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Color tunability of intense upconversion emission from Er3+–Yb3+ co-doped SiO2–Ta2O5 glass ceramic planar waveguides Jefferson Luis Ferrari,ab Karmel de Oliveira Lima,a Edison Pecoraro,d Rute A. S. Ferreira,c Luıs D. Carlosc and Rogeria Rocha Gonc¸alves*a Received 23rd January 2012, Accepted 11th March 2012 DOI: 10.1039/c2jm30456b This work reports on the infrared-to-visible CW frequency upconversion from planar waveguides based on Er3+–Yb3+-doped 100–xSiO2–xTa2O5 obtained by a sol–gel process and deposited onto a SiO2–Si substrate by dip-coating. Surface morphology and optical parameters of the planar waveguides were analyzed by atomic force microscopy and the m-line technique. The influence of the composition on the electronic properties of the glass-ceramic films was followed by the band gap ranging from 4.35 to 4.51 eV upon modification of the Ta2O5 content. Intense green and red emissions were detected from the upconversion process for all the samples after excitation at 980 nm. The relative intensities of the emission bands around 550 nm and 665 nm, assigned to the 2H11/2 / 4I15/2, 4S3/2 / 4 I15/2, and 4F9/2 / 4I15/2 transitions, depended on the tantalum oxide content and the power of the laser source at 980 nm. The upconversion dynamics were investigated as a function of the Ta2O5 content and the number of photons involved in each emission process. Based on the upconversion emission spectra and 1931CIE chromaticity diagram, it is shown that color can be tailored by composition and pump power. The glass ceramic films are attractive materials for application in upconversion lasers and near infrared-to-visible upconverters in solar cells.
1. Introduction The development of rare earth (RE) doped materials has attracted great interest because of their potential technological applications. Among the RE doped hosts, those with low phonon energy are noteworthy due to their photoluminescent properties, which make them interesting for use in photonic devices.1–3 Particularly, Er3+ ion-doped materials find potential application as optical amplifiers operating in the near infrared region. The utilization of Er3+-doped fiber and planar waveguide amplifiers (EDFAs and EDWAs) in telecommunication systems has been proposed in the literature. Apart from the near infrared emission, frequency upconversion (UC) processes can also take place, thereby generating visible luminescence after excitation at the near infrared region. Even though these processes limit the performance of a
Departamento de Quımica, Faculdade de Filosofia, Ci^ encias e Letras de Ribeira˜o Preto, Universidade de Sa˜o Paulo, Av. Bandeirantes, 3900, CEP 14040-901 Ribeira˜o Preto/SP, Brazil. E-mail: rrgoncalves@ffclrp. usp.br b Grupo de Pesquisa em Quımica de Materiais—(GPQM), Departamento de Ci^ encias Naturais, Universidade Federal de Sa˜o Joa˜o Del Rei, Campus Dom Bosco, Prac¸a Dom Helv ecio, 74, 36301-160 Sa˜o Joa˜o Del Rei, MG, Brazil. E-mail:
[email protected];
[email protected] c Physics department and CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal d Laborat orio de Materiais Fot^ onicos, Instituto de Quımica, UNESP, Caixa Postal 355, 14801-970 Araraquara/SP, Brazil
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optical amplification at 1.5 mm, there is the possibility of producing lasers,4–8 energy upconverting devices,9–11 and biophotonic materials,11–14 not to mention that the efficiency in solar cells is improved.14,15 In this sense, frequency UC can be an excellent alternative for the production of shorter wavelength solid-state lasers operating in the visible region using near infrared lasers as a pump, mainly because nowadays high power and lowcost diode lasers are being produced, which are widely used in telecommunication networks. Although numerous studies on UC processes have been reported since 1960, today they are still being extensively exploited, aiming at the development of new Er3+–Yb3+ co-doped phosphors for application as optical markers, short wavelength lasers, optical amplifiers, energy converters for solar cells, etc.4–15 The UC phenomenon was first demonstrated for Er3+, and the pioneering academic studies were performed on Er3+-doped fluoride crystals.16 A key requirement for the achievement of efficient upconversion mechanisms is to place the Er3+ ions in a host with low cutoff phonon energy, which allows for large lifetime of the excited state of the RE ions. Besides that, hosts with a wide band gap range are necessary, in order to avoid a quenching effect on the emission of the rare earth ion. A large number of UC investigations have been reported, most of which refer to halide crystals, glasses, and glass ceramic fibers and bulks.16–31 Rare earth and alkali metal halides such as J. Mater. Chem., 2012, 22, 9901–9908 | 9901
