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Energy transfer and frequency upconversion in Yb 3 + – Er 3 +-doped PbO-GeO 2 glass containing silver nanoparticles
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kassab, Luciana Reyes Pires Bomfim, F. A. Martinelli, José Roberto Wetter, Niklaus Ursus Neto, Jonas Jakutis Ara'ujo, Cid B. De |
| Copyright Year | 2008 |
| Abstract | We report the infrared-to-visible frequency upconversion in Er3+–Yb3+-codoped PbO-GeO2 glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the F5/2 → F7/2 transition of Yb3+ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er3+ transitions. The simultaneous influence of the Yb3+ → Er3+ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra. PACS 42.70.Ce · 78.55.-m · 78.55.Qr · 78.90.+t · 81.05.Pj Phenomena related to the interaction of light with rare-earthdoped (RE-doped) glasses containing metallic nanoparticles (NPs) have been attracting large attention due to their relevance for a variety of applications such as colored displays, optical amplifiers as well as optical sensors. In particular, L.R.P. Kassab Laboratório de Vidros e Datações da FATEC-SP, CEETEPS/UNESP, 01124-060 São Paulo, SP, Brazil F.A. Bomfim · J.R. Martinelli Centro de Ciências e Tecnologia de Materiais—IPEN, 05508-900 São Paulo, SP, Brazil N.U. Wetter · J.J. Neto Centro de Lasers e Aplicações—IPEN, 05508-900 São Paulo, SP, Brazil C.B. de Araújo ( ) Departamento de Física, Universidade Federal de Pernambuco, 50670-901 Recife, PE, Brazil e-mail: cid@df.ufpe.br the surface-plasmon-enhanced (SP-enhanced) luminescence observed in RE-doped glasses containing metallic NPs has been studied by many authors [1–11]. Among the systems of interest the heavy metal oxide (HMO) glasses are known as excellent materials for photonic applications because of their specific characteristics such as: large mechanical resistance, high chemical durability and thermal stability, large transmission window (from 400 to 4500 nm), high refractive index, large nonlinear optical response, and low cutoff phonon energy (400– 800 cm−1) [1]. In the past years the synthesis and the characterization of HMO glasses doped with RE ions and containing metallic NPs were reported [7–11]. For instance, down-conversion luminescence enhancement was reported for Pr3+-doped lead-germanate oxide (PGO) glass containing silver NPs [7]. The increase of Eu3+ luminescence due to the presence of gold NPs in tellurite glasses was observed and characterized [8]. Studies of SP-enhanced frequency upconversion (UC) in Pr3+-doped tellurium-oxide glasses with silver NPs were reported in [9]. Also, recently we have demonstrated enhancement of ∼100% in the infrared-to-green UC process in Er3+-doped PGO glass containing silver NPs [10]. The excitation was made at 980 nm and the growth of the UC intensity at 530 and 550 nm was attributed to the increased local field in the proximity of the NPs. Unfortunately, even in the presence of the NPs the UC luminescence of the Er3+-doped PGO is limited by the relatively small absorption cross-section of Er3+ ions at 980 nm. The red luminescence at 662 nm having wavelength far from the SP resonance wavelength, λSP, is less influenced by the presence of spherical silver NPs. However, it may be intensified if particles with different shapes and aggregates of NPs are present. On the other hand, PGO glass doped with Er3+ and Yb3+, without silver L.R.P. Kassab et al. Fig. 1 Absorption spectra of Er3+–Yb3+-codoped PbO-GeO2 glasses containing silver NPs for different heat-treatment durations NPs, present increased red luminescence due to the energy transfer (ET) from Yb3+ to Er3+ ions [12, 13]. In this letter we demonstrate improvement of the UC luminescence efficiency of Er3+ ions in PGO glass containing silver NPs by codoping the samples with Yb3+ ions. We show that the combined effect of the silver NPs and the efficient ET from resonantly excited Yb3+ ions contribute to the enhancement of the whole UC spectrum. The large absorption cross-section of the Yb3+ ions at 980 nm allows the increase of the red luminescence in such a way that it may reach intensities comparable with the green emissions. The PGO glass composition studied was 41 PbO-59 GeO2 (in mol%). For the experiments we added 0.5 wt% of Er2O3,3.0 wt% of Yb2O3, and 1.0 wt% of AgNO3 to the original composition. The samples were obtained by melting the starting oxide powders in an alumina crucible for 1 h at 1200°C, quenching and annealing in air in a preheated brass mold, and annealing at 420°C for 2 h. After the cooling the samples were polished and then annealed for different durations (τA = 24, 48, and 72 h) to reduce the Ag+ ions to Ag0, in order to nucleate silver NPs. Transmission electron microscopy (TEM) using a 200 kV equipment was performed to investigate the nucleation and growth of the NPs. The linear optical absorption spectra were measured at room temperature in the 300–1700 nm range and the frequency UC spectra (infrared-to-visible conversion) were excited with a CW diode laser (7 W, 980 nm). The luminescence signal was dispersed by a monochromator fitted with a photomultiplier. Figure 1 presents the absorption spectra of the Er3+– Yb3+-codoped PGO glass containing silver NPs in the visible and in the near-infrared regions. Absorption bands atFig. 