Materials Chemistry and Physics xxx (2012) 1e10
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NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties César dos Santos Cunha a, Jefferson Luis Ferrari b, Drielly Cristina de Oliveira a, Lauro June Queiroz Maia c, Anderson Stevens Leonidas Gomes d, Sidney José Lima Ribeiro e, Rogéria Rocha Gonçalves a, * a
Departamento de Química, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo, Av. Bandeirantes, 3900, 14040-901, Ribeirão Preto/SP, Brazil Grupo de Pesquisa em Química de Materiais e (GPQM), Departamento de Ciências Naturais, Universidade Federal de São João Del Rei, Campus Dom Bosco, Praça Dom Helvécio, 74, 36301-160, São João Del Rei, MG, Brazil c Grupo Física de Materiais, Instituto de Física, UFG, Campus Samambaia, Caixa Postal 131, 74001-970, Goiânia/GO, Brazil d Departamento de Física, Universidade Federal de Pernambuco, Cidade Universitaria, Recife/PE, 50670-901, Brazil e Laboratório de Materiais Fotônicos, Instituto de Química, UNESP, Caixa Postal 355, 14801-970, Araraquara/SP, Brazil b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
< Solegel high NIR luminescent nanostructured planar and channel waveguides. < Microstructured channels written by a femtosecond laser etching technique. < Transparent glass ceramic with rare earth-doped ZrO2 nanocrystals in a silica host. < Enhanced NIR luminescence, efficient energy transfer from the Yb3þ to the Er3þ ion. < New planar channel waveguides to be applied as EDWA in the C telecommunication band.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 January 2012 Received in revised form 30 May 2012 Accepted 21 June 2012
Optical and structural properties of planar and channel waveguides based on solegel Er3þ and Yb3þ co-doped SiO2eZrO2 are reported. Microstructured channels with high homogeneous surface profile were written onto the surface of multilayered densified films deposited on SiO2/Si substrates by a femtosecond laser etching technique. The densification of the planar waveguides was evaluated from changes in the refractive index and thickness, with full densification being achieved at 900 C after annealing from 23 up to 500 min, depending on the ZrO2 content. Crystal nucleation and growth took place together with densification, thereby producing transparent glass ceramic planar waveguides containing rare earth-doped ZrO2 nanocrystals dispersed in a silica-based glassy host. Low roughness and crack-free surface as well as high confinement coefficient were achieved for all the compositions. Enhanced NIR luminescence of the Er3þ ions was observed for the Yb3þcodoped planar waveguides, denoting an efficient energy transfer from the Yb3þ to the Er3þ ion. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Silica Nanostructure ZrO2 NIR luminescence Channel waveguides Femtosecond laser writing Solegel Photonic materials Planar waveguides
* Corresponding author. Tel.: þ55 16 36024851; fax: þ55 16 36338151. E-mail address:
[email protected] (R.R. Gonçalves). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
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1. Introduction Silica-based materials for the manufacture of erbium-doped planar waveguide amplifiers (EDWA’s) are attractive regarding their application in integrated optical devices operating in the Ctelecommunication band [1,2] for local area network (LAN). The combination of the minimum optical loss window of the SiO2 matrix [4] with the photophysical properties of the Er3þ ion in the near infrared region [3] justifies the use of this specific rare earth element for the preparation of photonic materials. Compared to the amount of rare earth ions present in optical fiber waveguides, higher erbium concentrations are required for the development of short-length optical channel or rib planar waveguide amplifiers, so that a satisfactory signal is obtained from the device. However, larger erbium concentrations can promote formation of rare earth clusters, and non-radiative processes such as energy migration between the Er3þ ions can take place thereby, reducing the quantum efficiency of the luminescence [5e7]. Therefore, the presence of clusters is a problem that has to be overcome, and this has motivated the development of new hosts that will improve the optical properties of these materials [5e7]. Another concern is the existence of competitive processes like upconversion phenomena [5], which