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fluoride and bromide are more frequently studied.16–18 Concerning UC in glasses and glass-ceramic, the majority of the publications is based on fluoride glasses, such as fluorozirconate19,20 and fluoroindate,21,22 germanate,23–25 phosphate26 and tellurite27,28 glasses, as well as on glass-ceramics29–31 containing rare earth doped fluoride and oxyfluoride nanocrystals. As for UC lasers, the first demonstration of stimulated UC emission was accomplished on an Er3+–Yb3+-doped BaY2F8 crystal at low temperature (77 K), using a lamp as the pump.32 However, great advances in both basic research and technological areas have occurred with the introduction of higher power excitation lasers in the market. Therefore, the availability of more efficient diodes and Ti:sapphire lasers operating from 800 to 1100 nm since the 1980s has prompted investigations on the UC dynamics, especially in the case of Er3+, thereby leading researchers to verify the possibility of producing UC lasers. In 1987, Silversmith et al. presented their results on the production of a continuous laser operating at 550 nm with pumping in the near infrared (NIR) on an Er3+ doped YAlO3 crystal at 77 K, with an output power of 1 mW for a pump of 200 mW.33 Significant advances have been made with respect to Er3+ or Er3+–Yb3+-doped LiYF4 and LiLuF4 crystals34–36 featuring laser emission in the green region, after excitation with a Ti:sapphire laser. A CW UC laser operating in the green region was obtained by using an Er3+-doped LiLuF4 crystal, which generated an output power of 213 mW (pumping with 2.6 W).34 In 2001, Smith et al. reported results on UC laser emission in the green region for Er3+-doped LiKYF5 excited by a diode laser.37 Recently, Heumann et al. obtained an output power of 1 W for a UC laser system, by employing a diode laser as the pump source, which represents the highest power output recorded for a continuous UC laser operating in the green region.38 There are countless studies on UC in glasses, glass ceramic fibers, bulks, and powders in the literature, and a limited number of investigations are related to films and planar waveguides. The application of the RE-doped upconverter film in solar cells for enhanced efficiency has been reported.39–42 Er3+-doped upconverters are expected to promote increased silicon solar cell efficiency by utilization of the low energy region of the solar spectrum.15 An important parameter to take into account is the selection of the matrix to synthesize a UC efficient material. Among the hosts, those with low phonon energy are noteworthy in order to avoid multiphonon relaxation from the excited states of Er ions. Besides this, the distribution of rare earth ions in the matrix is crucial in order to prevent cluster formation, which can also lead to luminescence suppression. Planar waveguides with efficient UC are interesting for the production of compact laser systems emitting in the visible region, using low-cost diode lasers as the pump source. Polman has examined energy transfer processes and upconversion in Er3+-doped Al2O3 planar waveguides.43 Gonc¸alves et al. have published interesting results of NIR-to-visible UC in a binary Er3+-doped SiO2–HfO2 planar waveguide, which produced intense green and red emissions. The controlled distribution of rare earth ions in low-phonon energy sites justifies the high intensity of the UC emission in the visible range.44 SiO2–TiO2,45 SiO2–HfO2,46–49 SiO2–Ta2O5,50–52 and SiO2– ZrO253,54 binary oxides have been also employed as RE host lattices in sol–gel derived Er3+-doped planar waveguides. Most of 9902 | J. Mater. Chem., 2012, 22, 9901–9908
these oxide nanocomposites contain rare earth ions located in sites of low cutoff phonon energy. Particularly, Ta2O5 presents some interesting properties with respect to the rare earth host, such as low cutoff phonon energy, high refractive index and possibility of inserting a high concentration of rare earth ions in the matrix.55 Nd3+-doped Ta2O5 has been described for laser applications.56–58 Indeed, high-quality upconversion emission from Nd3+-doped Ta2O5 rib planar waveguides deposited onto the silica substrate has been detected in the blue, green, orange, and red spectral regions.59 Considering the binary system SiO2–Ta2O5, an intense multicolor visible light emission has been observed for Tm3+–Ho3+– Yb3+ co-doped Ta2O5 nanocrystals in the case of a silica-based nanocomposite synthesized by the sol–gel method.10,60 The UC emissions were attributed to the ESA (excited state absorption), and ETU (energy transfer upconversion) mechanisms.17,18 This work reports on the preparation and optical characterization of the green and red upconversion emission of co-doped Er3+–Yb3+ 100–xSiO2–xTa2O5 glass ceramic planar waveguides prepared by the sol–gel method. On the basis of the UC emission spectra and the 1931CIE chromaticity diagram, the tunability of the waveguides has been evaluated as a function of the laser power source and composition.
