Materials Letters 101(2013)21-24 Contents lists available at sciverse scienceDirect materials letters Materials letters ELSEVIER journalhomepagewww.elsevier.com/locate/matlet Mid-IR luminescence of Dy+ and Pr+ doped Gas Gez0Sb10S(Se)65 bulk glasses and fibers F. Charpentier F Starecki, J.L. Doualan P Jovari, P. Camy b, J. Roles a, S Belin, B. bureau V. Nazabal s Equipe Verres 8 Ceramiques-Institut des Sciences chimiques de rennes, UMR 6226-Universite de Rennes 1-CNRS, 35042 Renne Institute for Solid State Physics and Optics, wigner Research Centre, Hungarian Academy of Sciences, H-1525 Budapest, A0B4y,.n, france Centre de Recherche sur les lors, les Materiaux et la photonique(CIMAP). UMR 6252 CEA-CNRS-ENSICaen, Universite de Caen, 14050 Saint Aubin. 9119 92 Gif sur Yvette, france ARTICLE INF O A BSTRACT Received 22 December 2012 (mid-IR). The complex glassy network of these chalcogenide glasses were characterized by Raman attering and extended X-ray absorption fine structure, to put in evidence a structure combining Available online 21 march 2013 tetrahedra of Germanium and Gallium and trigonal pyramids of Antimony. The arrangement of these structural units permits the introduction of rare earth ions thanks, in particular, to a charge compensa- on generated by the Ga in a tetrahedral site. Consequently. Pr+ and Dy+ ions inserted in such low phonon energy glasses emit efficiently in mid-IR, between 3.5 and 5 um. Finally Dy+ and Pr+ doped EXAFS GasGe2oSb10S(Se)65 fibers were obtained from bulk glass preform. Efficient emission in mid-IR was Mid-IR emission obtained by pumping Dy+ doped Gas Sb1oSs5 and Pr+ doped Gas Ge2o Sb10 S(Se)65 fibers at 920 nm Dysprosium and 2 um, respectively. e 2013 Elsevier B.V. All rights reserved. Introduction scalability if the selenide double-clad fiber optical loss remains lower than 5 dB/m [5]. The Pr+ also offers several transitions Mid-infrared(mid-IR)radiation, often generated by black body which give a broad emission spectrum between 3.5 and 5.5 um sources in optical sensors, possesses low brightness influenced by [1-3, 6. Selected Ga-Ge-Sb-S(Se)glasses doped with RE can be the surrounding Black body sources can be efficiently replaced by shaped in optical waveguides, like rib waveguides or conventional emission from rare earth(RE)ions [1] for which the brightness is fibers 3, 8, 10-13] and light can be easily confined, thanks to their relatively temperature independent changing mainly according optical pump source scheme and matrix nature. For potential 1.55 um). Moreover, in such glasses, the incorporation of RE ions is applications in this spectral range, the re mid-IR emission require possible up to 1 wt by keeping the amorphous state of the the use of material host with low phonon energy as sulfide and materials which is favorable for applications such as highly bright selenide matrices for instance[1-6]. Disordered glassy network incoherent sources. In the present paper, the amorphous structure lower the Re site symmetry and facilitate the mixing of orbitals by of complex quaternary GasGe S(Se)65 glasses will be allowing f-f transitions of RE ions. Consequently, the emission is described in relation to the spectroscopic properties of Dy+ and broad and non-structured unlike glass-ceramics [7 which is t incorporated in mentioned glasses. favorable for differential detection techniques in optical sensors. Moreover, optical waveguides can be made of chalcogenide glasses which increase the brightness of the fluorescent source. Among all 2. Materials and methods RE, Dy+ presents an emission band centered at 4.35 um corre- nding to the transition between the H1 and H1/2 levels The selected glass composition is GasGez0Sb1oS(Se)6s doped by [2,8, 9l. which can be efficiently pumped by commercial laser Dy+ or Pr+ ions from 0.05 to 1 wt%. After sealing, the chemical diodes. It was proposed by numerical modeling that simultaneous reagents in the silica tul e slowly heated to 900C durin ing at 1.7 um significantly improves the efficiency and power The 300-400 um diameter fibers were obtained by drawing length which were doped with 0.05 or 0. 1 wt of RE [12 orresponding author. Tel:+33223235748: fax: +33223235611 Absorption spectra were measured on bulk glasses with Perkin Elmer spectrometer Lambda-1050. A Ti: Sapphire laser emitting at 0167-577x/S-see fr e 2013 Elsevier B v. All rights reserved
