JPH0448116B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a rewritable information multilevel recording system.

(Prior Art) Conventionally, magneto-optical recording methods and methods utilizing phase change of inorganic materials are known as rewritable information recording methods.

In the magneto-optical recording method, the direction of spin in the medium is controlled by an external magnetic field, and a slight rotation of the polarization direction is detected using the Kerr effect or the Faraday effect.

Methods that utilize phase changes in inorganic materials produce crystal-crystal transitions or crystal-amorphous transitions in the materials due to the difference between rapid cooling and slow cooling after heat application, and utilize the accompanying change in reflectance. has been done.

In addition, for the purpose of improving the S/N ratio and reducing manufacturing costs, an information recording method using polymeric liquid crystal as a recording layer and using a constant optical energy density for writing was published in Japanese Patent Application Laid-Open No. 59-10930. and JP-A-59-35989.

In this method, changes in reflectance between the uniformly oriented state and the non-oriented state of the liquid crystal groups are utilized.

(Problems to be Solved by the Invention) As we become an information society, information media and systems with high recording density are desired, but the recording density is limited by the beam diameter spread depending on the wavelength of currently available light beams. 1μm, and the maximum recording density of the above two-state recording method is
10 ⁹ BPI ² . The present invention provides a method for recording information in a multivalued state in one recording spot as a method for further improving this recording density. At this time, in the magneto-optical recording method, spin reversal is used, and the rotation angle is small in this case, and the S/N ratio is small, so it is difficult to apply this to multi-value recording. In addition, in the method using a phase change medium, the phase transition temperature region is narrow, so it is difficult to perform multi-leveling using intermediate reflectance changes in that region.

Therefore, polymer liquid crystals that have an appropriate viscosity and can control the intermediate state, especially cholesteric polymer liquid crystals that can utilize black reflection, which refers to cholesteryl-containing liquid crystals and chiral nematic polymer liquid crystals, are used. The recording density is improved by controlling the energy density of the irradiated light energy to at least two or more values in a multivalued manner.

(Means for solving the problem) In an optical recording method in which information is recorded on a cholesteric polymer liquid crystal layer formed on a support by irradiation with a light beam, the irradiation energy density of the light beam is multivalued into at least two types. of the cholesteric polymer liquid crystal layer by controlling the helical pitch length or the inclination of the helical axis of the cholesteric polymer liquid crystal corresponding to the irradiation energy density of the light beam, or by forming a non-oriented state or pits. This is an optical recording method characterized by recording information by changing the light reflectance multi-valued.

(Function) In the present invention, an optical recording medium containing a cholesteric polymer liquid crystal as a recording layer was irradiated with light beam pulses having various energy densities. FIG. 1 shows the change characteristics of reflectance with respect to the energy density of irradiated light. The threshold for reflectance change is 25mJ/
cm ² , and the reflectance increases in the range of 65 mJ/cm ² . The reflectance decreases rapidly at light energy of 65 mJ/cm ² or more. These reflectance changes are preserved even after pulse irradiation. The above was revealed through our experiments.

This phenomenon is thought to be due to the following mechanism. That is, the light energy generated by the irradiation of the light beam is converted into thermal energy by the light absorber, thereby increasing the temperature of the cholesteric polymer liquid crystal. Since it is known that the cholesteric pitch of cholesteric polymer liquid crystals changes with temperature, it is thought that a shift in the selective reflection spectrum based on the cholesteric pitch will contribute to changes in reflectance due to changes in cholesteric pitch. .

In Figure 1, the energy density is 25mJ/cm ²
There is a region where the reflectance increases from 65 mJ/cm ² to 65 mJ/cm 2 . Microscopic observation has revealed that the selective reflection spectrum is red-shifted in this region. As a result, the maximum selective reflection wavelength λ _MAX 570 nm at room temperature of the cholesteric polymer liquid crystal used was red-shifted, and the reflectance using light on the long wavelength side increased, so that the wavelength of the laser diode used for reflectance measurement was 830 nm. This is thought to be due to an increase in the reflectance at .

There is a relationship between the maximum selective reflection wavelength λ _MAX and the cholesteric pitch, and _the refractive index n
Assuming that 1.5 and no temperature change, the red shift is considered to be due to an increase in cholesteric pitch.

This increase in pitch is fixed by rapid cooling due to the temperature difference between the irradiation spot and its surroundings after the irradiation of the light beam is completed. By fixing the increase in pitch, therefore, the increase in reflectance is also preserved.

Next, in Figure 1, the energy density is
A sharp decrease in reflectance is observed in the region of 65 mJ/cm ² or more. According to microscopic observation of the obtained spot, it has been found that this spot has a pit shape and has a low reflectance.

From this, in the region where the optical energy density is 65 mJ/cm ² or more, flow occurs due to the decrease in viscosity and pits are formed, and at the same time non-orientation occurs due to changes in the helical axis, resulting in a decrease in selective reflectance. it is conceivable that.

It is thought that this pit formation and non-orientation cause a decrease in reflectance even at the wavelength of 830 nm of the laser diode used for reflectance measurement.

