- Email: [email protected]
Ag+ planar waveguides in novel Er–Yb silicate glasses
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267 www.elsevier.com/locate/jpcs
Ag+ planar waveguides in novel Er–Yb silicate glasses Stanislava Jana´kova´a,, Blanka Sˇvecova´a, Linda Salavcova´a, Jarmila Sˇpirkova´a, Martin Mı´ kab, Jiri Oswaldc a
Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic c Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 162 53 Prague, Czech Republic
b
Abstract Planar waveguides were prepared by silver 2 sodium ion-exchange in a set of specially designed europium–ytterbium-doped silicate glasses. In this study, we focused on the influence of co-doping glasses with ytterbium ions on their final behaviour. First, a suitable molar ratio between Er3+/Yb3+ ions was searched in order to maximize the efficiency of absorption of the pumping signal at 980 nm and consequently to increase the intensity of the emitted signal at 1550 nm. Transmission spectra at 980 and 1550 nm and emission spectra at 1550 nm of these new glasses are given. Second, effect of ytterbium ions on permeability of a glass network structure during the ionexchange process of waveguides fabrication was also studied. Properties of the fabricated planar waveguides were characterized by scanning electron microscopy (SEM) in order to gain information about chemical composition of the active layer prepared in Er3+/Yb3+ substrate and dark mode spectroscopy at 671 nm to obtain information about the optical properties (e.g. number of guided modes, refractive index profiles and depth of waveguide active layer). r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Glasses; D. Luminescence; D. Optical properties
1. Introduction Materials containing rare-earth (RE) ions are intensively studied for preparation of active optical layers, which could be used as waveguide lasers, optical amplifiers, optical sensors, etc. [1]. Stimulated emission of erbium ions at 1.5 mm is mostly used for the active function of the layers. This research presents another step in the development of optical materials. Pumping into the excited state 4 I11/2 using 980 nm radiation was proved as the most profitable to achieve an efficient population inversion between metastable level and ground state of the erbium ions. Co-doping with ytterbium ions to enhance the absorption of pumping signal at 980 nm and so the increase of the efficiency of erbium emitted signal at 1550 nm was already studied, e.g. in Al2O3 waveguides [2], where was described the energy transfer process between Yb3+ and Corresponding author. Tel.: +420 22044 4003; fax: +420 22044 4411.
E-mail address: [email protected] (S. Jana´kova´). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.02.015
Er3+. This effect was mentioned also in silicate glasses [3]. Excitation takes place around 980 nm and can excite both erbium (4I15/2–4I11/2 ) and ytterbium (2F7/2–2F5/2 ). Yb3+ can either decay to its ground state or transfer its energy to the Er3+ 4I11/2 level. Erbium decay of the 4I11/2 state to the 4 I13/2 state leads to building-up population in the metastable 4I13/2 level. Composition of the erbium-containing optical glasses was studied in our previous works [4,5], where for example the presence of Zn2+ instead of other bivalent cations like Ca2+ or Mg2+ revealed to have positive influence on the glass permeability. In this study, co-doping of erbiumcontaining silicate optical glasses with ytterbium was done, the optimal ratio between Yb3+ and Er3+ ions to maximize the emission at 1.5 mm was experimentally found and the influence of the RE ions on the resultant properties of the ion-exchanged waveguides were described. In the frame of the study of ion exchange fabrication of the optical waveguides, special attention was paid to defining fabrication conditions of few-modes waveguides
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
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(those supporting two or three modes at 671 nm), as they will be potentially suitable for real applications in photonics devices. 