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Abstract
Frequency upconversion (UC) luminescence was observed in yttrium oxide (Y2O3) powders doped with trivalent europium ions (Eu3+) when the samples were irradiated with a femtosecond laser system operating either at 1275 nm or 1500 nm. The samples, prepared by low temperature combustion synthesis, were characterized by X-ray diffraction, scanning electronic microscopy, spontaneous Raman scattering and photoluminescence. The UC luminescence, corresponding to the 5D0 → 7FJ (J = 1, 2, 3, 4) transitions of Eu3+, characterized by measuring the amplitude of the UC signal versus the laser intensity, is attributed to the simultaneous absorption of three, four and five infrared photons. The results indicate that Eu3+ doped Y2O3 powder is an efficient upconverter for excitation using femtosecond near-infrared lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The study of frequency upconversion (UC) luminescence in materials doped with rare - earth (RE) ions is attracting large interest for about five decades, because of their potential applications in areas such as solid-state lasers, optical temperature sensors, biological imaging and color displays [1–7]. Indeed, the RE ions present enhanced luminescent properties when doping a large variety of optical materials with emissions from the ultraviolet (UV) to the near-infrared (NIR) exhibiting narrow bandwidths (down to few nanometers) and long lifetimes (micro to millisecond range) at room temperature [1–11]. Particularly, materials doped with the trivalent europium ion (Eu3+) is receiving large attention since Eu3+ emits intense red light at $\approx $610 nm when excited by light sources emitting in the visible and infrared. Moreover, Eu3+ can be used as a sensitive probe to analyze its environment because the transition 5D0 → 7F2, corresponding to the red emission, is very sensitive to the crystallographic site symmetry [11]. Although Eu3+ is a priori unresponsive to low-intensity light in the range 900 nm - 1500 nm, some research teams have focused on an alternative approach to obtain infrared-to-visible conversion using large-intensity short-pulsed lasers [12–15]. In such cases, UC luminescence is achieved by simultaneous multi-photon absorption, i.e., electrons in the Eu3+ ground state are promoted to the excited states without intermediate resonances. However, the UC luminescence of Eu3+ - doped Y2O3 (Y2O3: Eu3+) excited by simultaneous multi-photon absorption was not reported, although Y2O3 presents large NIR transmission besides good thermal stability and chemical durability as demonstrated by several groups [15–18].
In the present work, we report on the UC luminescence of the Y2O3: Eu3+ powder prepared by combustion synthesis. By exciting the powder at 1275 nm (7843 cm−1) and 1500 nm (6667 cm−1) with a femtosecond Optical Parametric Amplifier (OPA), we observed luminescence from 580 nm to 720 nm, IUC, with high-order nonlinear dependence as a function of the infrared power, ${P_{IR}}$, i.e., ${I_{UC}} \propto P_{IR}^N$, with N > 3. In particular, the excitation wavelengths of 1275 nm and 1500 nm were chosen because they are in resonance, by multi-photon excitation, with Eu3+ upper manifold levels. In addition, for many photonic applications, it may be of key importance to obtain visible fluorescence by excitation of infrared radiation, such as, high resolution microscopy and enhanced near-infrared absorption for photovoltaic systems [19,20]. Based on our luminescence results, UC pathways were identified and the intensity parameters Ω2, Ω4 and quantum efficiencies were also calculated.
