Detection of Up-converted Persistent Luminescence in the Near Infrared Emitted by the ${\mathrm{Zn}}_3{\mathrm{Ga}}_2{\mathrm{GeO}}_8:{\mathrm{Cr}}^{3+}$, ${\mathrm{Yb}}^{3+}$, ${\mathrm{Er}}^{3+}$ Phosphor

Up-conversion luminescence and long-persistent luminescence are two well-studied, special luminescence processes. By combining the unique features of these two luminescence processes, here we design a new luminescence process called up-converted persistent luminescence (UCPL), which enables us to generate persistent luminescence having an emission energy higher than the excitation energy. Guided by the UCPL concept, we create the first UCPL phosphor ${\mathrm{Zn}}_3{\mathrm{Ga}}_2{\mathrm{GeO}}_8:1\%{\mathrm{Cr}}^{3+}$, $5\%{\mathrm{Yb}}^{3+}$, $0.5\%{\mathrm{Er}}^{3+}$ by incorporating an up-converting ion pair ${\mathrm{Yb}}^{3+}/{\mathrm{Er}}^{3+}$ into a ${\mathrm{Zn}}_3{\mathrm{Ga}}_2{\mathrm{GeO}}_8:1\%{\mathrm{Cr}}^{3+}$ near-infrared persistent phosphor. After being excited by a 980 nm laser, the phosphor emits long-lasting ($>24\mathrm{h}$) near-infrared persistent emission peaking at 700 nm. The UCPL concept and the associated phosphors are expected to have important implications for several fields such as biomedical imaging.

Figure 1

Schematic diagrams of up-conversion luminescence (UCL), up-converted persistent luminescence (UCPL), and long-persistent luminescence (LPL), with hypothetical energy level schemes. $G$, $M$, $E$ and $D$ represent the ground state, metastable state, emission state, and delocalized state, respectively. Straight-line arrows and curved-line arrows represent optical transitions and electron transfer processes, respectively.

Figure 2

(a) UCPL schematic diagram in the $\mathrm{ZGGO}:\mathrm{Cr}$, Yb, Er system. The straight-line arrows and curved-line arrows represent optical transitions and energy (or electron) transfer processes, respectively. The emission levels are marked with red solid spots. (b) UCL emission spectrum (curve with yellow shadow) under a 980 nm laser excitation and photoluminescence excitation spectrum of ${\mathrm{Cr}}^{3+}$ (blue solid-line curve) by monitoring the 700 nm emission. The red dashed-line circles indicate the positions of the characteristic excitation peaks of ${\mathrm{Er}}^{3+}$. In the emission spectrum, the ${\mathrm{Cr}}^{3+}$ emission was highlighted by the diagonal shadow. (c) UCPL emission spectrum of $\mathrm{ZGGO}:\mathrm{Cr}$, Yb, Er recorded at 5 s after the stoppage of a 980 nm laser excitation. The upper right inset is the UCPL decay curve monitored at 700 nm emission. The left inset shows the NIR images (false color) taken at different delay times (5 min to 24 h) after ceasing the 980 nm laser excitation using a digital camera that was connected to the eyepiece of an ITT PVS-14 Gen III night vision monocular. The signals were attributed to the NIR UCPL. The imaging parameter is manual/ISO $400/30\mathrm{s}$. For all the UCPL measurements, the samples were irradiated by a 980 nm laser for 10 min at a power of 600 mW.

Figure 3

(a) Contour plot of TL spectra obtained by illuminating a $\mathrm{ZGGO}:\mathrm{Cr}$, Yb, Er sample for 300 s with a 980 nm laser whose power was tuned between 200 and 700 mW. The monitoring wavelength is 700 nm. The magnitude of the TL intensity is shown by the color bar. (b) Normalized TL spectra obtained by illuminating a $\mathrm{ZGGO}:\mathrm{Cr}$, Yb, Er sample using a 980 nm laser (at 450 mW) for different exposure times from 10 to 1000 s. (c) Schematic representation of electron transfer under up-converted excitation (UCEX).