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Materials Chemistry and Physics 144 (2014) 349e354

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Materials Chemistry and Physics

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

Concentration effect of Nd3þ ion on the spectroscopic propertiesof Er3þ/Nd3þ co-doped LiYF4 single crystal

Peiyuan Wang a, Haiping Xia a,*, Jiangtao Peng a, Haoyang Hu a, Lei Tang a, Yuepin Zhang a,Baojiu Chen b,*, Haochuan Jiang c

aKey Laboratory of Photo-electronic Materials, Ningbo University, Ningbo, Zhejiang 315211, ChinabDepartment of Physics, Dalian Maritime University, Dalian, Liaoning Province 116026, ChinacNingbo Institute of Materials Technology and Engineering, The Chinese Academy of Sciences, Ningbo, Zhejiang 315211, China

h i g h l i g h t s

* Corresponding authors. Tel./fax: þ86 574 876007E-mail addresses: hpxcm@nbu.edu.cn (H. Xia), che

0254-0584/$ e see front matter � 2014 Elsevier B.V.http://dx.doi.org/10.1016/j.matchemphys.2014.01.001

g r a p h i c a l a b s t r a c t

� High quality LiYF4:Er3þ/Nd3þ singlecrystals are prepared by a Bridgmanmethod.

� Concentration effect of Nd3þ ion onthe spectroscopic properties arediscussed.

� The energy transfer efficiencies forthe two ions are calculated via thelifetimes.

� InokutieHirayama model is used toanalyze the cross relaxation process.

a r t i c l e i n f o

Article history:Received 24 March 2013Received in revised form11 December 2013Accepted 4 January 2014

Keywords:A. Optical materialsB. Crystal growthC. Photoluminescence spectroscopyD. Optical properties

a b s t r a c t

High quality Er3þ/Nd3þ:LiYF4 single crystals were grown by a Bridgman method. Their spectroscopicproperties were studied to understand the Nd3þ concentration effect upon excitation of an 800 nm laserdiode. The intensest 2.7 mm emission was observed in the LiYF4 crystal codoped with 0.99 mol% Er3þ and0.62 mol% Nd3þ. Meanwhile, the emission intensity for the green up-conversion and 1.5 mm down-conversion of Er3þ decreased with increasing of the Nd3þ concentration. The modified InokutieHirayamamodel was used to analyze the decay curves of the 1.06 (Nd3þ) and 1.5 (Er3þ) mm emissions. The resultsindicated that the energy transfer process (Er3þ:4I13/2 þ Nd3þ:4I9/2 / Er3þ:4I15/2 þ Nd3þ:4I15/2) is mainlydue to the electric dipoleedipole interaction. The energy transfer efficiencies between Nd3þ and Er3þ

ions were calculated. All results suggested that the Er3þ/Nd3þ:LiYF4 single crystals may have potentialapplications in mid-infrared lasers.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently, considerable work has been carried out on the mid-infrared spectroscopic and laser properties of Er3þ doped inor-ganic materials due to the fact that Er3þ is an ideal luminescentcenter for 3 mm mid-infrared emission corresponding to 4I11/2 / 4I13/2 transition. The mid-infrared lasers have potential

53.nmbj@sohu.com (B. Chen).

All rights reserved.

applications in laser surgery, ophthalmology, trace gas monitoring,infrared countermeasures, and free-space communications [1e3].In most cases, the operation of the mid-infrared lasers is difficult tobe achieved [4] because the fluorescence lifetime of lower level 4I13/2 is considerably longer than that of the upper level 4I11/2, and the4I11/2 / 4I13/2 transition of Er3þ ion is characterized as “self-ter-minating” [5]. To solve this problem, Nd3þ, Pr3þ, Tm3þ, Yb3þ, orHo3þ as sensitizers were incorporated into the Er3þ doped mate-rials. This approach was tested as a feasible alternative to depop-ulate the lower level 4I13/2 and enhance the mid-infrared emissionintensity.

1.0

1.5

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(a)

109404

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004103112

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Intensity/a.u.

