The present invention relates to the field of optical recording materials, in particular, fluorescent compounds and matrices suitable for use in optical memory systems, including three dimensional optical memory systems for READ ONLY MEMORY (ROM).

Background of the Invention Information storage and optical recording systems are of rapidly increasing importance in the modern society in view of the exponential rise in the number and complexity of the data which must be recorded and be retrievable so as to handle the increasing everyday business workload, and to assist efficiently enough in scientific development.

Many optical information storage systems, including those based on silver halide emulsions and the like, have contributed significantly to this development, generally because of the high packing density combined with good resolution of information retrieve inherent to such systems. The advantage of READ ONLY MEMORY devices, based on photo chemical generation of fluorescent compounds lies in the fact that storage of information in such systems is rapid and accurate, and there is no fixing required. Reading is also rapid, exceptionally sensitive and accurate and is not accompanied by degradation.

At the same time there is another requirement the modern optical storage systems should comply with. The reading process should be non destructive for the memory material so as to enable repeating of reading thousands of times without detectable deterioration of the medium material. The reading process can be carried out either by scanning point by point ofthe same layer, i.e. bit by bit of information, or by simultaneous reading from a 2-D layer carrying a medium material with the data recorded therein. In this manner a very fast access time to the stored information can be achieved. Reading of this type can be implemented by illumination of a 2-D layer by a fiber optics means. The intensity of illumination radiation should be sufficient for triggering fluorescence within the medium

material accompanied by subsequent fluorescent photo-emission from all sites of the layer where the data have been stored.

Various methods and systems of recording of information based on generation of fluorescent compounds obtained from non-fluorescent precursor have been proposed.

The known methods include, for example, UV irradiation of bis-diarylchloromethyl-1,3,4-oxadiazoles as described by Singh in U.S. Patent 3,869,363, herein incorporated for reference. There are known also many others systems as described by Zweig in the paper "Photochemical Generation of Stable Fluorescent Compounds", Pure and Applied Chemistry, vol. 33, pages 389-410 (1973), herein incorporated for reference.

The main common feature of the prior art optical memory systems is associated with the fact that reading in those systems can be implemented only by lasers having wavelength shorter than 500 nm. It can be appreciated that this requirement prevents employing of modern lasers with longer wavelength.

Furthermore, there exists another requirement to the optical memory systems. This requirement is associated with the fact that the intermediate photo-induced fluorescent product should be thermally and photo-chemically stable otherwise it can be destroyed by the reading process itself. Unfortunately the known in the art materials employed in optical memory systems are not sufficiently stable.

It is therefore the primary object of the present invention to provide a method of recording of information within a 3-D Read Only Memory devices enabling high efficiency of writing in terms of quantum yield of the photo-transformation and in terms of quantum yield of the fluorescence produced by the photo-induced form of the medium material.

The other object of the present invention is to provide for a medium material suitable for use in an optical storage memory system, which is defined by high thermal and photochemical stability.

Summarv of the invention Rhodamine B lactams have been known for a long time for use as coloring element in thermo- and piezo-sensitive papers, as described in US 3922283 by M. Iahargi, S.Horiuchi and in Japanese patent 11660 (1972) by M. Sadao, O. Misaoki and F. Mitsuru and in US patent US 4599630 by K. Ohtaki and H. Sakamoto herein incorporated for reference.

The Rhodamine-B lactams have been recognized as suitable coloring additive for chemical sensors however they were not considered in connection with optical recording of information. Despite the fact that Rhodamine B lactams are well known compounds there was not known about their ability to undergo photo-transformation under illumination by long wave electromagnetic radiation with subsequent formation of fluorescent derivatives and therefore about their suitability for optical storage memory systems.

In accordance with the present invention it has been surprisingly discovered that the transformation of a Rhodamine-B lactam into respective Rhodamine derivative can be successfully implemented for recording of information in various optical memory systems by virtue of the fact that Rhodamine derivatives exhibit induced fluorescence when illuminated by a long wave radiation. The induced fluorescence can be detected for example by a CCD camera.

