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Frequency upconversion (UC) luminescence was investigated in terbium (Tb3+) doped lutetium silicate powders when the samples were irradiated with femtosecond lasers operating either at 800 nm or 1500 nm. The samples with three different Tb3+ concentrations were prepared by the combustion synthesis method. Rietveld analysis of the X-ray powder diffraction data showed the predominance of monoclinic Lu2SiO5 phase. UC luminescence signals induced by three- and four-photon absorption were identified. The mechanisms that originate the anti-Stokes luminescence were discussed.
© 2013 Optical Society of America
1. Introduction
Crystalline powders doped with rare-earth (RE) ions are strong candidates for luminescence based devices such as displays, fluorescent lamps, imaging devices, and scintillators [1]. Among the RE ions, terbium (Tb3+) has attracted significant attention because, along with europium (Eu3+), it presents very large photoluminescence (PL) quantum yield [2–8]. Although Tb3+ ions are a priori insensitive to near-infrared light at 800 nm some research teams have focused on alternative approaches to obtain infrared-to-visible conversion using short-pulsed lasers. When irradiated with strong near-infrared lasers visible PL is achieved by simultaneous multi-photon absorption, i.e.: electrons in the ground state are promoted to the excited states without intermediate resonances. This process is different from excited-state-absorption, energy-transfer, cooperative upconversion, and photon-avalanche that are more usual processes [9]. For instance, three-photon induced luminescence has been demonstrated in Ce3+-doped YAlO3 crystal [10], Tb3+-doped multicomponent oxide glass [11] and Eu3+-doped LiNbO3 polycrystalline powder [12].
Presently there is large interest in the study of powdered lutetium silicates. Many studies of lutetium silicate doped with RE ions aim their application as luminescent materials motivated by the already proved large efficiency of these compounds as cathode ray phosphors and gamma ray detectors in nuclear medical diagnostic equipment. Lutetium orthosilicate (Lu2SiO5) - LSO - and lutetium pyrosilicate (Lu2Si2O7) – LPS – are the two most stable compounds in the Lu2O3-SiO2 system and powders of cerium (Ce3+) doped LSO and LPS have been studied as scintillators [13–15]. However the frequency upconversion (UC) properties of optically excited RE doped lutetium silicate powders were not reported, as far as we know.
There are many synthesis methods to produce RE-doped powders such as citrate-gel [16], solid-state reaction [17] and combustion synthesis (CS) [6, 7, 18, 19]. For instance, CS has been employed to prepare RE-doped low-cost high-yield oxide powders with many advantages over the other methods because it involves simple apparatus, direct crystallization of small particles, low processing temperature and short preparation time. The products obtained by the CS method are generally more homogeneous and have smaller amount of impurities than the powders prepared by conventional methods. However, as it involves a drastic exothermic reaction, it is difficult to control the final product in terms of particle’s sizes and shape, as well as the crystalline phase purity.
In the present work we used the CS technique to prepare Tb3+ doped lutetium silicate powders to study some of their UC properties. One excitation source used was a femtosecond optical parametric amplifier (OPA) operating at 1500 nm. In this case the UC luminescence was due to the absorption of four laser photons. Moreover, aiming to discover new uses of this material, we performed experiments with a femtosecond laser at 800 nm. For biological applications, such as cell imaging, 800 nm is more convenient than 1500 nm due to the large optical absorption of water at this wavelength. Therefore PL induced by absorption of three 800 nm laser photons was also studied.
2. Experimental details
The powders were synthesized using reagent-grade terbium nitrate, Tb(NO3)3-6H2O, lutetium nitrate, Lu(NO3)3-6H2O, silica fumed 0.014 μm and urea, CH4N2O, as the starting materials. To prepare the samples, the metal nitrates and silica fumed were mixed in a proper weight to produce Lu2SiO5 phase. The mixture was put on a small amount of de-ionized water and after that urea was added to the aqueous nitrate solution. The mixed solution was kept under constant stirring until it was transformed into a transparent viscous gel. Afterwards, the resultant gel precursor was placed in a ceramic crucible and introduced in a furnace, preheated at 500 °C, for about 20 minutes. Finally, the as-prepared powders were sintered at 1100 °C for 2 h in air atmosphere with a heating-rate of 200 °C/h. The as-prepared samples have Tb3+ concentrations of 0.5, 1.5 and 3.0% wt.
Surface morphology was investigated using a Hitachi TM-1000 tabletop Scanning Electron Microscope (SEM). The accelerating voltage was preset at 15 kV. Imaging was obtained by a backscattering electron detector with the preset charge-up reduction mode.
Structural analysis was performed by X-ray powder diffraction (XRPD) using a Siemens D-5000 diffractometer with Cu Kα radiation (λ = 0.15406 nm). The diffractometer optics used to collect the data consisted of a fixed divergent slit (1.0 mm), a scattered radiation slit (1.0 mm), a receiving slit (0.2 mm) and a graphite monochromator in front of the detector. The XRPD data was refined following the Rietveld method with the GSAS-II software [20].
Stokes luminescence spectra were obtained using a continuous wave lamp peaked at 255 nm (25 W) as the excitation source. UC luminescence spectra were obtained with a Ti: sapphire amplified system (800 nm, 1.0 kHz, 100 fs) in conjunction with an OPA operating at 1500 nm. Fresh powders were held between two microscope slides with an estimated powder thickness of ≈0.5 mm. The laser power reaching the sample was adjusted using a λ/2 plate followed by a polarizer and focused with a lens (100 mm focal length); the laser spot diameter at the sample was ≈5 mm. The PL was collected by an optical fiber connected to a spectrometer that has resolution of 1 nm. A glass filter (Corning 4-94) was used to block the 800 nm scattered laser light in front of the spectrometer. In the 1500 nm experiments the wavelength rejection of the spectrometer was enough to block the elastic scattered light. The signal was processed using a boxcar (gate delay: 0 ms, gate width: 100 ms; sampling average: 10) coupled to a computer. The luminescence dynamics was obtained by collecting the PL with a multimode fiber connected to a 0.25 m monochromator attached to a H5783 Hamamatsu photomultiplier. The signals were recorded using a digital oscilloscope (load ≈50 Ohms) connected to a personal computer. All measurements were performed at room temperature.