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P. Wang et al. / Materials Chemistry and Physics 144 (2014) 349e354350

In order to obtain high luminescent efficiency from Er3þ dopedmaterials, the low phonon energy of the host matrix is needed dueto the narrow energy difference between 4I11/2 and 4I13/2 levels [6].Most of the previous studies have been focused on the Er3þ dopedtellurite [6,7], silicate, phosphate, germinate [8], fluoride [9,10], andfluorophosphate [11] based glasses. However, Er3þ doped crystalshave some advantages in the applications of laser devices due totheir conspicuous chemical and mechanical stability, reliablethermal performance and superior spectroscopic properties incomparison with most glass hosts [12]. So far 3 mm luminescenceproperties of Er3þ-doped SrLaGa3O7 [13], BaY2F8 [14] and YSGG[15,16] crystals have been reported. The LiYF4 single crystal host is agood laser media due to its low phonon energy (around 424 cm�1),high doping concentration of trivalent rare-earth ion substitutingfor the Y3þ ions, andwide optical transmittancewindowup to 6 mm[17]. LiYF4 crystals doped with rare-earth ions have been proven tobe efficient tunable all-solid-state lasing media that cover variedwavelength regions.

In our previous work [18], we observed that the 2.7 mmemissionwas strongly enhanced, and the 1.5 mm downconversion (4I13/2 / 4I15/2) and 550 nm (4S3/2 / 4I15/2) upconversion emissionswere significantly decreased in an Er3þ/Nd3þ co-doped LiYF4 singlecrystal. In this paper, we continue to study the concentration effectof Nd3þ ion on the spectroscopic properties of Er3þ/Nd3þ co-dopedLiYF4 single crystals. Spectroscopic and dynamical properties ofLiYF4 single crystals with fixed Er3þ concentration but various Nd3þ

concentrations were studied. The crystal with optimum dopingconcentrations of Er3þ and Nd3þ for achieving intensest 2.7 mmemission was confirmed. Additionally, in order to understand theenergy transfer behavior between Er3þ and Nd3þ, a modified Ino-kutieHirayama (IeH) model was used to analyze the measuredfluorescence decays.

2. Experimental

The Bridgman technique was used to grow the Er3þ/Nd3þ co-doped LiYF4 crystal. ErF3 and NdF3 powders of high purity(>99.999%) were used as dopants. The molar composition of theraw material is 51.5LiFe(48.5 � 1 � c)YF3e1ErF3ecNdF3 (c ¼ 0, 1,1.5, 2, 3) and 51.5LiFe47.5YF3e1NdF3. The mixture was ground for1 h in a mortar. The raw fluoride was treated with anhydrous HF at780e800 �C for 8e10 h to remove the residual moisturecompletely. A seed of pure LiYF4 single crystal with the <100>direction and size of F10 � 20 mmwas placed at the bottom of thecrucibles. The detailed process for the crystal growth was describedin Refs. [19e22]. The crystals grown by the Bridgman method wereabout 100 mm in length and 10 mm in diameter as shown in Fig. 1.Along the growth direction there was a small pale opaque matterabout several centimeters in length at the top of the crystal, cor-responding to the final portion of the melt to freeze which mightcaused by the excessive LiF in the starting materials. The obtainedcrystals are transparent, and their body color is purple and becomesdeeper with the increase of Nd3þ concentration. The as-growncrystals were cut into small pieces and well polished to about2.0 mm thickness for optical measurements. The structure of thecrystals was investigated by x-ray diffraction (XRD) using a XD-98X

Fig. 1. The as grown Er3þ/Nd3þ co-doped LiYF4 single crystal and polished sample.

diffractometer (XD-3, Beijing). The absorption spectra weremeasured using a U-4100UV/VIS/NIR spectrophotometer in awavelength range from 400 to 1600 nm. The measuring accuracyfor the absorption spectra is 0.3%, the wavelength repeatability is�0.2 nm in ultraviolet and visible region and �0.5 nm in nearinfrared region, respectively. The up-conversion spectra weremeasured on a HITACHI F-4500 fluorescence spectrometer by usingan 800 nm laser diode as excitation source. Infrared emissionspectra originated from 4I11/2 / 4I13/2, and 4I13/2 / 4I15/2 weremeasured by using a monochromator (JY TP-550, France) equippedwith semiconductor detectors (InGaAs detector for 1.5 mm emis-sion, InSb detector for 2.7 mm emission). The fluorescence lifetimeswere tested with the FLSP920 fluorescence spectrophotometer. Theuncertainty for the fluorescence decays is estimated to be betterthan 5%. Meanwhile, the concentrations of Er3þ and Nd3þ ions inthe singly and doubly doped LiYF4 single crystals weremeasured byinductively coupled plasma atomic emission spectroscopy (ICP-AES, PerkinElmer Inc., Optima3000). All the measurements werecarried out at room temperature.