can considerably affect the emission around 1550 nm [4]. The addition of a co-doping sensitizer has been adopted by some authors, in order to enhance the NIR luminescence intensity significantly while maintaining relatively low Er3þ concentrations. In this sense, the preparation of Er3þ/Yb3þ co-coped materials can markedly increase the absorption at 980 nm this is because the Yb3þ ions have higher absorption cross-section, which culminates in a more efficient pumping mechanism and consequently enhances the Er3þ emission around 1550 nm [8e11]. For this reason, some efforts have been concentrated on the study of Er3þ and Yb3þ co-doped materials [12e14]. Another problem related to Er3þ-silica-based materials that must be solved is the occurrence of multiphonon relaxation due to the presence of OH groups. The vibration modes of these groups strongly quench the emission of the Er3þ ion [15]. Depending on the manufacturing process, it is difficult to eliminate OH groups completely, especially in the case of solegel-derived materials, in which specific annealing is required for the removal of these groups. Furthermore, the presence of pores must be controlled, since they can adsorb H2O molecules and, in the specific case of planar waveguides, they can influence the waveguiding properties by acting as a scattering center. Among the many techniques employed for the fabrication of planar waveguides, the solegel process [16,17] has emerged as one of the cheapest and most versatile methodologies for the preparation of Er3þ-silica-based planar waveguides for EDWA’s. The advantages of this process are that the precursors are potentially pure and can be mixed at the molecular scale, thereby promoting the formation of highly homogeneous multi-component materials, not to mention the possibility of controlling nanoparticle size and distribution. However, the hydrolytic solegel route generates OH groups, which have to be subsequently removed, to avoid luminescence quenching. The densification process and OH elimination have been investigated by some of us in the case of Er3þ-doped SiO2-derived planar waveguides by using vibrational spectroscopy and refractive index measurements [18]. There are many literature works on Er3þ-silica-based materials such as glasses [19e21], glass-ceramics [22e25], planar waveguides based on binary oxides [26e33], and channel waveguides [34]. These publications aim to obtain an ideal material for optical applications that can be applied in the area of C-telecommunication. Optical properties of Er3þ-doped silica-
derived materials based on SiO2eZrO2 [28,29], SiO2eTiO2 [32,35,36], SiO2eTa2O5 [18,34] and SiO2eHfO2 [26,27,33,37] have been reported, and emission around 1550 nm, which is interesting for photonic applications, has been described. Particularly, the Er3þ-doped SiO2eZrO2 planar waveguide has been shown to be a promising glass ceramic material for 1.5 mm applications, especially in the case of the nanocomposite 80SiO2e20ZrO2 which exhibits relatively low loss, high luminescence at 1.5 mm, and 4I13/2 lifetime of about 8.0 ms [29]. According to previous reports, the optical and structural properties depend on the zirconium concentration. A controlled phase separation with homogeneous distribution of the zirconium oxide nanocrystals in a glassy silica-based matrix has been observed. Furthermore, it has been noted that the Er3þ ions are preferentially in a ZrO2 nanocrystalline environment. The present work reports on the preparation of channel and planar waveguides based on Er3þ/Yb3þ co-doped SiO2eZrO2 by the solegel process for optical applications. Microstructured channels were written onto the surface of multilayered densified films deposited on SiO2/Si substrates by a femtosecond laser etching technique. The film densification process, optical properties, and the influence of the amount of zirconium oxide on these properties were also evaluated. Finally, the effect of the presence of Yb3þ ions on the Er3þ emission around 1550 nm under excitation of the nanocomposite at 980 nm was assessed. 