2. Experimental details
All chemicals used in the present investigation were of analytical grade (99.99%) and used as received without further purification. Y2O3:Eu3+ (0.5, 1.0 and 3.0 wt. %) powder was synthesized, by the combustion syntheses method [3,14], according to the following procedure: first, stoichiometric amounts of europium nitrate, Eu(NO3)3-6H2O, and yttrium nitrate, Y(NO3)3-6H2O, were dissolved in 15 mL of de-ionized water. After that urea, CH4N2O, was added to the aqueous nitrate solution. The mixed solution was kept under constant mechanical stirring for about 30 min, until it turns into a transparent liquid solution. Afterwards, the resultant precursor is placed inside a ceramic crucible and introduced in a furnace preheated to 520 °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 powders crystallinity was analyzed by X-ray powder diffraction (XRPD) using a Bruker AXS D8 Advance (Cu Ka radiation, 40 kV and 40 mA) equipment operated in a Bragg-Brentano θ/2θ configuration. The diffraction patterns were collected in a flat geometry and the X-ray diffraction data was refined following the Rietveld method using the GSAS software [21]. Morphological structure of the samples and energy dispersive X-ray spectroscopy (EDXS) were investigated using a VEGA 3 TESCAN Scanning Electron Microscope (SEM) coupled with a EDXS detector from Oxford Instruments (accelerating voltage preset at 15 kV). The images were obtained using an electron detector with the preset charge-up reduction mode. Spontaneous Raman scattering spectra of the heat-treated samples were measured using a Horiba LabRaman Spectrometer system. The Raman measurements were carried out using a CW laser (wavelength: 532 nm) with the spectrometer operating in a continuous scanning mode in the 300–700 cm−1 range. Excitation/emission spectra and the fluorescence decay time in the visible region were recorded using a spectrofluorometer (0.22 m, model: SPEX Fluorolog/1680) equipped with a Xe-lamp as the excitation source and a photomultiplier (Hamamatsu/R928) for detection. All measurements were performed at room temperature.
The UC spectra were obtained using an OPA pumped by a Ti: sapphire amplified laser system (800 nm, 1.0 kHz, 100 fs). All experiments were performed, at room temperature, with the OPA tuned either to 1275 nm or 1500 nm. The NIR beam power, at the sample, was adjusted using a λ/2 plate and a polarizer. The luminescence emission was collected using an optical fiber and analyzed in a compact spectrometer (Ocean Optics USB4000). The laser beam was focused on the sample with a 50 mm focal length lens with spot diameter estimated in ∼10-15 µm.
3. Results and discussion
3.1. Structural analysis
Figure 1(a) shows a SEM image of the obtained powder after heat-treatment at 1100 °C during 2h. The image contains brittle particles and porous of various sizes which is a consequence of the drastic character of the combustion synthesis procedure. Figure 1(b) presents the elemental analysis of chemical composition of the samples obtained by EDX spectroscopy. Characteristic peaks associated to yttria, europium and oxygen are identified in the Figs. 1(b)-(d).
Fig. 1. (a) SEM image of the Y2O3: Eu3+ (1.0 wt. %) powder prepared by combustion synthesis and heat-treated at 1100 °C for 2 h. (b), (c) and (d), respectively, EDX spectrum of the sample of Y2O3: Eu3+ doped at 0.50, 1.0 and 3.0 wt.
The XRPD diagram of the heat-treated Y2O3: Eu3+ (1.0 wt. %) sample is shown in Fig. 2(a). Complete crystallization of the thermally-treated sample is observed with representative diffraction peaks corresponding to the standard pattern of the Y2O3 space group Ia3 (206). It is noted that the peaks labeled as (222), (400), (440) and (622), corresponding to the b.c.c. structure, are the most prominent peaks. Additional phases or impurity peaks involving other species are not observed. Results of the Rietveld refinement is in good agreement with the JCPDS card number 86-1107.
Fig. 2. (a) XRPD data of Y2O3: Eu3+ (1.0 wt. %) powder heat-treated at 1100 °C for 2 h. The red line represents the Rietveld fitting trace. (b) Spontaneous Raman spectrum of the heat-treated Y2O3: Eu3+ (1.0 wt. %) powder.
Spontaneous Raman scattering spectra of the heat-treated Y2O3:Eu3+ powders were obtained in order to characterize the phonon energies. Figure 2(b) displays the Stokes Raman spectrum between 300 and 700 cm−1. Notice a prominent peak at 380 cm−1 and three weaker peaks centered around 335 cm−1, 470 cm−1 and 597 cm−1. The spectrum is in agreement with previous measurements of Raman spectrum performed with Y2O3 sintered at 1100 °C [22].
