Study on the effect of heat-annealing and irradiation on spectroscopic properties of Bi:α- BaB 2 O 4 single crystal

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1 Study on the effect of heat-annealing and irradiation on spectroscopic properties of Bi:α- BaB 2 O 4 single crystal Jun Xu, 1,* Hengyu Zhao, 1,2 Liangbi Su, 1 Jun Yu, 1,3 Peng Zhou, 1,2 Huili Tang, 1 Lihe Zheng, 1 and Hongjun Li 1 1. Key Laboratory of Transparent and Opto-functional Inorganic Materials, Chinese Academy of Sciences, Shanghai Institute of Ceramics, Shanghai , China 2. School of Materials Science and Engineering, Shanghai University, Shanghai , China 3. Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang Province, , China *xujun@mail.shcnc.ac.cn Abstract: The absorption, excitation, and ultrabroadband near-infrared luminescence spectra of Bismuth were investigated in H 2 -annealed and γ- irradiated Bi:α-BaB 2 O 4 (α-bbo) single crystals, respectively. Energy-level diagrams of the near-infrared luminescent centers were fixed. The electronic transition energies of near-infrared active centers are basically consistent with the multiplets of free Bi + ions. The minor difference of the energy-level diagrams of Bi + ions in H 2 -annealed and γ-irradiated Bi:α- BaB 2 O 4 crystals can be ascribed to the difference of the local lattice environments. The involved physical and chemical processes were discussed. The effect of Ar-, air-annealing and electron-irradiation on Bi:α- BaB 2 O 4 crystal were also investigated Optical Society of America OCIS codes: ( ) Fluorescent and luminescent materials; ( ) Spectroscopy, fluorescence and luminescence. References and links 1. A. A. Kaminskii, Modern developments in the physics of crystalline laser materials, Phys. Status Solidi 200(2), (2003) (a). 2. L. F. Mollenauer, N. D. Vieira, and L. Szeto, Optical properties of the Tl 0 (1) center in KCl, Phys. Rev. B 27(9), (1983). 3. M. Fockele, F. Lohse, J.-M. Spaeth, and R. H. Barturam, Identification and optical properties of axial lead centres in alkaline-earth fluorides, J. Phys. Condens. Matter 1(1), (1989). 4. L. F. Mollenauer, N. D. Vieira, and L. Szeto, Mode locking by synchronous pumping using a gain medium with microsecond decay times, Opt. Lett. 7(9), (1982). 5. U. Keller, Recent developments in compact ultrafast lasers, Nature 424(6950), (2003). 6. T. Udem, R. Holzwarth, and T. W. Hänsch, Optical frequency metrology, Nature 416(6877), (2002). 7. G. G. Paulus, F. Grasbon, H. Walther, P. Villoresi, M. Nisoli, S. Stagira, E. Priori, and S. De Silvestri, Absolute-phase phenomena in photoionization with few-cycle laser pulses, Nature 414(6860), (2001). 8. A. Rousse, C. Rischel, S. Fourmaux, I. Uschmann, S. Sebban, G. Grillon, Ph. Balcou, E. Förster, J. P. Geindre, P. Audebert, J. C. Gauthier, and D. Hulin, Non-thermal melting in semiconductors measured at femtosecond resolution, Nature 410(6824), (2001). 9. Y. Fujimoto, and M. Nakatsuka, Infrared luminescence from bismuth-doped silica glass, Jpn. J. Appl. Phys. 40(Part 2, No. 3B), (2001). 10. H. P. Xia, and X. J. Wang, Near infrared broadband emission from Bi 5+ -doped Al 2 O 3 GeO 2 X (X=Na 2 O, BaO, Y 2 O 3 ) glasses, Appl. Phys. Lett. 89(5), (2006). 11. J. Ren, J. Qiu, D. Chen, C. Wang, X. Jiang, and C. Zhu, Infrared luminescence properties of bismuth-doped barium silicate glasses, J. Mater. Res. 22(7), (2007). 12. X. G. Meng, J. R. Qiu, M. Y. Peng, D. P. Chen, Q. Z. Zhao, X. W. Jiang, and C. S. Zhu, Near infrared broadband emission of Bismuth-doped aluminophosphate glass, Opt. Express 13(5), (2005). 13. V. G. Truong, L. Bigot, A. Lerouge, M. Douay, and I. Razdobreev, Study of thermal stability and luminescence quenching properties of Bismuth-doped silicate glasses for fiber laser applications, Appl. Phys. Lett. 92(4), (2008). (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3385