2 TEM images of the samples annealed for 48 h at 420°C tributed to the 4f –4f transitions of Er3+ ions corresponding to the transitions starting from the ground state (I15/2) to the excited states are observed. The intense absorption band at ≈980 nm is mainly due to the F7/2 → F5/2 transition of Yb3+ ions that overlaps with the weaker I15/2 → I11/2 transition of the Er3+ ions. The absorption band related to the SP is not observed because the amount of the NPs is not enough to originate a strong band. With the basis on the dielectric function of silver [14] and the refractive index of PGO glass (≈2), we estimated λSP to be located in the range of ∼420 to ∼500 nm. However, the presence of the silver NPs in the PGO samples is confirmed by electron microscopy. Figure 2 shows a TEM image corresponding to the sample annealed for 48 h at 420°C. The metallic NPs have average diameter of 2.5 nm but we also observed the presence of larger particles and aggregates. The samples heat-treated for shorter time intervals present NPs with smaller average diameter. Figure 3a shows the emission spectra of the Er3+–Yb3+codoped PGO samples for different anneal durations, τA. Emission bands centered at 525, 550, and 662 nm that correspond to the transitions H11/2 → I15/2, S3/2 → I15/2, and F9/2 → I15/2, respectively, are observed. Figure 3b shows that the relative intensity of the UC bands can be adjusted by an appropriate choice of τA that controls the amount of silver NPs formed in the sample. Figure 4 presents a simplified energy level diagram for the Er3+ and the Yb3+ ions. Because the laser wavelength is in resonance with the transition F5/2 → F7/2 (Yb3+) and Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver Fig. 3 (a) Frequency UC spectra for excitation at 980 nm. (b) Normalized integrated UC intensity as a function of the annealing time because its oscillator strength is much greater than the one corresponding to the transition I15/2 → I11/2 (Er3+), we attribute the excitation of levels S3/2 and F9/2 in Er3+ ions mainly to the ET from Yb3+ ions. The ET efficiency depends on the relative concentration of Er3+ and Yb3+ and the lifetime of the levels participating in the ET process. There are two possible efficient ET pathways [12, 13] as indicated in Fig. 4 by the dashed lines 1, 2, and 3. The green emissions are mainly originated by the following process: Yb3+ ions are excited from the ground state to the F5/2 multiplet under 980 nm pumping; the excited ions transfer the stored energy to the Er3+ ions that are promoted from the ground state to the I11/2 state. Then, a second ET event promotes the Er3+ ions from the I11/2 level to the F7/2 level (steps (1) and (2)—dashed lines in the Fig. 4). Following this transition, the Er3+ ion relaxes nonradiatively to the H11/2 and S3/2 levels. Afterwards, the H11/2 → I15/2 and S3/2 → I15/2 transitions originate the green emissions. The proximity between the green luminescence wavelengths and λSP favors the intensity enhancement that is due to the Fig. 4 Energy level diagram of Er3+ and Yb3+ ions illustrating possible UC pathways for the Er3+–Yb3+-codoped glasses. The solid straight lines with upward and downward arrows indicate optical transitions; dotted lines and wavy arrows denote ET processes and nonradiative relaxation, respectively increased local field resulting from the mismatch between the dielectric functions of the silver NPs and the glass. The red emission at 662 nm is due to the F9/2 → I15/2 transition. Besides the feeding of the F9/2 level through the nonradiative relaxation from the S3/2 level, the ET event represented by 3 in Fig. 4 may occur following the nonradiative transition I11/2 → I13/2. The longer lifetime of the I13/2 level compared with the lifetime of the I11/2 level [1] makes channel 3 dominant over channels 1 and 2; consequently, the red emission is enhanced. Moreover, as shown in Fig. 3, the red emission is also intensified for larger values of τA. This is attributed to the nucleation of larger silver NPs and aggregates as observed for Pr3+ in [7, 9]. Notice that the red intensity illustrated in Fig. 3 becomes comparable with the intensity of transitions H11/2 → I15/2 and S3/2 → I15/2 for τA ≈ 40 h. We also note from Fig. 3b that the ratio UCI/UCII between the integrated intensities of transitions centered at 525 and 550 nm changes from 0.77 (for τA ≈ 0) to ∼1.0 (for τA ≈ 50 h). This behavior is understood considering that the transition at 525 nm is closest to λSP than 550 nm. In summary, with the present results we demonstrated the simultaneous exploitation of the enhanced local field contribution due to silver NPs and energy transfer processes between two different rare-earth (RE) ions in order to control the luminescence spectrum |
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| Alternate Webpage(s) | https://www.ipen.br/biblioteca/2008/13155.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