2. Experiments and measurement Samples of the bulk glasses were fabricated by mixing the major components of glass (SiO2, Al2O3, Na2O), with the network intermediates (ZnO) and RE oxides (Er2O3 and Yb2O3). Every particular glass contained various ratios of the RE ions while the content of Na2O was kept constant at 10 wt%. The glasses MM 42 and MM 37 contained Er3+ ions only (for the glass composition see Table 1). Molar rations Er3+/Yb3+ were: MM 55 (2/1), MM 56 (1/1), MM 57 (2/3). The batches were prepared to make 25 g of glass using substances of high purity that were thoroughly mixed and then melted in a Pt crucible at 1430 1C. After the melting and mechanical mixing, the homogeneous melt was cast into a stainless steel mould obtaining glass bars. The cubes of the glasses were cut to rectangular parallel epipeds and then both sides polished to dimensions 8 mm 8 mm 3 mm. The waveguides were fabricated in the glass substrates by Ag+2Na+ ion exchange using reaction bath made of eutectic mixture of sodium and potassium nitrates with addition of 24 wt% of silver nitrate. The salts were melted in platinum crucible, in which also occurred the ion exchange reaction. Substrate wafers were thoroughly precleaned in a series of solvents and placed into the oven set on the temperature of 280 1C for the Ag+2Na+ exchange and let heated prior being immersed into the reaction bath. This acted as a preheating stage to avoid cracking of the wafers. The ion exchange occurred after immersing of the pre-heated wafers into the reaction bath for specified times varying between 5 and 30 min. After the desired times, the samples were pulled out of the bath, then, after cooling to room temperature, washed with distilled water and ethanol to remove residuals of the reaction bath and dried in air. Photoluminescence spectra of the bulk glasses were collected in the region of 500–1600 nm at room temperature with the back-side excitation using the 488 nm continuous wave output of an Ar ion laser. A double grating 0.5 m monochromator with spectral resolution 3.2 nm/mm of a slit was used to analyse the emitted light, which was detected by a Ge detector. Transmission spectra Table 1 Content of Na and RE oxides in the used ZnO–SiO2 glasses Used substrate
MM MM MM MM MM
42 37 55 56 57
Content (mol%) Na2O
Er2O3
Yb2O3
14.7 14.1 13.6 13.4 13.1
0.4 3.3 3.3 3.2 3.2
0 0 1.6 3.2 4.8
for lem ¼ 1550 nm and emission spectra with lexc ¼ 980 nm were collected from samples included both, Er and Yb ions to describe the effect of Yb ions on an intensity of emission at 1550 nm and to optimise the ration between Er and Yb ions. Changes in chemical composition of the surface layers of the substrates, i.e. concentration profiles of the exchanged ions, Ag+2Na+, incurred during the fabrication of the waveguides were studied by scanning electron microscopy (SEM) with energy dispersive spectrometer (EDAX). The charge-up between the electron beam and the nonconducting glasses was avoided by thin graphite layer deposited on the substrates. Dark mode spectroscopy at 671 nm, employing the inverse WKB method [6], was used to characterize waveguiding properties of the fabricated layers, i.e. number of guided modes, refractive index vs. depth profile, total refractive index increment in the surface layer and effective depth of the optical (functional) layer. Surface refractive index is derived from the depth of the first, the shallowest, guided mode by the WKB method, the refractive index of the deepest guided mode is considered to be a refractive index of the glass matrix.
3. Results 3.1. Properties of the glass substrates Incorporation of Yb into Er-containing optical glasses resulted in increase of the efficiency of absorption in the vicinity 980 nm (both width and intensity of the absorption band increased, see Fig. 1), which is supposed to be one of the wavelength mostly used for excitation in the final optical components. It is obvious that the absorption (the effectiveness of the pumping at 980 nm) increased significantly when ytterbium ions were involved in the glass matrix and that it further increased with increasing content of Yb2O3 (from 8 to 21 wt%). Photoluminescence spectra of the used (Er+Yb)-containing glasses in the vicinity of 1550 nm are given in Fig. 2. In this case, the composition of the glass had different effect on behaviour of the substrates. Ytterbium ions do not emit in this region, only the emission bands of erbium ions are observed. The effect of addition of Yb3+ ions into the glass is evident (increased intensity of Er emission due to the energy transfer process between Yb3+ and Er3+). Glass MM 57 (with Er3+/Yb3+ molar ratio 2/3) exhibits the highest emission intensity. However, the emission yield of this glass is almost the same as that of MM 56 glass (Er3+/Yb3+ molar ratio 1/1). Therefore, it may be deduced that the optimal concentration of Yb3+ ions in the ZnO–SiO2 glasses to achieve maximum intensity of emission at 1550 nm is close to the concentration of Er3+ ions. The content of RE ions in the glass also affected their refractive indexes. As shown in Fig. 3, the refractive index at 671 nm increased for every particular substrate with
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
1.0 Phtotoluminescence intensity [a.u.]
1600
0.8 transmission (a.u.)