3. Results and discussion

3.1. X-ray diffraction

In order to identify the crystal phase of the samples, the powderX-ray diffraction measurement of Er3þ/Nd3þ co-doped LiYF4 crys-tals were performed .The result of powder XRD is shown in Fig. 2.The diffraction peak positions of the crystal sample match perfectlywith the XRD pattern of LiYF4 in the JCPD card (77-0816), indicatingthat the co-doping with Nd3þ and Er3þ ions has a negligible effecton the crystal structure. The concentrations of Er3þ and Nd3þ weremeasured via ICP technique, and the results are listed in Table 1. It isfound that the Er3þ concentration (around 0.99 mol%) is very closeto the designed value, but Nd3þ concentrations largely deviate fromthe designed ones. Thus, the measured concentrations are used inthe context below.

3.2. Absorption spectra

The absorption spectra of 0.99%mol Er3þ doped, 0.62%mol Nd3þ

and 0.99% mol Er3þ/0.62% mol Nd3þ co-doped LiYF4 single crystalsranging from 400 to 1750 nm are shown in Fig. 3, and the corre-sponding transitions of Er3þ and Nd3þ starting from ground state tohigher levels are labeled. The absorption spectra of the Er3þ/Nd3þ

20 40 60 800.0

0.5

2θ(degree)

Fig. 2. (a) XRD pattern plotted using the data reported in JCPDS card No. 77-0816 forLiYF4 powders. (b) XRD pattern of the studied Er3þ/Nd3þ co-doped LiYF4 single crystal.

Table 1Concentration of Er3þ and Nd3þ ions in the crystals (mol. %).

Sample i ii iii iv v vi

Er3þ concentration 0.99 0.99 0.98 0.99 0.98 0Nd3þ concentration 0 0.62 0.93 1.23 1.90 0.62

Fig. 4. 2.7 mm emission spectra for the Er3þ singly doped and Er3þ/Nd3þ co-dopedLiYF4 single crystal. The inset shows the dependence of integrated emission in-

3þ

P. Wang et al. / Materials Chemistry and Physics 144 (2014) 349e354 351

codoped samples consist of the absorption of Er3þ and the ab-sorption of Nd3þ. Apparently, the co-doped samples do not changethe level positions and the shapes of the absorption bands whencompared with the singly doped one. In the Er3þ/Nd3þ co-dopedsamples, an enhanced absorption band, which is more intensethan that of Er3þ doped sample at around 800 nm corresponding tothe overlap of Er3þ:4I15/2 / 4I9/2 and Nd3þ:4I9/2 / 4F5/2, 2H9/2transitions is observed. This means that the Nd3þ-introduction cangreatly increase the absorption of the 800 nm pumping light inEr3þ/Nd3þ codoped crystal. The absorption spectra are similar forall Er3þ/Nd3þ co-doped LiYF4 single crystals in this study. The ab-sorption spectra of the Er3þ/Nd3þ:LiYF4 is similar to that of theEr3þ/Nd3þ co-doped tellurite glasses [23].

tensity on the Nd concentration.

3.3. Fluorescence spectra and energy transfer

Fig. 4 shows the 2.7 mm emission spectra of Er3þ/Nd3þ co-dopedcrystals with different Nd3þ concentrations and Er3þ single-dopedLiYF4 crystal pumped by 800 nm LD, which were measured underthe same experimental conditions to get comparable results. Theinset in Fig. 4 shows the Nd3þ concentration dependence of 2.7 mmintegrated emission intensity. One can find that the Er3þ singlydoped crystal displays very weak emission at around 2.7 mm.However, an intense emission at the same wavelength is detectedin the Er3þ/Nd3þ co-doped samples, indicating that the introduc-tion of Nd3þ ions enhances the luminescence intensity of 4I11/2/

