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Nanostructured glass-ceramics as active media for eye-safe lasers Yordanova A.S. 1 Kosseva I.I 1 , Nikolov V.S. 1 , Iordanova R.S 1 , Milanova M.K 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bld. 11, 1113 Sofia, Bulgaria e-mail: [email protected] Acknowledgements The authors thank the project “Supporting the development of PhD students, post graduate students and young scientists” ( contract no. BG051PO001-3.3.06 – 0050) for the financial support. Contemporary status The creating of femtosecond laser requires a laser media with active ion, emitting in wide range (at least 100nm). The only active ion in this region (1.1-1.6 μm) is Cr 4+ having tetrahedral sites. All of the rare earth elements have emissions in narrow band, not exceeding 50nm (Yb 3+ ). Ions of Cr 3+ and Ti 3+ emit in 0.7-1.0μm region. Transition element`s emission requires very low temperature for stimulated radiation. Despite of numerous study for finding suitable media for laser emission in this range, the only operating are chromium doped crystals of Mg 2 SiO 4 (forsterite) and Y 3 Al 5 O 12 (yttrium aluminum garnet). However, using these media continues to be associated with several problems. Both materials have very high melting temperatures - for Mg 2 SiO 4 is 1890 o C and for Y 3 Al 5 O 12 is 1950 o C, which make single crystal growing too expensive. The existence of octahedral sites in both structures leads to exceeding concentration of Cr 3+ to concentration of Cr4+( which is in tetrahedral sites). The ratio of Cr 3+ /Cr 4+ is very sensitive to the preparation conditions of monocrystals. This fact leads to problems in reproducibly of laser`s characteristics. In addition, Mg 2 SiO 4 is characterized by sharply decreasing efficiency, with increasing temperature, so additional measurements are necessary to ensure efficient cooling of the laser medium. Actuality Compared to the other femtosecond lasers, these possessing emissions in area 1.1-1.6 μm have two very important advantages: ¾This emission area is safe as possible during an impact on human tissues(eye-safe lasers) ¾The optimum wave length for telecommunication is 1.3μm Research strategy Obtaining glass-ceramics from four structurally different compositions: -group 1- forsterite compounds with space group Pbmn, general formula Ме 2 МеО 4 , where Ме 2+ is Mg 2+ or Ca 2+ , and Me 4+ - Si 4+ or Ge 4+ . The coordination of Me 2+ in these compounds is octahedral, and of Me 4+ is tetrahedral. -group 2- compounds having hexagonal structure, space group R3, general formula LiMe 3+ Me 4+ O 4 , where Ме 3+ is Al 3+ or Ga 3+ , and Ме 4+ - Si 4+ or Ge 4+ . The coordination of all compounds is tetrahedral. -group 3-compounds having monoclinic structure, space group P21/n, general formula Li 2 Me 2+ Me 4+ O 4 , where Me 2+ is Mg 2+ or Zn 2+ , and Me 4+ - Si 4+ or Ge 4+ . These compounds have four different deformed tetrahedrons, one of which is Me 4+ . -group 4- compounds having monoclinic structure, space group P21/m, general formula Li 4 Me 4+ O 4 , where Me 4+ is Si 4+ or Ge 4+ . These compounds have seven different deformed and sized tetrahedrons. Two chemical compositions were used, having excess of SiO 2 , comparing to the Li 2 MgSiO 4 compound(N1,N2). The same compositions were doped with 0.2w% Cr 2 O 3 (N3,N4). The initial chemicals were subjected for thermal treatment in platinum crucible at 1450 o C. The obtained glass was quickly quenched to room temperature, between two metal plates. Received glass was additionally subjected to thermal treatment at 800 o C for 2 hours. The obtained glass-ceramics plates are shown at figure 1. Fig.1 Photographs of the obtained glass-ceramic plates from the initial compositions N1,N2,N3,N4 Fig 2. X-ray diffraction patterns of pure(N1,N2) and chromium doped glasses(N3,N4) Sample number Initial glass compounds (w%) SiO 2 Li 2 O MgO Cr 2 O 3 N1 60 22 18 - N2 55 21 24 - N3 55 21 24 0,2 N4 60 22 18 0,2 XRD powder diffraction of the as glass-ceramics are shown at figure 2. As can be seen the crystalized phases into the glass are Li 2 MgSiO 4 (main phase) and additional phases: MgO, Li 2 SiO 3 , SiO 2 for glass N1, SiO 2 for N2 and Li 2 SiO 3 , SiO 2 for N4. Into the glass N3, only crystallized phase is Li 2 MgSiO 4 . The optical absorption spectra of the glass – ceramics N3 were carried out with the use of VARIAN CARY-5E-UV-VIS-NIR-500 Scan spectrophotometer. Fig.3 Absorption spectra of the glass-ceramics N3 - Li 2 MgSiO 4 doped with Cr 4+ . The received absorption spectra shows absorption peaks specific for Cr 4+ . Conclusion The first experiments show that the obtaining of glass-ceramic from Cr 4+ doped silicates is possible and this is one alternative for obtaining media for femtosecond lasers emitting in wide range (region 1.1-1.6 μm). Grounds for program of research 1. The actuality of femtosecond lasers emitting in wide range (region 1.1-1.6 μm) and their increasing application in different areas, such as medicine, ecology, instrumentations. 2. Existing problems in obtaining of monocrystals from chromium doped Mg 2 SiO 4 and Y 3 Al 5 O 12 lasers . 3. Existing disadvantages concerning optical and laser characteristic of Mg 2 SiO 4 и Y 3 Al 5 O 12 and their strong characteristic`s sensitivity from obtaining conditions. 4. The opportunity these monocrystals to be replaced by glass-ceramic, emitting in the same area. The obtaining conditions of these materials would be technologically simplified and having well defined characteristics. 5. The opportunity by varying the compositions of the crystalizing phase, its amount in glass, shape, size distribution and homogeneity to optimize the laser characteristics. First results For the obtaining of glasses were used initial chemicals: SiO 2 , Li 2 CO 3 , MgO and Cr 2 O 3 , having purity over 99%. The chemical composition of glasses is shown in table 1.
