Recent developments in the new spectroscopic technique concerning photochemical hole burning (PHB) and its possible application to the wavelength-multiplexed high density optical memory systems are reviewed. The basic concept of PHB is the site-selective optical bleaching usually observed in the solid systems doped with organic dye molecules at liquid helium temperature region. The molecular conditions and the fundamental mechanisms for the appearance of the holes are discussed in relation to the ordinary photochromic molecules. The review also includes discussions on the present research activities on PHB materials for future optical storage systems.
