AP08_5.vp 1 Introduction In recent years, Rare Earth (RE) ions containing photon- ics materials have attracted much attention for their potential applications for full color displays, optical sources and laser systems such as optical amplifiers [1–6]. A list of the RE ele- ments with some of their basic properties is shown in Table 1. Most research has focused on RE ions which can emit in the visible region. Steckl et al. reported in [7] about the prop- erties of GaN layers doped with Eu3�, Er3�, and Tm3� ions. They obtained (for the first time) photoemission from higher excited RE states in GaN covering the entire visible spectrum: light emission in the green (from Er at 537/558 nm), red (Pr at 650 nm and Eu at 621 nm), and blue (Tm at 477 nm) spectral regions. A second major field of study deals with RE ions which can emit in the infrared region. For these purposes, the RE ions are most often studied for telecommunica- tions systems. For telecommunication systems operating at 1300 nm investigations are made of RE ions such as Nd3�, Pr3� and Dy3� [8–11]. For telecommunications applications at 1530 nm, investigations are made of Er3� and Tm3� ions [12–14]. In the last decade there have also been investigations of sensitizers to produce more efficient RE doped sources. The most often used sensitizer is Yb3� for Er3�-doped optical amplifiers [15, 16]. In addition to Yb3+ other RE ions are nowadays examined as sensitizers, such as Ho3� for Tm3� [17, 18] or Ho3� for Yb3� [19] doped photonics materials, etc. [20]. Optical materials such as semiconductors, glass and opti- cal crystals doped with RE ions are conventional materials for accomplishing lasing action. Recently there has been consid- erable interest in the development of new photonics materials such as polymers [21, 22], which have better properties and a lower price. In this paper, we present the fabrication and properties of Er and Er/Yb doped polymer layers. As a polymer material we chose Polymethylmethacrylate (PMMA), due to its low optical absorption, simple synthesis and low cost. These characteris- tics make it a suitable host material for RE ions [23, 24]. Er3� ions were chosen due to fact that Er3� ions now play a key role in long-distance optical communication systems. Yb3� co- -doping was applied because it was previously shown that the addition of ytterbium ions increased the intensity of the luminescence at 1530 nm [16]. 2 Experiment Small pieces of PMMA (Goodfellow) were left to dissolve in chloroform for a few days before being used in the fabrica- tion of PMMA layers. The PMMA layers were fabricated by spin coating on silicon substrates or the polymer was poured into a bottomless mold placed on a quartz substrate and left to dry. For RE doping anhydrous ErCl3 and YbCl3 or ErF3 and YbF3 or erbium(III) tris(2,2,6,6-tetramethyl-3,5-heptanedio- 14 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 5/2008 Optical Properties of Erbium and Erbium/Ytterbium Doped Polymethylmethacrylate V. Prajzler, V. Jeřábek, O. Lyutakov, I. Hüttel, J. Špirková, V. Machovič, J. Oswald, D. Chvostová, J. Zavadil In this paper we report on the fabrication and properties of Er3� and Er3�/Yb3� doped Polymethylmethacrylate (PMMA) layers. The reported layers were fabricated by spin coating on silicon or on quartz substrates. Infrared spectroscopy was used for an investigation of O-H stretching vibration. Measurement were made of the transmission spectra in the wavelength ranges from 350 to 700 nm for the Er3� doped samples and from 900 to 1040 nm for the Yb3� doped samples. The refractive indices were investigated in the spectral range from 300 to 1100 nm using optical ellipsometry and the photoluminescence spectra were measured in the infrared region. Keywords: polymer, polymethylmethacrylate, Erbium, Ytterbium, optical properties. Atomic number Element Electron configuration RE3� Ground term RE3� 58 Cerium – Ce 4f 15s25p6 