Mid-IR luminescence of Dy3þ and Pr3þ doped Ga5Ge20Sb10S(Se)65 bulk glasses and fibers F. Charpentier a , F. Starecki b , J.L. Doualan b , P. Jóvári c , P. Camy b , J. Troles a , S. Belin d , B. Bureau a , V. Nazabal a,n a Equipe Verres & Céramiques—Institut des Sciences chimiques de Rennes, UMR 6226-Université de Rennes 1-CNRS, 35042 Rennes, France b Centre de Recherche sur les Ions, les Matériaux et la Photonique (CIMAP), UMR 6252 CEA-CNRS-ENSICaen, Université de Caen, 14050 Caen, France c Institute for Solid State Physics and Optics, Wigner Research Centre, Hungarian Academy of Sciences, H-1525 Budapest, POB 49, Hungary d Synchrotron SOLEIL, L'Orme des Merisiers, Saint Aubin, 91192 Gif sur Yvette, France article info Article history: Received 22 December 2012 Accepted 11 March 2013 Available online 21 March 2013 Keywords: Chalcogenide glass Fiber EXAFS Mid-IR emission Praseodymium Dysprosium abstract Dy3þ and Pr3þ doped Ga5Ge20Sb10S(Se)65 glasses provide good emission efficiencies in the mid-infrared (mid-IR). The complex glassy network of these chalcogenide glasses were characterized by Raman scattering and extended X-ray absorption fine structure, to put in evidence a structure combining tetrahedra of Germanium and Gallium and trigonal pyramids of Antimony. The arrangement of these structural units permits the introduction of rare earth ions thanks, in particular, to a charge compensation generated by the Ga in a tetrahedral site. Consequently, Pr3þ and Dy3þ ions inserted in such low phonon energy glasses emit efficiently in mid-IR, between 3.5 and 5 μm. Finally, Dy3þ and Pr3þ doped Ga5Ge20Sb10S(Se)65 fibers were obtained from bulk glass preform. Efficient emission in mid-IR was obtained by pumping Dy3þ doped Ga5Ge20Sb10S65 and Pr3þ doped Ga5Ge20Sb10S(Se)65 fibers at 920 nm and 2 μm, respectively. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Mid-infrared (mid-IR) radiation, often generated by black body sources in optical sensors, possesses low brightness influenced by the surrounding. Black body sources can be efficiently replaced by emission from rare earth (RE) ions [1] for which the brightness is relatively temperature independent changing mainly according to optical pump source scheme and matrix nature. For potential applications in this spectral range, the RE mid-IR emission require the use of material host with low phonon energy as sulfide and selenide matrices for instance [1–6]. Disordered glassy network lower the RE site symmetry and facilitate the mixing of orbitals by allowing f–f transitions of RE ions. Consequently, the emission is broad and non-structured unlike glass-ceramics [7] which is favorable for differential detection techniques in optical sensors. Moreover, optical waveguides can be made of chalcogenide glasses which increase the brightness of the fluorescent source. Among all RE, Dy3þ presents an emission band centered at 4.35 mm corresponding to the transition between the 6 H11/2 and 6 H13/2 levels [2,8,9], which can be efficiently pumped by commercial laser diodes. It was proposed by numerical modeling that simultaneous lasing via 6 H11/2-6 H13/2 and 6 H13/2-6 H15/2 transitions by pumping at 1.7 μm significantly improves the efficiency and power scalability if the selenide double-clad fiber optical loss remains lower than 5 dB/m [5]. The Pr3þ also offers several transitions which give a broad emission spectrum between 3.5 and 5.5 μm [1–3,6]. Selected Ga–Ge–Sb–S(Se) glasses doped with RE can be shaped in optical waveguides, like rib waveguides or conventional fibers [3,8,10–13] and light can be easily confined, thanks to their high refractive index (2.25 for sulfides and 2.58 for selenides at 1.55 mm). Moreover, in such glasses, the incorporation of RE ions is possible up to 1 wt % by keeping the amorphous state of the materials which is favorable for applications such as highly bright incoherent sources. In the present paper, the amorphous structure of complex quaternary Ga5Ge20Sb10S(Se)65 glasses will be described in relation to the spectroscopic properties of Dy3þ and Pr3þ incorporated in mentioned glasses. 2. Materials and methods The selected glass composition is Ga5Ge20Sb10S(Se)65 doped by Dy3þ or Pr3þ ions from 0.05 to 1 wt%. After sealing, the chemical reagents in the silica tube were slowly heated to 900 1C during several hours, followed by water quenching and annealing [12,13]. The 300–400 mm diameter fibers were obtained by drawing Ga–Ge–Sb–S(Se) glass preforms of 12 mm diameter and 100 mm length which were doped with 0.05 or 0.1 wt % of RE [12]. Absorption spectra were measured on bulk glasses with Perkin Elmer spectrometer Lambda-1050. A Ti:Sapphire laser emitting at Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters 0167-577X/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matlet.2013.03.062 n Corresponding author. Tel.: þ33223235748; fax: þ33223235611. E-mail address: virginie.nazabal@univ-rennes1.fr (V. Nazabal). Materials Letters 101 (2013) 21–24
F Charpentier er al./ Materials Letters 101(2013)21-24 920 nm and an OPo (pumped at 832 nm by Ti: Sapphire laser) for SbS3] trigonal pyramids even if the EXAFS fit for Sb and Ga K-edge umping at 1300 nm were used to excite a 20 cm long Dyt doped are not absolutely perfect. In detail, the raman spectrum of the gla Ga-Ge-Sb-S(Se)fibers and bulk glasses. A homemade Tm: YAG is dominated by a broad band from 250 to 450 cm", which is formed laser pumped the F2 level of Pr+: Ga-Ge-Sb-S(Se)fibers and by an overlap of several bands as the symmetric stretching modes of bulk glasses absorbing around 2 um. The mid-IR light focused [GeSaz] Td centered at 342 cm". Gasap] Td at 320 cm"or [Sbs3p] the monochromator slit with a Sapphire lens was detected by a trigonal pyramids at 293 cm. The band located at 433 cm reveals cooled InSb detector. The structure of Ga-Ge-SbS(Se)glasses were that (Gesapl Td are mainly connected by their corners. The presence investigated using an HR800(Horiba-Jobin-Yvon) micro-Raman of a weak band at 476cm-l, attributed to the presence of S-s bonds spectrophotometer with 785 nm laser diode. Extended X-ray may explain the presence of a weak band at 269 cm-associated absorption fine structure(EXAFS)measurements were performed with the presence of homopolar bonds( S3 Ga-GaS3)[23]. In the case on Dy+: Gas Gez0Sb10S65 glasses. EXAFS of powdered glass and of selenide glasses, structural entities are similar according to raman several crystalline compounds, ie Dy2 S3. Ga2S3, Ba Ga2S4 CaGa2S4, spectroscopy although the homopolar (Se-Se and Ge-Ge)bonds Sb2S3, GeS2 were recorded at K-edge of Ge, Ga, Sb and Lur-edge of proportion and the presence of Td linked by edges appear to be Dy-at SAMBA beamline at SoLEIL The EXAFS measurements were greater [10. The EXAFS studies on pure and re doped selenide recorded in transmission or fluorescence modes at room tempera- glasses will be carried out to support this hypothesis. The [GaS(Se)a ture except for Ga K-edge and Dy Liur-edge for which a liquid N2 Td in stoichiometric glass are proposed to be distributed in the glass cryostat was used, respectively. Raw intensities were converted network to enable a good dispersion of REs forming bridges Ga-S into xk) curves by the viper program [14. Coordination numbers, (Se-Re to offset the partial negative charge of Gas(Se)4 