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Deki, Superbroadband near-ir nano-optical source based on bismuth-doped high-silica nanocrystalline zeolites, Opt. Lett. 34(8), (2009). 25. J. Ruan, L. B. Su, J. R. Qiu, D. P. Chen, and J. Xu, Bi-doped BaF 2 crystal for broadband near-infrared light source, Opt. Express 17(7), (2009). 26. L. B. Su, P. Zhou, J. Yu, H. J. Li, L. H. Zheng, F. Wu, Y. Yang, Q. H. Yang, and J. Xu, Spectroscopic properties and near-infrared broadband luminescence of Bi-doped SrB 4 O 7 glasses and crystalline materials, Opt. Express 17(16), (2009). 27. L. B. Su, J. Yu, P. Zhou, H. J. Li, L. H. Zheng, Y. Yang, F. Wu, H. P. Xia, and J. Xu, Broadband near-infrared luminescence in γ-irradiated Bi-doped alpha-bab 2 O 4 single crystals, Opt. Lett. 34(16), (2009). 28. H. T. Sun, T. Hasegawa, M. Fujii, F. Shimaoka, Z. H. Bai, M. Mizuhata, S. Hayashi, and S. Deki, Significantly enhanced superbroadband near infrared emission in bismuth/aluminum doped high-silica zeolite derived nanoparticles, Opt. Express 17(8), (2009). 29. J. E. Sansonetti, and W. C. Martin, Handbook of Basic Atomic Spectroscopic Data, J. Phys. Chem. Ref. Data 34(4), (2005). 30. International Atomic Energy Agency, Absorbed dose determination in photon and electron beams, an international code of practice, 2 nd ed. Vienna: IAEA, Technical Reports Series No (1997). 1. Introduction Exploring new laser materials with novel active ions or lasing channels is one of the longterm themes in the field of lasers. Hundreds of crystalline host materials with diverse structures have been extensively developed since the birth of solid-state laser based on a synthetic ruby in However, the popular active ions are limited in transition metals and rare-earth ions [1]. Only a few main group metal ions such as Tl and Pb have been studies as laser-active ones in 1980s [2 4]. In fact, the main group metal ions doped materials own ultrabroad emission bands, crucial to the production of ultrafast lasers [5], with extensive applications such as fundamental researches on the relaxation processes and chemical reaction dynamics, etc., optical coherence tomography, and optical frequency metrology [6 8]. However, no main group metal ions doped materials have been practically used for the application of lasers until now. Recently, Bi-doped glasses and fibers have attracted great interest since 2001 [9 17], owing to its ultra-broad infrared luminescence properties in the wavelength region from 1000 nm to 1700 nm with FWHM over 200 nm, covering the whole telecommunications window. Such broad emission band is also favorable for the production of laser with ultra-short pulse width. Furthermore, the wavelength-tunable laser outputs in the region from 1150 nm to 1300 nm have been realized in Bi-doped fiber at room temperature [18,19]. However, the nature of the active center emitting in the near-infrared (NIR) is still unknown in the Bidoped glass or fibers. In the previous publications, the emission has been attributed to electronic transition of Bi 5+ [9,10], Bi 2+ [11], Bi + [12 14], color centers [15], Bi clusters [17], Bi - 2 and Bi 2-2 dimmer [20] or Bi atom in glass [21], but the origin is still controversial. (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3386

