Test of an optical memory cell. 

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... advances in the sol-gel based gel-glass technology have resulted in the development of new advanced materials that find applications in the fields of photonics and optical computing. 1 Materials possessing two stable states, each with distinct optical characteristics are usually sought for digital optical memory devices. This property can, for example, be manifested by the existence of two distinct degrees of absorption ͑ transmission ͒ at a particular optical waveband. Transitions between states in these materials can be achieved by light irradiation at specific wavelengths 2 ͑ photochromic materials ͒ or by changing the material temperature ͑ thermochromic materials ͒ . When used as optical storage media, long term material stability, ease of reading and writing and high information storage capacity are some of the desired characteristics sought in these substances. Previously optical memories based on photochromic materials were reported. 3,4 Here a thermochromic material is demonstrated as an optical information storage media. The sol-gel process has been described elsewhere. The dopant used in the preparation of the thermochromic material (C 19 H 18 N 2 O 3 ) contains photoactive organic molecules. 6 Casting the doped material into the molds be- fore completion of the curing process resulted in fabrication of several experimental devices. Generally, the shape of these devices can be adapted to their intended applications, such as optical fibers or waveguides. Coupling with the silica-based fibers is a straightforward procedure, since in- dex matching and adhesion are provided by the sol-gel material itself. The insert of Figure 1 shows an experimental setup used for the measurements of thermal and optical properties of thermochromic material. The sample holder is a rectangular glass container measuring 10 mm in both height and width and 1.5 mm in depth. The material under study is placed inside the container through the uncovered top opening. The external surface of the plates is coated with a thin layer of ITO ͑ indium tin oxide ͒ , which is used as an optically transparent electrical resistance. The temperature of the sample is varied by applying a dc voltage to the ITO electrodes. Temperature control is achieved by an electronic system with a temperature sensor residing inside the material. This setup enables the collection of spectral response data at different temperatures, including a dy- namic response of the material to thermal variations. The spectral characteristics of the thermochromic material as a function of sample temperature were measured by a Perkin- Elmer Lambda2 spectrophotometer. Figure 1 shows a series of absorption spectra as a function of material temperature. There clearly exist two absorption bands, one of which is located in the ultraviolet ͑ UV ͒ and the second in the green ͑ G ͒ regions. Material behavior in each one of these regions is distinct. Absorption in the UV remains almost constant, independent of the temperature. It is mainly due to the sol-gel gel-glass host matrix. On the other hand, absorption in the G region increases with the rise of the material temperature. The peak of the absorption in the G is in the 514 to 540 nm wavelength band. The thermochromic substance under study has two stable states, each of which can be identified by the material color: colorless and colored ͑ purplish red ͒ , which correspond to low and high absorption conditions, respectively. Material behavior as a function of temperature in the green part of the spectrum was further studied. In particular, absorption as a function of temperature was measured at the wavelength of 514.5 nm. This wavelength also corresponds to one of the emission lines of an argon laser, which was employed in the optical memory experiment, as described later. Figure 2 shows a family of absorption curves corresponding to the heating and cooling cycles of the material. The threshold temperature, at which the trans- fer of material from colorless to a colored state begins has a value of around 24°C. Also, on the cooling cycle material does not return to the initial colorless state, but rather remains in the colored one. It will stay colored as long as kept in the light-free environment. Figure 3 shows material absorption long time after its exposure to temperatures above the threshold. In particular, this is an absorption curve of a sample whose temperature has been raised to 30°C and later cooled to room temperature. One can observe ͑ Figure 2, curve b ͒ that immediately after the sample temperature was lowered back to 20°C the absorption was 0.079. However, after a period of 20 h ͑ Fig. 3 ͒ it reached the maximum value of 0.18. All the samples whose temperatures were raised above the threshold value and later lowered to the room temperature eventually reached this particular maximum absorption value. The time required to reach this value depends on the maximum temperature reached during the heating cycle. Let us take, for example, curve c in Fig. 2, where temperature was raised to 40° C. At this temperature the absorption value was 0.12 and, therefore, took much less time, 13 h ͑ not shown in the figure ͒ to arrive to a maximum absorption. Having two absorption bands, in the UV and G regions of the spectrum, the thermochromic material also exhibits photochromic properties, i.e., the transition between two stable states can be achieved by applying light irradiation at a particular wavelength. It has been demonstrated experi- mentally that by applying UV light to the colorless material, the colored state is achieved. 6 The reverse is produced by a G light. The highest absorption in the G of 0.18 was obtained by exposing material to the UV light and is equivalent to the maximum absorption value reached by a thermal process. Note that because the transition between colorless and colored states in the substance under study can be achieved either by heating or UV irradiation, the material exhibits both thermochromic and photochromic qualities. This does not imply that all the thermochromic materials are also photochromic, or vice versa. Also, the physical mechanism of the material state change due to heating differs significantly from the photon-induced change. The explanation of the thermal phenomenon is not straightforward. It may be attributed to a simple inertial mechanism or to a three state energy system, where a second, intermediate, state is metastable. This issue is currently under investigation. The properties of the thermochromic material, such as the existence of two stable states with distinct optical characteristics, long term stability and ease of achieving transition ͑ light or temperature ͒ between colored and colorless states make this sol-gel gel-glass substance a promising candidate for optical memories. Previously, optical memories based on photochromic materials were reported. 3,4 Here thermochromic material is demonstrated as an optical information storage media. A photochromic-material-based optical storage device was also fabricated to be compared with the thermochromic-based device. Two optical memory cells containing sol-gel gel-glass doped with photochromic and thermochromic materials were prepared. Their construction is similar to the containers used for material characterization. The dimensions of the writing area are 1.5 ϫ 2.5 cm. The writing procedure for the thermochromic memory cell ͑ TMC ͒ is the following. Material is placed in the colored state by either exposing the TMC to UV light or by applying heat. A similar technique is used for the erasing procedure. In the case of a photochromic memory cell, UV exposure is the only material coloring means. The time it takes to place material into the colored state depends on whether a photoprocess or a thermal process is utilized. Typically, a thermal technique is slower. However, in each of the cases, the dynamics depend on the amount of energy delivered into the system; that is, for the UV-based coloring, it is the light intensity that matters, the heating rate and the finally achieved temperature control the rate of the thermal technique. Writing is done by placing a photolithographic mask over the colored TMC and exposing it to the beam of an argon laser at 514.5 nm. In fact, the writing process is really a bleaching of the material. Figure 4 shows the re- sulting pattern written over the cell ͑ Optical Memory Demo . Similar to coloring, the dynamics of a writing process depend on the intensity of the argon laser. The readout process can be done by a low intensity argon light or just by a low power conventional light bulb. The photochromic memory cell ͑ PMC ͒ reading and writing procedure is similar to that of TMC, except that a thermal erasure technique can not be used. The written pattern can survive for a very long time ͑ weeks or more ͒ if the sample is kept in a dark environment at temperatures below a cer- tain threshold. Even under the exposure to UV light, the pattern remained visible for at least an hour. A newly developed thermochromic sol-gel gel-glass based material was studied from the point of view of its optical memory properties. The availability of two distinct absorption-based stable states and easy transition methods by means of light and temperature, make this material to be a good candidate for applications such as writable and eras- able optical memories. Samples of memory elements were fabricated and showed a good degree of stability and con- trast. Currently a more quantitative study of the thermochromic material properties is under way, and the results will be published elsewhere. We would like to thank Professor M. A. Muriel for his assistance and support. This work was supported by the Spanish CICYT research grants TIC-95-0631-C04-02, MAT95-0040-C02-01 and ...