Intense ultraviolet upconversion luminescence from Tm 3 + Yb 3 + : - Y F 3nanocrystals embedded glass ceramicDaqin Chen, Yuansheng Wang, Yunlong Yu, and Ping Huang Citation: Applied Physics Letters 91, 051920 (2007); doi: 10.1063/1.2767988 View online: http://dx.doi.org/10.1063/1.2767988 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/5?ver=pdfcov Published by the AIP Publishing

This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.3.157.48 On: Sat, 30 Nov 2013 18:03:57

Intense ultraviolet upconversion luminescence from Tm3+/Yb3+:�-YF3nanocrystals embedded glass ceramic

Daqin Chen, Yuansheng Wang,a� Yunlong Yu, and Ping HuangState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter,Chinese Academy of Sciences, Fuzhou, Fujian 350002, People’s Republic of China

�Received 7 June 2007; accepted 9 July 2007; published online 2 August 2007�

The infrared to ultraviolet upconversion emissions of Tm3+ 1I6→ 3F4 �346 nm� and 1D2→ 3H6�362 nm� transitions, originating from the five- and four-photon upconversion processes,respectively, were observed in the Tm3+/Yb3+ codoped precursor glass and glass ceramic containing�-YF3 nanocrystals. The ultraviolet luminescence of the glass ceramic is 30 times stronger than thatof the precursor glass, which could be attributed to the decreased probability of the 3F2→ 3F4transition and the increased cross relaxation of 3F2+ 3H4→ 3H6+ 1D2 resulted from the partition ofrare earth ions into nanocrystals. © 2007 American Institute of Physics. �DOI: 10.1063/1.2767988�

In recent years, short-wavelength solid-state lasers in theultraviolet �UV� to green spectral range have attracted muchattention due to their wide applications including high-density optical data storage, color display, infrared sensor,and so on. Up to now, visible light emissions by frequencyupconversion �UC� processes with the help of intrinsic en-ergy level matching of certain rare-earth �RE� ions have beenextensively investigated.1–4 However, efficient infrared toUV UC emissions at room temperature were rarelyreported.5,6

Transparent oxyfluoride glass ceramics have been inten-sively investigated as hosts for the visible upconversionluminescence.7–19 These nanocomposites may combine thefavorable properties from both fluoride crystal and oxideglass matrix, i.e., low phonon energy and high mechanical,chemical stabilities. The nanostructured transparent glass ce-ramic is achieved by the controlled crystallization of the pre-cursor glass with appropriate chemical composition, and thekey factor for the efficient luminescence is the partition ofthe optically active ions into the precipitated fluoride nano-crystals. Indeed, there have been many investigations on theoptical performances of the RE-doped glass ceramics con-taining PbxCd1−xF2 nanocrystals.7–14 Since lead and cadmiumare toxic elements and could not be used extensively fromthe view of environmental protection, the glass ceramicscontaining ReF3 �Re=La,Y� or MF2 �M=Ca,Sr,Ba� nano-crystals were then developed subsequently.15–19 Recently, ef-ficient UV emission in the glass ceramic containingTb3+/Yb3+:CaF2 nanocrystals was first achieved through thethree-photon upconversion processes with the help of coop-eration energy transfer from Yb3+ to Tb3+.20 In this letter, wereport a Tm3+/Yb3+:�-YF3 nanocrystals embedded glass ce-ramic which yields intense UV UC luminescence through thefour—or five-photon processes.

The material was prepared with the followingcomposition �in mol %�: 43.4SiO2–28Al2O3–17NaF–11YF3−xTmF3−yYbF3 �x, y=0.1, 0.2, 0.4, and 0.5�. Thechemicals were mixed thoroughly and melted in a covered Ptcrucible at 1400 °C for 30 min in the ambient atmosphere.The melt was poured into a 300 °C preheated copper mold

and cooled down naturally to room temperature to form theprecursor glass, which was then cut into 3 mm2 coupons andheat treated to 670 °C with a heating rate of 10 K/min, thenhold for 2 h to form glass ceramic through crystallization. Toidentify the crystallization phase and determine the meansize of crystallites, x-ray diffraction �XRD� analysis was car-ried out with a powder diffractometer �DMAX2500 Rigaku�using Cu K� radiation ��=0.154 nm�. The microstructuresof the samples were studied using a transmission electronmicroscope �TEM� �JEM-2010�. The upconversion emissionspectra at room temperature were detected using theHamamatsu R943-02 photomultiplier tube and the Spex1000M monochromator under 976 nm Ti: sapphire laser ex-citation.

