Optical Materials 36 (2013) 455–457

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Optical Materials

journal homepage: www.elsevier .com/locate /optmat

Nd3+:CaF2 crystal with controlled photoluminescence spectroscopicproperties by codoping Y3+ ions

0925-3467/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.optmat.2013.10.009

⇑ Corresponding authors. Tel.: +86 21 69987573.E-mail addresses: Su_lb@163.com (L. Su), xujun@mail.shcnc.ac.cn (J. Xu).

Qingguo Wang a,b, Liangbi Su a,⇑, Fengkai Ma a, Yaoyu Zhan a,b, Dapeng Jiang a, Xiaobo Qian a,Jingya Wang a, Lihe Zheng a, Jun Xu a,⇑, Witold Ryba-Romanowski c, Piotr Solarz c, Radoslaw Lisiecki c

a Key Laboratory of Transparent and Opto-Functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, Chinab Graduate School of the Chinese Academy of Science, Beijing 100039, Chinac Institute of Low Temperature and Structure Research, Polish Academy of Sciences (ILTSR), Okolna, 50-422 Wroclaw, Poland

a r t i c l e i n f o

Article history:Received 17 June 2013Received in revised form 19 September 2013Accepted 6 October 2013Available online 30 October 2013

Keywords:Calcium fluorideFluorescence quenchingBuffer ionsLaser crystal

a b s t r a c t

High optical quality Nd,Y-codoped CaF2 crystals with diameter 3-inch were obtained by TGT method.Absorption and emission spectra were recorded at 5 K, 12 K, and 300 K respectively. Judd–Ofelt theorywas applied to obtain standard parameter Xt (t = 2, 4, 6) and the fitting result of experimental lifetimeof Nd3+ 4F3/2 ?

4I11/2 transition. Broad and flat 1.06 lm emission spectra of Nd3+ ions were obtained withY3+-codoping. The largest emission bandwidth of Nd3+ 4F3/2 ?

4I11/2 transition is up to 28 nm. The resultsrevealed that codoping Y3+ ions in Nd:CaF2 crystal can effectively modulate the spectra properties of Nd3+

ions, in addition to the reduction of fluorescence concentration-quenching.� 2013 Elsevier B.V. All rights reserved.

1. Introduction

As one of the most famous host crystals, early studies of rareearth-doped CaF2 crystals have been reported in the sixties of thepast century already. Sorokin and Stevenson reported the laseroperation in U:CaF2 crystal in 1960 [1], which is the second laserhost after Cr:Sapphire. The first LD-pumping laser operation wasalso achieved in U:CaF2 [2]. The first transparent ceramics laserwas made of Dy-doped CaF2 ceramic [3]. Therefore, the spectro-scopic properties and laser performance of Re3+-doped CaF2 at-tracted more and more attention [4–6]. Especially, diode-pumped99 fs Yb:CaF2 oscillator was achieved in 2009 [7]. In the same year,terawatt laser output was obtained from Yb:CaF2 provided by ourgroup [8]. Nd3+-doped CaF2 as a laser system has been abandonedsince a long time because of a very detrimental concentrationquenching effect which results from the clustering of the rare-earth ions and some cross-relaxation type energy transfers whichkill their emission quantum efficiency [9]. So, the buffer ions M3+

(such as Gd3+, Y3+ and Sc3+) were codoped in CaF2 to break the[Nd3+–Nd3+] clusters, and the laser operation of the Nd3+-dopedCaF2–MF3 disordered systems under Xe-lamp pumping were dis-cussed [10–13]. Recently, laser operations of Nd:SrF2 [14] and

Nd,Y:CaF2 [15] single crystals were achieved with diode andTi:spahhire laser pumping, respectively.

In this letter, Y3+ was selected as buffer ions to be added into theNd3+:CaF2 crystal to prevent formation of [Nd3+–Nd3+] clusters.Moreover, Y3+ ions can also modulate spectroscopic properties ofNd3+ ions because of the formation of new [Nd3+–Y3+] clusters.Although room temperature absorption spectra, emission spectraand J.O. parameters of 2.0 at%Nd3+, x(x = 0–6.0) at%Y3+:CaF2 crys-tals were discussed in Ref. [15], low temperature spectra andluminessence spectra at 300 K of these crystals and crystals withhigher Nd3+ concentration of 3 at% were presented and comparedin our work.

2. Experiment

The equipment of TGT for crystal growth of rare-earth-dopedCaF2 single crystals is. Nd,Y:CaF2 (Nd3+: 2.0 at%-Y3+: 0 at%, 2.0 at%,6.0 at% ; Nd3+: 3.0 at%-Y3+: 0 at%. 3.0 at% ;) crystals with high opti-cal quality were obtained from Temperature Gradient Technique(TGT) method, as described in Ref. [16]. The crystals were grownon the h111i-CaF2 seeds at a cooling rate of 1.5 �C/h from1400 �C to 1200 �C. The concentrations of Nd3+ and Y3+ in the crys-tals were measured by inductively coupled plasma atomic emis-sion spectroscopy (ICP–AES) analysis, pointing at the segregationcoefficients for Nd3+ and Y3+ ions of about 0.82 and 0.90.