2. Experimental procedure Er3þ and Yb3þ co-doped SiO2eZrO2 films deposited onto silica on silicon (10 mm SiO2eSi (100) p-type) substrates and containing 0.3 mol% Er3þ ions and 1.2 mol% Yb3þ ions were prepared by the solegel process. The starting solutions obtained by mixing tetraethylorthosilicate (TEOS; Fluka, 98% purity), ethanol (Synth, 99.5% purity), deionized water, and 0.27 mol L1 hydrochloric acid were pre-hydrolyzed for 1 h at 65 C. The TEOS:HCl:H2O molar ratio was 1:0.01:2. In parallel with this procedure, an ethanolic colloidal suspension was prepared using ZrOCl2$8H2O (Fluka, 99% purity) as precursor. Different amounts of this suspension were then added to the TEOS solution, resulting in Si:Zr molar ratios of 90:10, 85:15, 80:20, and 75:25. Erbium and ytterbium ions were added as ErCl3 and YbCl3 ethanolic solutions pre-standardized with 0.01 mol L1 EDTA. The quantities of added Er3þ and Yb3þ were 0.3 and 1.2 mol%, respectively, in relation to the total number of Si þ Zr moles. The sols were filtered through a 0.2 mm Millipore filter and maintained under room temperature in an ultrasound bath, followed by magnetic stirring for 16 h. Then, the multilayered films were deposited onto previously cleaned silica on silicon (10 mm SiO2eSi (100) p-type) substrates by the dip-coating technique, with a deposition rate of 30 mm min1. Before further coating, each layer was annealed for 60 s at 900 C until 35 layers had been achieved. The remaining solution was maintained at 60 C in an oven, in order to obtain xerogels. The xerogels were annealed at 900 C for 8 h, forming the nanocomposite powders. The refractive indexes (at 532, 632.8, and 1538 nm) of the planar waveguide deposited onto the silica on silicon (10 mm SiO2eSi (100) p-type) substrate were measured on an m-line Metricon 2010 apparatus using the prism coupling technique. Measurements were accomplished every 15 min for the films treated at 900 C. On the basis of the refractive indexes at 532 nm obtained in the TE mode throughout the heat treatment, it was possible to verify the evolution of the densification process. The surface of the films 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 3.03 version.
Please cite this article in press as: C.S. Cunha, et al., NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
C.S. Cunha et al. / Materials Chemistry and Physics xxx (2012) 1e10
Fig. 1. Refractive index as a function of the annealing time of the films containing (A) 10, (B) 15, (C) 20 and (D) 25 mol% zirconium deposited onto the silica (100) substrate.
Fig. 2. Thickness as a function of the annealing time of the films containing (A) 10, (B) 15, (C) 20 and (D) 25 mol% zirconium deposited onto the silica (100) substrate.
Please cite this article in press as: C.S. Cunha, et al., NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
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Table 1 Number of TE and TM modes, and refractive index for the 532, 632.8 and 1538 nm of the 1.2 mol% Yb3þ and 0.3 mol% Er3þ-co-doped SiO2eZrO2 planar waveguides films annealed at 900 C after full densification. Waveguide composition
90SiO2e10ZrO2
85SiO2e15ZrO2
80SiO2e20ZrO2
75SiO2e25ZrO2
TE
TM
TE
TM
TE
TM
TE
TM
Number of modes
@532.0 nm @632.8 nm @1538 nm
2 1 1
2 1 1
2 2 1
2 1 1
2 2 1
2 2 1
3 2 1
3 2 1
Refractive index (h)
@532.0 nm (0.0005) @632.8 nm (0.0005) @1538 nm (0.005)
1.5112
1.5091
1.5455
1.5434
1.5721
1.5672
1.6033
1.5901
1.5066
1.5050
1.5399
1.5381
1.5661
1.5611
1.5964
1.5838
1.4883
1.4884
1.5236
1.5236
1.5472
1.5442
1.5722
1.5589
@532.0 nm (0.0005) @632.8 nm (0.0005) @1538 nm (0.005)
0.0021
0.0021
0.0049
0.0132
0.0016
0.0018
0.0050
0.0126
0.0001
0.0000
0.0030
0.0133
Dh (hTE hTM)
Images of the 80SiO2e20ZrO2 nanocomposites annealed at 900 C for 8 h were obtained by high-resolution transmission electron microscopy (HRTEM) using a Philips CM 200 microscope. The photoluminescence (PL) spectra in the infrared region were acquired on a spectrofluorometer Fluorolog 3-222 (FL3-222) Horiba Jobin Yvon at room temperature, and excitation at 980 nm was carried out by using a diode laser. The emission spectra were collected in the 1300e1700 nm range, power 500 mW, and filter cut-off Schott RG695. Microstructured channels were written onto the surface of multilayered densified films deposited on SiO2/Si substrates by means of a femtosecond laser etching technique. To this end, an 800-nm Ti-sapphire laser emitting 150 fs, 76 MHz mode-locked pulses was focused on the film surface with the aid of an objective lens. The spot size of the focused beam was 5 mm at the focal point, and the average power of the laser was 20 mW. The scanning speed was 0.5 mm s1. 3D surface profilometry of the channel waveguide was measured on a FORMTRACER profilemeter from Taylor Hobson Precision model SV-C525.
