Optical Materials xxx (2014) xxx–xxx

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Optical Materials

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Efficient enhanced 1.54 lm emission in Er/Yb: LiNbO3 crystal codopedwith Mg2+ ions

http://dx.doi.org/10.1016/j.optmat.2014.01.0330925-3467/� 2014 Published by Elsevier B.V.

⇑ Corresponding author. Tel.: +86 21 69918480; fax:+86 21 69918607.E-mail address: yhang@sion.ac.cn (Y. Hang).

Please cite this article in press as: P. Zhang et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.01.033

Peixiong Zhang a,b, Jigang Yin a, LianHan Zhang a, Youchen Liu a, Jiaqi Hong a,b, Kaijie Ning a, Zhi Chen a,b,Xiangyong Wang a,b, Chunjun Shi a,b, Yin Hang a,⇑a Key Laboratory of High Power Laser Materials, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, Chinab University of Chinese Academy of Sciences, Beijing 100039, China

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Laser materialInfrared optical properities1.54 lm enhanced fluorescence

a b s t r a c t

Single crystals of Er: LiNbO3 and Er/Yb: LiNbO3 co-doped with and without Mg2+ ion were successfullyprepared. The quality of the as-grown crystals were characterized. Yb3+ was demonstrated to increasean absorption efficiency at the range of 900-1000 nm. The use of Mg2+ codoping for enhancement ofthe Er3+:4I13/2!4I15/2 1.54 lm emissions were investigated. It was found that Mg2+ ion greatly increasedEr3+ 1.54 lm emission by increasing nonradiative relaxation rate of 4I11/2!4I13/2 transition of Er3+ ion inMg/Er/Yb: LiNbO3 crystal. The Mg2+ codoping also resulted in lengthening the lifetime of the 4I13/2 mul-tiplet by 9% for the Mg/Yb/Er: LiNbO3 crystal when compared with the Yb/Er: LiNbO3 crystal. The nonra-diative relaxation rate of 4I11/2!4I13/2 transition of Er3+ ion increases from 935 s-1 in Er/Yb: LiNbO3 crystalto 1040 s-1 in Mg/Er/Yb: LiNbO3 crystal. It was also found that the intensity of maximum phonon energyof Er: LiNbO3 at 872 cm-1 significantly strengthen by introducing the Mg2+ ion, leading to a more favour-able condition for the nonradiative relaxation of 4I11/2!4I13/2 transition in Er3+ ion. We propose that theMg/Er/Yb: LiNbO3 crystal may be a promising material for 1.54lm laser applications.

� 2014 Published by Elsevier B.V.

1. Introduction

Lithium niobate (LiNbO3, LN) crystal has been studied for manydecades, as it has very excellent electro-optic, acousto-optic, andnonlinear optical properties, which has important applications inseveral aspects, such as nonlinear optics, integrated optics, andholographic data storage [1–5]. Moreover, periodically poled LNstructures can also be used to achieve frequency conversion byquasi-phase matching (QPM) technique for the integrated devices[6–8]. LN has been extensively doped with rare-earth ions, inwhich laser oscillation, electro-optic and nonlinear optics effectscan be integrated within just one crystal [9–11]. Er3+ doped LNcrystal has been widely investigated for it operates at the eye-safe1.54 lm, a primary carried wavelength can be used in numerousfields, including range finding, lidar, and telecommunications[12–14]. Previous studies on Er: LN offer many possibilities forthe miniaturization of integrated optics. However, the absorptioncross-section of Er3+ ion in 980 nm is significantly low, limitingthe pump efficiency. To solve this problem, Yb3+ ion was intro-duced as an efficient sensitizer for Er3+ ion to increase optical

pumping efficiency due to large absorption cross-section of Yb3+

and efficient energy transfer from Yb3+ to Er3+ [15–20]. Cantelaret al. [21] reported the growth of LN co-doped with Er3+/Yb3+,and obtained that Yb3+ co-doping could increase the absorptionefficiency near the range of 980 nm. However, the photorefractiveeffect of Er: LN crystal not only affects the performances of practi-cal applications, but also limits both the pumping and operatingwavelengths, and hence hinders further demonstration on moreefficient applications. To suppress this detrimental effect, anti-photorefractive ions Mg2+ (4.6 mol.%) has been introduced toimprove the photorefractive damage resistance [22]. Moreover,L. Sun et al [23] reported that the co-doped with Mg2+ ions couldalso enhance the 1.54 lm emission and simultaneously suppressgreen upconversion emission in Er: LN crystal.