1265
MM37 MM55 MM56 MM57
0.6
0.4
0.2
MM 55 MM 56 MM 57 MM 37 T=20 ° C λex=980 nm
1400 1200 1000 800 600 400 200
0.0 800
1000
1200 1400 wavelength (nm)
0
1600
1450
1500
1550 wavelength [nm]
1600
1650
1.0
transmission (a.u.)
0.8
Phtotoluminescence intensity [a.u.]
1600 MM37 MM55 MM56 MM57
0.6
0.4
0.2
0.0 800
1000
1200 1400 wavelength (nm)
1600
Fig. 1. Transmission spectra of Er3+- and Yb3+/Er3+- silicate glass substrates.
increasing amount of the RE ions in the glass matrix. The effect of Er and Yb ions was in this case similar. 3.2. Optical waveguides Planar optical waveguides were prepared in the (Er+Yb)-doped glass substrates by Ag+3Na+ exchange and the obtained results were compared with those obtained in the Er only containing glasses, i.e., substrates MM 37 and MM 42. As the depth of the prepared layer and the amount of exchanged ions strongly influence optical properties of the waveguide, knowledge about diffusion process in the substrates in dependence on their composition is very important. Fig. 4 presents distribution of the Ag+ ions in three waveguides prepared by the same technological procedure (20 min exchange at 280 1C) in a set of substrates that differ in content of the RE ions (Er3+ and Yb3+). Total amount of the in-diffused silver ions decreased in the case of the glasses with higher total content of the RE ions. These results indicate that content of the RE ions in the substrate strongly affects the process
MM 55 MM 56 MM 57 MM 37 T=20 ° C λex=980 nm
1400 1200 1000 800 600 400 200 0 1450
1500
1550
1600
1650
wavelength [nm] Fig. 2. Emission spectra of the MM glasses (lexc ¼ 980 nm).
of ion exchange. With increase in content of RE ions, the glass substrate became less permeable for the exchanging ions. Higher content of the RE ions makes the density and the viscosity of the glass increase as well, and so does the compactness of the glass. Increasing compactness of the material results in decreased permeability for diffusing ions. Concerning the glass permeability, there was no difference found between doping the glasses with Er or Yb ions; it was their total amount that mattered. Similar effect of the RE ions on permeability of the glass substrate, i.e., on velocity of the ion-exchange process, was observed when the prepared layers were characterized by dark mode spectroscopy (see Table 2A). The waveguides prepared using the same fabrication conditions in the glass with a higher content of the RE were shallow and guided less modes (in the case of MM57 after 20 min of IE no waveguiding effect was found at all) compared to those prepared in the glasses containing lower amount of the RE. To increase the number of the guided modes in these samples, it is necessary to prolong duration
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
1.590
Table 2 Waveguiding properties and refractive index changes
1.575
Used Content of substrate Er3++Yb3+ (mol%)
1.560 1.545 1.530 1.515 0
2
5
4
8
content of Er2O3 + Yb2O3 in glass (mol.%)
Fig. 3. Dependence of the refractive index values of the MM glasses on their content of rare earth ions. (dark mode spectroscopy at 671 nm).
42 37 55 56 57
0.4 3.3 4.9 6.5 8.0
20 20 20 20 20
7 3 2 1 0
7.0 2.7 1.8 1.0 —
0.09356 0.09036 0.08687 0.07557 —
B. MM MM MM MM MM
42 37 55 56 57
0.4 3.3 4.9 6.5 8.0
5 20 20 25 30
3 3 2 2 2
3.6 2.7 1.8 1.4 1.3
0.09861 0.09036 0.08687 0.08164 0.07773
A: the waveguides fabricated after 20 min of the ion exchange. B: few-modes waveguides suitable for further application. Dn – differences in the refractive index between the surface refractive index of prepared layer and the refractive index of the glass matrix; determined by dark mode spectroscopy at 671 nm, TE polarisation.
