2504 OPTICS LETTERS / Vol. 34, No. 16 / August 15, 2009

Broadband near-infrared luminescence in�-irradiated Bi-doped �-BaB2O4 single crystals

Liangbi Su,1,4 Jun Yu,1,3 Peng Zhou,1 Hongjun Li,1 Lihe Zheng,1 Yan Yang,1 Feng Wu,2

Haiping Xia,3 and Jun Xu1,5

1Key Laboratory of Transparent and Opto-Functional Inorganic Materials, Chinese Academy of Sciences, ShanghaiInstitute of Ceramics, Shanghai 200050, China

2Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China3Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang Province, 315211, China

4 su_lb@163.com5 xujun@mail.shcnc.ac.cn

Received June 1, 2009; revised July 9, 2009; accepted July 16, 2009;posted July 29, 2009 (Doc. ID 112224); published August 13, 2009

Spectroscopic properties of as-grown and �-irradiated undoped and Bi-doped �-BBO �BaB2O4� single crys-tals were investigated. Bi2+ and color centers in Bi:�-BBO crystals were investigated to be nonluminescentin the near-infrared (NIR) region. Broadband NIR luminescence at 1139 nm with a FWHM of 108 nm anda decay time of 526 �s was realized in Bi:�-BBO crystal through � irradiation. Bi+ was attributed to beresponsible for the NIR emission, which can be bleached by thermal annealing. The involved physical pro-cesses in Bi:�-BBO crystal during the courses of irradiation and heat annealing were tentativelyestablished. © 2009 Optical Society of America

OCIS codes: 160.2540, 300.6280.

Laser-active solid-state materials have been exten-sively developed since the birth of laser in ruby in1960. However, popular active ions are limited intransition metals and rare-earth ions [1]. Only a fewmain group metal ions, such as Tl and Pb, were stud-ied as laser-active ones in the 1980s [2–4]. Recently,Bi-doped glasses and fibers have attracted great in-terest since 2001 [5–12], owing to their ultrabroad IRluminescence properties in the wavelength regionfrom 1000 to 1700 nm with a FWHM over 200 nm.Such a broad emission band is favorable for the pro-duction of a laser with ultrashort pulse width. More-over, the laser outputs have been realized in Bi-dopedfiber at room temperature [13,14]. However, the na-ture of the active center emitting in the near-infrared(NIR) is still unknown in the Bi-doped glass or fibers.In previous publications, the emission has been at-tributed to electronic transition of Bi5+ [5,6], Bi2+ [7],Bi+ [8,9], Bi clusters [10], or color centers [11], but theorigin is still controversial.

Obviously, it is much easier to interpret the spec-troscopic properties and understand the nature of theBi-related active centers in crystal hosts than glassesor fibers, because of the ordered and rigid crystal lat-tice. Generally, the crystalline structure can also pro-vide much higher quantum efficiency than glasses orfibers. However, few works [15–18] have been re-ported in Bi-doped crystalline materials until now. Inthis Letter, we report the spectroscopic properties ofBi-doped �-BBO �BaB2O4� single crystals with atrigonal structure. The space group and lattice pa-rameters of an �-BBO crystal are R3̄c, a=b=0.7235nm, c=3.9192 nm, �=�=90°, and �=120°. Broad-band emission peaking at about 1139 nm with aFWHM over 100 nm was observed in �-irradiatedBi:�-BBO crystal. The nature of the center emittingin NIR was proposed to be Bi+.

