Rare earth (RE) trivalent praseodymium ion (Pr3+) possesses abundant energy levels showing intensive emissions in ultraviolet (UV), visible, and infrared (IR) regions upon multiwavelength excitations. The characteristic emissions enable Pr3+ ions to act as functional centers in various optical materials for different applications, such as in solid-state lasers, fiber amplifiers, flat panel displays, scintillation detectors, and various phosphors^{[1–7" target="_self" style="display: inline;">–7]}. Therefore, photoluminescence (PL) properties of Pr3+ have always been investigated as a focused interest for researchers in a wide range of works^{[5–10" target="_self" style="display: inline;">–10]}. We have more recently reported the UV and visible PL properties of Pr3+ in strontium phosphate glasses, and demonstrated that upon 448 nm excitation the relative intensity of the blue emission (PJ3→H43) to that of the composite orange one (D21→H43) is highly sensitive to the Pr3+ concentration^[10]. This phenomenon is closely related to the interaction of Pr3+ ions with lattice phonons of the host; that is, the phonon-aided P03→D21 nonradiative relaxation (NRR) process likely occurs, thus increasing the emission intensity of the PJ3→H43 transition relative to that of the D21→H43. In other words, the monochromic orange emission originating from the D21 level of Pr3+ ions could be controlled by increasing the probability of the P03→D21 NRR process. As matter of fact, there have been relevant reports in this respect on glasses containing Pr3+ ions^{[10–15" target="_self" style="display: inline;">–15]}, but there still have been many interesting phenomena waiting for us to discover. In the present paper, we investigate PL properties of zinc strontium phosphate (ZSP) glasses doped with a relatively low Pr3+ concentration, and discuss the mechanism involved for realizing the monochromatic orange emission. As an optical material, glass is very competitive owing to its lower cost of synthesis, ease of fabrication, and flexibility of applications^{[16–20" target="_self" style="display: inline;">–20]}. Therefore, this Letter will be significant for the possibility of enormous value in many fields such as lighting sources and lasers.

In the present work, the ZSP glass was used as the host and sodium zinc tellurite (NZT) and barium gadolinium germinate (BGG) glasses as the reference hosts for doping Pr3+ ions of different concentrations ranging from 0.05 to 1.0 mol.% (Table 1). Glass samples were prepared by the conventional melt-quenching method using analytical-grade reagents of ZnO, NH4H2PO4, SrCO3, Na2CO3, and BaCO3, and Gd2O3 (99.95%), GeO2 (99.999%), TeO2 (3N), and Pr6O11 (99.9%) as initial materials. The batches were well mixed and ground, then melted in aluminum crucibles. The experimental melting conditions for phosphate glasses were same as our previous work^[10], while germinate and telluride glass samples were melted at 1550°C for 3 h and 850°C for 2 h, respectively^[21,22]. Glass melts were poured onto the preheated steel mould and quenched in air. The prepared glass samples were annealed at a temperature close to Tg for 3 h to remove the inner stress induced during quenching. All of the samples were cut into 2.5 mm-thick square pieces and were well polished to a good optical quality.

The UV and visible absorption spectra of the samples were obtained by a Unico UV-2102 PC spectrophotometer. PL spectra and decay curves were recorded by a Fluorolog-3-P UV-vis-NIR fluorescence spectrophotometer (Jobin Yvon, Longjumeau, French). All measurements were carried out at room temperature.

Figure 1 presents the absorption spectrum of the ZSP (G1) sample (containing 1.0 mol% Pr3+). The ZSP glass shows a good transparency from the visible to the deep UV region with a cutoff edge close to 210 nm, similar to the strontium phosphate glass, as reported^[10]. Four obvious absorption bands are distinguished in the visible region located at 442, 466, 480, and 590 nm, which are associated with the transitions of Pr3+ ions: H43→Pj3 (j=0,1,2) and H43→D21, respectively. Moreover, a weak UV absorption band around 290 nm is detectable, which should be assigned to the interconfiguration of the f−d transition of Pr3+ ions (H43→4f15d1), according to previous work^[10].

Fig. 1. Absorption spectrum of G1 sample (1.0 mol% Pr3+).

