent publications include the following, each of which is hereby incorporated herein by reference: T. Nishimura, E. Yagyu, M. Yoshimura, N. Tsukada and T. Takeyama,
SPIE Proceedings on Photochemistry and Photoelectrochemistry of Organic and Inorganic Molecular Thin Films
, eds: A. Frank, M. F. Lawrence, S. Ramesesha, C. C. Wamser, Vol. 1436, 31 (1991); H. Lin, T Wang and T. W. Mossberg,
Optics Lett
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Optics Lett
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J. Phys. Chem
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J. Chem. Phys
. 104, 793 (1996); T. Reinot, W. H. Kim, J. M. Hayes and G. J. Small,
J. Opt. Soc. Am
. B 14, 602 (1997); S. A. Basun, M. Raukas, U. Happek, A. A. Kapalyanskii, J. C. Vial, J. Rennie, W. M. Yen and R. S. Meltzer,
Phys. Rev. B
56, 12992 (1997).
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a medium for high density storage of information.
It is a further object of the present invention to provide a method for making a medium for high density storage of information.
It is another object of the present invention to provide such a method which uses fast holeburning.
Another object of the present invention is to provide a composition suitable for having holes burned therein in order to produce a medium for high density storage.
A further object of the present invention is to provide such a composition which is suitable for having holes burned rapidly therein.
The present invention is uniquely based on the application of the photon-gated holeburning process to II-VI materials doped with rare earth ions for achieving fast high density optical holeburning. According to the present invention, rare earth doped II-VI compounds are utilized for obtaining fast low power photon-gated high density rewritable memory. Persistent spectral holeburning is effectuated as the mechanism for information storage. Typical inventive practice prescribes a composition which has the following attributes: polycrystallinity; the presence of a host which is one or more narrow bandgap II-VI compounds; and, the presence of a dopant (with which the host is doped) which is one or more rare earth ions existing in at least two different valence states.
The designation “II-VI compound” is conventionally understood to refer to a compound which is the combination of a group (column) two (“II”) element and a group six (“VI”) element of the periodic table. Known group “IIA” elements are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra). Known group “IIB” elements are zinc (Zn), cadmium (Cd) and mercury (Hg). Known group “VIA” elements are oxygen (O), sulfur (S), selenium (Se), tellurium (Te) and polonium (Po). With regard to the present invention, experimentation has primarily involved the II-VI compounds MgS, CaS, BaS and SrS. Beside the II-VI sulfides (II-VI compounds including the group six element sulfur), other II-VI compounds such as II-VI selenides (II-VI compounds including the group six element selenium) appear to be promising and are being investigated.
The known “rare earth elements,” listed as follows, are conventionally understood to be those sixteen elements having an atomic number of 39 or between 57 and 71 inclusive: yttrium (Y, 39); lanthanum (La, atomic number 57); cerium (Ce, atomic number 58); praseodymium (Pr, atomic number 59); neodymium (Nd, atomic number 60); promethium (Pm, atomic number 61); samarium (Sm, atomic number 62); europium (Eu, atomic, number 63); gadolinium (Gd, atomic number 64); terbium (Tb, atomic number 65); dysprosium (Dy, atomic number 66); holmium (Ho, atomic number 67); erbium (Er, atomic number 68); thulium (Tm, atomic number 69); ytterbium (Yb, atomic number 70); lutetium (Lu, atomic number 71). With regard to the present invention, experimentation has primarily involved the rare earth element europium (Eu); other rare elements have also been tried, notably cerium (Ce), neodymium (Nd), samarium (Sm) and terbium (Tb).
According to many embodiments, the present invention provides a composition which is characterized by polycrystallinity and which comprises at least one II-VI compound and a
Hasan Zameer Ul
Mathur Veerendra Kumar
Kaiser Howard
The United States of America as represented by the Secretary of
Tucker Philip
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