Optical logic element and methods for respectively its...

Optical: systems and elements – Optical computing without diffraction

Reexamination Certificate

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C359S900000, C365S106000, C365S112000

Reexamination Certificate

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06219160

ABSTRACT:

FIELD OF THE INVENTION
The invention concerns a multistable optical logic element and more particularly a proximity-addressable multistable optical logic element, comprising a light-sensitive organic material which can undergo a photocycle by irradiation light of one or more suitable wavelength, wherein the photocycle in addition to the physical ground state comprises one or more metastable physical states, wherein the physical state of the logic element is changed in the photocycle by causing a transition from a metastable state to another metastable state or by causing a transition from the ground state to a metastable state and vice versa, wherein a physical state is assigned a determined logical value, and wherein a change of the physical state of the logic element causes a change in its logic value and takes place by addressing the logic element optically for writing, reading, storing, erasing and switching of an assigned logic value. The invention also concerns a method for preparation of a light-sensitive organic material which can undergo a photocycle by irradiation with light of one or more suitable wavelengths, wherein the photocycle in addition to the physical ground state comprises one or more metastable physical states, and wherein the light-sensitive organic material is used as a switchable or data storage medium in a multistable logic element as stated above.
The invention also comprises a method for optical addressing of an optical logic element as stated above and with light-sensitive organic material prepared by method as stated above, such that the optical logic element is in an initial metastable physical state, and wherein the optical addressing comprises steps for writing, reading, storing, erasing and switching of a logic value assigned to the optical logic element.
Finally the invention concerns the use of the multistable optical logic element and the method for optical addressing of the multistable optical logic element.
DESCRIPTION OF RELATED ART
Digital computers are today essentially based on the use of semiconductor technology, i.e. electronic circuits which are driven and switched by electric currents. In case there are desired a particular fast storage and retrieval of data, there are for this purpose used storage mediums which also are based on semiconductor technology. For mass storage of data there have over decades been used magnetic storage mediums which have the advantage that the stored data quickly may be erased and new data once more stored in these storage mediums. For storage of large data volumes which shall be stored only once and then thereafter only read, optical storage mediums of the type WORM (Write Once Read Many Times) have in the later years seen an increasing use. Examples of such media are CD-ROM and the laser disk which not only have been used for storing databases and other substantially information-carrying files, but which have seen a wide distribution and popularity as storage medium for program sources used in audio-visual media, for instance for storing of music and movies.
In the later years there has also been proposals to store and process data by means of optical devices only. Optical dataprocessing has generally a number of advantages over data processing based on common semiconductor technology such as silicon technology. Optical dataprocessing may in any case theoretically increase the capacity for processing and storing of data with at least one order of magnitude beyond what is possible with today's conventional semiconductor technology. Optical dataprocessing further is expected to give increased error safety and processing speed, while it comports the possibility of a fast processing of data, not least by that devices for optical data storage and dataprocessing may be made substantially smaller.
In order to realise the potential which optical data storage and optical dataprocessing seem to imply, it is necessary to find media which makes it possible to realise the technology in a suitable manner. Particularly the interest has been concentrated on light-sensitive organic materials, for instance different proteins, and in this connection there shall generally be referred to an article of Robert R. Birge, “Protein-Based Computers”, Scientific American, March 1995, pp. 2-7. An instance of a light-sensitive organic material of this kind is bacteriorhodopsin which is a biologically generated protein compound and discussed in detail in the above-mentioned article by Birge. It may be produced in large scale by fermentation and has a high chemical and physical stability. When bacteriorhodopsin is illuminated with light of a suitable wavelength, it goes through changes which manifest themselves in different ways, included as modifications of the optical absorption properties. These changes are as described in Birge's article, connected with transition from a ground state bR and to various distinct energy states, denoted K, L, M etc. which each has a residence time which may be influenced thermally and/or optically. The sequential progression from the ground state and further through a set of such states is in the following denoted as the photocycle of the bacteriorhodopsin.
The possibility for optical data storage and optical data processing in bacteriorhodopsin was realised several years ago. A method for storage of data in that connection would be to transfer the bacteriorhodopsin from the bR state which, e.g. may represent a logic 0, to an intermediate state in the photocycle and with long lifetime, as this intermediate state may be denoted as a metastable state and for instance represent a logical 1. When natural bacteriorhodopsin is stimulated by light, it goes, however, through the whole photocycle and returns to the ground state bR in milliseconds, the longest-living intermediate state being the M state. A great research effort has been devoted to modify the original BR-molecule such that its lifetime in the M state is increased, for instance by experiments with mutants and the use of chemical modifications. This has resulted in that the lifetime of the M state has been increased to several seconds and even minutes, yet this is all too short time in relation to what is regarded as necessary for archival storage of data, namely many years, or even decades. Nevertheless the M state has been investigated with the aim of use in optical data storage wherein information is to be stored and retrieved during short intervals or wherein the stored data constantly is subjected to a refresh. It is also possible to interrupt the residence time in the M state by illumination with blue light, something which efficiently corresponds to a delete or erase operation. The limited lifetime in M state is, however, a negative factor in regard of practical uses, as the latter would imply a relatively complicated hardware and all the same give a limited capacity. Another disadvantage with basing schemes for data storage on the M state of the bacteriorhodopsin is the time delay which is inherent in the photocycle, corresponding to a delay of typically 100 &mgr;s from when the ground state bR for instance is excited with a light pulse and till the M state is reached.
Attempts have also been made to develop suitable media for optical data storage by modifying the light-sensitive material such that it undergoes an irreversible change when illuminated at a suitable wavelength. Such materials may make a simple write operation possible followed by an arbitrary number of read operations, but cannot be erased and written once more. They will hence be able to realise optical memories of the types ROM and WROM, but not of the type “erasable”.
Recently it has been proposed, as it is evident from the above-mentioned article by Birge, to base schemes for optical date storage on a branching process in the photocycle of the bacteriorhodopsin. With starting point in the ground state bR a short pulse of yellow/green light initiates the photocycle, whereafter the bacteriorhodopsin spontaneously undergoes a sequence of states in the photocycl

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