A nonlinear light absorption material includes a compound represented by the following formula (1) as a main component:

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in the formula (1), R.sup.1 to R.sup.6 are each independently a hydrocarbon group.

A nonlinear light absorption material includes a compound represented by the following formula (1) as a main component:

##STR00001##

in the formula (1), R.sup.1 to R.sup.10 are each independently a hydrogen atom, a halogen atom, a saturated hydrocarbon group, a halogenated alkyl group, an unsaturated hydrocarbon group, a hydroxyl group, a carboxyl group, an alkoxycarbonyl group, an aldehyde group, an acyl group, an amide group, a nitrile group, an alkoxy group, an acyloxy group, a thiol group, an alkylthio group, a sulfonate group, an acylthio group, an alkylsulfonyl group, a sulfonamide group, a primary amino group, a secondary amino group, a tertiary amino group, or a nitro group.

Long term optical memory includes a storage medium composed from an array of silicon nanoridges positioned onto the fused silica glass. The array has first and second polarization contrast corresponding to different phase of silicon. The first polarization contrast results from amorphous phase of silicon and the second polarization contrast results from crystalline phase of silicon. The first and second polarization states are spatially distributed over plurality of localized data areas of the storage medium.

Long term optical memory includes a storage medium composed from an array of silicon nanoridges positioned onto the fused silica glass. The array has first and second polarization contrast corresponding to different phase of silicon. The first polarization contrast results from amorphous phase of silicon and the second polarization contrast results from crystalline phase of silicon. The first and second polarization states are spatially distributed over plurality of localized data areas of the storage medium.

An optical disc is formed by bonding a first information recording medium to a second information recording medium. The first information recording medium includes a first substrate having opposed first and second surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second information recording medium includes a second substrate having opposed third and fourth surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second surface of the first substrate is bonded to the fourth surface of the second substrate. At least one of the second surface or the fourth surface includes a depression and a projection. The depression is formed in a thickness direction of the optical disc. The projection is provided inside the depression at a predetermined distance or longer from an outer circumferential edge of the depression in a radial direction of the optical disc.

An optical disc is formed by bonding a first information recording medium to a second information recording medium. The first information recording medium includes a first substrate having opposed first and second surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second information recording medium includes a second substrate having opposed third and fourth surfaces, at least one dielectric film, at least one recording film, and a cover layer. The second surface of the first substrate is bonded to the fourth surface of the second substrate. At least one of the second surface or the fourth surface includes a depression and a projection. The depression is formed in a thickness direction of the optical disc. The projection is provided inside the depression at a predetermined distance or longer from an outer circumferential edge of the depression in a radial direction of the optical disc.

A recording medium includes a recording layer. The recording layer includes an aliphatic polymer, and a multiphoton absorption compound containing at least one bond selected from the group consisting of a carbon-carbon double bond, a carbon-nitrogen double bond, and a carbon-carbon triple bond, and having a multiphoton absorption characteristic. When the thickness of the recording layer is 100 ?m, the transmittance of the recording layer in the thickness direction with respect to light having a wavelength of 405 nm is greater than or equal to 80%.

A reading method for reading contents information with a laser from an optical disc including a plurality of recording layers includes positioning a focus position of the laser at a first recording layer and reading a first part of the contents information recorded in the first recording layer. If a first link information recorded in the first recording layer is detected, judging whether a first condition is fulfilled or not. If the first condition is fulfilled, positioning the focus position of the laser at a second recording layer and reading a first link destination information recorded in the second recording layer. If the first condition is not fulfilled, reading a second part of the contents information recorded in the first recording layer.

A reading method for reading contents information with a laser from an optical disc including a plurality of recording layers includes positioning a focus position of the laser at a first recording layer and reading a first part of the contents information recorded in the first recording layer. If a first link information recorded in the first recording layer is detected, judging whether a first condition is fulfilled or not. If the first condition is fulfilled, positioning the focus position of the laser at a second recording layer and reading a first link destination information recorded in the second recording layer. If the first condition is not fulfilled, reading a second part of the contents information recorded in the first recording layer.

An optical recording medium includes a light-receiving surface that receives incident light and at least three information signal layers on which information signals are to be optically recorded. Among the at least three information signal layers, the information signal layer closest to the light-receiving surface has a reflectance in a range of 4% to 11%.