The XRD patterns of the samples are presented in Fig.1�a�. The precursor glass is amorphous with no any sharppeaks. After crystallization, the XRD pattern shows intensediffraction peaks assigned to the orthorhombic �-YF3crystals.21 The mean size of the crystals was calculated to beabout 20 nm by the Scherrer formula. The volume fraction ofthe crystalline phase estimated by the ratio of the integratedarea of the peaks and the total XRD pattern is about 32%.The TEM image of the glass ceramic �Fig. 1�b�� demon-strates that nanoparticles sized 18–23 nm are distributed ho-mogeneously among the glass matrix with their selected areaelectron diffraction �SAED� rings well indexed to the ortho-rhombic �-YF3.

Figure 2 shows the room temperature upconversionemission spectra of the precursor glass and glass ceramicwith the pump power of 200 mW. Both spectra exhibit the

a�Author to whom correspondence should be addressed; electronic mail:yswang@fjirsm.ac.cn

FIG. 1. �Color online� �a� XRD patterns of the precursor glass and the glassceramic; �b� TEM image and the SAED pattern of the glass ceramic.

APPLIED PHYSICS LETTERS 91, 051920 �2007�

0003-6951/2007/91�5�/051920/3/$23.00 © 2007 American Institute of Physics91, 051920-1 This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.3.157.48 On: Sat, 30 Nov 2013 18:03:57

ultraviolet and blue emission bands of Tm3+ originated fromthe following four transitions: 1I6→ 3F4��346 nm�, 1D2→ 3H6��362 nm�, 1D2→ 3F4��451 nm�, and 1G4→ 3H6��478 nm�. In comparison with those of the precursor glass,the UV and blue emissions of the glass ceramic are greatlyenhanced �to about 30 times for UV emissions�. It is obviousthat, for the glass ceramic, the ultraviolet luminescence isstrong because the blue one with an equivalent intensity canbe clearly seen by the naked eyes, as shown by the left insetof Fig. 2. The influence of different Yb3+ and Tm3+ concen-trations on the ultraviolet upconversion luminescence is ex-hibited by the right inset of Fig. 2. For the codoped glassceramics with a fixed Tm3+ content of 0.1 mol %, the upcon-version emissions at 346 and 362 nm get stronger with in-creasing Yb3+ content from 0.1 to 0.5 mol %; while for thesamples with a fixed Yb3+ content of 0.5 mol %, both theultraviolet emissions initially strengthen with Tm3+ contentincreasing from 0.1 to 0.2 mol %, and they turn to be weak-ened obviously when Tm3+ exceeds 0.2 mol % due to theconcentration quenching.

The UC emission intensity �Iup� depends on the excita-tion power �P� following to the relationship of Iup�Pn, wheren is the number of the pumping photons required to exciteRE ions from the ground state to the emitting excited state.22

The power dependence of the aforementioned four emissiontransitions is presented in Fig. 3 by a log-log plot. Remark-ably, the data could be divided into two groups for all thefour transitions. When P�110 mW, the slopes of the linearfittings are 4.54 for 1I6→ 3F4, 3.79 for 1D2→ 3H6, 3.83 for1D2→ 3F4, and 2.64 for 1G4→ 3H6, indicating that five, four,and three pumping photons are required to populate the 1I6,1D2, and 1G4 emitting levels, respectively. While when P�110 mW, intensity saturation occurs and the correspond-ing slopes of the linear fittings reduce to 1.97, 1.43, 1.58, and1.15 respectively, attributing to the competition between thelinear decay and the upconversion processes for the deple-tion of the intermediate excited states.22–24

Figure 4 shows the energy level diagrams of Tm3+ andYb3+ ions, as well as the proposed mechanisms to explain theUV luminescence. The pump photons of 976 nm laser onlyexcite Yb3+ ions, then three successive energy transfers �ETs�from Yb3+ to Tm3+ populate the 3H5, 3F2, and 1G4 levels of

Tm3+.25 Since the 1D2 level of Tm3+ cannot be populated bythe fourth photon from Yb3+ via ET to the 1G4 due to thelarge energy mismatch �about 3500 cm−1� between them, thecross relaxation process of 3F2+ 3H4→ 3H6+ 1D2 betweenTm3+ ions may alternatively play an important role in popu-lating 1D2 level24–26 from which the UV band at 362 nmarises. After the 1D2 level was populated, the 3P2 level canbe excited by another ET process from Yb3+ to Tm3+, andthen it nonradiatively decays to the 1I6 level from which theother UV band at 346 nm occurs.