List of drawings Fig. 1 shows fluorescence spectra of a Rhodamine B derivative in accordance with the present invention.

Fig.2 represents experimental set-up suitable for two-photon writing of information into 3-D memory system.

Detailed description of the preferred embodiments Thus the active medium in accordance with the present invention includes a material existing in a first, non-fluorescent form, capable of photo-transformation into a second, fluorescent form. A longer wavelength irradiation is then applied to cause the

second form to fluoresce and thus to display an image which can be detected, for example by a CCD camera.

For this purpose there are employed suitable compounds disclosed further and designated for the sake of brevity as D. Among those compounds are Rhodamine B lactams having general formula: where Y is phenyl ; o-, m- and p-halogenphenyl, o-, m- and p-alkylphenyl (alkyl - C1 -C18) In accordance with the present invention it has been empirically found that a group of compounds existing in a form D undergo photo-transformation into a fluorescent form E according to the following equation: The preparation of compounds D is described in details in M. Iahargi,; S.Horiuchi in U. S.

Patent 3,922,283 1975: For synthesis of a compound D to the mixture of 102 g Rhodamine B, 408 g toluene and 204 g o-chloroaniline 60.7 g of phosphorus chloroxide had been added. The temperature <BR> <BR> of the solution was increased up to 1000 C. The mixture was mixed for 1 hour at 1000 C, after that the extra quantity of toluene and o-chloroaniline had been separated by

distillation with water vapor, the rest had been extracted with 100 ml of toluene, rinsed with water solution of NaOH, and the extra quantity of toluene had been separated by distillation. The rest had been diluted with 300 ml of methanol and 100 g of o-chloroanilide of Rhodamine B had been isolated after drying out for 2 hr at 70" C.

It should be realized that the above described method is only an example. Any other methods suitable for synthesis of Rhodarnine-B lactams can be used by those skilled in the art.

Three different lactams designated correspondingly D 1 ,D2,D3 were prepared by similar manner and their respective fonnulae are presented below.

In the following non-limiting table 1 below are summarized spectral and luminescent properties of non-fluorescent lactams D together with similar properties of their corresponding fluorescent derivatives E.

Table 1 Spectral and Luminescent Properties of Lactams and respective Rhodamine derivatives in chloroform at 25°C.

Compound #maxD #maxD #maxE #DE #F1E (nm) (mol-1 1 (nm) cm-1) D1 320 12880 574 0.14 0.72 Et2N D2 320 12400 574 0.08 0.99 Et2N X D3 320 12880 578 0.18 0.68 Et2N NEt2 NIi)½½I/)\I In the following non-limiting table 2 below are summarized properties of the above compounds D, incorporated within a polymer film.

Table 2 Spectral and Luminescent Properties of the Rhodamine B lactams and respective Rhodamines in polystyrene film at 25°C.

<BR> <BR> <BR> <BR> <BR> <BR> Compound #maxD #maxE #DE #F1E<BR> <BR> <BR> <BR> <BR> <BR> <BR> (nun) (nun) Do 313 590 - 0.34

Et2N So D2 308 600 - 0.64 Et2NONEt2 D3 312 590 - 0.39 Et2N NEt2 In the above tables #maxD and #maxE are absorption maxima of form D and E, 8 axD iS extinction coefficient of form D, (pDE is quantum yield of the photo-transformation of form D into E for the irradiation wavelength of 320 nm and (pFsE is quantum yield of the fluorescence of form E at excitation with 590 nm.

Now with reference to the following non-limiting examples it will be described how an active medium containing the above mentioned compounds can be manufactured.

Example 1

In this example a solution of 0.0086 g of lactam D1 was mixed with the solution of 0.02 g polystyrene in chloroform. The ready composition have been deposited on lavsan slide with the aim of applicator and was allowed to evaporate until the complete drying out for 1 hour at 700C. The ready film having thickness 3 microns shows no fluorescence at 570 nm. After irradiation for 3 minutes with high pressure Hg-lamp (200 W) equipped with UFS glass filter the very contrast red photo-coloration has been got. When the film was irradiated at 320 nm the compound D1 was converted into compound El, which exhibits a bright fluorescence.

