Analysis of the thermal behavior of photoluminescent optical memory based on CaS:Eu,Sm media

Abstract

The temperature dependence of the photoluminescence (PL) and infrared stimulated luminescence (ISL) exhibited by CaS doped with both europium (Eu) and samarium (Sm) ions is reported for temperatures in the range between 100 and 700 K. Behavior indicative of strong electron-phonon coupling is observed in the PL emission of the Eu2+ ion and two processes with activation energies (ΔE1,ΔE2) are recognized in the quenching of the PL intensity. The smaller of these (ΔE2=0.056eV) is identified as the energy separation between Eu2+ excited states and the conduction band of CaS while ΔE1, which has a value of 0.55 eV, is attributed to some intervening quenching state. On the other hand, analysis of the integrated ISL intensity as a function of temperature prompts the conclusion that the separation (Δε) between the lowest excited energy levels of Eu2+ ions and that of Sm2+ ions is approximately 28 meV at low temperature. When it is considered for an optical memory medium, recording experiments performed at different temperatures show the writing process to be more efficient when conducted at higher temperatures rather than lower ones. Moreover, two different time constants, that are themselves linear functions of temperature, are observed to govern the readout process in which electrons trapped at the Sm ions are depleted under continuous (IR) irradiation. Finally, studies of the ISL intensity versus storage time at different temperatures show the reading output to be lowered for longer storage times due to a thermally assisted decay route facilitating the release of electrons trapped at Sm2+ ions back to Eu2+ ions. © 2005 The American Physical Society.