In a method of high density (superior to 10?12 bits/cc) optical recording of information in a three-dimensional multilayered format, information is written in a photopolymer (10) as submicron volume elements of altered index of refraction. The index change results from alteration of the photopolymer (10) induced by two-photon excitation of the photo-sensitizer at the waist of a highly focused beam (20) from a subpicosecond colliding pulse modelocked laser (16). Quadratic dependence of two-photon excitation on incident instantaneous intensity serves to confine crosslinking of the polymer (10) to the focal volume; hence, bit array layers may be written with thicknesses on the order of the confocal parameter. As an example, eight well resolved bit planes can be written in a polymer film about 30 microns thick. Information written in this manner may be read with sufficient axial resolution by differential interference contrast (DIC) or confocal microscopy. This technique may be employed in multilayered write once read many (WORM) optical storage discs, for example.
METHOD FOR THREE-DIMENSIONAL OPTICAL