Introduction

The reflection and refraction of light at plane surfaces and interfaces, and the absorption of light by thin-film media play important roles in optical data storage. The reflection of polarized light from the surface of a magnetic material, for instance, is accompanied by a change of the state of polarization, thus carrying information about the state of magnetization of the material. The interaction is known as the magneto-optical (MO) Kerr effect and is generally used in MO data storage systems. The absorption of light by the storage medium and the subsequent creation of a hot spot is another common occurrence in optical recording. Such hot spots are typically needed for effecting the recording process, be it ablation in WORM-type media, structural phase transformation in phase-change media, or thermomagnetic recording in MO media.

Both read and write processes can benefit from the incorporation of the storage layer in a multilayer (optical-interference-type) structure. In the case of readout, the multilayer enhances the signal contrast (or the signal-to-noise ratio) by utilizing constructive interference among the various information-carrying beams reflected at the interfaces. In the case of writing and erasure, antireflection structures improve the sensitivity of the medium and promote efficient utilization of the available laser power. These benefits are not mutually exclusive and, in fact, the multilayers in use today are designed to have good recording sensitivity as well as enhanced readout signal-to-noise ratio.