Provided is an objective lens which is used so that more information can be accumulated in a large-capacity optical disk and which has a further enhanced numerical aperture NA. The objective lens is a single lens having the numerical aperture NA and a refractive index n, and is configured so as to satisfy: NA≥0.91 and 1.61≤n<1.72.

A recording medium according to one embodiment of the present disclosure includes a recording layer and an optical thin film. The recording layer includes a heat-sensitive color-developing composition and a photothermal conversion material. The photothermal conversion material absorbs a wavelength in an infrared region and generates heat. The optical thin film is provided on one surface of the recording layer. The optical thin film reflects the wavelength in the infrared region and transmits a wavelength in a visible region.

Provided is an objective lens which is used so that more information can be accumulated in a large-capacity optical disk and which has a further enhanced numerical aperture NA. The objective lens is a single lens having the numerical aperture NA and a refractive index n, and is configured so as to satisfy: NA≥0.91 and 1.61≤n<1.72.

A recording medium according to one embodiment of the present disclosure includes a recording layer and an optical thin film. The recording layer includes a heat-sensitive color-developing composition and a photothermal conversion material. The photothermal conversion material absorbs a wavelength in an infrared region and generates heat. The optical thin film is provided on one surface of the recording layer. The optical thin film reflects the wavelength in the infrared region and transmits a wavelength in a visible region.

A data storage medium for storing digital data. The medium includes a mixture of different nano-sized materials, each of the nano-sized materials having a respective optical transition profile characterizing an optical transition of the nano-sized material and covering a respective wavelength range, wherein a combined optical transition profile of the mixture covers an extended wavelength range as compared to the respective wavelength ranges of the respective optical transition profiles of the different nano-sized materials, and wherein one or more of the different nano-sized materials is photo-reactive to selectively vary a respective absorption/emission band upon irradiation to encode digital data in the combined optical transition profile of the mixture.

In a recording technique in which a plurality of light spots are simultaneously formed by using an ultra-short pulse laser and a spatial phase modulator, and a plurality of recording dots having refractive indexes different from those of the vicinities thereof are formed inside a recording medium, it is hard to make recording quality and a recording density compatible. Therefore, a plurality of dots are recorded at a predetermined dot pitch, and then other dots are recorded between the recorded dots.

A compound is represented by Formula (1) below,

##STR00001##

in Formula (1) above, R.sup.1 to R.sup.22 each independently contain at least one atom selected from the group consisting of H, B, C, N, O, F, Si, P, S, Cl, I, and Br, and L.sup.1 and L.sup.2 each independently represent a single bond or CC.