2F5/2 59 Praseodymium – Pr 4f25s25p6 3H4 60 Neodymium – Nd 4f35s25p6 4I9/2 61 Promethium – Pm 4f45s25p6 5I4 62 Samarium – Sm 4f55s25p6 6H5/2 63 Europium – Eu 4f65s25p6 7F0 64 Gadolinium – Gd 4f75s25p6 8S7/2 65 Terbium – Tb 4f85s25p6 7F6 66 Dysprosium – Dy 4f95s25p6 6H15/2 67 Holmium – Ho 4f105s25p6 5I8 68 Erbium – Er 4f115s25p6 4I15/2 69 Thulium – Tm 4f125s25p6 3H6 70 Ytterbium – Yb 4f135s25p6 2F7/2 Table 1: The rare earth elements and some of their properties nate) (Sigma-Aldrich) and ytterbium(III) tris(2,2,6,6-tetra- methyl-3,5-heptanedionate) (Goodfellow) were dissolved in C5H9NO or C2H6OS (Sigma-Aldrich). The layers were fabricated in such a way that the content of erbium in the solutions varied from 1.0 at. % to 20.0 at. %, and were then added to the polymer. The samples containing 1.0 at. % of erbium were co-doped with ytterbium (Er3�/Yb3� samples) in amounts varying from 1.0 at. % to 20.0 at. %. The molecular structure of PMMA is shown in Fig. 1. The molecu- lar structure of ErCl3 shown in Fig. 2a, the structure of ErF3 in Fig. 2b, and that of erbium(III) tris(2,2,6,6- tetramethyl-3,5- -heptanedionate) and ytterbium(III) tris(2,2,6,6-tetramethyl- -3,5-heptanedionate) are shown in Fig. 2c. 3 Results and discussion The samples were characterized by infrared spectroscopy (FT-IR) using a Bruker IFS 66/v FTIR spectrometer equipped with a broadband MCT detector, to which 128 interferograms were added with a resolution of 4 cm�1 (Happ-Genzel apo- dization). Fig. 3 displays the FT-IR spectra of PMMA layers doped with Er3� ions (ErCl3) in the wavelength range from 4000 to 2600 cm�1. © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 15 Acta Polytechnica Vol. 48 No. 5/2008 O n CH3 C CH3 CH2 C O Fig. 1: Molecular structure of PMMA Me O O O O O O C(CH3)3 C(CH3)3 C(CH3)3 C(CH 3 ) 3 C(CH3)3 C CC C C C Me = Er3+, Yb3+ CH CHCH C(CH 3 ) 3 a) c)b) Er Cl Cl Cl Er F F F Fig. 2: Molecular structure of a) ErCl3, b) ErF3 and c) erbium(III) and ytterbium(III) ions 3600 3400 3200 3000 2800 2600 0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 0% Er 1% Er 5% Er 10% Er 15% Er 20% Er A b s o rb a n c e (- ) Wavenumber (cm ) �1 O-H 3349 C-H 2994 C-H 2953 C-H 2843 Fig. 3: Infrared spectra of PMMA samples doped with Er3� using ErCl3 The three strong broad bands occurring at 2994 cm�1, 2953 cm�1, 2843 cm�1 correspond to the aliphatic C-H bands. These bands are assigned to the stretching vibrations of CH3 and CH2, and indicate a high content of hydrogen-rich CHx. The absorption band at 3349 cm�1 corresponds to the O-H stretching vibrations of the PMMA layers. Fig. 3 also shows that increasing the Er3� content also increased the intensity of the O-H vibrations. This can be explained by the fact that ErCl3, as very hygroscopic substance, not only dopes the poly- mer samples also but bring a certain amount of water. It is a well-known fact that the presence of O-H groups in a matrix containing rare earth ions unfortunately causes problems by hindering emission in the infrared region. The transmission measurements were performed using a UV-VIS-NIR Spectrometer (UV-3600 Shimadzu) in the spec- tral range from 350 to 700 nm. The transmission spectra of the Er3� doped polymer (erbium(III)) in the spectral range from 350 nm to 700 nm are shown in Fig. 4. In the samples containing 10.0 at. % of Er3�, two bands appeared that were attributed to the following transitions: 4G11/2 (377 nm) and 2H11/2 (519 nm). In the samples containing 20.0 at. % of Er 3� one more band appeared at 4F9/2 (650 nm). We did not ob- serve bands 2G7/2 (355 nm), 2G9/2 (363 nm), 2H9/2 (405 nm), 4F3/2 (441 nm), 4F5/2 (448 nm), 4F7/2 (485 nm) and 4S3/2 (539 nm). The same results were obtained for samples doped with ErCl3 and ErF3 solution. The transmission spectra of the Er3� (1.0 at. %) doped polymers co-doped with Yb3� ions using erbium(III) and ytterbium(III) (from 1.0 at. % to 20.0 at. %) in the spectral range from 900 nm to 1040 nm are shown in Fig. 5. The sample containing 20.0 at. % of Yb3� ions has a typical Yb3� 2F5/2 transition with maxima at 977 nm. The samples 16 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 5/2008 350 400 450 500 550 600 650 700 0 5 10 15 20 25 30 35 2 G 9/2 2 G 7/2 4 F 3/2 2 H 11/2 T ra n s m is s io n (a .u .) Wavelength (nm) 1% Er 5% Er 10% Er 20% Er4G 11/2 4 F 7/2 4 F 5/2 4 F 9/2 4 S 3/2 Fig. 4: Transmission spectra of the Er3� doped PMMA using erbium(III) 900 920 940 960 980 1000 1020 1040 2 0. 