3.2. Stokes luminescence analysis
The excitation and emission spectra of the Eu3+ doped Y2O3 powder are presented in Fig. 3. The excitation spectrum, shown in Fig. 3(a), was obtained by monitoring the emission line at $\approx $611 nm (5D0 → 7F2 transition). The broad band with a maximum at $\approx $260 nm is attributed to the charge transfer (transition O2 → Eu3+) from the ligand to the metal [16–18]. The narrow bands observed in the spectrum are ascribed to the Eu3+ f–f transitions 7F0 → 5H6 (300 nm), 7F0 → 5H3 (320 nm), 7F0 → 5D4 (360 nm), 7F0 → 5GJ (385 nm), 7F0 → 5L6 (395 nm), 7F1→5D3 (416 nm) and 7F0 → 5D2 (464 nm).
Fig. 3. (a) Excitation spectrum of Y2O3:Eu3+ sample measured at 300 K. (b) Stokes luminescence spectrum of Y2O3:Eu3+ powder for excitation at 260 nm.
Figure 3(b) illustrates the luminescence of the Y2O3:Eu3+ powder under UV excitation (wavelength: 260 nm). Almost all bands in the spectrum are due to transitions starting from the 5D0 state. For this particular excitation condition, the Eu3+ emissions from the 5D0 state, occurs due to the ligand-to-metal (O2−to-Eu3+) charge transfer mechanism [16–18]. As indicated in the Fig. 3(b) these emission bands correspond to 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transitions. The most intense emission at $\approx $611 nm, due to the 5D0 → 7F2 forced electric dipole transition, strongly depends on the symmetry of the Eu3+ environment. On the other hand, the magnetic dipole transition 5D0 → 7F1 hardly changes with the crystal field of the host material.
Figure 4(a) shows the luminescence decay of state 5D0 (transition 5D0 → 7F2 centered at $\approx $611 nm). The excitation was performed by using a pulsed xenon lamp (pulse duration of 40 µs), with emission centered at 260 nm. The data was well fitted by a single-exponential function, with the decay-time of state 5D0 determined using the expression $I(t )= {I_0}\textrm{exp}({ - t/\tau } )$, where $I(t )$ is the luminescence intensity as a function of time t, ${I_0}$ is the intensity at $t = 0$, and $\tau = 1.45$ ms. Figure 4(b) shows the energy level diagram for Eu3+ in the prepared Y2O3 crystal, this diagram is based on the results presented in Fig. 3.
Fig. 4. (a) Temporal behavior of the fluorescence at 611 nm (transition: 5D0→7F2 for excitation at 260 nm). The data points were fitted using the expression $I(t )= {I_0}\textrm{exp}({ - t/\tau } )$. (b) Energy level diagram, based on the results of Fig. 3, showing the proposed UC pathways. The solid lines represent radiative processes (excitation and luminescence) and dotted lines are nonradiative decay channels.
The luminescence behavior was investigated using the Judd-Ofelt (JO) intensity parameters $({{\Omega _\lambda };\lambda = 2,\; 4,\; 6} )$, that contain contributions from the forced electric dipole and dynamic coupling mechanisms influencing the Eu3+ behavior [23]. It is well known that the Judd-Ofelt intensity parameters of Y2O3: Eu3+ powders were already investigated in other works [15,24–26]. However, a large variety of synthetic approaches has been used to produce and to improve the luminescence properties of Eu3+ doped Y2O3 ceramic powders. Each chemical route offers different thermodynamics and kinetics of the reactive solutions. As a result, the fabricated powders present a large variety of morphologies, chemical homogeneity and impurity. Particularly, the JO parameter (Ω2, Ω4) are closely related to hypersensitive transitions of Eu3+ which are strongly dependent on the local symmetry around the RE ion. As a consequence, the values of the JO parameters (Ω2, Ω4) for Eu3+ doped Y2O3 powders obtained in this work, shown in Table 1, differ from those obtained following different synthetic approaches [15,24–26].