0146-9592/09/162504-3/$15.00 ©

The Bi-doped �-BBO single crystals were grown bytraditional Czochralski method in a N2 atmosphere.The raw materials were BaCO3 (99.9%), H3BO3(99.9%), and Bi2O3 (99.999%). The Bi concentrationin the melt was 5.0 mol.% relative to Ba. To compen-sate the volatility of B2O3 during the process of crys-tal growth, the amount of H3BO3 in the starting ma-terials should be overweight, 1.0–2.0 mol.%. Thecrystal growth procedure was the same as that de-scribed in [19]. The concentration of Bi in the �-BBOcrystal was detected by inductively coupled plasma-atomic emission spectroscopy. Undoped �-BBO andBi:�-BBO crystals were exposed to � rays from a60Co source with a dose rate of 100 Gy/h for totaldoses of 10 kGy at the room temperature. The opticalabsorption spectra were recorded by a Jasco V-570spectrophotometer. The visible and IR luminescencespectra were obtained with a Fluorolog-3 (JobinYvon, France) spectrofluorometer upon exciting witha 450 W Xe lamp and 808 nm (or 980 nm) laser diode(LD). Emission decay curves were measured with aTektronix TDS 3020 oscilloscope. The measurementswere performed at room temperature, and all theemission spectra were corrected for the setup charac-teristic.

The concentration of Bi in the Bi:�-BBO singlecrystal was checked out only 0.23 mol.%, owing to thevolatilization of Bi2O3 and low segregation coeffi-cient. There is a broad absorption band covering thewavelength region from 400 to 800 nm in the absorp-tion spectrum of Bi:�-BBO crystal, as shown in Fig.1(a). For an as-grown undoped �-BBO crystal, thereis one absorption band at 286 nm, which should beascribed to color centers of Ba2+ vacancies [20,21],that was greatly increased by Bi2O3 doping, as shownin Fig. 1(b). One vacancy of Ba2+ should be produced,

3+

accompanied by the entering of two Bi ions, in or-

2009 Optical Society of America

August 15, 2009 / Vol. 34, No. 16 / OPTICS LETTERS 2505

der to compensate the charge imbalance induced byBi3+ substituting Ba2+. The process can be expressedas 2Bi3++3Ba2+→2BiBa+VBa�, where BiBa is Bi3+

substituting the lattice site of Ba2+ and VBa� is thevacancy of Ba2+ with two negative charges. So the ab-sorption band at 286 nm in the �-BBO crystal can betentatively assigned to the lattice defects of VBa�,which will grow in number with doping of Bi2O3. Ac-cording to earlier research [22,23], the absorptionbands of Bi3+ generally lie at energies higher than400 nm, which might be masked by the strong ab-sorption band at 286 nm, while the absorption in the400–800 nm wavelength range can be ascribed toBi2+ [24,25]. To further identify the Bi3+ and Bi2+ cen-ters, the visible emission and the corresponding exci-tation spectra were measured, as shown in Fig. 2.

Fig. 1. (a) Absorption spectra of as-grown �-BBO crystal(curve 1), �-irradiated �-BBO crystal (curve 2), Bi:�-BBOcrystal (curve 3), and �-irradiated Bi:�-BBO crystal (curve4). (b) Zoom of the UV absorption spectra of as-grown�-BBO and Bi:�-BBO crystals. (c) Additional absorption(AA) spectra of undoped and Bi-doped �-BBO crystal by�-irradiation. (d) Absorption spectra of �-irradiatedBi:�-BBO crystals before (curve 1) and after N2-annealed(curve 2).

Fig. 2. (Color online) Visible luminescence and corre-

sponding excitation spectra of Bi:�-BBO crystal.

Under UV excitation, there are two main emissionbands at 426 and 590 nm, which can be assigned toBi3+ and Bi2+, respectively, as identified in [22–25].Three excitation bands at 370, 445, and 543 nm existfor the emission at 590 nm. It should be noted thatthe emission at 590 nm of Bi2+ can also be excited by370 nm of Bi3+. This indicates that the energy can betransferred from Bi3+ to Bi2+.

