Thus far, high-capacity optical disks of various kinds have been achieved by reducing the size of an information mark that is written on a disk track and also by making a wavelength of the laser beam for use in recording/playback shorter and adopting an objective lens of a larger numerical aperture, to reduce the size of a focus spot on a focusing surface.
In, for example, a CD (compact disc), its disk substrate serving as an optical transmissive layer (a transparent protective layer formed on an information recording layer, which is also called a transparent substrate) has a thickness of about 1.2 mm, a laser beam wavelength of about 780 nm and a numerical apertures (NA) of 0.45 of an objective lens are selected, with its recording capacity being 650 MB.
In a DVD (digital versatile disc), its optical transmissive layer has a thickness of about 0.6 mm, and a laser beam wavelength of about 650 nm and an NA of 0.6 are selected, resulting in a recording capacity of 4.7 GB.
In a higher-density BD (Blu-ray Disc), an optical disk whose optical transmissive layer thickness is 0.1 mm is used to determine the laser beam wavelength to be about 405 nm and the NA to be 0.85, thereby achieving a high capacity of 25 GB per layer.
Besides these discs, there is an HD DVD (high-definition digital versatile disc) and the like in which an optical disk whose optical transmissive layer thickness is 0.6 mm that is the same as that of the DVD is used to determine the laser beam wavelength to be about 405 nm and the NA to be 0.65, thereby achieving a high capacity of 18 GB or more.
In the field of optical recording, a high-density recording scheme has been researched in recent years which uses a super-definition optical disk on which a super definition mask layer is formed having a nonlinear optical absorption characteristic or a nonlinear optical transmission characteristic where an index of refraction varies depending on light intensity. In this scheme, by causing changes in the index of refraction in a localized high temperature zone or a localized high intensity zone in the focus spot of an optical disk, marks can be played back that is smaller than a diffraction limit λ/(4NA) that is determined by optical elements of an optical disk apparatus—i.e., the numerical aperture NA of a converging lens and optical wavelength λ (for instance, refer to Non-patent Document 1).
In such a super resolution optical disk, however, because a mask layer thereof absorbs light, larger playback energy is needed in comparison to that in a conventional optical disk. It is known that as a result, a low frequency noise level (or disk noise) contained in playback signals increase (refer to Non-patent Document 2, for example).
Non-patent Documents 1 and 2 relates to Super-RENS (super resolution near field structure) scheme, which is for a typical super resolution optical disk. In addition to this disk, another super resolution optical disk is proposed which is formed of a material having a nonlinear optical absorption characteristic or nonlinear optical transmission characteristic in which an index of refraction of the recorded information mark varies depending on light intensity (refer to Non-patent Document 3, for example). These are hereinafter collectively called super resolution optical disk.    [Non-patent Document 1]    “Observation of Eye Pattern on Super-Resolution Near-Field Structure Disk with Write-Strategy Technique”, Jpn. J. Appl. Phys., Vol. 43, No. 7A, pp. 4212-4215 (2004)    [Non-patent Document 2]    “Low Frequency Noise Reduction of Super-Resolution Near-Field Structure Disc with Platinum-Oxide Layer,” ODS, Technical Digest, ThC3 (2005)    [Non-patent Document 3]    “Sub-Terabyte-Data-Capacity Optical Discs Realized by Three-Dimensional Pit Selection,” Jpn. J. Appl. Phys., Vol. 45, No. 4A, pp. 2593-2597 (2006)