0.08 Ag2O/SiO2 [at.%/at.%]
Time Number of Depth Dn of IE guided modes* (mm)*
A. MM MM MM MM MM
MM 55 MM 56 MM 57
0.06
0.04
0.02
0.00 0.0
0.5
1.0
1.5
2.0 2.5 3.0 depth [μm]
3.5
4.0
4.5
refractive index (at 671 nm) [-]
refractive index (at 671nm) [-]
1266
4. Discussion and summary Co-doping of Yb3+ ions into the already presented [4] Er-containing silicate glasses was done; three types of the glasses were designed with a varying Er/Yb molar ratio. A noticeable improvement of the absorption efficiency in the vicinity of 980 nm, which is one of the expected excitation wavelength, was achieved by incorporating ytterbium in the glass matrix. The photoluminescence spectra of these new glasses revealed that the emission intensity at 1550 nm also increased after addition of the ytterbium ions into the glass, confirming the increased
1.66 MM 42 MM 37 MM 55 MM 56 MM 57
1.64 1.62 1.60 1.58 1.56 1.54 0.0
refractive index (at 671 nm) [-]
Fig. 4. Ag+ ions concentration depth-profiles in different substrates. (SEM-EDAX).
of the ion exchange. In order to obtain few-modes waveguides in all types of the substrate glass, duration of the ion exchange was modified as shown in Table 2B. The refractive index vs. depth profiles in the prepared waveguiding layers obtained by dark mode spectroscopy (see Fig. 5) corresponded well with the pertinent depth profiles of the Ag+ ions concentration (Fig. 4).
1.68
0.5
1.0
1.5 depth [μm]
2.0
2.5
3.0
1.68 1.66 MM 42 MM 37 MM 55 MM 56 MM 57
1.64 1.62 1.60 1.58 1.56 1.54 0.0
0.5
1.0
1.5 depth [μm]
2.0
2.5
3.0
Fig. 5. Depth-profiles of waveguiding layers. Each line represent different substrate, dots marked depths where each mode is guided Derived by WKB inverse method, dark mode spectroscopy at 671 nm.
efficiency of excitation of the Er3+ ions. However, only a slight positive change was observed by increasing the Yb content in the Er/Yb ratio from 1/1 to 2/3 demonstrating thus that the equimolar ratio is sufficient to achieve the maximum emission of the Er3+ ions.
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
Effect of the presence of the RE ions in the glass matrix on the properties important for the fabrication of optical waveguides was also studied. We found out that the refractive index of the substrate glasses increased with the increased amount of the RE ions involved in the matrix and, at the same time, the permeability of the glass matrix for the exchanged ions decreased. It means that the incorporation of Ag+ ions into the surface layer of the substrate is more difficult (to obtain a similar amount of the exchanged ions a prolonged duration of the ionexchange process is needed). Deteriorated conditions for the migrating ions indicate that in the silicate glasses the RE ions have more-less ionic properties, similarly as the presence of calcium ions slowed down or even inhibited the ion exchange process, that is the fact well known to those using the ion exchange to fabricate optical waveguides and that was also mentioned in our paper [5]. This interesting effect will be farther investigated in more details. As expected, no explicit differences in behaviour of ytterbium in comparison to erbium were found. 5. Conclusion A series of newly designed glasses was examined from the point of view of few-mode waveguide fabrication. The
1267
experimental results showed that to maximize the efficiency of Er excitation to maximize the absorption of pumping signal at 980 nm and so to increase the intensity of the emission at 1550 nm equimolar ratio between erbium and ytterbium is sufficient. Increasing the total content of REs in the glass results in increased refractive index of the glass and in decreased permeability for diffusing ions.
Acknowledgement The research has been supported by the GACR Grant 106/05/0706 and MSM project 6046137302.
References [1] A.J. Kenyon, Prog. Quant. Electron. 26 (2002) 225. [2] C. Strohhofer, A. Polman, Opt. Mater. 21 (2003) 705–712. [3] R.M. Almeida, A.C. Marques, J. Non-Cryst. Solids 352 (2006) 475–482. [4] L. Salavcova´, B. Sˇvecova´, S. Jana´kova´, O. Kolek, M. Mika, J. Sˇpirkova´, A. Langrova´, Ceram—Silikaty 49 (1) (2005) 53–57. [5] P. Capek, M. Mika, J. Oswald, P. Tresnakova, L. Salavcova, O. Kolek, J. Schrofel, J. Spirkova, Opt. Mater. 27 (2004) 331. [6] J.M. White, P.F. Heinrich, Appl. Opt. 15 (1976) 151.
Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267 www.elsevier.com/locate/jpcs
Ag+ planar waveguides in novel Er–Yb silicate glasses Stanislava Jana´kova´a,, Blanka Sˇvecova´a, Linda Salavcova´a, Jarmila Sˇpirkova´a, Martin Mı´ kab, Jiri Oswaldc a
Department of Inorganic Chemistry, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic Department of Glass and Ceramics, Institute of Chemical Technology, Technicka 5, 166 28 Prague, Czech Republic c Institute of Physics, Czech Academy of Sciences, Cukrovarnicka 10, 162 53 Prague, Czech Republic
b
Abstract Planar waveguides were prepared by silver 2 sodium ion-exchange in a set of specially designed europium–ytterbium-doped silicate glasses. In this study, we focused on the influence of co-doping glasses with ytterbium ions on their final behaviour. First, a suitable molar ratio between Er3+/Yb3+ ions was searched in order to maximize the efficiency of absorption of the pumping signal at 980 nm and consequently to increase the intensity of the emitted signal at 1550 nm. Transmission spectra at 980 and 1550 nm and emission spectra at 1550 nm of these new glasses are given. Second, effect of ytterbium ions on permeability of a glass network structure during the ionexchange process of waveguides fabrication was also studied. Properties of the fabricated planar waveguides were characterized by scanning electron microscopy (SEM) in order to gain information about chemical composition of the active layer prepared in Er3+/Yb3+ substrate and dark mode spectroscopy at 671 nm to obtain information about the optical properties (e.g. number of guided modes, refractive index profiles and depth of waveguide active layer). r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Glasses; D. Luminescence; D. Optical properties
1. Introduction Materials containing rare-earth (RE) ions are intensively studied for preparation of active optical layers, which could be used as waveguide lasers, optical amplifiers, optical sensors, etc. [1]. Stimulated emission of erbium ions at 1.5 mm is mostly used for the active function of the layers. This research presents another step in the development of optical materials. Pumping into the excited state 4 I11/2 using 980 nm radiation was proved as the most profitable to achieve an efficient population inversion between metastable level and ground state of the erbium ions. Co-doping with ytterbium ions to enhance the absorption of pumping signal at 980 nm and so the increase of the efficiency of erbium emitted signal at 1550 nm was already studied, e.g. in Al2O3 waveguides [2], where was described the energy transfer process between Yb3+ and Corresponding author. Tel.: +420 22044 4003; fax: +420 22044 4411.
E-mail address: [email protected] (S. Jana´kova´). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.02.015
Er3+. This effect was mentioned also in silicate glasses [3]. Excitation takes place around 980 nm and can excite both erbium (4I15/2–4I11/2 ) and ytterbium (2F7/2–2F5/2 ). Yb3+ can either decay to its ground state or transfer its energy to the Er3+ 4I11/2 level. Erbium decay of the 4I11/2 state to the 4 I13/2 state leads to building-up population in the metastable 4I13/2 level. Composition of the erbium-containing optical glasses was studied in our previous works [4,5], where for example the presence of Zn2+ instead of other bivalent cations like Ca2+ or Mg2+ revealed to have positive influence on the glass permeability. In this study, co-doping of erbiumcontaining silicate optical glasses with ytterbium was done, the optimal ratio between Yb3+ and Er3+ ions to maximize the emission at 1.5 mm was experimentally found and the influence of the RE ions on the resultant properties of the ion-exchanged waveguides were described. In the frame of the study of ion exchange fabrication of the optical waveguides, special attention was paid to defining fabrication conditions of few-modes waveguides
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
1264