Even in this region, pits and non-orientation are preserved by rapid cooling.

As described above, different states of reflectance are maintained depending on the energy density of each irradiated light.

Thereby, by controlling the energy density of the irradiated light in a multivalued manner, it becomes possible to record information by changing the light reflectance of the optical recording medium in a multivalued manner.

Next, we will discuss erasing information. The recording point of the irradiation spot prepared as described above is reheated, and this may be the entire recording medium or just that spot.
It has been found that upon gradual cooling, the reflectance of the irradiated spot becomes the same as that of the surrounding area, and is also the same as the initial reflectance.

This is thought to be because the cholesteric polymer liquid crystal undergoes a predetermined reorientation during reheating and slow cooling, and as a result, the reflectance returns to its initial state, meaning that the information is erased and can be rewritten or rewritten. Become.

The rewriting method can be performed in the same manner as the initial writing method described above.

(Example) Next, an example of the present invention will be described with reference to FIGS. 1 to 3.

FIG. 1 is a diagram showing the change in reflectance obtained with respect to the optical energy density when the cholesteric polymer liquid crystal (shown in FIG. 3) used in one example was irradiated with a light beam. Here, the light beam is pulsed light with a width of 80 μs, and a laser diode with a wavelength of 830 nm is used.

The energy density of the light beam is 25 mJ/ ^cm2 , which is the threshold for writing energy.
The reflectance increases in the range up to 65mJ/ ^cm2 .
When an energy density of 65 mJ/cm ² or more is applied, the reflectance decreases. The spot diameter is 10 μm.

Next, the medium for the embodiment will be explained using Figure 2.

A vanadium oxide film is deposited as a photorespirator 3 on a support substrate 1, and a cholesteric polymer liquid crystal 5 is spin-coated thereon. Then, to form an air gap cell having an air gap layer 9, a spacer 7 and a transparent protection plate 11 are provided.
Use. A circular optical plate 15 is provided on this. DRAW is traditionally used for writing and reading.
You can use a type of optical head. However, light beam 1
3 is circularly illuminated, and it is desirable that the direction of rotation coincides with the direction of helical rotation of the cholesteric polymer liquid crystal. This is because the selective reflection of a cholesteric polymer liquid crystal is effective only for light having the same rotation direction as its helical rotation direction.

When a non-polarized light beam 13 is used, the S/N ratio during reproduction can be increased by providing a circular light plate on the top or bottom surface of the transparent protection plate 11 that matches the helical direction of the cholesteric polymer liquid crystal. The cholesteric polymer liquid crystal layer used in the examples is bulky at room temperature and does not need to be sealed unlike low molecular liquid crystals. In addition, the maximum peak wavelength λ _MAX of the selective reflection spectrum at room temperature is 570 nm, and the half-width
It is 100nm. The half-width is a measure of the orientation order of cholesteric polymers, and a half-width of 100 nm does not indicate very high orientation order. It is easily assumed that writing and reading sensitivity will be improved if the film has a high orientational order and the maximum peak wavelength is appropriately close to the wavelength of the writing/reading light. The reflectance of the cholesteric polymer liquid crystal used in the examples is 5%, and the circular direction of the laser beam is the same as the direction of rotation of the helix of the cholesteric polymer, which is left-handed in the examples.

The optical energy density of the reading light is 10mJ/
^cm2 , and no deterioration of the medium is observed.

(Effect of the invention) In the example, the light energy density was set to 25 mJ/
cm ² , 50 mJ/cm ² , 60 mJ/cm ² , and 75 mJ/cm ² , it is now possible to record 2 bits of information without applying an electric field, whereas previously only 1 bit of information could be recorded per spot. It was possible to write, erase, and rewrite. Furthermore, the playback S/N ratio is
It was 69dB. It is expected that the S/N ratio and recording density will be improved by setting the selected wavelength position of the recording medium and improving the alignment uniformity.

As described above, according to the present invention, multilevel recording becomes possible and the amount of recorded information can be increased.

[Brief explanation of the drawing]

FIG. 1 shows the reflectance characteristics of a cholesteric polymer liquid crystal used to explain the operating principle of the present invention. FIG. 2 is a sectional view of a recording medium used in an embodiment of the present invention. FIG. 3 is a diagram showing the molecular structure of the cholesteric polymer liquid crystal used in the examples. In the figure, 1...support substrate, 3...light absorber,
5... Cholesteric polymer liquid crystal, 7... Spacer, 9... Air gap layer, 11... Transparent protective plate,
13...Light beam, 15...Circularly polarizing plate.

Claims (1)

[Claims] 1. An optical recording method for recording information on a cholesteric polymer liquid crystal layer formed on a support by irradiation with a light beam, in which the irradiation energy density of the light beam is multivalued controlled into at least two types, The light reflectance of the cholesteric polymer liquid crystal layer can be varied by changing the helical pitch length or the inclination of the helical axis of the cholesteric polymer liquid crystal corresponding to the irradiation energy density of the light beam, or by forming a non-oriented state or pits. An optical recording method that records information by changing its value.