Optical Background for Near-Field Recording

Abstract

The diffraction limited spot size in optical recording can be decreased by use of the Solid Immersion Lens (SIL). The simplest form of the SIL utilizes a hemispherical lens with the rays of a focused bean entering along its radii to form a spot at the flat surface of the hemisphere. The spot size is decreased from that in free space by a factor 1/n, where n is the refractive index. If an optical disk is placed close enough to the SIL, this reduced spot size is obtained at the disk. In this case the effective numerical aperture of the lens can be greater than one. The SIL is used in optical recording in two different ways. In the first example the SIL is mounted on a floating head which maintains the air gap automatically to be of the order of 100 nm. This method is now being developed for a removable disk drive with over 10 Gigabytes storage by TeraStor. A second technique, demonstrated by Sony, uses an effective numerical aperture less than one with a relatively large air gap of the order of 50 micrometers. This method gives relatively low spherical aberration and is suitable for use with multilayer systems.