3. Results and discussion
Figure 1 shows the SEM image and the XRPD pattern for one of our prepared samples. The SEM image shows a highly porous material with a complex sponge-like structure containing aggregated particles. This is characteristic of powders prepared by CS where a large amount of gases are released during ignition. The X-ray diffraction peaks are consistent with those of standard monoclinic Lu2SiO5 with space group I2/a (JCPDS card No. 41-0239) and monoclinic Lu2Si2O7 with space group C2/m (JCPDS Card No. 34-0509). The samples do not present the phase Lu2O3 because the heat-treatment temperature was larger than 1000 °C [21]. Rietveld analysis was performed in the XRPD data shown in Fig. 1 and the results are presented in Table 1. As expected, Lu2SiO5 is the predominant phase.
Fig. 1 (a) SEM image and (b) XRPD pattern for one of the samples studied in this work. The SEM image full scale bar: 20 μm.
Table 1. Rietveld refinement results of our lutetium silicate powder prepared by combustion synthesis.
Figure 2 shows the Stokes spectra obtained upon excitation at 255 nm. The PL bands are due to the Tb3+ inner-4f shell electronic transitions from excited states to lower energy states: 5D4 → 7FJ (J = 1, 3, 4, 5, 6) and (5D3,5G6) → 7FJ (J = 3, 4, 5, 6) [22]. PL bands in the range 350-450 nm appear only in the sample with low Tb3+ doping concentration (0.5% wt) indicating quenching processes in the samples with larger Tb3+ concentrations, probably due to cross-relaxation involving states 5D3 and 5D4 [4]. Strong emissions in the range 470 - 650 nm are observed for all samples.
Figures 3(a) and 3(b) show the UC spectra of samples containing 0.5, 1.5 and 3.0% wt of Tb3+ ions excited at 800 nm and 1500 nm, respectively. Both, Fig. 3(a) and 3(b), show similar results to the ones observed in Fig. 2 with the strongest emission located at ≈549 nm.
Fig. 3 Anti-Stokes luminescence spectra of the lutetium silicate samples excited at 800 nm (a) and at 1500 nm (b).
The nonlinear character of the PL under near-infrared excitation is confirmed by determining the number of laser photons involved in the generation of each UC photon. Assuming that the UC process is not saturated we expect that the PL intensity obeys the relationship IUC α ILN where N is the number of laser photons required to populate the UC emitting states and IL is the laser intensity. The value of N may be found determining the slope of the best-fit line in a log-log plot of IUC versus IL. Figure 4 shows the PL signals obtained at 549 nm from one of the samples as a function of excitation intensity. The values determined for N indicate that four and three laser photons contribute for the UC processes for excitation at 1500 and 800 nm, respectively. Similar results were obtained for the other two samples.
In order to understand the anti-Stokes PL process a simplified energy levels diagram of the Tb3+ ions and the UC excitation pathways are represented in Fig. 5. The direct one-photon absorption at 255 nm and three-photon absorption (with laser wavelength at 800 nm) fall into the Tb3+ ion 4f75d1 manifold of states from where, after successive nonradiative relaxation steps, the excited ions decay to the emitting states 5D3 and 5D4. The off-resonance two-photon absorption of transition 7F6 → 5D3 excited with the laser at 800 nm is ruled out because the energy mismatch between the two photons energy and the 5D3 level is large (≈1500 cm−1). Accordingly, the results presented in Fig. 4 show that the emission from state 5D4 is due to three-photon absorption at 800 nm. The UC process observed under 1500 nm excitation, also depicted in Fig. 5, is more complex. The proposed scheme considers a resonant one-photon absorption 7F6 → (7F0, 7F1) followed by three-photon absorption to state 5D2, without intermediate resonance. The energy mismatch is compensated by phonons that participate in the process. The results of Fig. 4 show that a four-photon process dominates.
Fig. 5 Tb3+ energy level diagram illustrating the single-photon and multi-photon absorption and luminescence.
Further evidence for the energy-transfer process involving Tb3+ ions in the 5D4 state are presented in the semi-logarithmic plot of Fig. 6 showing the luminescence decay of level 5D4 following the excitation at 800 nm. In Lu2SiO5, RE ions occupy two different crystallographic sites of C1-symmetry. It was observed that the RE ions present comparable lifetimes in these two sites [23]. In this case, it was reasonable to estimate the PL decay time in our samples by use of the equation [24]:
where I(t0) is the maximum intensity at the initial time t0. The values found were 1.25, 1.10 and 0.84 ms for 0.5, 1.5 and 3.0 wt %. The shortening in the PL decay time indicates quenching of population as the doping concentration is raised.4. Summary
In summary, we reported Stokes and anti-Stokes photoluminescence spectra of Tb3+ doped lutetium silicate powders prepared by combustion synthesis. For excitation we used femtosecond lasers at 800 nm and 1500nm and the results show that blue, green and red emissions can be obtained. The multi-photon absorption process that generates infrared-to-visible luminescence upconversion involves three and four incident laser photons. The present results indicate that RE-doped lutetium silicate is a promising infrared-to-visible upconverter material.
Acknowledgments
This work was supported by the National Institute of Photonics (INCT de Fotônica) granted by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and by FACEPE (Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco).
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