(those supporting two or three modes at 671 nm), as they will be potentially suitable for real applications in photonics devices. 2. Experiments and measurement Samples of the bulk glasses were fabricated by mixing the major components of glass (SiO2, Al2O3, Na2O), with the network intermediates (ZnO) and RE oxides (Er2O3 and Yb2O3). Every particular glass contained various ratios of the RE ions while the content of Na2O was kept constant at 10 wt%. The glasses MM 42 and MM 37 contained Er3+ ions only (for the glass composition see Table 1). Molar rations Er3+/Yb3+ were: MM 55 (2/1), MM 56 (1/1), MM 57 (2/3). The batches were prepared to make 25 g of glass using substances of high purity that were thoroughly mixed and then melted in a Pt crucible at 1430 1C. After the melting and mechanical mixing, the homogeneous melt was cast into a stainless steel mould obtaining glass bars. The cubes of the glasses were cut to rectangular parallel epipeds and then both sides polished to dimensions 8 mm 8 mm 3 mm. The waveguides were fabricated in the glass substrates by Ag+2Na+ ion exchange using reaction bath made of eutectic mixture of sodium and potassium nitrates with addition of 24 wt% of silver nitrate. The salts were melted in platinum crucible, in which also occurred the ion exchange reaction. Substrate wafers were thoroughly precleaned in a series of solvents and placed into the oven set on the temperature of 280 1C for the Ag+2Na+ exchange and let heated prior being immersed into the reaction bath. This acted as a preheating stage to avoid cracking of the wafers. The ion exchange occurred after immersing of the pre-heated wafers into the reaction bath for specified times varying between 5 and 30 min. After the desired times, the samples were pulled out of the bath, then, after cooling to room temperature, washed with distilled water and ethanol to remove residuals of the reaction bath and dried in air. Photoluminescence spectra of the bulk glasses were collected in the region of 500–1600 nm at room temperature with the back-side excitation using the 488 nm continuous wave output of an Ar ion laser. A double grating 0.5 m monochromator with spectral resolution 3.2 nm/mm of a slit was used to analyse the emitted light, which was detected by a Ge detector. Transmission spectra Table 1 Content of Na and RE oxides in the used ZnO–SiO2 glasses Used substrate
MM MM MM MM MM
42 37 55 56 57
Content (mol%) Na2O
Er2O3
Yb2O3
14.7 14.1 13.6 13.4 13.1
0.4 3.3 3.3 3.2 3.2
0 0 1.6 3.2 4.8
for lem ¼ 1550 nm and emission spectra with lexc ¼ 980 nm were collected from samples included both, Er and Yb ions to describe the effect of Yb ions on an intensity of emission at 1550 nm and to optimise the ration between Er and Yb ions. Changes in chemical composition of the surface layers of the substrates, i.e. concentration profiles of the exchanged ions, Ag+2Na+, incurred during the fabrication of the waveguides were studied by scanning electron microscopy (SEM) with energy dispersive spectrometer (EDAX). The charge-up between the electron beam and the nonconducting glasses was avoided by thin graphite layer deposited on the substrates. Dark mode spectroscopy at 671 nm, employing the inverse WKB method [6], was used to characterize waveguiding properties of the fabricated layers, i.e. number of guided modes, refractive index vs. depth profile, total refractive index increment in the surface layer and effective depth of the optical (functional) layer. Surface refractive index is derived from the depth of the first, the shallowest, guided mode by the WKB method, the refractive index of the deepest guided mode is considered to be a refractive index of the glass matrix.
3. Results 3.1. Properties of the glass substrates Incorporation of Yb into Er-containing optical glasses resulted in increase of the efficiency of absorption in the vicinity 980 nm (both width and intensity of the absorption band increased, see Fig. 1), which is supposed to be one of the wavelength mostly used for excitation in the final optical components. It is obvious that the absorption (the effectiveness of the pumping at 980 nm) increased significantly when ytterbium ions were involved in the glass matrix and that it further increased with increasing content of Yb2O3 (from 8 to 21 wt%). Photoluminescence spectra of the used (Er+Yb)-containing glasses in the vicinity of 1550 nm are given in Fig. 2. In this case, the composition of the glass had different effect on behaviour of the substrates. Ytterbium ions do not emit in this region, only the emission bands of erbium ions are observed. The effect of addition of Yb3+ ions into the glass is evident (increased intensity of Er emission due to the energy transfer process between Yb3+ and Er3+). Glass MM 57 (with Er3+/Yb3+ molar ratio 2/3) exhibits the highest emission intensity. However, the emission yield of this glass is almost the same as that of MM 56 glass (Er3+/Yb3+ molar ratio 1/1). Therefore, it may be deduced that the optimal concentration of Yb3+ ions in the ZnO–SiO2 glasses to achieve maximum intensity of emission at 1550 nm is close to the concentration of Er3+ ions. The content of RE ions in the glass also affected their refractive indexes. As shown in Fig. 3, the refractive index at 671 nm increased for every particular substrate with
