Under 980 nm excitation, unusual 3P2→3H6(~264 nm) and 3P2→3F4(~309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished due to the efficient cross-relaxation of 1G4+3H4→3F4+1D2(Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.
© 2008 Optical Society of America
Frequency upconversion (UC) from infrared (IR) to visible/ultraviolet (UV) by materials doped with trivalent rare-earth (RE) ions, such as Tm3+, Er3+, Ho3+, Nd3+, and Pr3+, has attracted intensive attention for more than 30 years [1,2]. Its potential applications include detection of infrared radiation, optical storage, color display, UC lasers, biosensor, and so on [3-7]. Most of these applications have been achieved with the help of fluorides due to their low phonon energy and high quantum efficiencies as luminescent hosts . Among RE ions, Tm3+ has attracted increasing attention because it has metastable levels (Fig. 1) suitable for emitting blue and UV UC luminescence [9-11]. Especially, UV UC luminescence from Yb3+-sensitized Tm3+ in fluorides has become one hot topic of optical functional materials in recent years . Our group has reported strong UV UC emissions from Yb3+/Tm3+-codoped fluoride film and attributed the UV enhancement to the decrease of Judd-Ofelt parameter Ω2, which reflects the symmetry of the crystal field . Wang et al. have reported intense UV UC luminescence from Yb3+/Tm3+-codoped YF3 nanocrystals embedded glass ceramic . Chen et al. demonstrated six-photon and five-photon UV UC luminescence in Yb3+/Tm3+-codoped ZBLAN fluoride glass . Despite the fact that UV UC emissions have been widely investigated and some novel spectral phenomena have been reported, the study of new approaches to obtain efficient UV luminescence as well as the mechanism of the enhancement is still a challenge for the development of short-wavelength solid-state lasers .
In this study, intense UV UC luminescence was presented in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals under 980 nm excitation. The 3P2→3H6 and 3P2→3F4 emissions were observed for the first time. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished.
Y2O3, Yb2O3, and Tm2O3 (purity ≥ 99.999 %) were supplied by Shanghai Chemical Reagent Company. EDTA, NaF, and HNO3 were supplied by Beijing Chemical Reagent Company, and were of analytical grade. Deionized water was used to prepare solutions. Ln2O3 (Ln=Y,Yb, and Tm) were dissolved in dilute HNO3 by heating to prepare the stock solution of Ln(NO3)3.
In a typical synthesis, 1 mL of 0.5 M Ln(NO3)3 aqueous solution and 0.5 mmol of EDTA were dispensed into 20 mL of deionized water and magnetically stirred for 1 h, forming a chelated Ln-EDTA complex. Then 16 mL of 0.5 M NaF aqueous solution was added to the solution. After vigorous stirring for 1 h, the mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. The autoclave was sealed and maintained in an oven at 160 °C for 18 h, and then cooled down slowly to room temperature. Subsequently, the suspension was centrifuged at 8000 rpm for 10 min. The resultant product was then washed thoroughly and dried in vacuum at 80 °C. To improve the crystallinity of the nanocrystalline powder, the product was annealed at 400 °C for 2 h.
The crystal structure was analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using a nickel-filtered Cu-Ka radiation (λ=1.4518Å). The size and morphology were investigated by scanning electron microscope (SEM, KYKY 1000B). UC luminescence spectra were recorded with a Hitachi F-4500 fluorescence spectrophotometer (2.5 nm for spectral resolution (FWHM) of the spectrophotometer and 400 V for PMT voltage) at room temperature.
Figure 2(a) shows the XRD pattern of the microcrystals. All the diffraction peaks can be indexed to the pure hexagonal NaYF4 (JCPDS 16-0334). No other impurity peaks were detected. The corresponding SEM image (Fig. 2(b)) shows that the NaYF4:Yb3+/Tm3+ microcrystals are hexagonal pillars.