2. Experimental procedure Er3+-doped and Yb3+ co-doped SiO2–Ta2O5 planar waveguides containing 0.3 mol% Er3+ and 1.2 mol% Yb3+, deposited onto silica on silicon (10 mm SiO2–Si (100) p-type) substrates, were prepared by the sol–gel method. A solution with a total concentration of Si + Ta ¼ 0.445 mol L1 was obtained at Si–Ta molar ratios of 70 : 30, 60 : 40, and 50 : 50. Tantalum ethoxide (Aldrich—99.98%) and tetraethoxysilane (Merck—98%) were used as precursors. Er3+ and Yb3+ ions (0.3 mol% Er3+ and 1.2 mol% Yb3+ in relation to the Si + Ta ions) were added from their chloride solutions, which were obtained by dissolving Er2O3 and Yb2O3 in 0.1 mol L1 HCl aqueous solution, followed by solvent exchange with ethanol. A mixture of tetraethylorthosilicate (TEOS), anhydrous ethanol, and concentrated hydrochloric acid (solution 1) was firstly prepared. The TEOS–HCl molar ratio was 50 : 1. Tantalum ethoxide, 2-ethoxyethanol, and Er3+ ions (solution 2) were mixed in a separate container. Then, solutions 1 and 2 were mixed. The final solution was kept under stirring at room temperature for 30 minutes. Next, an aqueous hydrochloric acid solution (0.27 mol L1) was added to the final solution at a TEOS–HCl ratio of 1 : 0.007. Subsequently, the solution was filtered through a 0.2 mm Millipore filter and left to stand for 16 h, for film deposition. The film was deposited by dipcoating at a dipping rate of 30 mm min1. Before further coating, each layer was annealed for 60 s at 900 C, until 50 layers were achieved. After that, the planar waveguides were heat treated at 900 C for different time periods, so that full densification would be achieved and possible OH and CH groups, as well as H2O present in the material pores, which act as quenchers, would be eliminated.52 The film surface was characterized by atomic force microscopy (AFM) using a Shimadzu microscope SPM 9600 operating in the contact mode, in order to investigate surface roughness. Images were collected using the SPM software version 3.03. This journal is ª The Royal Society of Chemistry 2012
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The upconversion emission spectra were recorded in the reflection mode, at room temperature, by means of a Fluorolog-3 Horiba Scientific (Model FL3-2T) apparatus equipped with a modular double grating excitation spectrometer (fitted with 1200 grooves mm1 grating blazed at 330 nm) and a TRIAX 320 single-emission monochromator (fitted with 1200 grooves mm1 grating blazed at 500 nm, reciprocal linear density of 2.6 nm mm1). The excitation source was a 980 nm Thorlabs semiconducting diode laser (LDM2). Diffuse reflectance spectra were registered between 215 and 315 nm, at room temperature, using a JASCO V-560 instrument. The refractive indexes (at 532 and 632.8 nm) and losses (at 632.8 nm) of the planar waveguide deposited onto the silica on silicon (10 mm SiO2–Si (100) p-type) substrate were measured by an m-line Metricon 2010 equipment on the basis of the prism coupling technique.61
3. Results and discussion The optical parameters and the thickness of the films obtained by means of the prism coupling technique at 532 and 632.8 nm for TE and TM polarizations are summarized in Table 1. It can be noted that the refractive index increases as a function of the Ta content. The refractive index of the planar waveguide is in agreement with the one calculated at 532 nm by the Lorentz– Lorenz equation using 1.4603 and 2.1852 for the refractive indices of SiO2 and Ta2O5, respectively. The refractive index and the number of the deposited layers were optimized, so as to obtain a monomode planar waveguide in the near infrared region. Low optical losses of around 0.9 dB cm1 were measured