3 Obviously, it is much easier to interpret the spectroscopic properties and understanding the nature of the Bi-related active centers in crystal hosts than glasses or fibers, because of the ordered and rigid crystal lattice. Furthermore, the crystalline structure can also provide much higher quantum efficiency than glasses or fibers. Thus, study on the NIR luminescence properties of Bi-doped crystals is very attractive and promising to understand the nature of the luminescent centers and developing new all-solid-state broadly-tunable or ultrashortpulse lasers. Up to now, several works [22 28] on the NIR luminescence properties of Bidoped crystalline materials have been reported. In Ref. 22, the broadband NIR luminescence peaking at 1080 nm with the lifetime of 140 μs was observed in Bi-doped RbPb 2 Cl 5 crystal and Bi + ion was proposed to be responsible for the NIR luminescence. The NIR luminescences of Bismuth were also demonstrated in Bi-doped BaF 2 and α-bab 2 O 4 (BBO) crystals, as well as SrB 4 O 7 polycrystalline by the current authors [25 27]. All the experimental results placed weights on the assignment of Bi + as the NIR luminescent center. In the previous work [26], the three-dimensional [BO 4 ] network structure of SrB 4 O 7 crystal was proved to be too rigid to accommodate Bi +, while the quasistable phase of α- BaB 2 O 4 crystal has relatively relaxed lattice structure composed of two-dimensional [B 3 O 6 ] triangles, is appropriate for accommodating Bi + ions. In Ref [27], broadband NIR luminescence was demonstrated in γ-irradiated Bi:α-BaB 2 O 4 crystals. Under γ-irradiation, the electrons trapped in the defect centers of V Ba were released and drifted freely in the crystal lattice. Then Bi 3+ ions, as originally doped in α-bab 2 O 4 crystal, captured free electrons and turned to Bi 2+, further Bi +. The process of Bi 3+ to Bi + can be translated as: V V + 2e (1) '' Ba 3 Bi + + 2e Bi + (2) In Bi:α-BaB 2 O 4 crystal, Bi 3+ ions substituted for Ba 2+ ions in the crystal lattice. In this work, the effect of thermal treating in various atmosphere(h 2, Ar and air atmosphere) and irradiation with γ-irradiation or electron-irradiation on Bi:α-BaB 2 O 4 single crystals were studied to identify the NIR luminescent center. The spectroscopic properties of γ-irradiated and H 2 -annealed samples were investigated and the energy-level structures were fixed. 2. Experiment The Bismuth-doped α-bab 2 O 4 single crystals were grown by conventional Czochralski method in N 2 atmosphere, the same as described in Ref [27]. Heat-annealing was carried out at 700 C for 3 hours in H 2, Ar and air atmosphere. Then the samples were cooled to room temperature with a slow rate of 20 C/h. The electron-irradiation was carried out with dose of 140KGy at room temperature. The optical absorption spectra were recorded by a Jasco V-570 UV/VIS/NIR spectrophotometer. The infrared luminescence spectra were obtained with a ZOLIX SBP300 spectrofluorometer with an InGaAs detector excited with 808 nm LD. The excitation and emission spectra, and the fluorescence decay curves in both visible and infrared regions were recorded by using a FLS920 compressor attachment. The measurements were performed at room temperature. 3. Results and discussion The samples of Bi:α-BaB 2 O 4 crystal were annealed up to 800 C in air, Ar and H 2 atmosphere, respectively. Only heat-annealing in H 2 atmosphere succeeded in producing broadband NIR luminescence in Bi:α-BaB 2 O 4 crystal, as shown in Fig. 1, with center wavelength of 985 nm and FWHM of 187 nm under excitation of 808 nm LD. The decay time of the emission at 985 nm was measured to be 408 μs. For comparison, the central wavelength, FWHM, and lifetime of γ-irradiated Bi:α-BaB 2 O 4 crystal were 1140 nm, 108 nm, and 526 μs, respectively. Ba (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3387

4 Fig. 1. NIR emission spectra of H 2 -annealed and γ-irradiated Bi:α-BaB 2 O 4 crystals under excitation of 808 nm LD. Fig. 2. Absorption, excitation, and emission spectra of (a) γ-irradiated Bi:α-BaB 2 O 4 crystal and (b) H 2 -annealed Bi:α-BaB 2 O 4 crystal. The excitation source is 808 nm LD. The absorption, excitation, and NIR emission spectra of the γ-irradiated and H 2 -annealed Bi:α-BaB 2 O 4 crystals are shown in Fig. 2(a) and Fig. 2(b), respectively. The two broadband NIR emissions of the two crystals both possess three intense bands in their corresponding excitation spectra. And the counterparts of the three excitation bands can be distinguished in the corresponding absorption spectra. According to the excitation and emission spectra, the electronic energies of multiplets of the NIR luminescent centers in the γ-irradiated and H 2 - annealed Bi:α-BaB 2 O 4 crystals can be derived, as listed in Table 1, along with those of free Bi + ions. One can see that the energies of the electronic states of the NIR luminescent centers in the two crystals are basically consistent with those of multiplets of free Bi + ions [22,29], from the ground state of 3 P 0 to the third excited state 1 D 2. The minor difference among them should be contributed by the crystal fields. Higher excited states couldn t appear in the absorption or excitation spectra due to the limit of the bandgap of α-bab 2 O 4 crystal. According to the electronic energies listed in Table 1, the schematic energy-level diagrams of Bi + in H 2 -annealed and γ-irradiated Bi:α-BaB 2 O 4 crystals can be fixed, as shown in Fig. 3. The distinct differences among the electronic energies of Bi + ions in H 2 -annealed and γ-irradiated Bi:α-BaB 2 O 4 crystals should be due to the different physical and chemical mechanisms for creation of Bi + ions. Under γ-irradiation, the electrons should be easily released from the defect centers of V Ba and would freely drift in the lattice, where V Ba is the vacancy of Ba 2+ with two negative charges, resulted from charge compensating for two (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3388