4I13/2 transition of Er3þ ions. Similar phenomena have also beenfound in our previous study [18] and Er3þ/Nd3þ co-doped glasses[7,22,23]. When the Nd3þ doping concentration increases from 0 to0.62 mol %, the 2.7 mm emission intensity increases dramatically,and then decreases with further increase of Nd3þ concentration butremains much stronger than that of Nd3þ free sample. This effect isclearly shown in the inset in Fig. 4. These results indicate that Nd3þ

Fig. 3. Absorption cross section of Er3þ doped (a), Nd3þ doped (b) and Er3þ/Nd3þ co-doped (c) LiYF4 single crystals.

is a good sensitizer for Er3þ to enhance 2.7 mm emission in LiYF4single crystal.

The 1.5 mm down-conversion emission and the 550 nm up-conversion emission spectra are shown in Fig. 5(a) and (b). In or-der to achieve better look, the emission intensities for Er3þ/Nd3þ

co-doped crystals were magnified by a factor of 3. The insets inFig. 5 shows the Nd3þ concentration dependence of 1.5 mm down-conversion emission and the 550 nm up-conversion integratedintensity (which is the real intensity without enlargement). Fromthe insets in Fig. 5(a) and (b), the 1.5 mm (Er3þ:4I13/2 / 4I15/2) and550 nm (Er3þ: 2H11/2, 4S3/2 / 4I15/2) emission intensities decreasewith increasing Nd3þ concentration and keeping the Er3þ concen-tration constant.

From above results, it can be again deduced that Nd3þ is asuitable sensitizer to enhance the 2.7 mm emission in Er3þ dopedLiYF4 single crystal because Nd3þ can enhance the absorption ofpump energy and transfer its energy to Er3þ, increase the popula-tion of 4I11/2 energy level of Er3þ. As a consequence, the 2.7 mmemission from Er3þ increases significantly, but the population ofthe 4I13/2 energy level of Er3þ decreases, leading to the depression of550 nm and 1.5 mm emissions of Er3þ. One can confirm that the 4I13/2 level of Er3þ is quenched by the Nd3þ in the co-doped samples,and the depopulation of 4I13/2 level of Er3þ is beneficial to the2.7 mm laser operation. Therefore, Nd3þ ion is a proper sensitizerimproving 2.7 mm emission performance in Er3þ doped LiYF4crystal.

The possible mechanisms of the energy transfer processes be-tween Nd3þ and Er3þ have been reported in previous works [18,24].Here we attempt to quantitatively investigate the energy transfersbetween Er3þ and Nd3þ. The energy level diagram for the Er3þ/Nd3þ co-doped LiYF4 crystal is shown in Fig. 6. In the Er3þ dopedsample, the Er3þ ions are first excited by 800 nm into 4I9/2, and thennonradiatively relax to 4I11/2. The Er3þ ions in 4I11/2 can radiativelyand nonradiatively relax to 4I13/2, the radiative transition generates2.7 mm emission, however 2.7 mm emission in the Er3þ dopedsample is very weak. The 2.7 mm emission corresponding to 4I11/2 / 4I13/2 transition is weak in the Er3þ singly doped sample,indicating that nonradiative transition is the main route fordepopulating 4I11/2. The Er3þ in 4I13/2 radiatively relaxes to groundstate and generates 1.5 mm (4I13/2 / 4I15/2). Meanwhile, an Er3þ in4I13/2 accepts the energy from another Er3þ in 4I9/2, namely, energytransfer upconversion (ETU) or absorbs the energy of excitation

Fig. 5. 1.5 mm down-conversion (a) and 550 nm up-conversion (b) emission spectra for the studied crystals with fixed Er3þ concentration but varied Nd3þ concentration. The insertsshow the relationships between the integrated down- and up-conversion intensities and the Nd3þ concentration.

P. Wang et al. / Materials Chemistry and Physics 144 (2014) 349e354352

photons, namely, excited state absorption process (ESA) to get intothe 2H11/2. The 2H11/2, 4S3/2 are in a thermal equilibrium so the550 nm emission can be observed.

