Spectral Data Storage Using Rare-Earth-Doped Crystals
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pend on a material property referred to as inhomogeneous absorption line broadening. Materials exhibiting this property contain active atoms or molecules that individually respond to (absorb) very specific frequencies of light, but the collective response of all of the material's active atoms or molecules Covers a spec tral region that is broad compared with the response of a particular active atom or molecule. Inhomogeneous absorption line broadening is caused by local variations in the structure of the host, which in turn lead to variations in the electronic levels of the active atoms or molecules. The absorption linewidth of an individ ual absorber is referred to as the homogeneous linewidth r h , and the absorption width of a collection of inhomogeneously broadened absorption centers is referred to as the inhomogeneous linewidth F,. Application of monochromatic light to such a material has the effect of exciting only a very small subset of active ab sorbing atoms—those residing in the illuminated spatial volume within a homogeneous width of the exciting light's specific frequency. If the frequency of the imposed light is shifted, a different subset of active absorbing atoms in the il luminated volume responds. The process of spectral hole-burning is illustrated in Figure 1. Figure la shows a material's inhomogeneous absorption profile, which is a collection of individ ual atomic (or molecular) absorptions. Figure lb shows the homogeneous absorp tion profile of a particular active atom or molecule. If such a material is excited by narrow-band light, active atoms or mole cules whose homogeneous spectrum and
physical position overlap with the narrow band excitation light undergo an optical transition to an excited State. This transference of population results in a dip in the bulk absorption spectrum, that is, a spectral hole, as shown in Figure lc. The persistence time of the spectral hole is re ferred to here as the data-storage time. The number of individually addressable frequencies in such a material is given by the ratio ri/Ih. In the most promising ma terials, several million discrete subsets of active absorbing atoms can be separately addressed in each distinct illuminated volume. In this article, we concentrate on spec tral hole-burning in crystalline solids doped with rare-earth (RE) ions. Of par ticular interest for spectral hole-burning are the 4/-4/ transitions of the RE ions.' Table I lists measured values of the homo geneous and inhomogeneous linewidths, as well as the data-storage time, for a variety of RE-doped crystals. Although the detailed behavior of a given material depends on both the host crystal and the RE dopant, the differences observed upon exchanging the host are relatively small compared with the differences observed upon exchanging the dopant. As a class, the RE-doped crystals display many de-
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