Table 1. Parameters associated to the 5D0 energy level: luminescence lifetime (τ), radiative (AR) and nonradiative (ANR) transitions rates and luminescence quantum efficiency (ηQE). Judd-Ofelt intensity parameters: Ω2, Ω4. Excitation performed at 260 nm.
In fact, according to the JO theory, the intensity parameters can be calculated from the luminescence spectra [23,27] and then they can be used to estimate the spontaneous (radiative) decay rate of the emitting level ${}_{}^5{D_0}$ . The integrated emission intensities (areas below the luminescence bands) are associated to the radiative emission rates as
Similarly, the luminescence quantum efficiency (ηQE) of the radiative level ${}_{}^5{D_0}$ can be estimated by ${\eta _{QE}} = {A_R}/({{A_R} + {A_{NR}}} )\equiv {A_R} \cdot \tau $, and its value is depicted in Table 1. The nonradiative decay rates can be estimated using $\tau $ and ${A_R}$ and the values found, for all the samples, are shown in Table 1.
3.3 Anti-Stokes luminescence analysis
The red luminescence from Eu3+ doped materials excited by infrared photons of wavelengths longer than 900 nm was previously reported but co-doping with another RE ions as, for example, Yb3+ [29–31]. In those cases, the UC process was due to a cooperative effect in which the Yb3+ absorb light at $\approx $1 µm, and subsequently a pair of excited Yb3+ transfer their energy to a nearby Eu3+. The excitation of transparent matrices singly doped with Eu3+ using NIR light in the range 900-1500 nm, is not possible via a direct one-photon absorption process, because there is a large energy gap between the Eu3+ electronic states 7F6 (∼ 5000 cm−1) and 5D0 (∼ 17000 cm−1). However, if ultrashort laser pulses in the NIR are employed, the Eu3+ may be excited via multi-photon absorption processes due to the high peak power of the femtosecond pulses. Hence, Fig. 5 shows the UC spectra of the sample Y2O3: Eu3+ (1.0 wt. %) excited with wavelengths at 1275 nm and at 1500 nm. The reported results show the typical reddish photoluminescence from Eu3+ doped Y2O3 obtained when excited under UV (wavelength: 260 nm). The main emission lines, located at $\approx $580, $\approx $588, $\approx $595, $\approx $600, $\approx $611, $\approx $650, and $\approx $710 nm, are clearly identified as the 5D0 → 7FJ ($\textrm{J}\, = \,0{\ -\ }4$) transitions.
Fig. 5. (a) Upconversion luminescence spectra of Y2O3: Eu3+ (1.0 wt. %) powder for two excitation wavelengths: 1275 nm and 1500 nm. (b) Upconversion luminescence of the Y2O3 powder doped with Eu3+ (0.5, 1.0 and 3.0 wt. %) under femtosecond laser excitation (laser wavelength: 1500 nm; pump peak power: 0.22 GW).
Figure 5(a) shows that the 5D0 → 7F0 emitted intensity increases, for the excitation wavelength at 1500 nm. This behavior may be due to the different incident power or to some geometrical alignment factor. Unfortunately an integrated sphere was not available to clarify this point. Figure 5(b) shows that the UC spectra for all samples present similar behavior. The ratios between the bands’ amplitudes of the samples should be according to the relative Eu3+concentration, although, slight misalignments and laser intensity fluctuations prevent a precise comparison.
