No-phonon spectral hole-burning produced by condensed organic impurity large molecules under laser irradiation is not only narrow in width, high in sensitivity, but lasts a long time. It therefore has a bright application future.
One very exciting application of laser spectroscopy would be to use the high-resolution capabilities of today’s lasers and the phenomenon of persistent spectral hole-burning to form a frequency domain optical storage system.1 In such a system, bits of information would be stored by locally modifying (in the frequency domain) the shape of an inhomogeneously broadened absorption line of an absorber in a solid at low temperatures. Because typical homogeneous widths are 1000X (or more) narrower than typical inhomogeneous widths, as many as 1000 bits could be written in one diffraction-limited 1-μm spot In a sample, yielding the impressive storage density of 1011 bits/cm2.
Recently there has been considerable interest in frequency domain optical memories based on the cryogenic phenomenon of photochemical hole burning.1 This phenomenon occurs whenever persistent photochemical changes can be induced by exposure to narrowband optical radiation tuned to a frequency within the inhomogeneously broadened zero phonon line of a photoactive material contained In a host. The narrowband radiation selectively excites that fraction of molecules whose homogeneous linewidth overlaps the excitation frequency. The ensuing photochemical reaction results in a persistent narrow dip or hole in the line shape. Since typical ratios of inhomogeneous- to-homogeneous widths are of the order of 103, 103 resolvable holes can be burned into the inhomogeneous line at each spatial location in the sample. Thus the storage density in such a frequency domain optical memory might ultimately be 103 higher than in a conventional optical memory.
Photochemical hole burning (PHB) is a topic of attracting increasing interest from spectroscopic viewpoints as well as from the practical prospects for ultra-high-density frequency-domain optical storage. Recent advances in PHB study are reviewed mainly focusing on the effects of chemical structure of guest ions and molecules as well as host matrices on the efficiency of hole formation and its temperature dependence, low energy excitation modes and Debye-Waller factors in amorphous matrices, and the mechanism of photon-gated PHB.