The ever increasing demand for high data storage capacity compels the two-photon (2P) excitation based three-dimensional (3D) bit by bit optical data storage system, where the information is recorded as a localized physical or chemical change inside the volume of recording media. Even though the highly confined property of 2P induced optical data storage has led to a higher recording density, it can only reach a theoretical density up to a few Terabytes per disc, which is far below the requirement for the next generation of information storage devices. This book introduces the emerging field...