5 Bi 3+ substituting two Ba 2+. Then, Bi 3+ ions could capture free electrons and turn into Bi 2+, further Bi +, expressed as V Ba V Ba + 2e and Bi e Bi +. So, the whole process of γ- irradiation is a physical one. However, under the reducing effect of H 2 -annealing, Bi + ions should be directly produced from Bi 2+ ions accompanied with the creation of O 2- vacancies (V O 2- ). So, the chemical reaction was taken place in the process of H 2 -annealing. Therefore, the different local lattice environments of Bi +, associated with V Ba or V O 2- by irradiation or annealing, respectively, resulted in the difference of the energy-level splitting of Bi +. Furthermore, Bi + ions in H 2 -annealed Bi:α-BaB 2 O 4 crystals have higher thermal stability than those of the γ-irradiated. The NIR luminescence of the latter was thoroughly bleached after annealing at about 500 C in N 2 atmosphere, as reported in Ref [25]. This indicates that the trapped electrons in Bi + ions in α-bab 2 O 4 crystal induced by γ-irradiation can be released by thermal activation, which would be recaptured by V Ba centers. However, for the H 2 - annealed crystal, V O 2- acts as the stabilizer of Bi + in the lattice of α-bab 2 O 4. Table 1. Energies of multiplets of Bi + ions in α-bab 2 O 4 crystals and free Bi + ions [29] Energy (cm 1 ) of Bi + Multiplet Free ion H 2 -annealed Bi:α- BaB 2 O 4 γ-irradiated 3 P 0 0 0, , P P D Fig. 3. Schematic energy-level diagrams of Bi + in the Bi:α-BaB 2 O 4 crystals after H 2 -annealed and γ-irradiated. To testify the explanation of Bi + center as the nature of NIR luminescence, Bi:α-BaB 2 O 4 crystal was irradiated with electron beam. Different from γ-irradiation which excites freeelectrons from the vacancies in the crystal lattice, the electron beam directly provides electrons for Bi 3+ center and reduces Bi 3+ to low-valence. According to the electron absorption rule [30], the 7 mm-thick Bi:α-BaB 2 O 4 crystal can be irradiated with 140 KGy electron beam at room temperature without any dose loss. The sample remained colorless after electron irradiated. Absorption spectra before and after electron irradiated were shown in Fig.4, the new-born 680 nm absorption band, which corresponded to the excitation band (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3389

6 at 685 nm in γ-irradiated sample, should be attributed to Bi+ ions. In Ref [26], Bi:SrB 4 O 7 (2.0% mol and 5.0% mol Bismuth-doped) glasses also exhibited a ~680 nm absorption band. Fig. 4. Absorption spectra of electron-irradiated Bi:α- BaB 2 O 4 crystal (solid line) and asgrown Bi:α-BaB 2 O 4 crystal (dash line). Fig. 5. Near-infrared emission spectra of electron-irradiated Bi:α-BaB 2 O 4 crystals under excitation of 808 nm LD The electron irradiated Bi:α-BaB 2 O 4 crystal exhibited a NIR emission with central wavelength of 1131 nm and the FWHM of 98 nm, as shown in Fig. 5. The same experiment was performed on a thinner sample with the thickness of 1mm. However, no NIR emission was observed even with irradiation dose of 280 KGy. The absence of NIR luminescence indicates that the thin sample is incapable to capture electrons which can easily pass through the crystal. 4. Concluison In conclusion, γ-irradiated, electron-irradiated and H 2 -annealing are effective methods to produce NIR luminescence in Bi:α-BaB 2 O 4 crystal. The minor difference among the energylevel diagrams of the NIR luminescent centers in γ-irradiated and H 2 -annealed samples should be ascribed to the crystal fields. γ-irradiation, electron-irradiation and H 2 -annealing produce free electrons in crystal lattice to reduce Bi 3+ ions to low-valence. The electronic transition energies of NIR active centers were basically consistent with the multiplets of free Bi + ions. (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3390

7 Acknowledgments This work was supported by the National Natural Science Foundation of China under the number of and , Shanghai National Natural Science Foundation under the number of 08ZR , and Hundred Talents Project of the Chinese Academy of Science. The authors are grateful to Xiaoming Fang and Prof. Xinnian Li for their help in performance of electron irradiation. (C) 2010 OSA 15 February 2010 / Vol. 18, No. 4 / OPTICS EXPRESS 3391