Fig. 1. The low-temperature absorption and emission spectra of 2%Nd,6%Y:CaF2

crystal.

Table 1Energies of the crystal field levels of the 4I9/2, 4I11/2, 4I13/2 and 4F3/2 multiplets of Nd3+

in CaF2 crystal.

Crystals 4E of 4F3/2

(cm�1)4E of 4I9/2

(cm�1)4E of 4I11/2

(cm�1)4E of 4I13/2

(cm�1)

2%Nd:CaF2 98 935 618 4952%Nd,6%Y:CaF2 102 816 511 4943%Nd:CaF2 68 1037 636 5593%Nd,3%Y:CaF2 98 886 519 403

Table 2The standard parameters Xt (t = 2, 4, 6) and the spectroscopic quality factor (X4/X6)of the grown crystals.

Crystal X2 (10�20 cm2) X4 (10�20 cm2) X6 (10�20 cm2) X4/X6

2.0%Nd:CaF2 1.3437 2.8688 4.9040 0.582.0%Nd,2.0%Y:CaF2 1.1692 3.6507 6.3027 0.602.0%Nd,6.0%Y:CaF2 0.6967 3.2858 4.9142 0.673.0%Nd:CaF2 1.2204 2.7304 5.1657 0.533.0%Nd,3.0%Y:CaF2 0.9871 2.7908 5.1757 0.54

456 Q. Wang et al. / Optical Materials 36 (2013) 455–457

The Nd,Y:CaF2 samples with thickness of 3.0 mm were preparedand polished for spectra measurement. Low-temperature absorp-tion spectra were recorded with a Varian Model 5E UV–VIS–NIR(UV–visible-near-IR) absorption spectrophotometer. Spectralbandwidth employed was 0.1 nm in the UV–VIS and 1.0 nm inthe NIR. A sample was placed into an Oxford Model CF 1204 con-tinuous-flow liquid helium cryostat equipped with a temperaturecontroller.

3. Results and discussion

The low-temperature absorption and emission spectra under808 nm LD pumping of 2%Nd,6%Y:CaF2 crystal and 3%Nd,3%Y:CaF2

were shown in Figs. 1 and 2. Even in the low temperature of 12 K,Nd,Y:CaF2 crystals still have broadband emission spectra. In thewavelength region around 1.06 lm, Nd,Y:CaF2 crystals have moreemission peaks than Nd:CaF2, indicating that Y-codoping makesthe sites of Nd3+ more complicated. The energies of crystal fieldlevels of Nd3+ multiplets relevant for laser operation were deter-mined from low-temperature absorption and emission spectra.The data are shown in Table 1. The derived Nd3+ 4F3/2 mutipletsplitting increased with Y3+ codoping, whereas Nd3+ 4I11/2, 4I13/2

and 4F3/2 multiplets splitting decreased.

Fig. 2. The low-temperature absorption and emission spectra of 3%Nd,3%Y:CaF2

crystal.

To assess laser potential of Nd,Y:CaF2 system the standardJudd–Ofelt treatment of room-temperature absorption spectrawas performed. The three phenomenological Judd–Ofelt parame-ters Xt (t = 2, 4, 6) and the spectroscopic quality factor (X4/X6)of the grown crystals were shown in Table 2. The parameters ob-tained follow the trend: X6 > X4 > X2, which agrees with that ofother existing laser active media. It can be found that the spectro-scopic quality factor of the grown crystal increased gradually withincreasing the concentration of Y3+ ions which indicated the betteremission spectroscopic properties. As we all know, X2 is sensitiveto the asymmetry in vicinity of the rare earth ions (short-range ef-fects) [17].

Figs. 3 and 4 show the emission spectra of Nd3+ 4F3/2 ? 4I11/2

transition at room temperature. It can be found that the peak emis-sion intensity increased with Y3+ ions codoping. In addition, Y3+

ions codoping can also modulate the emission spectral structureof Nd3+ ions in CaF2 host. For Nd3+ single-doped CaF2, Nd3+ ions

Fig. 3. The room-temperature emission spectra of Nd3+ 4F3/2 ? 4I11/2 transition in2%Nd:CaF2 crystal codoped with Y3+ at different concentration.

Fig. 4. The room-temperature emission spectra of Nd3+ 4F3/2 ? 4I11/2 transition in3%Nd:CaF2 and 3%Nd,3%Y:CaF2 crystal.

Table 3Photoluminescence properties of Nd,Y:CaF2 crystals.