To study the variations of the valence states of Biions, � irradiation was carried out on the Bi:�-BBOcrystal. The significant changes of the absorptionspectrum can be observed after irradiation, as shownin Fig. 1(a), with the color of the Bi:�-BBO crystalturned to green from colorless. The AA spectrum canclearly represent the changes of the absorptionbands, as shown in Fig. 1(c), where the AA valueswere determined as ��2−�1�, where �2 and �1 are ab-sorption coefficients of Bi:�-BBO crystal after andbefore � irradiation, respectively. Two strong AAbands appeared in the UV and visible–NIR regions,respectively; one peaks at 335 nm, while the othercovers the wavelength region from 500 to 900 nm,peaking at 748 nm. The assignments for the two newbands will be discussed in the following. �-irradiationwas also carried out on as-grown undoped �-BBOcrystals. Different from Bi:�-BBO crystal, irradia-tion did not change the color of the colorless undoped�-BBO crystals. There is only one AA band peakingat 286 nm in the AA spectrum of undoped �-BBOcrystals, indicating the increase of VBa� centers withirradiation.

The NIR luminescences of the four kinds of �-BBOcrystals were measured under excitation of Xe lampand LDs of 808 and 980 nm. However, NIR emissionwas observed only in the irradiated Bi:�-BBO crys-tals, peaking at 1139 nm with an FWHM of 108 nm,accompanied with a shoulder at about 1350 nm, asshown in Fig. 3, under excitation of 808 or 980 nm.The decay curve of the emission at 1139 nm is shownin the inset of Fig. 3, which can be fitted with thefirst-order exponential mode with lifetime of 526 �s.

Fig. 3. (Color online) NIR emission spectra of the�-irradiated Bi:�-BBO crystal upon 808 nm and 980 nmLDs exciting, respectively. The inset is the decay curve of

the emission at 1139 nm.

2506 OPTICS LETTERS / Vol. 34, No. 16 / August 15, 2009

Since the broad NIR luminescence has been observedin Bi-doped RbPb2Cl5 crystals in [15] and irradiatedBi-doped �-BBO crystals in this work, the sugges-tions of Bi5+ or ��AlO4/2�−,Bi+� complexes should beignored as the mechanism of the NIR luminescentcenters. And Bi2+ and color centers have also beenruled out in this work, as discussed in the above. Sothe most probable mechanism for the NIR emissionin the � irradiated Bi:�-BBO crystal should be Bi+.Under � irradiation, the electrons can be easily re-leased from the defect centers of VBa� and wouldfreely drift in the lattice. Then Bi3+ ions could cap-ture free electrons and turn into Bi2+, further Bi+, ac-companied with the production of a new center ofVBa. Therefore the new absorption band at 335 nmcan be assigned to VBa centers, while the other at748 nm is assigned to Bi+ ions, responsible for theNIR luminescence. The involved process can be ex-pressed as VBa�→VBa+2e, Bi3++e→Bi2+, and Bi2+

+e→Bi+.To study the thermal stability of the NIR lumines-

cent centers (i.e., Bi+ ions), heat annealing in N2 at-mosphere at 500 °C was performed for 3 h on the�-irradiated Bi:�-BBO crystal. The absorption spec-trum of the �-irradiated Bi:�-BBO crystal after N2annealing was shown in Fig. 1(d). After annealing,the broad absorption band at 748 nm almost disap-peared, and the other at 335 nm remained only alittle. No NIR emission can be observed after anneal-ing under excitation of 808 or 980 nm LD. This indi-cates that the trapped electrons in Bi+ ions can be re-leased by thermal activation, which would berecaptured by VBa centers. That is to say, the Bi3+ in�-BBO crystal is a shallower electron trap relative toVBa centers. The involved processes would be takenplace in reverse to those during irradiation, whichcan be expressed as Bi+−2e→Bi3+ and VBa+2e→VBa�.

In conclusion, Bi3+, Bi2+, and color centers wereidentified in as-grown or �-irradiated undoped andBi-doped �-BBO single crystals, respectively. Bi2+

and color centers in �-BBO single crystals were dem-onstrated to be nonluminescent in NIR region.�-irradiation can produce the NIR broadband lumi-nescent centers in Bi:�-BBO crystals, which was pro-posed to be Bi+. The irradiation-induced Bi+ ions inBi:�-BBO crystal were thermally instable, owing toits shallower electron trap than VBa� centers. It canbe believed that the broadband emission at 1139 nmof �-irradiated Bi:�-BBO crystal with an FWHMmore than 100 nm and a lifetime of 520 �s should bepromising for the applications in all solid-state tun-able or ultrafast lasers with diode pumping.