Optical properties were evaluated by the prism coupling technique [38e40]. The refractive index values were fundamental for the monitoring of the densification process. Fig. 1 depicts the refractive index values of the 100-xSiO2exZrO2 films, with x ¼ 10, 15, 20, and 25, as a function of different annealing times at 900 C, under air atmosphere. In general, the refractive index increases with longer annealing time for all the compositions, until a constant value is reached. Full densification is achieved, and it is directly dependent on the amount of zirconium oxide. Fig. 1 clearly shows that the refractive index value undergoes an initial sharp decrease after 15 min of annealing at 900 C for samples with 10 (Fig. 1(A)), 15 (Fig. 1(B)), and 20 (Fig. 1(C)) mol% zirconium. This is due to the simultaneous elimination of OH groups and water molecules from the pores present in the nondensified film [28,29]. In other words, the OH groups, and
Fig. 3. Refractive index at 632.8 nm for TE mode for planar waveguide based on Er3þ/ Yb3þ-co-doped SiO2eZrO2 containing different percentages of ZrO2 in comparison to the value of refractive index obtained by the LorentzeLorenz equation.
Fig. 4. Dispersion curve of the refractive index for planar waveguides based on Er3þ/ Yb3þ-co-doped SiO2eZrO2 containing 10, 15, 20, and 25 mol% ZrO2 measured in TE polarization.
3. Results and discussion 3.1. Optical and waveguiding properties
Please cite this article in press as: C.S. Cunha, et al., NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
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Fig. 5. Calculated squared electric field profiles of the TE0 mode at 532, 632.8 and 1538 nm for planar waveguides based on Er3þ/Yb3þ-co-doped SiO2eZrO2 containing (A) 10, (B) 15, (C) 20, and (D) 25 mol% ZrO2.
