An optical disc such as a CD (Compact Disc), DVD (Digital Versatile Disc), or BD (Blu-ray Disc) is an information recording medium used for recording video data, audio data, or other information by irradiation with laser light and for reproducing the recorded information. Optical discs have increased in capacity through successive generations. For example, CDs achieve a 650 MB capacity with an approximately 1.2-mm thick light transmitting disc substrate layer, laser light having a wavelength of approximately 780 nm, and an objective lens having a numerical aperture (NA) of 0.45. DVDs, which were the next generation after CDs, achieve a 4.7 GB capacity with an approximately 0.6-mm thick light transmitting disc substrate layer, laser light having a wavelength of approximately 650 nm, and an NA of 0.6. DVDs have, for example, a structure about 1.2 mm thick, with two disc substrates about 0.6-mm thick bonded together. The standards for BDs, which have even higher recording densities, specify a light transmitting protective layer approximately 0.1 mm thick covering the information recording surface, laser light having a wavelength of approximately 405 nm, and a 0.85 NA; the capacities achieved are 25 GB for a single-layer disc and 50 GB for a dual-layer disc, enabling the recording of high-definition (High-Vision) video over an extended time period. In addition, capacities as large as 100 GB and 128 GB have been realized in multi-layer BDs having three and four layers, respectively, which have been reduced to practice on the basis of single-layer BDs having a single recording layer. The BD standards are disclosed in, for example, Non-Patent Document 1 (Blu-ray Disc White Paper 1.C, Physical Format Specification for BD-ROM, 5th Edition, March 2007).
It is anticipated that general users will be dealing with greatly increased amounts of data in next-generation video with higher definition than High-Vision, and in three-dimensional video, etc., so large capacity optical disc systems capable of storing larger amounts of data, exceeding the capacities of single-layer and multi-layer BDs, are being sought.
The above-described increasing capacities of optical discs have been achieved by greatly reducing the size of the light spot focused on the focal plane of the objective lens, thereby reducing the size of the marks recorded on the tracks in the recording layer, by shortening the wavelength of the laser light and increasing the NA of the objective lens. In reducing the size of the focused spot, however, there is a physical limit defined by the optical performance of the objective lens and the wavelength of the laser light. Specifically, the size limit of a reproducible recorded mark is said to be the diffraction limit λ/(4NA) determined by the wavelength λ of the laser light and the NA of the objective lens.
In recent years, optical discs (referred to below as super-resolution optical discs) having a super-resolution functional layer with an optical characteristic (such as an optical absorption characteristic or optical transmission characteristic) that varies nonlinearly according to the intensity of the laser light are attracting attention as a way of achieving high-density recording and reproduction beyond the physical limits. When the laser light is focused onto the super-resolution functional layer, the refractive index or another optical characteristic changes in a local area of high light intensity or high temperature within the illuminated spot, and the localized light (such as near-field light or localized plasmon light) generated in the local area is converted to propagating light by mutual interaction with the recorded marks in the recording layer. This makes it possible to use conventional BD optical heads to reproduce information from tiny recorded marks smaller than the λ/(4NA) diffraction limit. Accordingly, higher recording densities can be achieved even with a 405-nm laser light wavelength and 0.85 NA objective lens.
A super-resolution optical disc has the advantage that it can assure downward compatibility with single-layer BDs and multi-layer BDs; such compatibility is considered difficult to achieve with other large capacity recording and reproducing technologies such as holographic recording systems, or near-field light recording systems, which require an optical head structure using a solid immersion lens (SIL) having a numerical aperture of unity or greater. The structure of a super-resolution optical disc of this type is disclosed in, for example, Non-Patent Document 2 listed below.
Prior Art References
Patent References
Non-Patent Document 1: Blu-ray Disc White Paper 1.C Physical Format Specification for BD-ROM, 5th Edition, March 2007
Non-Patent Document 2: “Bit-by-Bit Detection on Super-Resolution Near-Field Structure Disc with Platinum Oxide Layer”, Jpn. J. Appl. Phys. Vol. 42 (2003) pp. L589-L591