Therefore, in order to achieve crystals with high optical stimu-lation efficiency, high photorefractive damage threshold, andhigh luminous efficiency, the studying of LN triple-doped withMg2+/Yb3+/Er3+ is important and necessary. In our previous work[24], high-quality Mg2+/Yb3+/Er3+ triple-doped LN crystal wasgrown for the first time and the fundamental optical propertieswere studied. In this letter, Er: LN and Er/Yb: LN crystals co-dopedwith and without Mg2+ ion were successfully prepared. Comparedwith the crystals without Yb3+ ion co-doping (Er: LN, Mg/Er: LN), the

2 P. Zhang et al. / Optical Materials xxx (2014) xxx–xxx

co-doping of Yb3+ ion (Yb/Er: LN, Mg/Yb/Er: LN) increased anabsorption efficiency at the range of 900-1000 nm, thus leads toan enhanced 1.54 lm luminescence emission. Mg2+ was demon-strated to greatly enhance the Er3+: 4I

13/2!4I15/2 1.54 lm emissionin Mg/Er/Yb: LN and Mg/Er: LN crystals by increasing nonradiativerelaxation rate of 4I11/2 !4I13/2 transition. The mechanism of Mg2+

ions codoping effect on 1.54 lm emission were both quantitativelyand qualitatively discussed based on the results of Er3+ ion energylevel scheme, fluorescent lifetime of 4I11/2 multiplet of Er3+, Judd-Ofelt theory, and Raman spectra.

Fig. 1. X-ray rocking curves of the as-grown crystals corresponding to the (006)diffraction peak of the c-cut wafer.

(a)

(b)

2. Experimental procedure

Er (0.6 mol.%): LiNbO3 and Er (0.6 mol.%)/ Yb (0.3 mol.%):LiNbO3 crystals codoped with and without 5 mol.% MgO weregrown by the Czochralski (Cz) method with medium frequencyinduction heating system from congruent LiNbO3 ([Li]/[Nb]=0.945) melt. The raw materials used for crystal growth wereLi2CO3 (99.999%), Nb2O5 (99.999%), Yb2O3 (99.999%), Er2O3

(99.999%), and MgO (99.999%), and they were mixed completelyfor 24 h. Then, the mixture was pressed into disks and heated inair at 750 �C for 5 h to remove CO2, and then further heated upto 1150�C for 8 h to form polycrystalline powers. Following, thepolycrystalline powers were followed loaded into a 60 mm diam-eter platinum crucible for crystal growth. The < 001 > orientedseed of dimension 5�5�30 mm3 was used for crystal growth.Pulling rate was in the range of 0.6-0.8 mm/h in the body part ofcylinder and the rotation rate was kept at 8-12 rpm. The grownfour crystals were transparent, crack-free, and pink, and they arenamed as Er-1, Mg/Er-2, Yb/Er-3, and Mg/Yb/Er-4, respectively.The as-grown crystals were poled between two platinumelectrodes at 1200�C with a current density of 5-8 mA/cm2.

The concentrations of Yb3+, Er3+, and Mg2+ ions in the as-growncrystals were detected by inductively coupled plasma-atomicemission spectrometry (ICP-AES), and the results were shown inTable 1. The X-ray rocking curves of the as-grown crystals weremeasured by the PANalytical x’Pert PRo x-ray diffraction system.The absorption spectrums of the as-grown crystals were recordedby a UV-vis-NIR spectrophotometer (ModelV-570, JASCOCo.). TheRaman spectrum of the sample was recorded in back-scatteringgeometry by using a Renishaw invia Micro-Raman spectrometer.The fluorescence spectrum and lifetimes of the crystals at roomtemperature were acquired by a TRIAX550 spectrophotometerwith a laser diode (LD) as the pump source (excited at 980 nm).To compare the emission intensity between different samples,the measure experiments were done at the same excitation power,the samples have the same size and were subjected the samepolishing procedure. All the measurements were taken at roomtemperature.

3. Results and discussion

The X-ray rocking curve is often used to characterize the crys-tallinity of a crystal. Fig. 1 showns the X-ray rocking curves of

Table 1Concentration of Yb3+, Er3+, and Mg2+ ions in melt and crystal.