fully densified films. A refractive index at 632.8 nm and TE polarization of 1.5066, 1.5399, 1.5661, and 1.5964 for planar waveguides containing 10, 15, 20, and 25 mol% zirconium oxide are achieved after thermal treatment at 900 C for 540, 360, 170, and 20 min, respectively. The higher the ZrO2 concentration, the shorter the period required for pore elimination. According to previously reported structural properties [28], cross-linkage of the structure increases with zirconium content. As a consequence, the films are thicker and display higher densification degree. As illustrated in Fig. 2, the annealing time significantly influences the thickness of the films; i.e., longer annealing times result
adsorbed water are eliminated in this first stage, but no significant sintering occurs. At this point, it is important to remember that the refractive index of water is 1.33 [41], whereas the refractive index of air is 1.00. Thus, water elimination promotes a reduction in the refractive index value. For the planar waveguides containing 25 mol % zirconium (Fig. 1(D)), a shorter time of about 5 min only is required for the removal of the OH groups and water molecules, as compared to the samples with lower zirconium content. As the heat-treatment continues, the refractive index rises. After some time, which depends on the content of zirconium oxide, the refractive index values remain practically constant, resulting in well
Table 2 Confinement coefficient at 532, 632.8 and 1538 nm for TE and TM mode in the 1.2 mol% Yb3þ and 0.3 mol% Er3þ-co-doped SiO2eZrO2 planar waveguides films annealed at 900 C after full densification. Waveguide composition
Confinement coefficient
At 532 nm At 632.8 nm At 1538 nm
90SiO2e10ZrO2
85SiO2e15ZrO2
80SiO2e20ZrO2
75SiO2e25ZrO2
TE
TM
TE
TM
TE
TM
TE
TM
0.95 0.92 0.40
0.95 0.92 0.30
0.97 0.94 0.60
0.97 0.94 0.48
0.97 0.96 0.67
0.97 0.96 0.58
0.99 0.98 0.82
0.99 0.98 0.76
Table 3 A, B and C fitting variables to origin the dispersion refractive index curves using the Cauchy equation for the Er3þ/Yb3þ co-doped 100-xSiO2exZrO2 planar waveguides, for TE and TM polarization. Planar waveguide
90SiO2e10ZrO2 85SiO2e15ZrO2 80SiO2e20ZrO2 75SiO2e25ZrO2
A
B
TE
TM
TE
1.4831 1.5194 1.5422 1.5656
1.4836 1.5200 1.5656 1.5518
1.2934 1.0159 1.2226 1.6364
C TM
104 104 104 104
1.1798 8.7485 1.6364 1.7562
TE
104 103 104 104
1.4085 7.822 1.0670 1.6091
TM
109 108 109 109
1.3002 109 6.0237 108 1.6091 109 1.9037 109
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Fig. 6. 3D AFM images for 0.3 mol% Er3þ and 1.2 mol% Yb3þ-doped (A) 90SiO2e10ZrO2, (B) 80SiO2e20ZrO2 and (C) 75SiO2e25ZrO2 films prepared by the solegel process, on silica (100) substrate.
in films with reduced thickness. This is valid for annealing times at which the densification is not complete. In the specific case of planar waveguides containing 10, 15, 20, and 25 mol% zirconium oxide, the film thickness progressively declines after thermal treatment at 900 C for 500, 360, 160, and 20 min respectively. The densification augments and the thickness diminishes, because of the elimination of residual pores. A constant thickness value is finally reached, which is in agreement with the variation in the refractive index discussed above. Table 1 summarizes the optical properties of the films at 532, 632.8 and 1538 nm after achievement of full densification. The number of modes increases as a function of the refractive index. A thickness of about 1.0 mm was adjusted, so that single mode planar waveguides in the near infrared region could be obtained. First of all, the refractive index of the waveguides containing between 10 and 25 mol% ZrO2 agrees with the value calculated at 633 nm by the LorentzeLorenz (Eq. (1)) equation using h ¼ 1.4575 and 2.13 for the refractive index of SiO2 and ZrO2, respectively.
h2SiO2 1 h2ZrO2 1 h2 1 þ f ¼ f ZrO SiO2 2 2 2 h2 þ 2 hSiO2 þ 2 hZrO2 þ 2
(1)
The refractive index, measured in the TE and TM polarization modes, rises as a function of the nominal zirconium concentration, as reported by Gonçalves et al. [28,29]. Accordingly, it is an indication that a high densification degree has been obtained for all the waveguides. Fig. 3 brings the experimental and calculated
refractive index values, where the calculated refractive index has been estimated by the LorentzeLorenz equation (Eq. (1)). Moreover, there is a difference between the refractive index measured in the TE and TM modes for all the wavelengths, with a Dh (Dh ¼ hTE hTM) of around 0.001 up to 0.0136, depending on the ZrO2 content. More precisely, as the zirconia content rises, the birefringence becomes more pronounced. The introduction of ZrO2 into the silica network induces some phase separation [28], with consequent crystallization of tetragonal zirconium oxide nanocrystals. The dependence of the nanocrystals on volume and size distribution upon introduction of larger zirconium oxide amounts will be known further. The higher the ZrO2 concentration, the more significant the crystalline volume and the larger the size of the nanocrystals. This can contribute to the appearance of some birefringence in the planar waveguides. As reported by some of us, a similar behavior has been detected for the binary silica-derived planar waveguides containing hafnium oxide [26,27]. Fig. 4 corresponds to the dispersion curve h(l) for the planar waveguide obtained in the TE and TM modes. The refractive index measured (h) by the prism coupling technique was used for construction of the dispersion curves, which are based on the Cauchy equation (Eq. (2)).
