Eu3+ -and Sm 2+ -ion-doped glasses have been attracting much interest because they exhibit room-temperature persistent spectral hole-burning (PSHB) properties and have great potential for use in high-density frequency-domain optical data storage. The primary object of this strategy involves the preparation of glasses that exhibit highly efficient PSHB at room temperature. Recently, we found that Al 3+ ions in glasses have the effect of increasing the PSHB efficiency of the rare-earth ions, though the formation mechanism of the burned holes remains unclear. Here, for the first time, we observe that the stable and persistent spectral holes are formed by two-step electron transfer through an activated Eu 3+ ion during laser irradiation. This new observation could be very significant when considering the hole-burning mechanism and developing PSHB materials. When irradiated by a laser beam, a spectral hole is formed at the position of the laser energy in the absorption band, the presence or absence of which can be used to encode a digital 1 or 0 as a memory unit.[1] This type of memory is a good candidate for ultra-high-density frequency-domain optical data storage. Many materials doped with rare-earth ions have been shown to display the property of PSHB; however, in most of them PSHB is limited at low temperature. The Sm 2+ and Eu 3+ions are a special case as they show PSHB at room temperature [2][3][4][5][6][7][8][9] and have a high potential for use in practical memories. As a host material for these rare-earth ions, glass is more favorable than crystals because of its ability to widely distribute rare-earth ions, high transparency, compositional variety, and ease of mass production. · 10 SiO 2 (in mol %), hereafter denoted as 1E-10A, 1E-40A, and 29E-60A, respectively, were prepared using a melt-quenching technique. After heating at 800°C in air, they were irradiated with an X-ray source. The obtained glasses are X-ray amorphous and do not contain the Eu 2+ ion.The spectral holes were burned on the Figure 1 shows typical examples of multiple holes burned in the 1E-10A glass using laser beams with five different wavelengths. It is evident that the presence or absence of holes is shown in the spectrum and this material is a good candidate for ultra-density frequency-domain data memory. The typical PSHB spectra for the three glasses prepared in this study are shown in Figure 2. The persistent holes are clearly observed at the burning position for the samples of 1E-10A and 1E-40A. Figure 1. PSHB spectrum of 1E-10A glass before and after hole-burning. Holes were burned at five wavenumbers. The difference between the fluorescence intensities before and after burning is shown at the top.