The nonlinear character of the UC luminescence under NIR 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 UC intensity, IUC, is proportional to the power N of the infrared excitation intensity obeying the relationship ${I_{UC}} \propto P_L^N$, where N is the number of laser photons required to populate the UC emitting states and PL is the laser power. The value of N is determined by the slope of the best-fit line in a log-log plot of IUC versus ${P_L} $. Figure 6 shows the results for ${I_{UC}}$ versus ${P_L} $ corresponding to the emission centered at 611 nm and the N values. Notice that, for excitation at 1275 nm, we obtained $N \approx 3.5\, \pm \,0.2$, which suggests that simultaneous three- and four-photon absorption may occur. In this case, the numerical values obtained for the slopes differ by less than 0.1 from one result to another. On the other hand, for excitation at 1500 nm, we observed in Figs. 6(d,e,f) the slope varying in the range of $({4.2\, \pm \,0.4} )< N < ({4.6\, \pm \,0.4} )$, that suggest simultaneous four- and five-photon absorption.
Fig. 6. Log-log plot of the UC intensity versus the excitation laser power. The linear best-fit is displayed with the experimental points. Excitation wavelength: 1275 nm (a-c) with maximum peak power of 0.34 GW; 1500 nm (d-f) with maximum peak power of 0.28 GW.
Accordingly to the slopes in Fig. 6 the possible UC pathways can be described as follows: For excitation at 1275 nm, Eu3+ levels are near-resonance via three- and four-photon absorption to the transitions 7F1→5D3 (416 nm) and 7F0 → 5H3 (320 nm), respectively. For excitation at 1500 nm, Eu3+ levels are near-resonance, via four- and five-photon absorption, to the transitions 7F0 → 5GJ (375 nm) and 7F0→5H6 (300 nm), respectively. In both cases, after successive nonradiative relaxation steps, the excited ions decay to the emitting state 5D0. The near-resonance transitions may participate in the UC process because the multi-photon transitions may be phonon-assisted. The mismatch among the photons energy and the excited energy levels are compensated by emission or annihilation of phonons. In addition, another reason which may mitigate any apparent off-resonance process in the multi-photon transitions is the fact that the ultrafast pulsed source used in the experiments is broadband in order to support ∼100 fs of pulse duration, exhibiting a bandwidth of ∼ 15 nm. In order to better visualize the proposed anti-Stokes luminescence route, the excitation pathways involved in the UC process were indicated by the arrows in the diagram of the Eu3+ energy levels of Fig. 4(b).
Finally, another important aspect is that, due to the long lifetime (1.45 ms) of the 5D0 state, and due to the repetition rate of our lasers (1 kHz), from one pulse to the subsequent pulse some Eu3+ remains in the excited state 5D0, and excited state absorption may takes place. However, this excited state absorption may not contribute to the results of Fig. 6.
4. Conclusions
In summary, we investigated in this work the Stokes and anti-Stokes photoluminescence spectra of yttrium oxide powders doped with Eu3+. The samples were prepared by combustion synthesis and were characterized by electron microscopy, X-rays diffraction, spontaneous Raman scattering, excitation and luminescence techniques. For UC luminescence we used as an excitation source a femtosecond OPA tuned at two different NIR wavelengths (1275 nm and 1500 nm) and the results show in both cases an intense red emission due to Eu3+: 5D0 → 7F2 transition. The data analysis reveals that the multi-photon absorption (MPA) process that generates the infrared-to-visible UC luminescence involves three to five incident photons. Nowadays, MPA plays an important role for the new techniques of high-resolution optical microscopy and, as a promising approach, to enhance near-infrared absorption for photovoltaic systems. Thus, the herein reported results indicate a good potential of Eu3+ doped Y2O3 powders as infrared-to-visible UC material that may be interesting for scientific and technological applications.
Funding
Conselho Nacional de Desenvolvimento Científico e Tecnológico; Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior; Fundação de Amparo à Pesquisa do Estado da Bahia.
Acnowledgements
We thank the financial support from the Brazilian Agencies: Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (FACEPE), and Coordenação de Aperfeiçoamento de Pessoal de Ensino Superior (CAPES). This work was performed in the framework of the National Institute of Photonics (INCT de Fotônica) and PRONEX- Center of Excellence Program. Dr. Nikifor Rakov also acknowledges financial support from FAPESB.
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