3.1. Intense UV and weak visible emissions
Under 980 nm excitation (~320 W/cm2), unusually intense UV and weak visible emissions were observed in NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals, as shown in Fig. 3. These emissions come from the following transitions of Tm3+ ions: 3P2→3H6 (~264 nm) , 1I6→3H6 (~291 nm), 3P2→3F4 (~309 nm), 1I6→3F4 (~347 nm), 1D2→3H6 (~363 nm), 1D2→3F4 (~454 nm), 1G4 3H6 (~475 nm), 1D2→3H5 (~508 nm), 1D2→3H4 (~574 nm), 1G4→3F4 (~642 nm), 3F3→3H6 (~689 nm), and 3H4→3H6 (~795 nm). In comparison with usual UC emission spectra of Tm3+ [13,14], the 1G4→3H6 and 3H4→3H6 emissions almost vanished. Although Noginov et al. have predicted that the back energy transfer of 1G4 (Tm3+)+2F7/2 (Yb3+)→3H4 (Tm3+)+2F5/2 (Yb3+) could lead the 1G4 level to be quenched , this phenomenon has never been observed in experiments so far.
3.2. Unusual 3P2→3H6 (~264 nm) and 3P2→3F4 (~309 nm) emissions
This is the first report on the 3P2→3H6 and 3P2→3F4 emissions in Yb3+/Tm3+-codoped systems under 980 nm excitation. In our previous observations [8,13], the 3P2 level usually relaxed to 1I6 level and emitted 291 and 347 nm UV UC fluorescence. Radiative transitions from the 3P2 level are difficult to occur due to the nearby 3P0,1 and 1I6 levels offer the routes for rapidly nonradiative relaxation. The nonradiative relaxation of 3P2→3P1→3P0→1I6  is greatly influenced by the phonon energy and competes with the radiative transitions of 3P2→3H6 and 3P2→3F4. Presumably, two mechanisms might be responsible for 264 and 309 nm emissions. (1) Low phonon energy helps to decrease the nonradiative transition of 3P2→3P1→3P0→1I6 , resulting in the 3P2→3H6 and 3P2→3F4 emissions. However, the low phonon energy is unfavorable to phonon-assisted energy transfer from Yb3+ to Tm3+, leading the population of the 3P2 level to decrease. Therefore, 264 and 309 nm emissions can be observed only when the doped host has appropriate phonon energy, on which no final conclusion has yet been reached. (2) The efficient population of the 3P2 level causes the 264 and 309 nm emissions. Here, the luminescence intensity of 1I6→3H6 (~291 nm) is ~6 times stronger than that of 1G4→3H6 (~475 nm), indicating that the population of 3P2 is high enough to populate efficiently the 1I6. Corresponding, the enhanced population of 3P2 causes the 264 and 309 nm emissions, which will be described later.
3.3. Power dependence of UC luminescence.
For nearly any UC mechanism, the emission intensity is proportional to the n-th power of the excitation intensity, and the integer n is the number of photons absorbed per upconverted photon emitted . In the present case, the results are different. Figure 4 shows the double logarithmic plots of the emission intensity as a function of excitation power for the 3H4→3H6, 1G4→3H6, 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions. The corresponding slopes (n) of the dependencies are listed in Table 1.
In the initial stage (0.09-0.14 W), n=1.66, 2.80, 3.74, and 4.57 for the 3H4→3H6, 1G4→3H6, 1D2→3H6, and 1I6→3H6 emissions, respectively. This means that the populations of the states 1I6, 1D2, 1G4, and 3H4 come from five-photon, four-photon, three-photon, and two-photon UC processes, respectively. However, with the increase of excitation power, the values of n changed significantly. In the range from 0.15 to 0.26 W, n=1.23, 1.86, 2.87, 3.14, and 3.17 for the 3H4→3H6, 1G4→3H6, 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions, respectively. In the range from 0.26 to 0.31 W, the UC luminescence deviates from a linear relationship. In the final stage (0.31-0.39 W), n=0.78, 1.78, 3.16, 3.69, and 3.87 for the 3H4→3H6, 1G4→3H6, 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions, respectively. Obviously, when the excitation power is higher than 0.15 W, the slopes (n) of the 3H4→3H6 and 1G4→ 3H6 emission at low laser power are larger than those at high laser power, while the slopes (n) of the 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions at low laser power are smaller than those at high laser power.