hðlÞ ¼ A þ B=l2 þ C=l4
(2)
where h is the refractive index, l is the wavelength and, A, B and C are listed in Table 3.
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Fig. 7. (A) and (B) 2D surface profile for the channels constructed on Er3þ/Yb3þ co-doped 80SiO2e20ZrO2 planar waveguides prepared by the solegel process on SiO2/Si substrate and (C) 3D surface profilometry image of the channel waveguide.
Fig. 8. TEM images of the nanocomposite obtained at 900 C for 8 h containing (A) 10, (B) 15, (C) 20 and (D) 25 mol% zirconium.
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Fig. 11. Photoluminescence spectra relative to the 4I13/2 / 4I15/2 Er3þ ions transition in Er3þ/Yb3þ-co-doped 75SiO2e25ZrO2 and Er3þ doped 75SiO2e25ZrO2 films upon excitation at 980 nm.
Fig. 9. Histograms of the nanocomposite with ratio between SiO2eZrO2 of (A) 85/15, (B) 80/20 and (C) 75/25 obtained at 900 C for 8 h. Statistical histograms were obtained from the HRTEM images.
On the basis of the parameters obtained by m-line measurements, the confinement coefficient of the films was estimated. Fig. 5 shows the squared electric field profiles of the TE0 modes of the planar waveguides containing 10 (Fig. 5A), 15 (Fig. 5B), 20 (Fig. 5C), and 25 (Fig. 5D) mol% ZrO2, calculated at 532, 632.8, and 1538 nm. Table 2 lists the confinement coefficient at 532, 632.8, and 1538 nm for the TE and TM modes. The values represent the ratio of
the integrated intensity, i.e., the square of the electric field in the waveguide to the total intensity for each wavelength. The total intensity also includes the squared evanescent fields. The modeling indicates some differences in the confinement of the exciting light as a function of the composition. Because the thickness is almost the same, the higher the refractive index, the larger the confinement coefficient. A light confinement above 92% can be seen for the waveguides excited at 532 and 632.8 nm. For the 1538 nm wavelength, the TE0 confinement coefficients are 0.40, 0.60, 0.67, and 0.82 for the samples containing 10, 15, 20, and 25 mol% ZrO2, respectively. This indicates that an efficient injection at 1538 nm is possible, particularly for the waveguides with zirconium content higher than 20 mol%. As for the waveguides with lower concentration, the thickness must be increased, in order to obtain a well confined single mode at 1538 nm, which is the wavelength of interest in integrated optics and telecommunication applications. 3.2. Morphological surface properties of the planar and channel waveguides Homogeneous and crack-free films were obtained for all the compositions. Fig. 6 shows the surface morphology of the planar waveguides containing different amounts of zirconium oxide. The images shown here are representative of the other ones and do not exhibit any texture under AFM inspection. The AFM images evidence smooth surface films with mean surface of 0.6 nm (0.1) very low roughness, and high surface quality, without cracks. Surface roughness is generally considered an important source of losses in planar waveguides, but its contribution to the losses in the present waveguides can be deemed negligible. As reported before, optical losses are not considered to be directly correlated
Table 4 Values of bandwidth of the emission band around 1528 nm for the nanocomposites doped with Er3þ and co-doped with Er3þ/Yb3þ. Nominal composition
4
4
3þ
Fig. 10. Photoluminescence spectra relative to the I13/2 / I15/2 Er ions transition in Er3þ/Yb3þ-co-doped 80SiO2e20ZrO2 and Er3þ doped 80SiO2e20ZrO2 films upon excitation at 980 nm.
