Okuno TSUYOSHI (奥野 剛史)

Abstract

Optical read-out data storage materials based on upconversion phosphors were studied to achieve low-energy consumption. Meanwhile, all-optical data write-in and read-out can be realized using photo-stimulated luminescence (PSL)-based optical data storage. In this study, PSL of Zn ₂ GeO ₄ :Mn ²⁺ (ZGO:Mn) and Zn ₂ GeO ₄ :Mn ²⁺ ,Ca ²⁺ (ZGO:Mn,Ca) powder samples synthesized using a solid-state method was measured and the saturation of the integrated PSL intensity against the excitation intensity was analyzed. From the integrated intensity ratio of excitation and PSL at the saturation point, the PSL trap densities were calculated to be N _T ≃ 10 ¹⁶ cm ^–3 . The excitation time dependence of the integrated PSL intensity and photoluminescence (PL) initial rise in ZGO:Mn and ZGO:Mn,Ca were also measured. The time constants of charging in the PSL trap were τ _ex = 22.1 s (ZGO:Mn) and 36.2 s (ZGO:Mn,Ca), while those of the PL initial rise were τ ₁ = 0.977 s, τ ₂ = 10.7 s (ZGO:Mn) and τ ₁ = 2.53 s, τ ₂ = 24.0 s (ZGO:Mn,Ca). These results should contribute to advancing the design of all-optical data storage devices that consume less energy.

Ca2Si5N8:Eu2+ is known to exhibit a long red afterglow when codoped with Tm3+. In this study, the effects of Tm3+ codoping on the afterglow properties of Ca2Si5N8:Eu2+ are investigated in detail using time-resolved fluorescence spectroscopy and electron spin resonance (ESR). Ca2Si5N8:Eu2+, Tm3+ shows three types of emission: broad blue emission (450 nm), blue-line emission of Tm3+ (460 and 480 nm), and red emission of Eu2+ (615 nm). The broad blue emission is related to the defect levels in the host crystal. Eu2+ and Tm3+ decay with an intrinsic time (0.8 and 80 μs, respectively), thereafter Eu2+ emits afterglows with the same slope as the defect luminescence. The afterglow decay curve from 1 to 1000 s is linear in a log–log plot. Thermoluminescence measurements show trapped levels at 0.04 and 1.4 eV for Ca2Si5N8:Eu2+, and the addition of Tm3+ increases the number of electrons trapped in levels from 0.3 to 0.8 eV. In ESR measurements, nitrogen vacancies in silicon nitride crystals can be detected by the signal intensity of Si dangling bonds. The intensity of nitrogen vacancies is decreased by codoping of Tm3+. Hyperfine signal intensity from Eu2+ nuclear magnetic moments is also decreased. These results suggest that Tm3+ is located close enough to affect the electronic configuration of nitrogen vacancies and Eu2+. Nitrogen vacancies and Tm3+–Eu2+ pairs are considered to interact with each other and energy is transferred among them, which leads to a long afterglow.

Abstract

Defects in phosphors affect not only luminescence intensity but also emission peak width, decay time, and afterglow. The green phosphor β-SiAlON:Eu²⁺ exhibits the green emission of Eu²⁺ at 520 nm and the blue emission of nitrogen vacancies at 460 nm in time-resolved fluorescence measurements. The decay time of the intrinsic Eu²⁺ transition is 0.7 μs, but afterglow is detected from 50 μs to 0.01 s. This afterglow decay curve is the same for the green emission of Eu²⁺ and the blue emission of nitrogen vacancies, suggesting that the defect levels of the nitrogen vacancies affect the Eu²⁺ transition. The afterglow decay curves were analyzed using the formula of the general-order kinetics, ${\left( {1 + t/{\tau _{\text{B } } } } \right)^{ - n } }$, where $n$ is the decay power and ${\tau _{\text{B } } }$ is the decay time. This equation is generally used when analyzing afterglow on the order of seconds to hours but has not been examined systematically applied in samples with different concentrations of Eu²⁺ and temperatures on the order of nanoseconds to milliseconds. The decay power $n$ is approximately 1 for all Eu²⁺ concentrations (x = 0.001–0.3) and undoped β-SiAlON. The decay time ${\tau _{\text{B } } }$ is correlated with the density of the nitrogen vacancies determined by electron spin resonance. Furthermore, the value of $n$ is approximately 1 for 50 μs to 0.01 s and 0.3 for 1–1000 s. Thus, the luminescence mechanism of Eu²⁺ can be discussed by comparing $n$ and ${\tau _{\text{B } } }$ obtained from the decay curves. In addition, several different Eu²⁺-doped phosphors, namely SrAl₂O₄:Eu²⁺, Dy³⁺, CaAlSiN₃:Eu²⁺, and CaS:Eu²⁺, Tm³⁺, are studied.

Abstract

In afterglow phosphors, luminescence appears and can be observed with the naked eye for minutes to hours or more, even after photoexcitation ceases. Red afterglow and photostimulated luminescence (PSL) at 650 nm are studied in CaS:Eu²⁺, Mn²⁺ phosphors. Infrared light at 980 nm from a laser diode induces the red PSL for 990 s. Two types of trap states are found to be present in the phosphors by using thermoluminescence (TL). Deep trap states are reflected in a TL peak in the temperature region of 520 K, and are related to PSL. Shallow trap states reflected in the other TL peak at 250 K are related to afterglow. The intensity dependence of photoexcitation on PSL shows that carriers are more easily accumulated in the deep trap states than shallow trap states. Experiments of electron paramagnetic resonance are conducted to discuss the possible origins of PSL and the afterglow.