4. Mechanism analysis
4.1. Analysis on the nearly vanished radiative transitions from 1G4 and 3H4
A careful analysis of the luminescence dynamics of NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals can reveal the physical mechanism behind the nearly vanished radiative transitions from 1G4 and 3H4. Figure 1 shows the detailed UC processes of populations, relaxations, and emissions in the microcrystals. According to our previous report , the pump light excites only the Yb3+ ions, and three successive energy transfers from Yb3+ to Tm3+ populate 3H5, 3F2, and 1G4 levels. The 1D2 level of Tm3+ cannot be populated by the fourth photon from Yb3+ via energy transfer to the 1G4 due to the large energy mismatch (about 3500 cm-1) between them . The cross-relaxation between Tm3+ ions may alternatively play an important role in populating 1D2 level. Usually, there are primarily two cross-relaxation processes in populating the 1D2 level: 3F2+3H4→3H6+1D2  and 1G4+3H4→3F4+1D2 . They will become stronger with increasing Tm3+ concentration and lead the populations of the 1G4 and 3H4 to decrease. To further verify the mechanism related to the cross-relaxation processes, the dependence of the UC luminescence on the Tm3+ concentration was studied (Fig. 5). From the bottom to the top in Fig. 5, the doped concentrations of Tm3+ ions are 0.01mol%, 0.1mol%, 0.5 mol%, 1 mol%, and 1.5 mol%, respectively. We can see that the UC emissions from the 1I6 and 1D2 get strong and strong with Tm3+ concentration increasing, and the UC emissions from the 1G4 and 3H4 get weaker. When the Tm3+ concentration reaches to 1.5 mol%, the 1G4 and 3H4 emissions nearly vanished.
The populated 1D2 level radiatively relaxes to the ground-state and inter-states, which cause 363, 454, 508, and 574 nm emissions, as shown in the inset of Fig. 3. On the other hand, the 1D2 state may be excited to the 3P2 state via another energy transfer from excited Yb3+, producing 264 and 309 nm emissions, simultaneously. Before this work, there has not been being any experimental evidence to verify which of 3P0,1,2 and 1I6 levels is the terminal level from the 1D2 level for the five-photon UC process of Tm3+ ions. The UC emissions of 3P2→3H6 (~264 nm) and 3P2→3F4 (~309 nm) provide evidence for the energy transfer from Yb3+ to Tm3+ populating 3P2. Of course, the UC emissions from the 1I6 are still dominating in the five-photon UC processes due to the rapidly nonradiative relaxation of 3P2→3P1→3P0→1I6. For a relative high doping concentration, the microcrystals therefore emitted intense 347 and 291 nm fluorescence under the NIR excitation. Furthermore, since the photon (980 nm) exactly matches the energy separation between 1D2 and 3P2, the single-photon absorption of 1D2→3P2 might be exist.
4.2. Competition between linear decay and UC processes
For any UC mechanism, although the n-photon process corresponds to a slope approximately equal to n, the dependence of UC luminescence intensity on pump power is also expected to decrease in slope. Pollnau et al. attributed the decrease of n to the competition between linear decay and UC processes for the depletion of the intermediate excited states . They reported that the UC luminescence intensity for an n-photon energy transfer process was proportional to the n-th power of the pump power (Pn) in the limit of infinitely small UC rates, while the intensity was proportional to the pump power (P 1) in the limit of infinitely large UC rates. Therefore, the intensity of an UC luminescence excited by the sequential absorption of n photons has a dependence of Pβ on absorbed pump power P, with 1<β<n .
Although the population of the state 3H4 comes from a two-photon process, the slope of the 3H4→3H6 emission was estimated to be 1.23 of pump power in the range from 0.15 to 0.26 W. For the 3H4→3H6 emission, the intermediate excited state is 3F4. Therefore, The UC becomes the dominant depletion mechanism for 3F4 in NaYF4:Yb3+/Tm3+ microcrystals . For the 1G4→3H6 emission (a three-photon process), n=1.86 with a large discrepancy from n=3 is observed. It is reasonable to ascribe this to the increase of the nonradiative relaxation from the 3H4 state together with the reduction of n for the 3H4 state . Similarly, n=2.87 with a large discrepancy from n=4 for the 1D2→3H6 emission, n=3.14 with a large discrepancy from n=5 for the 1I6→3H6 (~291 nm) emission, and n=3.17 with a large discrepancy from n=5 for the 3P2→3H6 (~264 nm) emission are also observed.