90SiO2e10ZrO2 85SiO2e15ZrO2 80SiO2e20ZrO2 75SiO2e25ZrO2
FWHM (nm) 2 nm Er3þ
Er3þ/Yb3þ
35 28 22 21
28 27 24 24
Please cite this article in press as: C.S. Cunha, et al., NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
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with the amount of zirconium oxide, at least for concentrations lower than 25 mol% ZrO2 [28]. Similar results have also been obtained for other solegel silica-derived planar waveguides containing titania and hafnia. Fig. 7 represents the microstructured channel written onto the surface of Er3þ/Yb3þ co-doped 80SiO2e20ZrO2 planar waveguides on SiO2/Si substrate by a femtosecond laser etching technique. Fig. 7(A)e(C) reveals a surface profile with air grooves created after laser irradiation on the surface of the film. The rib waveguide can be noted between two air grooves with a width around 20 and 50 mm on the top and the base, respectively. The depth of the rib waveguide is the film thickness, since ablation occurs until a depth of 11.1 mm, thereby also reaching the SiO2 coating the Si substrate. The width and depth of the channels can be tailored by changing the spot beam is focused on and the power intensity, so as to produce mono or multimodal channel waveguides. The use of a femtosecond laser has been recently explored for the production of microstructured materials, including fibers and monoliths. There are few papers dealing with planar waveguides on structured films, so this subject is worth studying. Recently, we have published a work on microstructured channels that were successfully written onto solegel silica-derived planar waveguides containing tantalum oxide by a femtosecond laser irradiation etching [42]. 3.3. Structural and luminescence properties Fig. 8 contains the HRTEM images of the 100-xSiO2exZrO2 nanocomposite annealed at 900 C for 8 h. ZrO2 nanocrystals embedded in an amorphous phase are detected for the samples containing more than 15 mol% ZrO2, and an amorphous material is observed only for the 90SiO2e10ZrO2 composite. Homogeneous distribution of spherically-shaped nanoparticles can be noted with very narrow distribution and average size of about 4 nm. There is virtually no significant change in size with increasing zirconium content up to 25 mol%. Fig. 9 corresponds to the statistical histograms of the size distribution of ZrO2 nanocrystals. The volume of the crystals in the amorphous phase slightly increases with rising zirconium content, suggesting the occurrence of important nucleation during the crystallization process, simultaneously with the densification. Figs. 10 and 11 show the emission spectra of the Er3þ and Er3þ/ 3þ Yb co-doped 80SiO2e20ZrO2 and 75SiO2e25ZrO2 nanocomposites, respectively. The emission spectra display the 4I13/ 4 2 / I15/2 transition with a maximum around 1528 nm and shoulders at 1480 and 1560 nm for the samples doped with Er3þ and co-doped with Er3þ/Yb3þ. The emission bandwidth of the Er3þ and Er3þ/Yb3þ co-doped samples are summarized in Table 4. The higher the zirconium content, the narrower the emission band. A broader bandwidth is evident for the samples containing 10 mol% ZrO2, where the Er3þ ions are distributed in an amorphous system. For the compounds containing higher zirconium and rare earth concentration, the emission bandwidth is reduced, and the FWHM changes from 28 up to 21 nm. These results corroborate with previously reported data [28,29]. The addition of ytterbium ions induces some modification, which is especially evident for the system containing the lowest zirconium concentration. A narrowing of 7 nm can be verified when Yb3þ is added to the 90SiO2e10ZrO2 nanocomposite, suggesting some local change of the contribution to the nucleation process. Finally, the most important result concern the enhancement of the Er3þ emission in the presence of Yb3þ ions, which clearly demonstrates an efficient energy transfer process from the Yb3þ to the Er3þ ions in this system. An enhancement of 40-fold was observed for the samples co-doped with Er3þ and Yb3þ with respect to the nanocomposite doped only with Er3þ.
