JPH03100647A - Optical memory element - Google Patents

Abstract

PURPOSE:To prevent deterioration of reproduced signal quality by forming a recording medium allowing glass transition point of the recording medium, its temperature raised by photoirradiation at the time of recording, and its temperature raised by photoirradiation at the time of reproduction to satisfy specified conditions. CONSTITUTION:The recording medium 2 satisfies Tr<Tg<Tw, where Tg is its glass transition point, Tw is its temperature raised by photoirradiation at the time of recording, and Tr is its temperature raised by photoirradiation at the time of reproduction. The temperature Tr is lower than that Tr and mobility in the polymer chain of the polymer solid around the organic compound is suppressed and the photochromic reaction of the organic compound is restricted, thus permitting the difference of light absorbances between the recorded area and the nonrecorded area to be kept constant even at the time of repeating reproduction of the optical memory element, and accordingly, deterioration of quality of the reproduced signal to be avoided.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical memory element that can record, reproduce, or erase information using optical means.

[Conventional technology]

In recent years, the need for optical memory devices as high-density, large-capacity memory devices has been increasing year by year. Optical memory devices are generally classified into three types depending on their usage: read-only memory, additionally recordable memory, and rewritable memory.

Among these, the optical memory element used as additional recordable memory irradiates the recording medium in the optical memory element with light such as a laser beam, and causes the irradiated part of the recording medium to melt or decompose. Information is recorded in an optical memory element by forming a hole as a shape change.

In addition, in additionally recordable memory, by irradiating the recording medium in the optical memory element with light such as a laser beam, and crystallizing or non-crystallizing the recording medium in the irradiated area, reflections in the irradiated area can be generated. It records information by changing optical properties such as optical properties.

Next, as optical memory elements used as rewritable memories, there are known ones that utilize the magneto-optical effect and those that utilize the phase change of a recording medium. The former, which utilizes the magneto-optical effect, irradiates a recording medium containing a magnetic film that is magnetized in a direction perpendicular to the surface of the recording medium with light such as a laser beam, raising the temperature of the irradiated area to a temperature higher than the Curie temperature. By applying a magnetic field from the outside while it is raised, the direction of magnetization in that part is reversed from the direction of magnetization in the part that is not irradiated with light, thereby recording information. .

On the other hand, in the latter type of phase change, the recording medium is irradiated with light such as a laser beam, and the irradiated portion of the recording medium undergoes a phase transition from an amorphous state to a crystalline state, or from a crystalline state to an amorphous state. This records information. Note that the above methods for recording information in optical memory elements all utilize light such as laser light as a heat source, and therefore can be summarized as heat mode recording.

Incidentally, in addition to the heat mode recording described above, research and development of optical memory elements using a photon mode recording method has recently been actively conducted. As this photon mode recording, one that utilizes an organic compound exhibiting a photochromic phenomenon is known. Photochromic phenomenon is a phenomenon in which certain substances in a solid or liquid state reversibly change color when irradiated with light. Organic compounds that exhibit this phenomenon include, for example, hydrazone,
Many compounds include osazone, stilbene, salicylaldehyde, spiropyran, flucyto, azobenzene and its derivatives.

Among such compounds, an optical memory element using well-known spiropyran as a recording medium will be exemplified.

When spiropyran is irradiated with ultraviolet light and visible light, the intramolecular ring-opening reaction shown in Figure 3(b) and the ring-closing reaction shown in Figure 3(a) occur alternately, resulting in solid lines ■ and virtual lines ■ in Figure 4. It is known that changes in the absorption spectrum as shown in (2) occur reversibly.

Therefore, for example, by irradiating spiropyran with ultraviolet light in advance, it has the absorption spectrum shown by the solid line I above, and with this as an initial state, it is exposed to relatively high output recording light having a wavelength of around 52 Onm. Make a record. In the area irradiated with the recording light, spiropyran undergoes a ring-closing change and changes to the absorption spectrum shown by the virtual line (■). That is, minute regions with different absorption spectra are formed.

Therefore, for example, as shown in FIG. 5(a), the reproduction light, which is a weak light of around 52 Onm, is focused by the lens 5 and irradiated onto the recording medium 6 to remove the unrecorded area indicated by A. →When scanning in order from the recorded area indicated by B to the unrecorded area indicated by C, there will be a difference in absorbance indicated by ΔT, as shown by the solid line in Figure (b), so based on this, The recorded signal can then be played back.

Furthermore, when erasing a recorded signal, when the recorded portion is irradiated with ultraviolet light, spiropyran undergoes a ring-opening change and returns to the absorption spectrum shown by the solid line I above. As described above, since the photochromic phenomenon occurs reversibly, the recording medium can be used as an erasable optical memory element.