CaF2 Emissionwavelength(nm)

Emissionbandwidth(nm)

Radiativelifetime(ls)

Fluorescencelifetime (ls)

Peakemissioncross-section(10�20 cm2)

2.0%Nd 1065 28 642 13.2 1.92.0%Nd,2.0%Y 1054 27 470 181.2 2.52.0%Nd,6.0%Y 1054 25 566 227.1 2.43.0%Nd 1066 28 632 12.8 2.23.0%Nd,3.0%Y 1053 28 533 103.0 2.2

Q. Wang et al. / Optical Materials 36 (2013) 455–457 457

present the multi-peak emission spectroscopic characterization, at1061 nm, 1090 nm, and 1126 nm, respectively. Y3+ codoping de-pressed the emission peak at the long-wavelength range(1090 nm and 1126 nm). Then, the emission spectra were domi-nated by the band around 1.06 lm, which became more broadand flat. The largest FWHM of the peak emission wavelength at1.06 lm reached up to 26 nm which is higher than neodymium-doped phosphate glass (20 nm). It is promising to produce theultrashort pulse and tunable laser output.

Entire and detailed absorption spectra of these Nd,Y:CaF2 crys-tals were analyzed within the framework of Judd–Ofelt (J–O) for-malism. The 4F3/2 radiative emission lifetime of Nd ions in thesecrystals were calculated, as listed in Table 3, as well as measuredemission lifetimes at room temperature. Then, the emission cross

sections were calculated by using the usual Fuchtbauer–Ladenburgformula. Table 3 shows the photoluminescence properties of thefive Nd:CaF2 and Nd,Y:CaF2 crystals with different doping concen-trations. It is very clear that codoping Y3+ ions as buffer ions shar-ply increases the fluorescence lifetime of Nd ions. Moreover, theemission cross section was also enhanced by codoping Y3+, owingto the suppression of the emission bands at the long-wavelengthrange (1090 nm and 1126 nm).

4. Conclusions

Nd,Y-codoped CaF2 crystals with high optical quality weregrown by TGT method. Y3+ codoping markedly modulated thespectroscopic properties of Nd:CaF2 crystals, originating from thevariations of the local coordination structure of Nd3+ ions in crystallattice. Broadband and flat emission spectrum of Nd3+ 4F3/2 ?

4I11/2

transition was obtained in Nd,Y:CaF2 crystal with FWHM morethan 25 nm, profitable for the production of ultrashort lasers.

Acknowledgement

This work is supported by the National Natural Science Founda-tion of China (Grant Nos. 60938001, 61178056, 61205171 and91222112).

References

[1] P.P. Sorokin, M.J. Stevenson, Phys. Rev. Lett. 5 (1960) 557–559.[2] R.J. Keyes, T.M. Quist, Appl. Phys. Lett. 4 (1964) 50–52.[3] S.E. Hatch, W.F. Parsons, R.J. Eeagley, Appl. Phys. Lett. 5 (1964) 153–154.[4] C. Labbe, J.L. Doualan, P. Camy, R. Moncorge, M. Thuau, Opt. Commun. 209

(2002) 193–199.[5] P. Camy, J.L. Doualan, S. Renard, A. Braud, V. Menard, R. Moncorge, Opt.

Commun. 236 (2004) 395–402.[6] V. Petit, J.L. Doualan, P. Camy, V. Menard, R. Moncorge, Appl. Phys. B 78 (2004)

681–684.[7] F. Friebel, F. Druon, J. Boudeile, Diode-pumped 99 fs Yb:CaF2 oscillator, Opt.

Lett. 34 (2009) 1474–1476.[8] M. Siebold, M. Hornung, R. Boedefeld, S. Podleska, S. Klingebiel, C. Wandt, F.

Krausz, S. Katsch, R. Uecker, A. Jochmann, J. Hein, M.C. Kaiuza, Opt. Lett. 33(2008) 2770–2772.

[9] J. Fernandez, A. Oleaga, J. Azkargorta, I. Iparraguirre, R. Balda, M. Voda, A.A.Kaminskii, Opt. Mater. 13 (1999) 9–16.

[10] A.A. Kaminskii, V.V. Osico, A.M. Prochorov, Y.K. Voronko, Phys. Lett. 22 (1966)419–421.

[11] A.A. Kaminskii, R.G. Mikaelyan, I.N. Zygler, Phys. Stat. Sol. 31 (1969) K85–K86.[12] A.A. Kaminskii, N.R. Agamalyan, G.A. Denisenko, S.E. Sarkisov, P.P. Fedorov,

Phys. Stat. Sol. (a) 70 (1982) 397–406.[13] A.A. Kaminskii, Z.I. Zhmurov, V.A. Lomonov, S.E. Sarkisov, Phys. Stat. Sol. (a) 84

(1984) K81–K84.[14] O.K. Alimov, T.T. Basiev, M.E. Doroshenko, P.P. Fedorov, V.A. Konyushkin, A.N.

Nakladov, V.V. Osiko, Opt. Mater. 34 (2012) 799–802.[15] L.B. Su, Q.G. Wang, H.J. Li, G. Brasse, P. Camy, J.L. Doualan, A. Braud, R.

Moncorge, Y.Y. Zhan, L.H. Zheng, X.B. Qian, J. Xu, Laser Phys. Lett. 10 (2013)035804.

[16] Y.Z. Zhou, J. Cryst. Growth 78 (1986) 31.[17] S. Tanabe, T. Ohyagi, N. Soga, T. Hanada, Phys. Rev. B 46 (1992) 3305–3310.