To analyze the variation of the optical properties of thematerial after crystallization, the Judd-Ofelt analysis27,28 wascarried out. Generally speaking, the intensity parameter �2 issensitive to the environmental configuration symmetry of theRE ions, and it decreases with the host changing from oxidesto fluorides.29–32 The results presented in Table I show that�2 in the glass ceramic is much smaller than that in theprecursor glass, indicating that some Tm3+ and Yb3+ ions areincorporated into YF3 nanocrystals. The partition of RE ionsin the precipitated fluoride nanophase has also been observedin other oxyfluoride glass ceramics.7–19 With the help of theJudd-Ofelt parameters, the radiative transition probability be-

FIG. 2. �Color online� Upconversion emission spectra of �a� the glass ce-ramic and �b� the precursor glass; the inset of �a� shows the photograph ofthe glass ceramic with blue luminescence �left� and the variation of theultraviolet upconversion luminescence on the Yb3+ or Tm3+ concentration�right�.

FIG. 3. �Color online� Log-log plot of the upconversion emission intensityversus the pumping power.

FIG. 4. �Color online� Schematic diagram illustrating the mechanisms ofYb3+-sensitized Tm3+ upconversion luminescence in the glass ceramic.

051920-2 Chen et al. Appl. Phys. Lett. 91, 051920 �2007�

This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.3.157.48 On: Sat, 30 Nov 2013 18:03:57

tween states J and J� can be expressed as26,27

AJJ� =64�4e2

3h�2J + 1��3

n�n2 + 2�2

9 �i=2,4,6

�iUJJ�i , �1�

where UJJ�i is the reduced matrix element of the rank i unit

tensor between J and J�. The reduced matrix elements ofTm3+ for J= 1D2 and J�= 3F4 and 3H6 are listed in Table I.For Tm3+ ions, �362+�451�1, where �362 and �451 are thebranching ratios of the 1D2→ 3H6 and 1D2→ 3F4 transitions,respectively.33 Since UJJ�

2 of the 1D2→ 3F4 transition is large,the luminescence intensity ratio R= I451 nm/ �I362 nm+ I451 nm���451 will be largely influenced by the change of the envi-ronment configuration. From the result shown in Fig. 2, it isdetermined that R=0.56 for the precursor glass and R=0.36for the glass ceramic. The reducing of R verifies the decreas-ing of �2 after crystallization.

Based on the reduced matrix elements of the Tm3+: 3F2level and Eq. �1�, the radiative probability of the 3F2→ 3F4transition is precluded by the decrease of �2 �the branchingratio of the 3F2→ 3F4 transition is about 50% in the glassceramic�, which helps to populate the 1D2 level by the en-hanced cross relaxation of 3F2+ 3H4→ 3H6+ 1D2. An increaseof the 1D2 population makes the energy transfer ofYb3+: 2F5/2+Tm3+: 1D2→Yb3+: 2F7/2+Tm3+: 3P2 efficient,and thus enhances the 1D2→ 3H6 and 1I6→ 3F4 UC emis-sions.

In summary, a transparent oxyfluoride glass ceramiccontaining Tm3+/Yb3+:YF3 nanocrystals was developed.The ultraviolet upconversion fluorescence increases by abouta factor of 30 for the glass ceramic compared to its precursorglass. The Judd-Ofelt parameter �2 is obviously decreasedafter crystallization of YF3 nanocrystals due to the incorpo-ration of rare earth ions into them, which helps to reduce theprobability of the 3F2→ 3F4 transition and enhance the crossrelaxation of 3F2+ 3H4→ 3H6+ 1D2. The intense upconver-

sion emissions of the glass ceramic make it a good candidateapplicable as the ultraviolet and visible solid-state lasers orthe fiber lasers.

This work was supported by the National Natural Sci-ence Foundation of China �Grant No. 50672098� and theFund of National Engineering Research Center for Optoelec-tronic Crystalline Materials �2005DC105003�.

1F. Auzel, Chem. Rev. �Washington, D.C.� 104, 139 �2004�.2E. Heumann, S. Bär, K. Rademaker, G. Huber, S. Butterworth, A.Diening, and W. Seelert, Appl. Phys. Lett. 88, 061108 �2006�.

3F. Wetrone, J. Boyer, J. A. Capobianco, A. Speghini, and M. Bettinelli, J.Appl. Phys. 96, 661 �2004�.

4A. Patra, S. Saha, M. A. R. C. Alencar, N. Rakov, and G. S. Maciel,Chem. Phys. Lett. 407, 477 �2005�.

5F. Pandozzi, F. Vetrone, J. C. Boyer, R. Naccache, J. A. Capobianco, A.Speghini, and M. Bettinelli, J. Phys. Chem. B 109, 17400 �2005�.