2 5. 3 0. 3 5. 4 0. 4 5. 5 0. 5 5. 6 0. 6.5 1% Yb 5% Yb 10% Yb 20% Yb T ra n s m is s io n (a .u .) Wavelength (nm) 2 F 5/2 Fig. 5: Transmission spectra of Er3� (1.0 at.%) doped PMMA co-doped with Yb3� using erbium(III) and ytterbium(III) ions with a lower concentration have a weaker 2F5/2 transition, and the samples with concentration 1 at. % Yb3� ions have no visi- ble Yb3� (2F5/2) transition. The refractive indices were measured using variable angle spectroscopic ellipsometry (VASE, J.A.Woollam & co.) work- ing in rotating analyzer mode. The measurements were car- ried out in the spectral range from 300 to 1100 nm. Fig. 6a shows the dependence of the refractive indices of the Er3� doped PMMA (erbium (III)), and Fig. 6b shows the depend- ence of the refractive indices of the 1at. % Er3� (erbium (III)) doped PMMA with Yb3� co-doping (ytterbium (III)). It is obvious that increasing the content of Er3� and Er3�� Yb3� increases the refractive indices of the material. The refractive index value is, also a matter of the polariz- ability of the ions present in the material [26]. Polarizability (or ion deformation) is understood as a function of the size of the ions – the larger the size, the larger the polarizability, and vice versa. The presence of larger cautions in the substance (in this case rather large Er 3� and/or Yb3� the thin layer of poly- mer) usually raises the refractive index. The results are not surprising, but what is important is the exact refractive index value (of course depending on the wavelength) of the depos- ited material. Semiconductor laser excitation (P4300 operating at � x � 980 nm with Eex � 500 mW; room temperature) was used to detect sample luminescence in the range from 1450 to 1650 nm. The photoluminescence spectra of the Er3� doped samples (erbium (III)) are given in Fig. 7a, and the Er3� doped samples (ErF3) are given in Fig. 7b. In the case of erbium (III) only the samples with higher Er3+ concentra- © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 17 Acta Polytechnica Vol. 48 No. 5/2008 300 400 500 600 700 800 900 1000 1100 1 47. 1 48. 1 49. 1 50. 1 51. 1 52. 1 53. 1 54. 1.55 0 % Er 5 % Er 10 % Er 20 % Er In d e x o f re fr a c ti o n (- ) Wavelength (nm) a) 300 400 500 600 700 800 900 1000 1100 1 48. 1 50. 1 52. 1 54. 1 56. 1 58. 1 60. b) 0 % Er 1% Er 1% Yb 1% Er 20% Yb In d e x o f re fr a c ti o n (- ) Wavelength (nm) Fig. 6: Wavelength dependence of the refractive indices a) of the Er3� and b) Er3��Yb3� doped PMMA layers using erbium(III) and ytter- bium(III) tions showed very weak photoluminescence bands at 1530 nm attributed to the erbium transition 4I13/2 � 4I15/2. In the case of ErF3, the emission intensity is higher than that of erbium (III) (See Fig. 7). The highest emission inten- sity was found in sample (ErF3), which contained 10.0 at. % erbium. Fig. 8 shows the infrared emissions obtained for samples doped with 1.0 at.% erbium and co-doped with ytterbium ions in amounts varying from 1.0 at. % to 10.0 at. % (Fig. 8a erbium(III), ytterbium(III) and Fig. 8b ErF3, YbF3). The samples showed very weak emission at 1530 nm. Therefore co-doping with ytterbium ions had only a weak effect on the photoluminescence spectra. 