[Problem to be solved by the invention]

However, as mentioned above, recording by the photochromic phenomenon is performed in photon mode, so even if weak light is used during playback, the photochromic reaction of the weak light will occur and be repeated several times. Over time, ring-closing products accumulate in the spiropyran, and as shown by the broken line in FIG. 5(b), the difference in absorbance decreases from ΔT to ΔT', resulting in a decrease in the signal quality obtained. It has been pointed out that the disadvantage is that the

Furthermore, most materials exhibiting a photochromic phenomenon have the disadvantage that it is difficult to maintain a stable colored state, and the color easily fades due to heat.

[Means to solve the problem]

In order to solve the above problems, the optical memory element according to the present invention records, reproduces, or erases information by irradiating light onto a recording medium formed by dispersing an organic compound exhibiting a photochromic phenomenon in a polymer solid. In an optical memory element, the glass transition temperature of the recording medium is Tg, the temperature when the temperature of the recording medium increases due to light irradiation during recording is Tw, and the temperature when the temperature of the recording medium increases due to light irradiation during reproduction is Tr. In this case,
It is characterized by being equipped with a recording medium that satisfies Tr<Tg<Tw.

[For production]

According to the above configuration, the inadvertent progress of the photochromism reaction due to the reproducing light is reliably prevented. That is, the temperature Tr when the temperature of the recording medium increases due to light irradiation during reproduction.
is lower than the glass transition temperature Tg of the recording medium, and the movement of polymer chains in the polymer solid around the organic compound that is irradiated with light is suppressed, so the photochromic reaction of the organic compound is also suppressed. .

Therefore, no matter how many times reproduction is repeated for such an optical memory element, the difference in absorbance between the recorded portion and the unrecorded portion is kept constant, and the problem of deterioration of reproduced signal quality is resolved. Furthermore, the storage performance of recorded information against heat is also improved.

On the other hand, the temperature T of the recording medium when it is irradiated with light during recording
Since w is higher than the glass transition temperature Tg of the recording medium,
The movement of polymer chains in the polymer solid around the organic compound that is irradiated with light is allowed, the photochromic reaction of the organic compound is promoted, and a prescribed recording is performed.

〔Example〕

An embodiment of the present invention will be described below with reference to FIGS. 1 to 3. Note that the third
We will use the diagram again here.

As shown in FIG. 2, the optical memory element according to the present invention includes a substrate 1 made of plastic or glass, and a recording medium 2 formed on the surface of this substrate 1. .

The recording medium 2 is a polysulfone resin containing one type of organic compound that exhibits a photochromic phenomenon accompanied by ring opening and closing of molecules upon irradiation with light, such as stilbene derivatives, spiropyrans, flucyto compounds, and azo compounds. It has a structure in which it is dispersed in a polymeric solid (binder) made of polyamide resin, polyether sulfone resin, or polycarbonate resin. In such a recording medium 2, when Tg is the glass transition temperature, Tw is the temperature when the temperature rises due to light irradiation during recording, and Tr is the temperature when the temperature rises due to light irradiation during reproduction, Tr< It is formed to satisfy Tg<Tw.

Here, when recording light of 10 mW (wavelength around 520 nm) is focused to a beam diameter of 1 μm and irradiated onto the recording medium 2 for 100 ns, the thickness of the recording medium 2 is 1 μm, and the molar absorption coefficient is lXl0' 42/mol - ( m, the organic compound concentration is 0.1 mol/f, and the density of the recording medium 2 is 1.2 g/c.
d, assuming the specific heat to be 0.1 kcal/kg'C, approximately 20
Since the temperature increases by 0°C, the initial temperature of recording medium 2 is set to 40°C.
240°C, which is obtained by adding the above-mentioned 200°C, becomes the temperature Tw when the temperature of the recording medium 2 is increased due to light irradiation during recording.

On the other hand, if the above-mentioned light is reduced to 1 mW and used as the reproduction light, the temperature will rise by about 20°C, so if the initial temperature of the recording medium 2 is 40°C and 20°C is added to this, 60°C is the reproduction light. This is the temperature Tr when the temperature of the recording medium 2 increases due to light irradiation.

Therefore, the glass transition temperature Tg of the recording medium 2 is 60°C ~
It will be set between 240°C.

In the case of the recording medium 2 in which the organic compound is a spiropyran compound, the polymer film in which the spiropyran compound is dispersed is formed on the substrate 1 while being irradiated with ultraviolet light with an output of about 10 mW, for example. The subsequent spiropyran compound is exposed to the ultraviolet rays and is in a ring-opened state as shown in FIG. 3(b).

According to the above configuration, the inadvertent progress of the photochromism reaction due to reproduction light is reliably prevented. That is, the temperature Tr when the temperature of the recording medium 2 increases due to light irradiation during reproduction.
(60″C) is lower than the glass transition temperature Tg of the recording medium 2, and at this time, the movement of the polymer chains of the polymer solid around the spiropyran compound that is irradiated with light is suppressed, so the spiropyran compound The photochromic reaction, in this case the ring closure change of the spiropyran compound, will be inhibited.