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Emission spectra of ZSP glasses with different Pr3+ concentrations are shown in Fig. 2(a). Upon 442 nm excitation, emission bands due to the f−f transitions of Pr3+ ions between P1,03−H43 (481, 488 nm), P03−H53 (522 nm), D21−HJ3 (597, 610 nm), and P03−F23 (640 nm) levels are observed, which are identical to the previously published work^[10]. Much impressively, with the decrease in Pr3+ concentration, all emissions originating from the P1,03 levels are weakened greatly while that from the D21 level enhanced steadily. Taking the intensity ratio of orange/blue (599/482 nm) emissions as an example, it increases monotonously from 1.67 (G1) to 23.16 (G5), as shown in Fig. 2(b), indicating that the energy transfer from P1,03 to D21 dominates in the 442 nm excitation process on glasses with the lower Pr3+ concentration, leading to a monochromatic orange emission [inset of Fig. 2(b)]. The Pr3+ concentration dependence of the monochromatic orange emission is also evidenced by excitation spectra compared between samples G1 (1.0 mol% Pr3+) and G5 (0.05 mol% Pr3+). As seen in Fig. 2(c), three excitation bands at 442, 466, and 480 nm are observed and they are assigned to H43→Pj3(j=0,1,2) transitions corresponding to the absorption spectrum in Fig. 1. It is seen that excitation bands obtained by monitoring emissions at both 599 and 482 nm have a similar shape and location for both samples, however, their energy distribution is quite different. G5 shows much more intense excitation bands when monitoring at 599 nm than at 482 nm, while G1 does not show a similar evolution.

Fig. 2. (a) Emission spectra of ZSP samples, (b) ratio of 599/482 intensity as a function of the Pr3+ concentration and (c)  excitation spectra of G5 and G1 monitored at 599 or 482 nm.

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Further supporting evidences come from their decay curves (λex=442nm, λem=599nm), which are compared in Fig. 3. The curves can be well fitted by a second-order exponential decay model using the equation^[23]I=A1exp(−t/τ1)+A2exp(−t/τ2),(1)where I is the luminescence intensity, A1 and A2 are constants, t is the time, and τ1 and τ2 are the rapid and slow parts of the lifetime. To calculate the average lifetime (τ), we useτ=(A1τ12+A2τ22)/(A1τ1+A2τ2).(2)The calculated τ of the G1 and G5 samples are 14.61 and 15.27 μs, respectively. The prolonged decay time of G5 with the decreased Pr3+ doping concentration demonstrates the enlarged population of electrons at the D21 level, which is in accordance with the supposed ET process of P03→1D2.

Fig. 3. Decay curve of G1 and G5 samples (λex=442nm, λem=599nm).

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As illustrated in the simplified energy level diagram of Pr3+ ions in Fig. 4, the orange emission (D21→Hj3) under the 442 nm excitation depends on the phonon-aided Pj3→D21 NRR process resulting from the interaction between Pr3+ ions and the glass matrix. For the same host, the lower Pr3+ concentration may increase the probability of the NRR process due to less interaction between the Pr3+ ions themselves. Therefore, the G4 and G5 samples show around a 90% quenched blue emission that initiates from the upper Pj3 level and a greatly enhanced orange emission from the lower D21 level. In this regard, it is easy to infer that at the lower level of Pr3+ concentration, the phonon energy of the host becomes another factor influencing the Pr3+:Pj3→D21 NRR process^[24,25]. The phosphate glass possesses the higher phonon energy (Table 1); thus, it is reasonable to attribute the monochromatic orange emissions of the present Pr3+-doped ZSP glasses to the higher probability of the NRR process due to the joint effects of the lower Pr3+ concentration and the higher phonon energy of the host.

Fig. 4. Simplified energy level diagram of Pr3+.

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To clarify the above explanations, NZT and BGG glasses were taken as the reference hosts (G6, G7) for doping the same Pr3+ concentration as G5 (0.05 mol.%) and compared in Fig. 5 with G5 for their emission spectra on excitation at 442 nm. Different from G5, G6 and G7 show multiple emission bands where the blue emission is more intense than the orange one. Further comparison between G6 and G7 exhibits the much lower orange/blue intensity ratio of G6 (1.33) than that of G7 (2.10), as well as the sharp emission at 640 nm (G6) compared with the weak one (G7), corresponding well to the lower phonon energy of G6 than that of G7. Such differences agree with our expectations and demonstrate the lower probability of the Pr3+: Pj3→D21 NRR process in G6 and G7 than in G5, consequently making populated electrons from the Pj3 excited state to the D21 level insufficient, resulting in the weaker orange emission.

Fig. 5. Emission spectra of glass samples with different phonon energy upon 442 nm excitation.

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The monochromic orange emission is observed in the ZSP glass doped with the low Pr3+ concentration (0.05 mol.%). Excitation spectra indicate the impact of the Pr3+ concentration on the distribution of the excitation energy while the prolonged decay time of the sample with the decreased Pr3+ doping concentration demonstrates the enlarged population of electrons at the D21 level most likely due to the ET process from P03. NZT and BGG glasses with the lower phonon energy are taken as reference hosts for comparison, which suggests that the higher phonon energy of the host and the lower concentration of Pr3+ doping favor the monochromic emission from Pr3+ due to the beneficial phonon-aided Pj3→D21 NRR process.