A current-tuned GaAlAs-diode laser is utilized both to burn and to detect narrow photochemical holes in the inhomogeneously broadened 833-nm zero-phonon line of the R' color center in LiF. Applications for reading and writing data into frequency-domain optical memories based on photochemical hole burning are discussed.
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.
Many investigations have been done on the field of frequency domain optical memories based on photochemical hole burning (PHB). This effect offers an extra dimension for the organization of an optical memory. At cryogenic temperatures several solid state materials such as aggregate color centers in alkali halide crystals exhibit inhomogeneously broadened absorption lines. In such an absorption line permanent narrow holes can be burnt by photochemical processes induced by exposure to intense laser radiation with a wavelength within the inhomogeneously broadened line.