Er-1 Mg/Er-2 Yb/Er-3 Mg/Yb/Er-4

Mg2+ in melt (mol.%) 0 5 0 5Er3+ in melt (mol.%) 0.6 0.6 0.6 0.6Yb3+ in melt (mol.%) 0 0 0.3 0.3Mg2+ in crystal (mol.%) 0 6.1 0 5.9Er3+ in crystal (mol.%) 0.9 0.72 0.744 0.66Yb3+ in crystal (mol.%) 0 0 0.441 0.375

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the as-grown crystals corresponding to the (006) diffraction peakof the c-cut wafer. It is evident to see that the peak is highly sym-metrical without any splitting and the full width at half maximum(FWHM) are only 16.24

00, 16.20

00, 16.42

00, and 16.16

00for Er-1, Mg/Er-2,

Yb/Er-3, and Mg/Yb/Er-4, respectively. Such narrow FWHM0s indi-

cate that the crystallinity of the as-grown crystals is fairly high.The room-temperature absorption spectra of the as-grown crys-

tals in the near ultraviolet, visible and near infrared regions is pre-sented in Fig. 2(a). The absorption spectrum of the as growncrystals consist of 10 absorption bands centered at approximately

Fig. 2. (a) Absorption spectra of Er: LN, Mg/Er: LN, Yb/Er: LN, and Mg/Yb/Er: LNcrystals in the near ultraviolet, visible and near infrared regions. (b) Absorptionspectra of the as-grown crystals near the range of 980 nm.

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(a) (b)

(c) (d)

Fig. 3. Emission spectra of the as-grown crystals (excited at 980 nm) in the wavelength range of 1450-1700 nm. (a)Emission spectra of Mg/Er/Yb: LN and Mg/Er: LN crystals;(b)Emission spectra of Er/Yb: LN and Er: LN crystals; (c)Emission spectra of Mg/Er/Yb: LN and Er/Yb: LN crystals; (d)Emission spectra of Mg/Er: LN and Er: LN crystals.

Fig. 4. Measured fluorescence decays of the Mg/Er/Yb: LN and Er/Yb: LN crystals at 1.54 lm(excited at 980 nm).

P. Zhang et al. / Optical Materials xxx (2014) xxx–xxx 3

380, 406, 454, 492, 524, 548, 658, 804, 978, and 1528 nm, whichcorrespond to the transitions from 4I15/2 to 4G11/2, 2H9/2, 4F5/2,4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2, respectively. In therange of 850-1050 nm, a strong overlap between the transitionof Yb3+ ions from 2F7/2 to 2F5/2 and Er3+ ions from 4I15/2 to 4I11/2

for the crystals co-doped with Yb3+ ion (Yb/Er: LN, Mg/Yb/Er: LN)is clear shown in Fig. 2(b). It is expected that efficient energy trans-mission will occur from the 2F5/2 excited state of Yb3+ to the 4I11/2

state of Er3+. Two strong absorption peaks (953 nm and 978 nm) oc-cur, which allow the crystal to be pumped by LD for the possibility of

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generating laser output. By contrast, the absorption of the crystalscodoped without Yb3+ ion (Er: LN, Mg/Er: LN) in the range of850-1050 nm is significantly weak and LD pumping is highlydifficult to perform. Therefore, importing of Yb3+ ions results inan increased absorption efficiency at about 900-1000 nm, whichis beneficial to effectively enhance the pump efficiency.

Fig. 3(a) presents the 1.54 lm emission spectra of Er-1 andEr/Yb-3 crystals. It can be observed that the incorporation to Yb3+

ions to Er: LN crystal leads to an enhanced 1.54 lm luminescenceemission than that of Er: LN crystal undoped with Yb3+ ions. This

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Fig. 5. Simplified energy level diagram for Er3+, Yb3+ codoped system under 980 nmexcitation.

4 P. Zhang et al. / Optical Materials xxx (2014) xxx–xxx

enhanced 1.54 lm emission is also observed between the Mg/Er/Yb: LN crystal and Mg/Er: LN crystal (shown in Fig. 3(b)). Thesefacts that co-doped with Yb3+ ions results in an enhanced 1.54lm emission in Er/Yb: LN amd Mg/Er/Yb: LN crystals may be dueto the efficient energy transmission from Yb3+ to Er3+.