Td [24, 25 bond lengths and Debye-Waller factors were obtained by fitting to the extent that the ga cone n is greater at least by an order the xk) curves with the Viper program. Backscattering amplitudes of magnitude with respect to RE ions. and phases needed to calculate the model curves were obtained by In sulfide glasses, our EXAFS results suggest that Dy+ is the Feff program [15 surrounded by 7.9+0.5 sulfur atoms for a concentration of 1 wt of Dy+( Fig. 1)whereas for a lower concentration(0.1 wt%) more representative for the doping of optical fibers, the coordina 3. Results and discussion tion number appears to be lower, 6.3+0.5. It is reasonable to Rare earth custering is expected to influence fluorescen efficiency by energy transfers Even if the Re concentration is 0.04 Purged set up relatively low, the clustering effect on fluorescence can occur Unpurged set up depending on matrix and rare earth [16]. For crystalline compounds, 3 it is well known that MGa2S4(M=Ca, Sr or Ba)thiogallates 0.03 of rare earths. Gallium atoms are in the center of tetrahedra(td linked by corners, for example [17]. In the case of CaGa2S4, the rE ions can occupy a single type of sites with a coordination number of ight. In the case of chalcogenide glasses, the presence of gallium facilitates the incorporation and dispersion of RE [18, 19 and the ri ions seem to be surrounded by x7 sulfur atoms [20, 21]. The role of o the gat ion in glassy matrix based on Ges2 can probably be 80.01 pared to that played by the apt ion in silicate glass the ionic-covalent nature of bonds and crystal field around the H12H132 ons are not equivalent for these two matrices. Adding this element 000 enables to insert more rE in the glass while limiting the formation of 40004100420043004400450046004700 aggregates by the formation of RE-Al complexes [ 22]. In the case of GasGezoSb1oS65 glasses, the Raman and EXAFS studies confirm that wavelength [nm glassy network is mainly composed of [Gesa and Gasa Td and Fig. 2. Fluorescence spectra of Dy+: GaGeSbS fiber(0.1 wt%) 71250661%coy Gas Gez0 Sb10 Se65 Dy L edge EXAFS data =00137A b3四≥0cooo3o Gas Ge20 Sb10 S6 00051.015202530354045505560 r闪A Wavenumber [cm-1 Fig. 1.(a) Radial distribution function curves of Dy ions in Gas Gezo Sb1oSas(phase shifts not corrected) and results of the fitting of the first coordination shell around Dy+
920 nm and an OPO (pumped at 832 nm by Ti:Sapphire laser) for pumping at 1300 nm were used to excite a 20 cm long Dy3þ doped Ga–Ge–Sb–S(Se) fibers and bulk glasses. A homemade Tm:YAG laser pumped the 3 F2 level of Pr3þ:Ga–Ge–Sb–S(Se) fibers and bulk glasses absorbing around 2 mm. The mid-IR light focused on the monochromator slit with a Sapphire lens was detected by a cooled InSb detector. The structure of Ga–Ge–SbS(Se) glasses were investigated using an HR800 (Horiba–Jobin-Yvon) micro-Raman spectrophotometer with 785 nm laser diode. Extended X-ray absorption fine structure (EXAFS) measurements were performed on Dy3þ:Ga5Ge20Sb10S65 glasses. EXAFS of powdered glass and several crystalline compounds, i.e., Dy2S3, Ga2S3, BaGa2S4, CaGa2S4, Sb2S3, GeS2 were recorded at K-edge of Ge, Ga, Sb and LIII-edge of Dy-at SAMBA beamline at SOLEIL. The EXAFS measurements were recorded in transmission or fluorescence modes at room temperature except for Ga K-edge and Dy LIII-edge for which a liquid N2 cryostat was used, respectively. Raw intensities were converted into χ(k) curves by the Viper program [14]. Coordination numbers, bond lengths and Debye–Waller factors were obtained by fitting the χ(k) curves with the Viper program. Backscattering amplitudes and phases needed to calculate the model curves were obtained by the Feff program [15]. 