Therefore, no matter how many times reproduction is repeated for such an optical memory element, the difference in absorbance between the recorded portion and the unrecorded portion is kept constant, and the problem of deterioration of reproduced signal quality is resolved. Furthermore, the storage performance of recorded information against heat is also improved.

On the other hand, the temperature Tw (240°C) of the recording medium 2 when it is irradiated with light during recording is the glass transition temperature T of the recording medium 2.
Since it is higher than g, at this time, the movement of the polymer chains of the polymer solid around the spiropyran compound that is irradiated with light is allowed.

Therefore, the photochromic reaction of spiropyran compounds,
In this case, the ring-closing change of the spiropyran compound shown in FIG. 3(a) is promoted and predetermined recording is performed.

In addition, considering that the movement of polymer chains gradually becomes active from several tens of subcutaneous temperatures below the glass transition temperature Tg, and that information retention becomes more sensitive to heat, the glass transition temperature increases as the temperature Tw increases. The closer the temperature Tw
, is 240°C, the temperature Tg is 150°C ~ 2
Approximately 00°C is suitable.

Regarding increasing the temperature Tg, although the relationship between the temperature Tg and the polymer structure is not clear, it can be said that the temperature Tg is generally high for a structure in which the movement of molecules is restricted. For example, reducing the flexibility of a polymer chain by introducing a benzene ring has the effect of increasing the temperature Tg, and introducing a polar group has the effect of increasing the Tg because it increases the interaction between molecules.

Furthermore, the Tg can also be increased by introducing crosslinking.

Here, a recording medium made of a spiropyran compound dispersed in a polymer solid whose temperature Tg is set at around 150°C is initialized by irradiation with ultraviolet rays as described above, and then irradiated with visible light. Looking at the change in absorbance in the visible light range with respect to changes in the intensity of irradiation energy, we found that, as shown by the solid line in Figure 1, a rapid change in absorbance begins when a certain irradiation energy is exceeded. In other words, as shown by the broken line (conventional example) in the same figure, even when receiving relatively weak irradiation energy, a slight change in absorbance occurs, and the difference in absorbance between the recorded and unrecorded areas increases with repeated reproduction. It can be seen that the drawback of the photon mode, which is that it gradually becomes smaller, has been eliminated.

〔Effect of the invention〕

As described above, the optical memory element according to the present invention is an optical memory element that records, reproduces, or erases information by irradiating light onto a recording medium formed by dispersing an organic compound exhibiting a photochromic phenomenon in a polymer solid. When the glass transition temperature of the recording medium is Tg, the temperature when the temperature of the recording medium increases due to light irradiation during recording is Tw, and the temperature when the temperature of the recording medium increases due to light irradiation during reproduction is Tr. , Tr<Tg<T
This configuration includes a recording medium that satisfies W.

As a result, no matter how many times reproduction is repeated for such an optical memory element, the difference in absorbance between the recorded portion and the unrecorded portion is maintained constant, and the problem of deterioration of the reproduced signal quality is resolved. In addition, it also has the effect of improving the storage performance of recorded information against heat.

[Brief explanation of drawings]

1 and 2 show one embodiment of the present invention. FIG. 1 is a graph showing the relationship between visible light irradiation energy irradiated onto a recording medium and visible light absorbance in comparison with a conventional method. FIG. 2 is a sectional view of essential parts of the optical memory element. FIG. 3(a) is an explanatory diagram showing a ring-closed state of a spiropyran compound, and FIG. 3(b) is an explanatory diagram showing a ring-opening state of a spiropyran compound. FIG. 4 is a graph showing the relationship between irradiation light wavelength and absorbance for the ring-opened state and ring-closed state of the spiropyran compound. FIG. 5 shows a conventional example, and FIG. 5(a) is an explanatory diagram showing how the optical memory element is irradiated with laser light;
FIG. 5B is an explanatory diagram showing that the difference in absorbance between the recorded portion and the unrecorded portion becomes smaller than that at the beginning of recording. 1 is a substrate, and 2 is a recording medium. high diagram

Claims (1)

[Scope of Claims] 1. An optical memory element in which information is recorded, reproduced, or erased by irradiating light onto a recording medium formed by dispersing an organic compound exhibiting a photochromic phenomenon in a polymer solid, comprising: When the glass transition temperature is Tg, the temperature when the temperature of the recording medium increases due to light irradiation during recording is Tw, and the temperature when the temperature of the recording medium increases due to light irradiation during reproduction is Tr, Tr<
An optical memory element comprising a recording medium that satisfies Tg<Tw.