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
1.0 Phtotoluminescence intensity [a.u.]
1600
0.8 transmission (a.u.)
1265
MM37 MM55 MM56 MM57
0.6
0.4
0.2
MM 55 MM 56 MM 57 MM 37 T=20 ° C λex=980 nm
1400 1200 1000 800 600 400 200
0.0 800
1000
1200 1400 wavelength (nm)
0
1600
1450
1500
1550 wavelength [nm]
1600
1650
1.0
transmission (a.u.)
0.8
Phtotoluminescence intensity [a.u.]
1600 MM37 MM55 MM56 MM57
0.6
0.4
0.2
0.0 800
1000
1200 1400 wavelength (nm)
1600
Fig. 1. Transmission spectra of Er3+- and Yb3+/Er3+- silicate glass substrates.
increasing amount of the RE ions in the glass matrix. The effect of Er and Yb ions was in this case similar. 3.2. Optical waveguides Planar optical waveguides were prepared in the (Er+Yb)-doped glass substrates by Ag+3Na+ exchange and the obtained results were compared with those obtained in the Er only containing glasses, i.e., substrates MM 37 and MM 42. As the depth of the prepared layer and the amount of exchanged ions strongly influence optical properties of the waveguide, knowledge about diffusion process in the substrates in dependence on their composition is very important. Fig. 4 presents distribution of the Ag+ ions in three waveguides prepared by the same technological procedure (20 min exchange at 280 1C) in a set of substrates that differ in content of the RE ions (Er3+ and Yb3+). Total amount of the in-diffused silver ions decreased in the case of the glasses with higher total content of the RE ions. These results indicate that content of the RE ions in the substrate strongly affects the process
MM 55 MM 56 MM 57 MM 37 T=20 ° C λex=980 nm
1400 1200 1000 800 600 400 200 0 1450
1500
1550
1600
1650
wavelength [nm] Fig. 2. Emission spectra of the MM glasses (lexc ¼ 980 nm).
of ion exchange. With increase in content of RE ions, the glass substrate became less permeable for the exchanging ions. Higher content of the RE ions makes the density and the viscosity of the glass increase as well, and so does the compactness of the glass. Increasing compactness of the material results in decreased permeability for diffusing ions. Concerning the glass permeability, there was no difference found between doping the glasses with Er or Yb ions; it was their total amount that mattered. Similar effect of the RE ions on permeability of the glass substrate, i.e., on velocity of the ion-exchange process, was observed when the prepared layers were characterized by dark mode spectroscopy (see Table 2A). The waveguides prepared using the same fabrication conditions in the glass with a higher content of the RE were shallow and guided less modes (in the case of MM57 after 20 min of IE no waveguiding effect was found at all) compared to those prepared in the glasses containing lower amount of the RE. To increase the number of the guided modes in these samples, it is necessary to prolong duration
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
1.590
Table 2 Waveguiding properties and refractive index changes
1.575
Used Content of substrate Er3++Yb3+ (mol%)
1.560 1.545 1.530 1.515 0
2
5
4
8
content of Er2O3 + Yb2O3 in glass (mol.%)
Fig. 3. Dependence of the refractive index values of the MM glasses on their content of rare earth ions. (dark mode spectroscopy at 671 nm).
42 37 55 56 57
0.4 3.3 4.9 6.5 8.0
20 20 20 20 20
7 3 2 1 0
7.0 2.7 1.8 1.0 —
0.09356 0.09036 0.08687 0.07557 —
B. MM MM MM MM MM
42 37 55 56 57
0.4 3.3 4.9 6.5 8.0
5 20 20 25 30
3 3 2 2 2
3.6 2.7 1.8 1.4 1.3
0.09861 0.09036 0.08687 0.08164 0.07773
A: the waveguides fabricated after 20 min of the ion exchange. B: few-modes waveguides suitable for further application. Dn – differences in the refractive index between the surface refractive index of prepared layer and the refractive index of the glass matrix; determined by dark mode spectroscopy at 671 nm, TE polarisation.