6G. Y. Chen, G. Somesfalean, Z. G. Zhang, Q. Sun, and F. P. Wang, Opt.Lett. 32, 87 �2007�.

7Y. Wang and J. Ohwaki, Appl. Phys. Lett. 63, 3268 �1993�.8M. Takahashi, M. Izuki, R. Kano, and Y. Kawamoto, J. Appl. Phys. 83,3920 �1998�.

9R. S. Quimby, P. A. Tick, N. F. Borrelli, and L. K. Cornelius, J. Appl.Phys. 83, 1649 �1998�.

10H. Hayashi, S. Tanabe, and T. Hanada, J. Appl. Phys. 89, 1041 �2001�.11B. N. Samson, P. A. Tick, and N. F. Borrelli, Opt. Lett. 26, 145 �2001�.12M. Abril, J. Méndez-Ramos, I. R. Martín, U. R. Rodríguez-Mendoza, V.

Lavín, A. Delgado-Torres, V. D. Rodríguez, P. Múñez, and A. D. Lozano-Gorrín, J. Appl. Phys. 95, 5271 �2004�.

13K. Driesen, V. K. Tikhomirov, G. Görller-Walrand, V. D. Rodríguez, andA. B. Seddon, Appl. Phys. Lett. 88, 073111 �2006�.

14M. J. Dejneka, J. Non-Cryst. Solids 239, 149 �1998�.15S. Tanabe, H. Hayashi, T. Hanada, and N. Onodera, Opt. Mater. �Amster-

dam, Neth.� 19, 343 �2002�.16D. Chen, Y. Wang, F. Bao, and Y. Yu, J. Appl. Phys. 101, 113511 �2007�.17D. Chen, Y. Wang, Y. Yu, and Z. Hu, Mater. Sci. Eng., B 123, 1 �2005�.18D. Chen, Y. Wang, Y. Yu, E. Ma, and L. Zhou, J. Solid State Chem. 179,

532 �2006�.19X. S. Qiao, X. P. Fan, J. Wang, and M. Q. Wang, J. Appl. Phys. 99,

074302 �2006�.20L. Huang, T. Yamashita, R. Jose, Y. Arai, T. Suzuki, and Y. Ohishi, Appl.

Phys. Lett. 90, 131116 �2007�.21JCPDS Card No. 74-0911.22M. Pollnau, D. R. Gamelin, S. R. Lüthi, H. U. Güdel, and M. P. Hehlen,

Phys. Rev. B 61, 3337 �2000�.23X. Qin, T. Yokomori, and Y. Ju, Appl. Phys. Lett. 90, 073104 �2007�.24G. Qin, W. Qin, C. Wu, S. Huang, D. Zhao, J. Zhang, and S. Lu, Opt.

Commun. 242, 215 �2004�.25R. J. Thrash and L. F. Johnson, J. Opt. Soc. Am. B 11, 881 �1994�.26G. De, W. Qin, J. Zhang, J. Zhang, Y. Wang, C. Cao, and Y. Cui, J. Lumin.

122-123, 128 �2007�.27B. R. Judd, Phys. Rev. 127, 750 �1962�.28G. S. Ofelt, J. Chem. Phys. 37, 511 �1962�.29E. W. J. L. Oomen and A. M. A. van Dongen, J. Non-Cryst. Solids 111,

205 �1989�.30X. Zou and T. Izumitani, J. Non-Cryst. Solids 162, 68 �1993�.31Y. Nageno, H. Takebe, K. Morinaga, and T. Izumitani, J. Non-Cryst.

Solids 169, 288 �1994�.32M. Bettinelli, A. Speghini, M. Ferrari, and M. Montagna, J. Non-Cryst.

Solids 201, 211 �1996�.33S. Tanabe and T. Hanada, J. Appl. Phys. 76, 3730 �1994�.

TABLE I. Reduced matrix elements of the specific Tm3+ transitions and thecalculated Judd-Ofelt parameters of Tm3+ in the precursor glass and glassceramic.

Transitions of Tm3+ U2 U4 U6

1D2→ 3F4 0.5750 0.0961 0.22601D2→ 3H6 0 0.3156 0.09283F2→ 3H5 0 0.2915 0.58763F2→ 3F4 0.2995 0.0580 0.04303F2→ 3H6 0 0 0.2580

Judd-Ofelt parameters�10−20 cm−1� Glass Glass ceramic

�2 3.4188 1.8803�4 3.0692 3.8343�6 0.8955 0.2778

051920-3 Chen et al. Appl. Phys. Lett. 91, 051920 �2007�

This article is copyrighted as indicated in the abstract. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

152.3.157.48 On: Sat, 30 Nov 2013 18:03:57