The fluorescence spectrums near at the wavelength of 1.54 lm,which is assigned to the 4I13/2 !4I15/2 transition of Er3+ ion in theYb/Er-3 and Mg/Yb/Er-4 crystals, are shown in Fig. 3(c). Fig. 3(d)portrays the 1.54 lm emission spectra of Er-1 and Mg/Er-2 crys-tals. As shown in Fig. 3(c), it is evident that the emission intensityof the Mg/Yb/Er-4 crystal is two times that of the Yb/Er-3 crystal.The enhanced 1.54 lm emission intensity of the Mg/Er-2 crystalwith at least two times higher than that of the Er-1 crystal is alsorevealed from Fig. 3(d). The above emission results indicate thatthe co-doped with Mg2+ ions is an effective way to enhance the1.54 lm emission in Er: LN crystal, which may provide many moreadvantages for practical applications.

To further investigate the optical characteristics of 1.54 lmemissions, the time-resolved decays of 4I13/2 multiplet of Yb/Er-3and Mg/Yb/Er-4 crystals were measured under the excitation of980 nm, and shown in Fig. 4. The two experiment curves can bewell fitted to single exponential functions. The measured lifetimeof the 4I13/2 multiplet in the Mg/Yb/Er-4 crystal is 3.70 ms, which

Fig. 6. Time-resolved decays of the 4I11/2 !4I15/2 transition of Er3

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is lengthened about 9% when compared with the date for theYb/Er-3 crystal (3.38 ms). This lengthening of the measuredlifetime indicates that codoping of Mg2+ ions is an effective wayto improve the 1.54 lm spectroscopic properties of Er3+ ions inMg/Yb/Er: LN crystal.

Fig. 5 shows the simplified energy level diagram of the Er3+,Yb3+ codoped system. When the crystal is pumped by a 980 nmlaser diode, the excited Yb3+ ions may relax radiatively to theirground state 2F7/2, which gives a luminescence emission in therange of 920-1100 nm, or undergo an energy transmission process(ET1: 2F5/2Yb+4I15/2 Er!2F7/2Yb+4I11/2 Er) to the 4I11/2 level of Er3+

ions, thanks to the large absorption cross section around 980 nmof Yb3+ ion and the effective resonance energy transfer from Yb3+

to Er3+ occurred by the strong overlap of the luminescence spec-trum of donor Yb3+ ions and the absorption spectrum of acceptorEr3+ ions. Then, Er3+ in the 4I11/2 level, on one hand, some ionsundergo the ET2 upconversion process by receiving a second photonfrom Yb3+ in its neighbourhood (2F5/2Yb+4I11/2Er!2F7/2Yb+4F7/2Er),which was known as the excited state absorption (ESA) and mak-ing a contribution to the population of 4F7/2 level. On the otherhand, other ions in the 4I11/2level decay radiatively to 4I15/2 with1.0 lm emission, to 4I13/2 with 2.7 lm emission, or nonradiativelyto 4I13/2 level. Subsequently, ions in the 4I13/2 level relax radiativelyto their ground state 4I15/2, giving an emission near at 1.54 lm.

The experimental results that the co-doped with Mg2+ ionsleads to an enhancement of the 1.54 lm emission in Er/Yb: LNcrystal may be explained by comparing the nonradiative relaxationrate of 4I11/2!4I13/2 transition of Er3+ ion in Yb/Er-3 and Mg/Yb/Er-4crystals. The time-resolved decays of the 4I11/2!4I15/2 transition ofEr3+ ion at 1020 nm in the Yb/Er-3 and Mg/Yb/Er-4 crystals weremeasured, and shown in Fig. 6. The two experiment curves canbe well fitted to single exponential functions. The measured life-time of the 4I11/2 manifold in the Mg/Yb/Er-4 crystal is 596 ls,which is 15% shorter when compared with the date for the Yb/Er-3 crystal (687 ls). It is assuming that the effect of Yb3+ on thelifetime of 4I11/2 state (Er) in Yb/Er-3 and Mg/Yb/Er-4 crystals issimilar. Therefore, the Mg2+ codoping results in the shorteninglifetime of 4I11/2 state (Er). In theory, the nonradiative relaxationrate of 4I11/2 !4I13/2 transition of Er3+ ion can be estimated bythe following equation: WN=(srad-smeas)/sradsmeas, where WN isthe nonradiative relaxation rate, smeas is the measured lifetime,which were given in the Fig. 6, and srad is the radiative lifetime,which can be calculated by Judd-Ofelt (J-O) theory based on theroom-temperature absorption spectra of the Yb/Er-3 and Mg/Yb/

+ ion at 1020 nm in the Yb/Er: LN and Mg/Yb/Er: LN crystals.