When the Nd3þ ions are introduced, the Nd3þ ions can beeffectively excited by pumping light to 4F5/2, 2H9/2, and then relax to4F3/2 or transfer their energy to 4I9/2 of Er3þ ions (Nd3þ:4F3/2,Er3þ:4I15/2/Nd3þ:4I9/2, Er3þ:4I9/2) as seen the ET1 process in Fig. 6.The 4F3/2 of Nd3þ is a metastable level, in which the Nd3þ are de-excited via a radiative transition 4F3/2 / 4I11/2 generates 1.06 mm,or may also transfer their energy to Er3þ via the process ET1(Nd3þ:4F3/2 þ Er3þ:4I15/2)/ (Nd3þ:4I9/2þ Er3þ:4I9/2) or/and process(Nd3þ:4F3/2 þ Er3þ:4I15/2) / (Nd3þ:4I11/2 þ Er3þ:4I11/2). These en-ergy transfer processes from Nd3þ to Er3þ would increase thepopulation of Er3þ:4I11/2 so that the mid-infrared emission 2.7 mmcorresponding to 4I11/2 / 4I13/2 transition would increase in theEr3þ/Nd3þ samples. There are several reasons causing the nonlin-earity of the 2.7 mm emission intensity. Firstly, high Nd3þ concen-tration causes self-quenching. Such effect would decrease thepopulation of Nd3þ:4F3/2. Secondly, cross relaxation processes ET4(Er3þ:4I11/2 þ Nd3þ:4I9/2 / Er3þ:4I13/2 þ Nd3þ:4I13/2) and ET5(Er3þ:4I11/2 þ Nd3þ:4F3/2 / Er3þ:4I13/2 þ Nd3þ:4F9/2) with largeenergy mismatch may take place, which reduce the 2.7 mm emis-sion. In this paper, we get the maximum 2.7 mm emission intensityin the crystal sample doped with 0.99 mol% Er3þ and 0.62 mol%

Fig. 6. Energy level diagram for the Er3þ/Nd3þ co-doped

Nd3þ. The enhanced 2.7 mm emission will increase the populationof 4I13/2 and will lead to an increase of 1.5 mm emission; however,the decreased 1.5 mm emission is observed. This is due to thedepopulation of Er3þ:4I13/2 via an energy transfer process Nd3þ:4I9/2 þ Er3þ:4I13/2 / Nd3þ:4I15/2 þ Er3þ:4I15/2 (see ET2 in Fig. 6). TheNd3þ in 4I15/2 will relax to their ground state via cascade non-radiative processes. As mentioned above that the green up-conversion emission are caused by the ESA of 4I13/2 and the ETU(Er3þ:4I9/2þ Er3þ:4I13/2/ Er3þ:4I15/2þ Er3þ:4S3/2, 2H11/2), therefore,depopulation of Er3þ:4I13/2 will restrict both ESA and ETU process.Another energy transfer channel (Nd3þ:4I9/2 þ Er3þ:4S3/2, 2H11/

2 / Nd3þ:4G7/2 þ Er3þ:4I15/2) named ET3 as shown in Fig. 6 couldalso be a possible process quenching the green upconversionemission of Er3þ. Therefore, the green upconversion emissionsextremely decrease in the Er3þ/Nd3þ codoped samples. The Nd3þ

accepting energy from Er3þ will relax to 4F3/2 by the cascade non-radiative process, thus the Nd3þ recycles part of the upconversionenergy of Er3þ, which can contribute again to the 2.7 mm mid-infrared emission of Er3þ.

In order to quantificationally investigate the mutual energytransfer processes between Nd3þ and Er3þ, the fluorescence decaysfor 1.53 mm (4I13/2 / 4I15/2 of Er3þ) and 1.06 mm (4F3/2 / 4I11/2 ofNd3þ) were measured upon 800 nm pulsed excitation and areshown in Figs. 7 and 8. From the decay curves, the average

LiYF4 crystal and the energy transfer mechanisms.

Fig. 7. Fluorescence decay for 4I13/2 / 4I15/2 transition of Er3þ in LiYF4 single crystaldoped with (b) and without (a) Nd3þ pumped by 800 nm LD.