The authors thank Prof. Jianrong Qiu and Dr. JianRuan for meaningful discussions. The work was sup-ported by the National Natural Science Foundationof China (NSFC) under number 60778036 andShanghai National Natural Science Foundation un-der number 08ZR1421700.

References

1. A. A. Kaminskii, Phys. Status Solidi A 200, 215 (2003).2. L. F. Mollenauer, N. D. Vieira, and L. Szeto, Phys. Rev.

B 27, 5332 (1983).3. M. Fockele, F. Lohse, J.-M. Spaeth, and R. H.

Barturam, J. Phys. Condens. Matter 1, 13 (1989).4. L. F. Mollenauer, N. D. Vieira, and L. Szeto, Opt. Lett.

7, 414 (1982).5. Y. Fujimoto and M. Nakatsuka, Jpn. J. Appl. Phys.,

Part 2 40, L279 (2001).6. H. P. Xia and X. J. Wang, Appl. Phys. Lett. 89, 051917

(2006).7. J. Ren, J. Qiu, D. Chen, C. Wang, X. Jiang, and C. Zhu,

J. Mater. Res. 22, 1954 (2007).8. X. Meng, J. Qiu, M. Peng, D. Chen, Q. Zhao, X. Jiang,

and C. Zhu, Opt. Express 13, 1628 (2005).9. V. G. Truong, L. Bigot, A. Lerouge, M. Douay, and I.

Razdobreev, Appl. Phys. Lett. 92, 041908 (2008).10. M. Peng, J. Qiu, D. Chen, X. Meng, and C. Zhu, Opt.

Lett. 30, 2433 (2005).11. M. Yu. Sharonov, A. B. Bykov, V. Petricevic, and R. R.

Alfano, Opt. Lett. 33, 2131 (2008).12. Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, and S.

Khonthon, Appl. Phys. Lett. 90, 261110 (2007).13. E. M. Dianov, V. V. Dvoyrin, V. M. Mashinsky, A. A.

Umnikov, M. V. Yashkov, and A. N. Gur’yanov,Quantum Electron. 35, 1083 (2005).

14. I. Razdobreev, L. Bigot, V. Pureur, A. Favre, G.Bouwmans, and M. Douay, Appl. Phys. Lett. 90,031103 (2007).

15. A. G. Okhrimchuk, L. N. Butvina, E. M. Dianov, N. V.Lichkova, V. N. Zagorodnev, and K. N. Boldyrev, Opt.Lett. 33, 2182 (2008).

16. V. O. Sokolov, V. G. Plotnichenko, and E. M. Dianov,Opt. Lett. 33, 1488 (2008).

17. H. T. Sun, Y. Miwa, F. Shimaoka, M. Fujii, A.Hosokawa, M. Mizuhata, S. Hayashi, and S. Deki, Opt.Lett. 34, 1219 (2009).

18. J. Ruan, L. B. Su, J. R. Qiu, D. P. Chen, and J. Xu, Opt.Express 17, 5163 (2009).

19. G. Q. Zhou, J. Xu, X. D. Chen, H. Y. Zhong, S. T. Wang,and K. Xu, J. Cryst. Growth 191, 517 (1998).

20. W. Hong, L. E. Halliburton, D. Perlov, K. T. Stevens, R.K. Route, and R. S. Feigelson, Opt. Mater. 26, 437(2004).

21. S. C. Sabharwal, J. Cryst. Growth 187, 253 (1998).22. G. Blasse, and A. Bril, J. Chem. Phys. 48, 217 (1968).23. A. Novoselov, A. Yoshikawa, M. Nikl, J. Pejchal, and T.

Fukuda, J. Cryst. Growth 292, 236 (2006).24. A. M. Srivastava, J. Lumin. 78, 239 (1998).25. G. Blasse, A. Meijerink, M. Nomes, and J. Zuidema, J.

Phys. Chem. Solids 55, 171 (1994).