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Fig. 7. Raman spectra of Er: LN and Mg/Er: LN crystals at room temperature.

P. Zhang et al. / Optical Materials xxx (2014) xxx–xxx 5

Er-4 crystals (shown in Fig. 2(a)). The radiative lifetimes of 4I11/2

!4I13/2 transition of Er3+ ion in Yb/Er-3 and Mg/Yb/Er-4 crystalswere calculated to be 1921 ls and 1568 ls, respectively. Therefore,the values of WN in Yb/Er-3 and Mg/Yb/Er-4 crystals were calcu-lated to be 935 s-1 and 1040 s-1, respectively, indicating that thecodoping of Mg2+ can efficiently increase the nonradiative relaxa-tion of 4I11/2!4I13/2 transition of Er3+ ion. The increase of the non-radiative relaxation rate corresponds to the decrease of ESA from4I11/2 to 4F7/2 level of Er3+ ion, which results in the increase of pop-ulation of 4I13/2 state (Er), and leading to the enhanced 1.54 lmemission of the 4I13/2 !4I15/2 transition of Er3+ ion.

The behavior that Mg2+ doping induces the enhanced 1.54 lmemission in Er: LN crystal may also be understood by comparingthe maximum phonon energy of the Er-1and Mg/Er-2 crystalsbased on the Raman spectra. Fig. 7 shows the Raman spectraof the two crystals with the same thickness in the range of100-1000 cm-1. It is evident that with the codoping of Mg2+ ion,the intensity of the maximum phonon energy at 872 cm-1 signifi-cantly strengthen, but at the second maximum phonon energy(620 cm-1), it greatly reduce. The peak at 872 cm-1 correspondsto the stretching vibration of Nb-O in isolated groups, serve asthe traps to accept the energy transferred from the excited Er3+

ion. The energy transfer process increases the nonradiative transi-tion probability. Generally, after the Er3+ is stimulated to the 4I11/2

excited state, some ions undergo the ET2 upconversion process tothe 4F7/2 state by the ESA, which goes against the 1.54 lm emis-sion. The increased intensity of the maximum phonon energy at872 cm-1 means the increase of the maximum phonon content inthe Mg/Er-2 crystal, and hence results in a more energy transferfrom 4I11/2 excited state (Er3+) to the phonon of Nb-O stretchingvibration, which corresponds to a faster nonradiative relaxationof 4I11/2!4I13/2 transition of Er3+ ion and a reduced ESA rate. As aresult, the population of 4I13/2 state (Er3+) is increased and the1.54 lm emission in Mg2+ doped Er: LN crystal is enhanced.

4. Conclusion

In conclusion, Er: LN and Er/Yb: LN crystals codoped with andwithout Mg2+ ion were successfully prepared. The co-doping withYb3+ increases substantially the absorption band in the 900-1000nm region compared to Er-doped crystals, which would improvethe pump efficiency. Compared with the crystals without Mg2+

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ion codoping (Er: LN, Yb/Er: LN), the crystals co-doped with Mg2+

ion (Mg/Er: LN, Mg/Yb/Er: LN) present an enhanced 1.54 lm lumi-nescence emission under 980 nm excitation, which may providemany more possibilities for practical applications. The Mg2+ codop-ing also resulted in lengthening the lifetime of the 4I13/2 multipletby 9% for the Mg/Yb/Er: LN crystal when compared with the Yb/Er:LN crystal. The measured lifetime of 4I11/2 state (Er3+) indicates thatMg2+ ion codoping leads to a shortened lifetime from 687 ls inYb/Er: LN crystal to 596 ls in Mg/Yb/Er: LN crystal. The radiativelifetimes of 4I11/2!4I13/2 transition of Er3+ ion in Yb/Er: LN andMg/Yb/Er: LN crystals were calculated to be 1921 ls and 1568 lsby J-O theory. The corresponding nonradiative relaxation rate of4I11/2!4I13/2 transition of Er3+ ion increases from 935 s-1 to 1040s-1, which results in the enhanced 1.54 lm luminescence emission.It was also found that the intensity of maximum phonon energy ofEr: LN at 872 cm-1 significantly strengthen with the codoping ofMg2+ ion, which is favorable for the nonradiative relaxation from4I11/2 to 4I13/2 state (Er3+).

Acknowledgements

We would like to thank the State Key Program for BasicResearch of China (Grant No. 2010CB630703), and the NationalNatural Science Foundation of China (Grant No.51302283) for theirfinancial support.

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