New Mechanisms and New Systems of Hole Formation in Spectral Hole Burning

A variety of studies on persistent spectral hole burning (PSHB) have been carried out for the last twenty years [1-4], since the discovery of this phenomenon in 1974. The elucidation of electron-phonon interaction, the nature of a zero-phonon line and spectral diffusion in PSHB have been studied intensively together with the research aiming at its possible applicability to ultra-high density optical storage. However, as for the PSHB materials, most of the works have been carried out with porphyrins, phthalocyanines, and quinizarin including their derivatives, several organic dyes, samarium and some other inorganic ions. The number of molecules reported to show hole formation so far is restricted compared to a wide variety of chemical structure of organic molecules. This would be because it is usually thought that for the hole formation the existence of a zero-phonon line in guest molecules for SHB should be accompanied by the occurrence of some photochemical reaction of the guest molecules. A well-known exception on this point is the so-called non-photochemical hole burning (NPHB) [4]. Recently triplet-triplet energy transfer of guest molecule to a host photo-reactive matrix has been reported to be a new family [5] of PSHB systems with new mechanism.