4 Conclusion We have reported on the fabrication process and the properties of PMMA layers doped with Er3� and Er3� ions co-doped with Yb3� ions. � Polymer layers were fabricated by spin coating or by pour- ing the polymer into a bottomless mould placed on a glass substrate. � We observed the FTIR absorption band at around 3349 cm�1 corresponding to the O-H vibrations and three bands at 2994 cm�1, 2953 cm�1 and 2843 cm�1 corre- sponding to the aliphatic C-H bands. 18 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 5/2008 1450 1500 1550 1600 1650 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. P L In te n s it y (a .u .) Wavelength (nm) 4 I 13/2 4 I 15/2 a) 1% Er 10% Er 20% Er 1450 1500 1550 1600 1650 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. b) P L In te n s it y (a .u .) Wavelength (nm) 4 I 13/2 4 I 15/2 10% Er 5% Er 1% Er Fig. 7: Photoluminescence spectra of Er3� doped PMMA layers a) (erbium(III)), b) ErF3� ( �ex � 980 nm with Eex � 500 mW; room temperature) � The content of Er3� and Er3� � Yb3� ions had a significant effect on the transmission spectra. We observed three bands corresponding to the Er3� ions (4G11/2 – 377 nm, 2H11/2 – 519 nm, 4F9/2 – 650 nm and one band corre- sponding to the Yb3� ions (2F5/2 – 977 nm). These bands we observed in the samples doped with a higher Er and Yb concentration, and they almost disappeared in the background in the case of samples with a low Er and Yb concentration. � The refractive indices were investigated by spectroscopic ellipsometry and we found that increasing the content of the Er3� and Yb3� ions increases the refractive indices of the material. � The Er3� doped PMMA samples exhibited a typical emis- sion at 1530 nm, due to the Er3� intra-4f 4I13/2 � 4I15/2 only at samples with higher content of Er3� ions. The highest emission intensity was found in sample (ErF3) con- taining 10.0 at. % erbium. It was also found that the addi- © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 19 Acta Polytechnica Vol. 48 No. 5/2008 1450 1500 1550 1600 1650 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1.0 P L In te n s it y (a .u .) Wavelength (nm) 5% Yb 10% Yb 4 I 13/2 4 I 15/2 a) 20% Yb 1450 1500 1550 1600 1650 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. 0 0. 0 2. 0 4. 0 6. 0 8. 1 0. b) P L In te n s it y (a .u .) Wavelength (nm) 10% Yb 5% Yb 1% Yb 4 I 13/2 4 I 15/2 Fig. 8: Photoluminescence spectra of Er3��Yb3� doped PMMA layers (1.0 at. % Er) a) (erbium(III), ytterbium(III)), b) ErF3�, YbF3� ��ex � 980 nm with Eex � 500 mW; room temperature) tion of ytterbium did not substantially affect the 1530 nm luminescence. Acknowledgments Our research has been supported by the Grant Agency of the Czech Republic under grant number 102/06/0424 and re- search program MSM6840770014 of the Czech Technical University in Prague. References [1] Steckl, A. J., Zavada, J. M.: Optoelectronic Properties and Applications of Rare-Earth-Doped GaN. MRS Bulle- tin, Vol. 24 (1999), Issue 9, p. 33–38. [2] Steckl, A. J., Heikenfeld, J., Lee, D. 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Vitězslav Jeřábek, CSc. e-mail: Jerabek@fel.cvut.cz Department of Microelectronics Faculty of Electrical Engineering Czech Technical University in Prague Technická 2 166 27 Prague, Czech Republic 20 © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ Acta Polytechnica Vol. 48 No. 5/2008 Mgr. Oleksei Lyutakov Doc. Ing. Ivan Hüttel, DrSc. e-mail: Ivan.Huttel@vscht.cz RNDr. Jarmila Špirková, CSc. e-mail: Jarmila.Spirkova@vscht.cz Ing. Vladimír Machovič, CSc. Institute of Chemical Technology Technická 5, 166 27 Prague, Czech Republic Ing. Jiří Oswald, CSc. e-mail: Oswald@fzu.cz RNDr. Dagmar Chvostová e-mail: chvostov@fzu.cz Institute of Physics Academy of Sciences Czech Republic, v. v. i. Cukrovarnická 10/112 162 53 Prague 6, Czech Republic RNDr. Jiří Zavadil, CSc. e-mail: Zavadil@ufe.cz Institute of Photonics and Electronics Academy of Sciences Czech Republic, v. v. i. Chaberská 57 182 51 Prague, Czech Republic © Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 21 Acta Polytechnica Vol. 48 No. 5/2008