P. Wang et al. / Materials Chemistry and Physics 144 (2014) 349e354 353

fluorescent lifetimes can be calculated by using the following for-mula [25],

s ¼

ZIðtÞtdt

ZIðtÞdt

(1)

where I(t) represents the luminescence intensity as a function oftime t. For 0 mol % Nd3þ concentration, the decay curve followssingle exponential function. The lifetime of 4I13/2 level of Er3þ wasestimated to be 15.02 ms. However, the decay lifetimes decreaserapidly with the increase of Nd3þ concentration due to theappearance of the energy transfer Er3þ:4I13/2 þ Nd3þ:4I9/2 / Er3þ:4I15/2 þ Nd3þ:4I15/2 (ET2). In Fig. 7 the surged and dottedcurves show the experimental results.

The energy transfer efficiency for ET2 has been estimated fromthe lifetime values in Fig. 7 by following equation [26],

hET ¼ hx%Nd ¼ 1� sEr=NdsEr

(2)

where sEr/Nd and sEr are the lifetimes for Er3þ:4I13/2 / 4I15/2 tran-sition in the Er3þ/Nd3þ codoped and Er3þ singly doped crystals,respectively. The calculated values of hET for the ET2 process are

Fig. 8. Fluorescence decay for 4F3/2 / 4I11/2 transition of Nd3þ in LiYF4 single crystalwith fixed Er3þ concentration and various Nd3þ concentrations pumped by 800 nm LD.

listed in Table 2 where the lifetimes for Er3þ:4I15/2 and Nd3þ:4F3/2level were also listed. Similarly, the energy transfer efficiencies ofET1 are also shown in Table 2. According to Ref. [27], the lifetime ofNd3þ single doped LiYF4 is 525 ms.

As Nd3þ concentration increases, the distance between Nd3þ

and Er3þ becomes shorter, thus the energy transfer probabilitybetween Nd3þ and Er3þ would become higher. Fig. 9 shows therelationship between the energy transfer efficiencies and the Nd3þ

concentration in the co-doped samples. It can be seen that theenergy transfer efficiency from Er3þ to Nd3þ (ET2) ranges from95.14% to 98.74% as the Nd3þ dopant increases from 0.62 to 1.90.Meanwhile, the energy transfer efficiency from Nd3þ to Er3þ (ET1)increases from 39.05% to 71.43%. Therefore, the energy transferprocess fromEr3þ to Nd3þ has a positive influence on enhancing the2.7 mm emission. Another effect of Nd3þ introduction is thedepression of both the green up-conversion emission (2H11/2/4S3/2/

4I15/2) and 1.5 mmdown-conversion emission (4I13/2/ 4I15/2) ofEr3þ [18]. It is already known that both the up-conversion anddown-conversion emissions are related to the 4I13/2 level. One cansee that the decay lifetimes of 4I13/2 level decreases rapidly from15.02 ms to 0.19 ms as the Nd3þ ion concentration increases from0 to 1.90 mol %. It is a reasonable suggestion that the reverse energytransfer from Nd3þ to Er3þ ions restrains not only the 1.5 mmemission but also the 550 nm up-conversion emission of Er3þ.

It is well known that the non-exponential fluorescent decayfollows the InokutieHirayama (IeH) model [28,29], in which thefluorescent decay can be expressed as

IðtÞ ¼ Ið0Þexph� ðt=scrÞ3=s � t=sone�site

i(3)

where I(t) and I(0) represent the luminescence intensity duringdecay process and initial time t ¼ 0, respectively. scr is the temporalparameter of cross relaxation, and sone-site is the radiative and non-radiative decay parameter. Here s is a parameter to describe thedistance dependence of the cross relaxation process. s ¼ 6, 8, and10, correspond to the electric dipoleedipole, dipoleequadruple andquadrupleequadruple interactions between luminescent centers[30], respectively. The one-site decay constant, sone-site, is assignedas a fixed value of 15.02 ms from the decay of Er3þ singly doped(0.99 mol %) crystal. The fluorescence decays in Fig. 7 were fit to Eq.(3), and the solid curves in Figs. 7 and 8 show the fitting curves. Inthe fitting processes, the values of s are obtained to be 5.96, 6.02,5.98, and 5.97 for the Er3þ/Nd3þ co-doped samples when the Nd3þ

concentration increases from 0.62 mol % to 1.90 mol %, respectively.The s value is close to 6, which indicates that the energy transfer(Er3þ:4I13/2 þ Nd3þ:4I9/2 / Er3þ:4I15/2 þ Nd3þ:4I15/2) of electricdipoleedipole interaction is dominant in Er3þ/Nd3þ co-doped LiYF4single crystal.

