CaS:Eu2+, Tm3+ is a phosphor known to emit a long afterglow of red emission (650 nm) when excited by blue light (450 nm). It shows a long afterglow time of 700 s for Eu = 0.05% and Tm = 2%. The mechanism of this afterglow is investigated using time-resolved fluorescence (TR-F) spectroscopy from the nanosecond to millisecond region. At room temperature, it is not possible to investigate shallow levels because of the effects of thermal vibrations. The mechanism of the emission characteristics at room temperature would be affected by these levels that can be observed only at low temperatures. Therefore, the samples are cooled to 15 K for the TR-F measurements. The host material CaS emits blue light (420 nm) arising from sulfur defects, and the typical decay time is measured to be 6 ms. This blue emission becomes stronger when Tm3+ is doped. Furthermore, the doped Eu ions emit a broad red spectrum at 650 nm originating from the Eu2+-specific 4f65d1–4f7 transition. When the excitation is ceased, the red emission decays with a fast time constant of 0.6 μs. This value is a typical decay time for Eu2+. This red emission has multiple decay time constants, and a component with a decay time of 6 ms appears. This 6 ms decay time is the same as that of the blue emission from the sulfur defects, which have an important role on the red afterglow.

Red photostimulated luminescence at 650 nm appears under the excitation by using an infrared light-emitting diode at 940 nm in CaS:Eu2+ afterglow phosphors. The effect of doping of Dy3+ ions, alkali metal ions or Cl- ions in CaS:Eu2+ on afterglow or photostimulation is investigated. Afterglow temporal decays and thermoluminescence glow curves suggest that Dy3+ ions and alkali metal ions induce different types of defects in the phosphor, and enhance the afterglow independently. Doping of Cl- ions is found to enhance the photostimulation by the infrared excitation. Even if the phosphor is irradiated with infrared light for some seconds, the afterglow decay curve is the same as when it is not irradiated with infrared light. Trap states responsible for the photostimulation are different from those responsible for the afterglow. The red photostimulation appears under the excitation at 940 nm, after the phosphor is left in the dark for 60 minutes. Its intensity is 68% of the red photostimulated luminescence generated after being left in the dark for 10 minutes. It is considered that the PSL decreases little, when the sample is kept in the dark.