1. Field of the Invention
The present invention relates to an optical recording medium, in particular to a write once optical recording medium capable of recording information therein and reproducing the recorded information therefrom by causing the optical characteristics, such as transmittance and reflectance, of a recording material for use in the optical recording medium to change by the application of an optical beam thereto. In addition, the present invention also relates to a method of recording information in the above-mentioned optical recording medium.
2. Discussion of Background
The wavelength of a laser beam used for currently employed write once optical disk systems of a write once read many type (WORM), and a compact disc-recordable (CD-R) type is in the range of 770 nm to 790 nm, and a recording medium for the write once optical disk system is designed in such a fashion that recording and reproduction can be performed in the above-mentioned wavelength range.
Sooner or later, it will be inevitable that the capacity of such a recording medium must be significantly increased in accordance with the amount of information to be handled. It is also inevitable that the wavelength of the laser beam used for the recording and reproduction will be shortened.
In recent years, there have been proposed many types of write once optical disks for data recording and reproduction, using a cyanine dye or a phthalocyanine dye as a recording material.
For example, the write once optical disks using as the recording material a cyanine dye are disclosed in Japanese Laid-Open Patent Applications 57-62093, 58-56892, 58-112790, 58-114989, 59-85791, 60-83236, 60-89842, and 61-25886. Furthermore, the write once optical disks using as the recording material a phthalocyanine dye are disclosed in Japanese Laid-Open Patent Applications 61-150243, 61-177287, 61-154888, 61-246091, 62-39286, 63-37791, and 63-39888.
However, there has not yet been developed any recording material that has high light resistance and excellent preservation stability, and is capable of performing the recording and reproducing operation using an optical pick-up of a laser beam with a wavelength of 700 nm or less.
The currently employed CD-R is also designed in such a fashion that recording and reproduction can be performed by a laser beam with a wavelength in the range of 770 nm to 790 nm.
In the case of the above-mentioned CD-R system, the increase in capacity of the recording medium also becomes inevitable, and there is the trend toward shortening of the wavelength of the laser beam to be used for data recording and reproduction.
In CDs and CD-ROMs which are currently employed, aluminum (Al) is coated on the concave and convex portions of the substrate therefor and the wavelength-dependence of the reflectance of Al is so small that even if the wavelength of the laser beam used therefor is shortened in the future, reproduction of information recorded in such CDs and CD-ROMs is possible.
In contrast to this, the recording layer of the currently employed CD-R, which contains therein a dye with a maximum absorption wavelength in the range of 680 nm to 750 nm, is designed so as to exhibit a maximum reflectance when a laser beam with a wavelength of 770 nm to 790 nm is applied thereto, with the optical characteristics and constants and the thickness and structure of the recording layer being taken into consideration. Therefore, when a laser beam with a wavelength of 700 nm or less is applied to the currently employed CD-R, the reflectance exhibited by the recording layer is extremely small, so that the currently employed CD-R cannot cope with the shortening of the wavelength of the employed laser beam in the future. As a result, it will be highly possible that the information recorded in the currently employed CD-R system cannot be reproduced therefrom by such a future system.
Many recording materials, for example, the combination of cyanine dye/metal reflection layer, phthalocyanine dye/metal reflection layer, and azo metal chelate dye/metal reflection layer, are proposed for the conventional CD-R. To be more specific, as the recording material for the CD-R, the combination of a cyanine dye/metal reflection layer is proposed in Japanese Laid-Open Patent Applications 1-159842, 2-42652, 2-13656, and 2-168446; a phthalocyanine dye as the recording material is proposed in Japanese Laid-Open Patent Applications 1-176585, 3-215466, 4-113886, 4-226390, 5-1272, 5-171052, 5-116456, 5-69860 and 5-139044; and an azo metal chelate dye as the recording material is proposed in Japanese Laid-Open Patent Applications 4-46186, 4-141489, 4-361088, and 5-279580.
However, there is no write once optical recording medium that can solve the above-mentioned problems.