3. Results and discussion Rare earth clustering is expected to influence fluorescence efficiency by energy transfers Even if the RE concentration is relatively low, the clustering effect on fluorescence can occur depending on matrix and rare earth [16]. For crystalline compounds, it is well known that MGa2S4 (M¼Ca, Sr or Ba) thiogallates compounds have characteristics quite favorable for the fluorescence of rare earths. Gallium atoms are in the center of tetrahedra (Td) linked by corners, for example [17]. In the case of CaGa2S4, the RE ions can occupy a single type of sites with a coordination number of eight. In the case of chalcogenide glasses, the presence of gallium facilitates the incorporation and dispersion of RE [18,19] and the RE ions seem to be surrounded by ∼7 sulfur atoms [20,21]. The role of the Ga3þ ion in glassy matrix based on GeS2 can probably be compared to that played by the Al3þ ion in silicate glasses although the ionic–covalent nature of bonds and crystal field around the RE ions are not equivalent for these two matrices. Adding this element enables to insert more RE in the glass while limiting the formation of aggregates by the formation of RE-Al complexes [22]. In the case of Ga5Ge20Sb10S65 glasses, the Raman and EXAFS studies confirm that glassy network is mainly composed of [GeS4] and [GaS4] Td and [SbS3] trigonal pyramids even if the EXAFS fit for Sb and Ga K-edge are not absolutely perfect. In detail, the Raman spectrum of the glass is dominated by a broad band from 250 to 450 cm−1 , which is formed by an overlap of several bands as the symmetric stretching modes of [GeS4/2] Td centered at 342 cm−1 , [GaS4/2] Td at 320 cm−1 or [SbS3/2] trigonal pyramids at 293 cm−1 . The band located at 433 cm−1 reveals that [GeS4/2] Td are mainly connected by their corners. The presence of a weak band at 476 cm−1 , attributed to the presence of S–S bonds, may explain the presence of a weak band at 269 cm−1 associated with the presence of homopolar bonds (S3Ga–GaS3) [23]. In the case of selenide glasses, structural entities are similar according to Raman spectroscopy although the homopolar (Se–Se and Ge–Ge) bonds proportion and the presence of Td linked by edges appear to be greater [10]. The EXAFS studies on pure and RE doped selenide glasses will be carried out to support this hypothesis. The [GaS(Se)4] Td in stoichiometric glass are proposed to be distributed in the glass network to enable a good dispersion of REs forming bridges Ga–S (Se)–RE to offset the partial negative charge of [GaS(Se)4] Td [24,25] to the extent that the Ga concentration is greater at least by an order of magnitude with respect to RE ions. In sulfide glasses, our EXAFS results suggest that Dy3þ is surrounded by 7.970.5 sulfur atoms for a concentration of 1 wt % of Dy3þ (Fig. 1) whereas for a lower concentration (0.1 wt%), more representative for the doping of optical fibers, the coordination number appears to be lower, 6.370.5. It is reasonable to 125 150 175 200 225 250 275 300 325 200 250 300 350 400 450 500 Ga5 Ge20 Sb10 Se65 Ga5 Ge20 Sb10 S65 Reduced Intensity [a.u.] Wavenumber [cm-1] Fig. 1. (a) Radial distribution function curves of Dy3þ ions in Ga5Ge20Sb10S65 (phase shifts not corrected) and results of the fitting of the first coordination shell around Dy3þ ions and (b) Raman spectra of Ga5Ge20Sb10S(Se)65 glasses. Fig. 2. Fluorescence spectra of Dy3þ:GaGeSbS fiber (0.1 wt%). 22 F. Charpentier et al. / Materials Letters 101 (2013) 21–24
F Charpentier et aL/ Materials Letters 101(2013)21-24 a GeGaSbS: Pr"(500ppm) 0.006 GaSbse: Pr"(500ppm 0.004 co0≌Eu 0001 0.000 60080010001200140016001800200022002400 ength [nm Fig. 3. Pr+ doped Ga-Ge-Sb-S(Se)glasses absorption coefficient spectra. assume that the site around Dysprosium changes with increasing concentration without affecting the bond lengths, at least less than GeGaSbS: Pr l% of the latter. Without formation of re doped crystallites which GeGasbSe: Pr SeH would inevitably lead to a fluorescence spectrum structuring, the co Dyt coordination change tends to reveal environment variation hich may traduce