at 632.8 nm in the TE0 mode for the planar waveguide containing a Si–Ta molar ratio of 70/30. The scattering losses measured at 632.8 nm are not directly correlated to the Ta2O5 content. This is true at least for the planar waveguides with a Si–Ta molar ratio of up to 70 : 30, since no significant changes of this value were observed for the waveguides containing 20 and 10 mol% Ta. As reported before by some of us,52 a controlled phase separation can be obtained in a similar nanocomposite, where orthorhombic Ta2O5 nanocrystals are embedded in an amorphous silica-based matrix, with the nucleation and crystallization process being dependent on the Ta concentration. The influence of the Ta content on the crystal size and distribution, as well as on the volume ratio between the amorphous and nanocrystal phases has also been well described. According to the results, even if there is phase separation, there is homogeneous distribution of the nanocrystals with mean size less than 3.5 nm for the samples containing up to 30 mol% of Ta. So, it can be assumed that Rayleigh scattering due to the
nanocrystals does not contribute significantly to the optical losses, which is mainly due to inhomogeneities introduced by the multilayer process, as reported before for sol gel derived SiO2– TiO2 glass ceramic planar waveguides.45 However, the optical losses are more pronounced for the waveguides containing Si–Ta molar ratios of 60 : 40 and 50 : 50. This suggests that the volume scattering due to the presence of nanoparticles becomes more predominant as a result of the more marked phase separation in the case of these compositions. In order to obtain qualitative information about surface roughness, AFM was employed within a 3 3 mm scale limit for the planar waveguide with a Si–Ta molar ratio of 60 : 40, which is well representative of all the planar waveguides. The obtained images are depicted in Fig. 1, which shows a flat and crack-free planar waveguide surface that furnishes low mean surface roughness with values around 0.2 nm. Surface roughness is among the different sources contributing to optical losses. Hence, the high surface quality of the planar waveguides significantly avoids optical losses contribution, corroborating the low losses measured here, as light scattering in the films due to surface defects is minimized. The influence of composition on the electronic properties of glass-ceramic films was accompanied by reflectance spectra. The band gap and the cut-off were indirectly obtained from the reflectance spectra illustrated in Fig. 2.
Fig. 1 AFM images of the surface of the Er3+–Yb3+ co-doped planar waveguide prepared by the sol–gel method, with a Si : Ta molar ratio of 60 : 40.
Table 1 Optical parameters of the Er3+–Yb3+ co-doped planar waveguides, with Si : Ta molar ratio of 70 : 30, 60 : 40, and 50 : 50 @532 nm
Refractive index of 70Si : 30Ta Number of modes of 70Si : 30Ta Refractive index of 60Si : 40Ta Number of modes of 70Si : 30Ta Refractive index of 50Si : 50Ta Number of modes of 70Si : 30Ta
@632.8 nm
TE
TM
TE
TM
Thickness (mm)
1.6496(0.0001) 7 1.7428(0.0001) 8 1.8476(0.0003) 11
1.6460(0.0004) 7 1.7441(0.0001) 8 1.8390(0.0003) 11
1.6425(0.0060) 6 1.7301(0.0001) 7 1.8158(0.0003) 11
1.6432(0.0060) 6 1.7317(0.0001) 6 1.8145(0.0003) 11
2.1779(0.0039
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2.6483(0.0165) 3.1029(0.2570)
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Table 2 Optical band-gap values for the Er3+–Yb3+ co-doped planar waveguides with Si : Ta molar ratio of 70 : 30, 60 : 40, and 50 : 50
Fig. 2 Diffuse reflectance spectra of the Er3+–Yb3+ co-doped planar waveguides with Si : Ta molar ratio of 70 : 30, 60 : 40, and 50 : 50.