Similarly, the decay curves of 1.06 mm emission for Er3þ/Nd3þ

co-doped LiYF4 crystals were fit to Eq. (3) too, and the fitting resultsare presented as solid curves in Fig. 8. Through the fitting processesfor the 1.06 mm fluorescence decays, the s is 3.13, 2.86, 3.08, and3.05 for the Er3þ/Nd3þ co-doped samples. The value of s is very

Table 2Lifetimes and energy transfer efficiencies for the processes of ET2 and ET1 at thewavelength of 1.53 mm and 1.06 mm.

Concentration s/ms (1.53 mm) hET2/(%) s/ms (1.06 mm) hET1/(%)

x ¼ 0.99, y ¼ 0 15.02 / / /x ¼ 0.99, y ¼ 0.62 0.73 95.14 320 39.05x ¼ 0.98, y ¼ 0.93 0.52 96.54 310 40.95x ¼ 0.99, y ¼ 1.23 0.50 96.67 194 63.05x ¼ 0.98, y ¼ 1.90 0.19 98.74 150 71.43x ¼ 0, y ¼ 0.62 / / 525 /

Fig. 9. . Energy transfer efficiencies between Er3þ and Nd3þ in the Er3þ/Nd3þ co-dopedLiYF4 single crystal.

P. Wang et al. / Materials Chemistry and Physics 144 (2014) 349e354354

close to 3, which suggests that the energy transfer (Nd3þ:4F3/2,Nd3þ:4I9/2 / Nd3þ:4I9/2, Nd3þ:4F3/2) belongs to the exchangeinteraction.

4. Conclusions

In conclusion, Er3þ doped, Nd3þ doped and Er3þ/Nd3þ co-dopedLiYF4 single crystals with high quality have been grown by Bridg-man method. The concentration of Nd3þ significantly affects thespectroscopic properties. Nd3þ ion is an excellent sensitizer for Er3þ

in LiYF4 single crystals to achieve an intense 2.7 mm emissionbecause of the efficient energy transfer between Er3þ and Nd3þ.When the Nd3þ is introduced in the Er3þ samples, the 2.7 mmemission is enhanced. When the concentration of Er3þ is fixed, theoptimal concentration for the 2.7 mm mid-infrared is 0.99 mol%Er3þ and 0.62mol% Nd3þ in the current research. Additionally, Nd3þ

can also efficiently depopulate 4I13/2 level of Er3þ. The energytransfer efficiencies (hET2) for (Er3þ:4I13/2þNd3þ:4I9/2)/ (Er3þ:4I15/2 þ Nd3þ:4I15/2) increased from 95.14% to 98.74%. At the same time,the energy transfer efficiencies (hET1) for (Nd3þ:4F3/2 þ Er3þ:4I15/2) / (Nd3þ:4I9/2 þ Er3þ:4I9/2) increased from 39.05% to 71.43% asthe Nd3þ concentration increases from 0 to 1.90 mol %. Further-more, significant decreases in both the 550 nm up-conversion and1.5 mm down-conversion emissions are observed as Nd3þ concen-tration increases. The Er3þ/Nd3þ co-doped LiYF4 single crystalshave an enhanced emission at 2.7 mm under excitation of 800 nmLD and may have potential applications in mid-infrared lasers.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (Grant Nos. 51272109, 50972061, 11374044),the Natural Science Foundation of Zhejiang Province (Grant No.R4100364), the Natural Science Foundation of Ningbo City (GrantNo. 2012A610115), Research Funds for the Central University(3132013317) and K.C. Wong Magna Fund in Ningbo University.

References

[1] V.A. Serebryakova, E.V. Boiko, N.N. Petrishchev, A.V. Yan, J. Opt. Technol. 77(2010) 6e17.

[2] P. Amedzake, E. Brown, U. Hommerich, S.B. Trivedi, J.M. Zavada, J. Cryst.Growth 310 (2008) 2015e2019.