dipole-dipole distance shortening for high Dy concentrations For Dysprosium doped Gas GezoSb1oS(Se)65 fibers and glasses, 0,003 after optical pumping in the H1/ or an upper level, the mid-IR mission at 4.35 um with a FWHM of about 300 nm comes from 0002 the transition between H12 and H13/ levels of Dy+( Fig. 2) Several absorption bands could be used to pump Dy+ ion, but the absorption at 920 nm(oabs -1.10-20 cm)appears to be the best choice as versatile and efficient laser diodes exist at this wave- H一HH→H 0.000 length. Therefore focused on sulfide matrix, the phonon energy 4000 4500 300-350 cm"-)provides a dominant non-radiative relaxation of wavelength [nm the upper levels, so the Hsp level relaxes non-radiatively to the emitting level. The emission cross-section, determined by Fig 4. Fluorescence spectra of Pr+ doped Ga-Ge-Sb-S(Se)fibers. the Fuchtbauer-Ladenburg method, of H1/+H132 transition (avg=4.24 um)of Dy+ doped Gas Gezo Sb1oS65, is seven times higher(Gem(H1p-H13/2)=18. 1 x 10-2 cm2 at 4.3 um, trad(Hn/ relatively easily into the network(consisting essentially of [Ges 2)=2.3 ms, B=13 %)than that of l9p-l1/ transition (Se)apl tetrahedra and [Sbs(Se )3/2] trigonal pyramids), thanks to (aavg=4.53 um)in the sulfide glass doped with Er+[12] the presence of locally modified charge densities of [GaS(Se)a/2I For Pr+ doped Gas GezoSb1oS(Se )65, the absorption measure- tetrahedra. The rE should form stable complexes with Ga avoiding ments show three absorption bands at 1, 1.5, and 2 um corre- the cluster formation detrimental to the mid-IR emission insofar ponding to the electron transitions from the Ha ground state to as the re/ Ga ratio remains low: otherwise, structural changes low absorption cross-section of 3H4+'GA transition implies that Gas zo Sb1os6s by EXAFS analyses observed in case of Dy+ IGA, (F4. F3), and (H. F2)states, respectively( fig 3). The very seem to occur around the re as the pumping is performed at 2 um(H, F2), the closest states to emitting levels. The emission observed in mid-IR is very broad (3.5-5.5 um), composed of H6-+Hs and Hs -+Ha transitionsAcknowledgments (Figs. 3 and 4). The ratio between these two transitions depends transition. Chalcogenide glasses contain some impurities like This work was supported by ANR SEED 2012 CGSuLab SH (in Ga-Ge-Sb-S)and-Seh (in Ga-Ge-Sb-Se)that provi otion at 4.0 and 4.6 um, respectively(Fig. 4). Even if the References concentration of such entities remains lower than 100 ppm after purification process, they can easily cause non-radiative relaxation [11 Sanghera Is agarwal glass-fiber-based Mid-IR due to their quasi-resonant phonon energy for mid-IR transitions IEEE J Sel To [12, 26]. It is therefore necessary to further improve purity of 2 Schweizer T nid-infrared laser transitions in gallium glasses enabling a fluorescence signal as wide and flat as possible m sulphide glass. J Lumin [3 Park B). Seo HS, Ahn T, choi YG, Jeon DY, Chung W]. Mid-infrared (3.5-5.5 um) Conclusion bers. J Lumin2008:128:1617-22 [4 Prudenzano V. Nazabal V. Smektala F heoretical study of cascade laser in erbium-doped chalcogenide glass fibers. Gez0Sb1oS(Se)6 as studied focusing on the 4-5 um emission band. I [5 Quimby RS, Shaw LB, Sanghera JS, Aggarwal ID Modeling of cascade lasing in quaternary matrices, the re ions can be inserted nics Technol Lett 2008: 20: 123-5