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Besides the intense emission around the 1550 nm region, which represents a crucial point for WDM devices operating at C-telecommunication bands, the lifetime also plays an imperative role. The lifetime values obtained for the Er3þ and for the Er3þ/Yb3þ codoped 75SiO2e25ZrO2 nanocomposites are 7.8 and 8.2 ms, respectively. A single exponential profile can be detected, and nonradiative processes due to multiphonon relaxation have been assumed to be negligible in these hosts. 4. Conclusions Planar and channel waveguides based on Er3þ/Yb3þ co-doped SiO2eZrO2 were prepared by the solegel methodology using the dip-coating technique. Microstructured channels with high homogeneous surface profile were written onto the surface of multilayered densified films deposited on SiO2/Si substrates by a femtosecond laser etching technique. It was possible to tailor the width and depth of the channels by changing the spot of the beam was focused and the power intensity. The densification process of the planar waveguides was evaluated though the variations in the refractive index and thickness. Full densification was achieved at 900 C, after annealing for 23 up to 500 min, depending on the ZrO2 content. The amount of ZrO2 proved to be a key factor in the control of the annealing time, so that fully densified planar waveguides could be achieved. Crystal nucleation and growth took place concomitantly with densification, producing transparent glass ceramic planar waveguides containing rare earth doped spherical ZrO2 nanocrystals dispersed homogeneously in a silica-based glassy host, with a mean size of about 4 nm. Even when the phase separation was present, the size and distribution of the nanoparticles were homogeneously controlled. A mean size of about 4 nm was detected, with a slight increase in size distribution as a function of the zirconium content. The volume of the crystals in the amorphous phase also slightly increased with the zirconium content, suggesting an important nucleation during the crystallization process simultaneously with densification. Low roughness and crack-free surface and a high confinement coefficient were observed for all the compositions. Enhanced NIR luminescence of the Er3þ ions was detected for the Yb3þ co-doped planar waveguides, denoting an efficient energy transfer from the Yb3þ to the Er3þ ions. The optical properties for the channel and planar waveguides make them suitable materials for application in integrated optics, especially in EDWA and WDM devices. Acknowledgments 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. References [1] A. Polman, in: S. Jiang (Ed.), Rare-earth-doped Materials and Devices IV, Vol. 3942, 2000. [2] A. Polman, J. Appl. Phys. 82 (1997) 1e39. [3] G. Blasse, B.C. Grabmaier, Luminescent Materials, Springer, Berlin, 1994. [4] A. Polman, F.C.J.M.V. Veggel, J. Soc. Am. B 21 (2004) 871e892. [5] F. Auzel, F. Pelle, J. Lumin. 69 (1996) 249e255. [6] F. Auzel, G. Baldacchini, L. Laversenne, G. Boulon, Opt. Mater. 24 (2003) 103e109. [7] F. Auzel, P. Goldner, Opt. Mater. 16 (2001) 93e103. [8] C. Armellini, A. Chiasera, S. Dire, M. Ferrari, A. Martucci, M. Mattarelli, M. Montagna, E. Moser, S. Pelli, G.C. Righini, G.D. Soraru, L. Zampedri, J. Sol.Gel Sci. Technol. 32 (2004) 267e271. [9] L.A. Gomez, L.D. Menezes, C.B. de Araujo, R.R. Goncalves, S.J.L. Ribeiro, Y. Messaddeq, J. Appl. Phys. 107 (2010) 113508-1e113508-5.
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Please cite this article in press as: C.S. Cunha, et al., NIR luminescent Er3þ/Yb3þ co-doped SiO2eZrO2 nanostructured planar and channel waveguides: Optical and structural properties, Materials Chemistry and Physics (2012), http://dx.doi.org/10.1016/j.matchemphys.2012.06.040