4.3. Cross-relaxation processes and thermal effect at high laser power
In most cases, the values of n at lower excitation power are higher than those at higher excitation power due to typical saturation phenomenon and thermal quenching. (1) The typical saturation phenomenon is caused by the population exhaustion at the ground state; (2) The high non-radiative rates at high excitation densities lead to an increase of the temperature in the internal sample, inducing a thermal effect and which will lead to the quenching of UC luminescence [19,20]. However, for the 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions, the values of n in the range from 0.15 to 0.26 W are smaller than those in the range from 0.31 to 0.39 W. For the 3H4→3H6 and 1G4→3H6 emissions, although the values of n in the range from 0.15 to 0.26 W are higher than those in the range from 0.31 to 0.39 W, the log-log plots of UC luminescence versus excitation power are not typical saturation curves at all. The typical saturation curves should gradually bend downward.
We suggested that two mechanisms might cause this anomalous phenomenon. First, under different excitation power, different UC mechanisms are dominant . When the excitation power is high enough, the 1G4 and 1D2 are populated effectively, which are benefit for the cross-relaxation of 1G4+3H4→3F4+1D2 between the two neighboring Tm3+ ions . This cross-relaxation process can result in the increase of n for the emissions from 1D2, 1I6, and 3P2. Figure 6 shows the UC luminescence spectra under different excitation power density. When the excitation power increase, the effective populations of the 1G4 and the 3H4 intensively enhance the cross-relaxation mentioned above.
Second, this anomalous phenomenon we observed must be closely related to the effects of phonon density on the nonradiative transitions of Tm3+ levels and the phonon-assisted energy transfers between Tm3+ and Yb3+ ions. The observed values of n are results of competition between nonradiative relaxation (depopulation) and phonon-assisted energy transfer (population). It is well-known that the nonradiative relaxation rate of the excited state can be written as 
where, WNR(0) is the nonradiative relaxation rate at 0 K,ħω is the phonon energy, and <n> is the phonon density of states, which strongly depends on phonon energy and temperature 
where, k is Boltzmann’s constant and T temperature. The phonon density of states increases with increasing temperature. The increased phonon density of states not only results in the increased nonradiative relaxation rate and quenched UC luminescence, but also benefits the phonon-assisted energy transfers from Yb3+ to Tm3+. Therefore, we suggest that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation. At lower excitation power (0.15 – 0.26 W), the thermal effect can be negligible, and the log-log plots of UC luminescence versus excitation power are straight lines. With the increase of excitation power (0.26 – 0.31 W), the plots gradually bend downward, indicating that the nonradiative relaxations increase faster than the phononassisted energy transfers. When the excitation power is high enough (0.31 – 0.39 W), the slopes (n) of the 1D2→3H6, 1I6→3H6, and 3P2→3H6 emissions again increase, indicating that the energy transfers increase faster than the nonradiative relaxations. Of course, the underlying upconversion dynamics are complex and require further study.
Under 980 nm excitation, novel UC luminescent properties were presented in hexagonal NaYF4:Yb3+(20%)/Tm3+(1.5%) microcrystals. The 3P2→3H6 (~264 nm) and 3P2→3F4 (~309 nm) emissions were observed for the first time. In comparison with usual UC emission spectrum of Tm3+, the 1G4→3H6 and 3H4→3H6 emissions almost vanished. The dependence of the Tm3+ concentration on the UC luminescence indicated that the unusual phenomenon was caused by the efficient cross-relaxation of 1G4+3H4→3F4+1D2 (Tm3+) under high Tm3+ concentration.
In addition, the number of laser photons absorbed in one UC excitation process, n, changed in different excitation power range, which was theoretically explained considering different UC excitation mechanisms together with thermal effect under higher excitation power.
This research was supported by Natural Science Foundation of China (Grant Nos. 10474096 and 50672030).
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