Planar waveguide
Optical band-gap values (eV)
70Si–30Ta 60Si–40Ta 50Si–50Ta
4.51 4.48 4.35
at 526 and 548 nm and labeled as green), and 4F9/2 / 4I15/2 transitions (maximum intensity at 660 nm and labeled as red). The spectra exhibit a large bandwidth due to inhomogeneous broadening and Stark splitting of the Er3+ ions, probably because of the presence of Er3+ ions in many different sites in the structure of the nanocomposite, as observed before for the NIR luminescence.50–52 Previous studies on the structural properties of the
The reflectance edge is directly dependent on the Ta amount, and the lcutoff shifts from 370 to 394 nm as the Si–Ta molar ratio changes from 70 : 30 to 50 : 50. The band-gap values displayed in Fig. 3 were evaluated by the Kubelka–Munk model with Tauc,62 considering an indirect band-gap for all the systems. The obtained values are listed in Table 2. In accordance with the literature,55 the crystalline Ta2O5 bandgap values lies between 3.9 and 4.5 eV, and they depend directly on the preparation method, while the SiO2 band-gap value is around 8.5 eV. As a consequence, the band-gap energy of the SiO2–Ta2O5 planar waveguides changes from 4.51 to 4.35 as the Ta content increases. It is noteworthy that the planar waveguides exhibit an excellent transparence in the UV-VIS region, thereby contributing to the minimization of losses by absorption, as well as the exclusion of the quenching source. The UC emission spectra represented in Fig. 4 were measured in the visible range, specifically between 500 and 700 nm, with the excitation fixed at 980 nm and the power source varying from 0.012 to 0.573 W. The emission spectra display three broad Er3+ emission bands (two in the green region and one in the red one) assigned to the 2H11/2 / 4I15/2, 4S3/2 / 4I15/2 (maximum intensity
Fig. 3 Absorption coefficient as a function of incident photon energy in the near band gap region. Samples are identified and the calculated bandgaps are indicated in the figure.
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Fig. 4 Upconversion emission spectra of Er3+–Yb3+ co-doped planar waveguides with Si–Ta ratios of (A) 70 : 30, (B) 60 : 40, and (C) 50 : 50, after excitation at 980 nm. The excitation power source was changed from 0.012 to 0.573 W. The f–f transitions of Er3+ are attributed in the figure.
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SiO2–Ta2O5 nanocomposites demonstrated a controlled phase separation, with orthorhombic Ta2O5 nanocrystals embedded in a silicate based matrix.50,52 The presence of Ta2O5 spherical shape nanoparticles dispersed into the SiO2 amorphous matrix could be observed for the similar nanocomposite. It was clearly noted by HRTEM that the heat treatment at 900 C promotes not only the organic and OH groups elimination but also the densification and initial crystallization of the nanocomposite, with the presence of Ta2O5 nanocrystals with a mean size of around 2 and 4 nm dispersed into the SiO2-based host with a molar ratio between Si–Ta of 70/30 and 50/50 respectively. Additionally, there was strong indication that most of the Er3+ ions are preferentially localized in a Ta2O5-rich environment instead of the amorphous SiO2-based host.50–52 The complex structure of the Ta2O5 actually promotes very interesting and unique luminescent properties when it is doped with rare earth ions. Concerning the crystalline Ta2O5, as previously reported by Stephenson and Roth, the orthorhombic Ta2O5 structure contains twenty-two Ta atoms per unit cell, where a wide variety of coordination sites exist, and twelve different sites have been described. Therefore, if one assumes that the Er3+ ions are distributed in such a way that they replace the Ta5+ ions, many different symmetry sites occupied by the Er3+ ions can exist in the crystalline structure, which contributes to inhomogeneous broadening and may explain the broadband emission observed in this work. Moreover, many O2 vacancies are present in the Ta2O5 structure, as reported in the literature.38,42 Consequently, in order to ensure neutrality, many different sites are also formed due to distortion in the Ta2O5 structure. Hence, the RE ions added to Ta2O5 may occupy many different sites. They may substitute Ta5+ ions and/or be located in the interstitial space inside the structure of Ta2O5, thereby resulting in broad emission spectra. Albeit less probable, another factor that might be contributing to the appearance of new sites is the disruption of the silica network provoked by the presence of Ta2O5, with a consequent formation of SiO species, which can accommodate some Er3+ ions. So, it can be hypothesized that the broadband emission observed herein also receives a minimum contribution from some Er3+ ions accommodated in the silicate network, probably on the interface between Ta2O5 and the silicate-based matrix. The dynamics of the photophysical properties reported before indicates