assume that the site around Dysprosium changes with increasing concentration without affecting the bond lengths, at least less than 1% of the latter. Without formation of RE doped crystallites which would inevitably lead to a fluorescence spectrum structuring, the Dy3þ coordination change tends to reveal environment variation which may traduce dipole–dipole distance shortening for high Dy concentrations. For Dysprosium doped Ga5Ge20Sb10S(Se)65 fibers and glasses, after optical pumping in the 6 H11/2 or an upper level, the mid-IR emission at 4.35 mm with a FWHM of about 300 nm comes from the transition between 6 H11/2 and 6 H13/2 levels of Dy3þ (Fig. 2). Several absorption bands could be used to pump Dy3þ ion, but the absorption at 920 nm (sabs∼1.10−20 cm2 ) appears to be the best choice as versatile and efficient laser diodes exist at this wavelength. Therefore focused on sulfide matrix, the phonon energy (300–350 cm−1 ) provides a dominant non-radiative relaxation of the upper levels, so the 6 H5/2 level relaxes non-radiatively to the 6 H11/2 emitting level. The emission cross-section, determined by the Füchtbauer–Ladenburg method, of 6 H11/2-6 H13/2 transition (λavg¼4.24 mm) of Dy3þ doped Ga5Ge20Sb10S65, is seven times higher (sem( 6 H11/2-6 H13/2)¼18.1 10−21 cm2 at 4.3 mm, τrad( 6 H11/ 2)¼2.3 ms, β¼13 %) than that of 4 I9/2-4 I11/2 transition (λavg¼4.53 mm) in the sulfide glass doped with Er3þ [12]. For Pr3þ doped Ga5Ge20Sb10S(Se)65, the absorption measurements show three absorption bands at 1, 1.5, and 2 mm corresponding to the electron transitions from the 3 H4 ground state to 1 G4, (3 F4, 3 F3), and (3 H6, 3 F2) states, respectively (Fig. 3). The very low absorption cross-section of 3 H4-1 G4 transition implies that the pumping is performed at 2 mm (3 H6, 3 F2), the closest states to emitting levels. The emission observed in mid-IR is very broad (3.5–5.5 mm), composed of 3 H6-3 H5 and 3 H5-3 H4 transitions (Figs. 3 and 4). The ratio between these two transitions depends on the matrix and on the reabsorption due to the 3 H4-3 H5 transition. Chalcogenide glasses contain some impurities like – SH (in Ga–Ge–Sb–S) and –SeH (in Ga–Ge–Sb–Se) that provide absorption at 4.0 and 4.6 mm, respectively (Fig. 4). Even if the concentration of such entities remains lower than 100 ppm after purification process, they can easily cause non-radiative relaxation due to their quasi-resonant phonon energy for mid-IR transitions [12,26]. It is therefore necessary to further improve purity of glasses enabling a fluorescence signal as wide and flat as possible. 4. Conclusion The spectroscopy of Dy3þ or Pr3þdoped Ga5Ge20Sb10S(Se)65 glasses was studied focusing on the 4–5 mm emission band. In prepared quaternary matrices, the RE ions can be inserted relatively easily into the network (consisting essentially of [GeS (Se)4/2] tetrahedra and [SbS(Se)3/2] trigonal pyramids), thanks to the presence of locally modified charge densities of [GaS(Se)4/2] − tetrahedra. The RE should form stable complexes with Ga avoiding the cluster formation detrimental to the mid-IR emission insofar as the RE/Ga ratio remains low; otherwise, structural changes seem to occur around the RE as observed in case of Dy3þ: Ga5Ge20Sb10S65 by EXAFS analyses. Acknowledgments This work was supported by ANR SEED 2012 CGSμLab. References [1] Sanghera JS, Shaw LB, Aggarwal ID. Chalcogenide glass-fiber-based Mid-IR sources and applications. IEEE J Sel Top Quantum Electron 2009;15:114–9. [2] Schweizer T, Hewak DW, Samson BN, Payne DN. Spectroscopy of potential mid-infrared laser transitions in gallium lanthanum sulphide glass. J Lumin 1997;72-4:419–21. [3] Park BJ, Seo HS, Ahn JT, Choi YG, Jeon DY, Chung WJ. Mid-infrared (3.5-5.5 μm) spectroscopic properties of Pr3þ-doped Ge–Ga–Sb–Se glasses and optical fibers. J Lumin 2008;128:1617–22. [4] Prudenzano F, Mescia L, Allegretti L, Moizan V, Nazabal V, Smektala F. Theoretical study of cascade laser in erbium-doped chalcogenide glass fibers. Opt Mater 2010;33:241–5. [5] Quimby RS, Shaw LB, Sanghera JS, Aggarwal ID. Modeling of cascade lasing in Dy:chalcogenide glass fiber laser with efficient output at 4.5 μm. IEEE Photonics Technol Lett 2008;20:123–5. Fig. 3. Pr3þ doped Ga–Ge–Sb–S(Se) glasses absorption coefficient spectra. Fig. 4. Fluorescence spectra of Pr3þ doped Ga–Ge–Sb–S(Se) fibers. F. Charpentier et al. / Materials Letters 101 (2013) 21–24 23
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