With reference to fig. 1 one can see the absorption spectra of compounds D1 and El (curves 1,2) and the fluorescence spectrum of compound El (curve 3).

Example 2 In this example the mixture containing 0.0115 g of lactam D1, 0.0025 g of CBr4, 1.1 cm3 of toluene and 1 cm3 of 2% solution of polystyrene in CHIC13 have been deposited with the aim of applicator on the glass slide and was allowed to evaporate for 3-4 hours.

Afterwards the ready film having thickness 5 microns was irradiated by high pressure Hg-lamp with light 320-360 nm (glass filter UFS-1) for 2 minutes. The photoinduced photo-coloration with D56=l .08 has been got. The irradiated film was allowed to evaporate for 1 hour at 700C and for 30 minutes at 900C. After the treatment the film was kept 4 months in dark place at room temperature. The change in photo-induced optical density (D563=1.08) was less than 0.05%.

Thus writing of information can be implemented by causing the Rhodamine-B lactam to absorb the irradiated light and to transform into a Rhodamine derivative.

The process of reading of the recorded information can be implemented by detecting the fluorescence emitted from the Rhodamine derivative.

The advantage of the present invention lies in the fact that the reading process can be carried out by excitation of the recording medium at 600 nm, and thus the modern miniature diode lasers can be employed.

The active medium in accordance with the present invention can be advantageously implemented in the process of2 photon writing by virtue of a set-up as shown in fig. 2.

Two laser beams LIB 1 and LB2 from two respective laser sources (not shown) are selectively directed onto the layer of polymeric film F constituting a matrix carrying an active medium incorporated therein. The composition of this medium is chosen in accordance with the present invention. The first beam is produced by a cylindrical lens CL and the second beam is produced by a diode laser DL. The second beam can be deflected by virtue of deflectors Defl and Def2. The film is transparent to the electromagnetic radiation at least in some parts of the spectrum. The intersection of beam LIB 1 with the beam LB2 in a selected point of focus PF on the film is accompanied by a photo-chemical reaction in this selected location. In practice this location looks like a colored point. The laser beams can be displaced across the film and so the point of focus, which will describe a trajectory inside the polymeric film. The intersection of the trajectory with the active sites of the matrix is accompanied by writing of the binary data. The remaining part of material, that is the ones not intersected by this trajectory, is not affected since the irradiation intensity beyond the focus is less than the threshold required for transformation of the form D into form E.

In practice the writing was implemented by mode-locked Nd:Yag pico-second laser with the following parameters: energy E( 1 064nm)=0. 3 J; E(53 2nm)=0.03 C1J; P(1 064nm)=22W; P(532nm)=2W; A( 1 064nm)=3 x3 = 10~7cam2; A(5 3 2nm)=3 Oux 1 cm=3 10~3cam2, The beam at 532nm was focused by the cylindrical lens creating flat beam with uniform thickness app.30microns ( con-focal parameter in this case is about 1cm). The intersection of this beam with focused beam at 1064nm is followed by corresponding photo-transformation of form D into form E.

The photons emitted by form E can be deflected towards a CCD camera (not shown) and registered. This registration will constitute the process of reading. From a practical viewpoint the fluorescent response of the medium material presenting in the form E, with a written information recorded in it, would occur only if the intensity of the irradiation is sufficiently high. This second energy threshold, however, is lower than the threshold that should have been exceeded for writing.

The active medium in accordance with the present invention is characteristic by a very high photochemical stability: 180-hours irradiation of the film disclosed in Example 1 at the wavelength 580 nm (applied as an excitation source for reading) within the cell of the spectro-fluorimeter Shimadzu didn't lead to any visible changing in the intensity of the fluorescence of the photoinduced form E. Similar irradiation of the film carrying bis-di-m-anilinochloromethyl-1,3,4-oxadiazole as an active medium described in US patent 3869363 resulted in reducing the fluorescence intensity by 2.6 times.