[3] D.F. de Sousa, L.F.C. Zonetti, M.J.V. Bell, J.A. Sampaio, L.A.O. Nunes, App. Phys.Lett. 74 (1999) 908e910.

[4] M. Pollnau, S.D. Jackson, IEEE J. Quantum Electron. 7 (2001) 30e40.[5] Y. Tian, R.R. Xu, L.L. Hu, J.J. Zhang, J. Am. Ceram. Soc. 94 (2011) 2289e2291.[6] H.Y. Zhong, B.J. Chen, G.Z. Ren, L.H. Cheng, L. Yao, J.S. Sun, J. Appl. Phys. 106

(2009) 083114.[7] I. Jlassi, H. Elhouichet, M. Chtourou, M. Oueslati, Opt. Mater. 32 (2010) 743e

747.[8] R.R. Xu, Y. Tian, L.L. Hu, J.J. Zhang, Opt. Lett. 36 (2011) 1173e1175.[9] T. Sandrock, A. Diening, G. Huber, Opt. Lett. 24 (1999) 382e384.

[10] S.D. Jackson, T.A. King, M. Pollnau, J. Mod. Opt. 47 (2000) 1987e1994.[11] Y. Tian, R.R. Xu, L.L. Hu, J.J. Zhang, Opt. Lett. 36 (2011) 3218e3220.[12] J.X. Hu, H.P. Xia, H.Y. Hu, X.B. Zhuang, Y.P. Zhang, H.C. Jiang, B.J. Chen, Mater.

Res. Bull. 48 (2013) 2604e2608.[13] B. Simondi-Teisseire, B. Viana, A.M. Lejus, D. Vivien, J. Lumin. 72e74 (1997)

971e973.[14] S. Witter, M. Pollnau, R. Spring, W. Luthy, H.P. Weber, Opt. Commun. 132

(1996) 107e110.[15] B.J. Dinerman, P.F. Moulton, Opt. Lett. 19 (1994) 1143e1145.[16] R. Waarts, D. Nam, S. Saunders, J. Harrison, B.J. Dinerman, Opt. Lett. 19 (1994)

1738e1740.[17] A. Antognini, F.D. Amaro, F. Biraben, J.M.R. Cardoso, C.A.N. Conde, Opt. Com-

mun. 253 (2005) 362e374.[18] X.B. Zhuang, H.P. Xia, H.Y. Hu, J.X. Hu, P.Y. Wang, J.T. Peng, Y.P. Zhang,

H.C. Jiang, B.J. Chen, Mater. Sci. Eng. B 178 (2013) 326e329.[19] C.W. Lan, J. Cryst. Growth 229 (2001) 595e600.[20] H. Xia, J. Wang, X. Zeng, J. Zhang, Y. Zhang, J. Xu, Q. Nie, J. Funct. Mater. 36

(2005) 238e240.[21] T.A. Campbell, J.N. Koster, J. Cryst. Growth 147 (1995) 408e410.[22] X.Y. Yu, H.B. Chen, S.J. Wang, Y.F. Zhou, A.H. Wu, S.X. Dai, J. Inorg. Mater. 26

(2010) 923.[23] Y.Y. Guo, M. Li, Y. Tian, R.R. Xu, L.L. Hu, J.J. Zhang, J. Appl. Phys. 110 (2011)

013512.[24] L. Wen, J.R. Wang, L.L. Hu, L.Y. Zhang, Chin. Opt. Lett. 9 (2011) 121601.[25] S. Murakami, M. Herren, D. Rau, M. Morita, Chim. Acta 300 (2000) 1014.[26] W.T. Carnall, P.R. Fields, K. Rajnak, J. Chem. Phys. 49 (1968) 4412e4423.[27] R. James Ryan, Ray Beach, J. Opt. Soc. Am. B 9 (1992) 1883e1887.[28] M. Inokuti, F. Hirayama, J. Chem. Phys. 43 (1965) 1978.[29] J.X. Hu, H.P. Xia, H.Y. Hu, H.C. Jiang, J. Appl. Phys. 112 (2012) 073518.[30] Y.F. Zheng, B.J. Chen, H.Y. Zhong, J.S. Sun, L.H. Cheng, X.P. Li, J. Am. Ceram. Soc.

94 (2011) 1766e1772.