0.08 Ag2O/SiO2 [at.%/at.%]
Time Number of Depth Dn of IE guided modes* (mm)*
A. MM MM MM MM MM
MM 55 MM 56 MM 57
0.06
0.04
0.02
0.00 0.0
0.5
1.0
1.5
2.0 2.5 3.0 depth [μm]
3.5
4.0
4.5
refractive index (at 671 nm) [-]
refractive index (at 671nm) [-]
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4. Discussion and summary Co-doping of Yb3+ ions into the already presented [4] Er-containing silicate glasses was done; three types of the glasses were designed with a varying Er/Yb molar ratio. A noticeable improvement of the absorption efficiency in the vicinity of 980 nm, which is one of the expected excitation wavelength, was achieved by incorporating ytterbium in the glass matrix. The photoluminescence spectra of these new glasses revealed that the emission intensity at 1550 nm also increased after addition of the ytterbium ions into the glass, confirming the increased
1.66 MM 42 MM 37 MM 55 MM 56 MM 57
1.64 1.62 1.60 1.58 1.56 1.54 0.0
refractive index (at 671 nm) [-]
Fig. 4. Ag+ ions concentration depth-profiles in different substrates. (SEM-EDAX).
of the ion exchange. In order to obtain few-modes waveguides in all types of the substrate glass, duration of the ion exchange was modified as shown in Table 2B. The refractive index vs. depth profiles in the prepared waveguiding layers obtained by dark mode spectroscopy (see Fig. 5) corresponded well with the pertinent depth profiles of the Ag+ ions concentration (Fig. 4).
1.68
0.5
1.0
1.5 depth [μm]
2.0
2.5
3.0
1.68 1.66 MM 42 MM 37 MM 55 MM 56 MM 57
1.64 1.62 1.60 1.58 1.56 1.54 0.0
0.5
1.0
1.5 depth [μm]
2.0
2.5
3.0
Fig. 5. Depth-profiles of waveguiding layers. Each line represent different substrate, dots marked depths where each mode is guided Derived by WKB inverse method, dark mode spectroscopy at 671 nm.
efficiency of excitation of the Er3+ ions. However, only a slight positive change was observed by increasing the Yb content in the Er/Yb ratio from 1/1 to 2/3 demonstrating thus that the equimolar ratio is sufficient to achieve the maximum emission of the Er3+ ions.
ARTICLE IN PRESS S. Jana´kova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1263–1267
Effect of the presence of the RE ions in the glass matrix on the properties important for the fabrication of optical waveguides was also studied. We found out that the refractive index of the substrate glasses increased with the increased amount of the RE ions involved in the matrix and, at the same time, the permeability of the glass matrix for the exchanged ions decreased. It means that the incorporation of Ag+ ions into the surface layer of the substrate is more difficult (to obtain a similar amount of the exchanged ions a prolonged duration of the ionexchange process is needed). Deteriorated conditions for the migrating ions indicate that in the silicate glasses the RE ions have more-less ionic properties, similarly as the presence of calcium ions slowed down or even inhibited the ion exchange process, that is the fact well known to those using the ion exchange to fabricate optical waveguides and that was also mentioned in our paper [5]. This interesting effect will be farther investigated in more details. As expected, no explicit differences in behaviour of ytterbium in comparison to erbium were found. 5. Conclusion A series of newly designed glasses was examined from the point of view of few-mode waveguide fabrication. The
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experimental results showed that to maximize the efficiency of Er excitation to maximize the absorption of pumping signal at 980 nm and so to increase the intensity of the emission at 1550 nm equimolar ratio between erbium and ytterbium is sufficient. Increasing the total content of REs in the glass results in increased refractive index of the glass and in decreased permeability for diffusing ions.
Acknowledgement The research has been supported by the GACR Grant 106/05/0706 and MSM project 6046137302.
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