the minimum contribution of this latter factor to the inhomogeneous broadening. Regarding specifically the SiO2– Ta2O5 glass ceramics, they exhibit an unusual enlargement of the bandwidth, i.e., inhomogeneous broadening, as compared to other glass-ceramic and ceramic systems containing rare earthdoped nanocrystals.49,54 This can be explained by the complex structure discussed above. A similar behavior had been observed for the emission band of Tm3+–Ho3+–Yb3+-doped SiO2–Ta2O5 nanocomposites prepared by the sol–gel method,60 which displayed UC emission with intense and broad bands, generating white light. The UC emission of these planar waveguides after excitation at 980 nm is very intense, and the green emission is visible to the naked eye, even in the reflection configuration. Normally, to obtain intense UC emission in planar waveguides it is necessary to accomplish excitation in the coupled mode. However, it is noteworthy that the present case was carried out in the reflection mode. Er3+-doped planar waveguides containing This journal is ª The Royal Society of Chemistry 2012
the same Si and Ta contents were submitted to excitation at 980 nm, and no UC emission was verified with the naked eye in the planar waveguide configuration. This confirms the significant participation of the Yb3+ ions in the UC emission process via efficient energy transfer from Yb3+ to Er3+. The dependence on the excitation power of the emission intensities helps identify the number of photons involved in each UC transition. Fig. 5 shows the emission intensity I as a function of the excitation power P (in a log–log graph) for the green and red emissions. A linear behavior (I f Pn) is noted with a linear correlation coefficient around 0.997 and n values around 2. This indicates that two photons have to be absorbed for emissions to occur in different regions. The observed green and red emission UC effect in planar waveguides reported in this work may take place via excited state absorption (ESA), energy transfer
Fig. 5 Dependence of emission UC intensities as a function of the excitation power source at 980 nm of the Er3+–Yb3+ co-doped planar waveguides with different Si–Ta ratios: (A) 70 : 30, (B) 60 : 40, and (C) 50 : 50.
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upconversion (ETU) mechanisms,18 and the two photon absorption (TFA) process.63 The Er3+ and Yb3+ energy levels as well as the different mechanisms involved in the upconversion emission process under excitation at 980 nm are depicted in Fig. 6. The most possible mechanisms that can produce the green and red upconversion emission are ESA and ETU. In the framework of the ESA mechanism, one Er3+ ion is excited to the 4 I11/2 level by ground state absorption (GSA) followed by absorption of a second photon of the same energy bringing the excited ion to the 4F7/2 level. The excited ion decays non-radiatively to the 2H11/2 and 4S3/2, due to the fast multi-phonon nonradiative decay of 4F7/2 level to the 2H11/2 state. In the ETU mechanism, it is possible to have two Er3+ ions excited in the 4I11/2 state (GSA) or one Er3+ ion excited in the 4I11/2 state (GSA) and one Yb3+ ion in the 2F5/2 excited state (GSA). The two Er3+ ions excited in the 4I11/2 state (GSA) can exchange energy, and one of them decays to the ground state and the other one is excited to the 4F7/2 level. In the case of Er3+ and Yb3+ excited in the 4I11/2 state and 2F5/2 state respectively, the Yb3+ ion transfers energy to the Er3+ ion, which is excited to the 4F7/2 level. After this transfer, the ion in the 4F7/2 level relaxes non-radiatively to the 4S3/2 and 2 H11/2 levels. The ESA and ETU mechanisms populate the 4S3/2 and 2H11/2 and finally emission from these states occurs: 2H11/2 / 4 I15/2 and 4S3/2 / 4I15/2 (green emission). The 4S3/2 and 2H11/2 excited states relax also non-radiatively to the 4F9/2 and emission from this state gives rise to the red emission: 4F9/2 / 4I15/2 transition. The Er3+ ion excited to the 4I11/2 level by ground state absorption (GSA) can also decay non-radiatively to the 4I13/2 followed by absorption of a second photon bringing the excited ion to the 4F9/2level, with consequent 4F9/2 / 4I15/2 transition. Another mechanism that can take place is two photon absorption (TPA).63 The TPA process can be achieved by the simultaneous two photons absorption involving real or virtual levels and become more prominent using high excitation power. In the case of Er3+ ions, the TFA process using 980 nm as the excitation wavelength occurs through a real state. At high excitation power, the two photon absorption occurs, which populates the 4F7/2 very efficiently, originating highly efficient green emission.63 The red–green intensity ratio for each sample as a function of the excitation power can be seen in Fig. 7. This ratio decreases as
Fig. 6 Energy level diagram of the Er3+ and Yb3+ ions with the mechanisms for the UC process.
