Applications and theory of the solid immersion lens

The diffraction limited spot size in optical imaging recording and lithography 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 object to be imaged 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. Applications of the SIL to microscopy, lithography, and optical storage are discussed. The typical SIL is of the order of 2-3 mm diameter. However, we also raise the possibility of using lenses a few micrometers diameter made by micromachining techniques. The problem of tolerances of such lenses is discussed.