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Fig. 7 Red–green intensity ratio for each waveguide as a function of the excitation power.
a function of the excitation power, pointing out that the green emission is more intense when the samples are excited under higher power.63,64 Different Er3+ ion states, 4S3/2 (and 2H11/2) and 4F9/2, can be populated by the ESA and ETU mechanisms for the green and red upconversion emissions involving two-photon processes. However, the rate constants of the 4S3/2 and 4F9/2 are not the same, consequently the high power limit is not identical for both states. In a low power regime, for two-photon processes, the UC emission intensity increases with quadratic laser power dependence, but in an intermediate regime, the population of the 4S3/2 and 4F9/2 levels will result in different power dependence65 and consequently the color can be tuned with the laser power.63,64 Another point worthy of note is that the color observed upon modification of the power excitation may be directly associated with the Ta amount. Increasing Ta quantities may contribute to diminished non-radiative processes. Consequently, the non-radiative decay from 4S3/2 to 4S9/2 (responsible for the red emission) is reduced, and the green emission is favored. To estimate the emission color dependence on the excitation power, the x, y color coordinates were calculated on the basis of the UC emission spectra, Fig. 8. The emission color is tuned from x ¼ 0.151, y ¼ 0.173 to x ¼ 0.219, y ¼ 0.419 depending on the Ta amount and excitation power.
Fig. 8 1931 CIE chromaticity diagram coordinates of the Er3+–Yb3+ codoped planar waveguides with different Si–Ta ratios: (A) 70 : 30, (B) 60 : 40, and (C) 50 : 50, as a function of the power excitation source at 980 nm.
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4. Conclusions Planar waveguides based on Er3+–Yb3+ co-doped SiO2–Ta2O5 prepared by the sol–gel method using the dip-coating technique for deposition display upconversion emission in the visible region when excited by a laser at 980 nm. Uniform surface morphology with roughness less than 0.2 nm was evaluated by AFM, which evidenced the excellent quality of the system and contributed to yielding satisfactory optical properties. The surface morphology of the planar waveguides was analyzed by atomic force microscopy, which indicated low roughness even for the samples containing a Si–Ta molar ratio of 50 : 50. The refractive indexes (at different wavelengths), and the band-gap values changed as a function of the Ta amount. The Ta content was shown to be an important variable regarding the desirable optical properties. Low optical losses, around 0.9 dB cm1, were measured for the planar waveguide at 632.8 nm in the TE0 mode in the case of the sample containing 30 mol% Ta. Under excitation at 980 nm, at room temperature, two intense upconversion emission bands assigned to the 2H11/2 / 4I15/2, 4S3/2 / 4I15/2 and 4F9/2 / 4I15/2 transitions from Er3+ ions were observed with the naked eye. The observed green and red UC emission in planar waveguides reported in this work may take place via ESA and ETU. The planar waveguides obtained in this work possess optical qualities that make them promising candidates for shorter wavelength laser sources.
Acknowledgements The authors would like to thank FAPESP, CAPES, and CNPq for financial support. The authors also acknowledge Mrs Cynthia Maria de Campos Prado Manso and Dr Rafael I. Estrada Mejia for reviewing the text.
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