【摘要】采用Czochralski法生長(zhǎng)出一系列不同Ho3+（0.1，0.3，0.5 mol%）摩爾摻雜濃度的Ho：LiNbO3單晶。X射線衍射結(jié)果顯示了Ho：LiNbO3晶體的結(jié)構(gòu)特性。在550nm光源的激發(fā)下，在樣品中檢測(cè)到了由于3D3→5IJ（J=6和8）和5Gk→5I8（k=3，4，5和6）能級(jí)躍遷引起的紫外至紫光區(qū)域的上轉(zhuǎn)換發(fā)光。交叉弛豫5G4+5I8→5F4/5S2+5I6導(dǎo)致了發(fā)光強(qiáng)度隨著Ho3+摻雜濃度增加而減弱。晶體的紫外-可見(jiàn)光-進(jìn)紅外吸收光譜表明在Ho3+摻雜濃度為0.3mol%和0.5mol%時(shí)，Ho3+占據(jù)了LiNbO3晶體的Nb位。

【關(guān)鍵詞】Ho：LiNbO3晶體； 上轉(zhuǎn)換； 紫外和紫光

Abstract：Congruent LiNbO3 crystals doped with x mol% Ho3+ ions （x = 0.1， 0.3 and 0.5 mol%） （Ho：LiNbO3） are grown by Czochralski method. The structural properties of the grown crystals are confirmed by the powder X-ray diffraction （XRD） patterns. The ultraviolet and violet upconversion emissions， arising from the transitions of 3D3→5IJ（J=6and8） and 5Gk→5I8（k=3，4，5and6），respectively，were observed in Ho：LiNbO3 crystals under 550 nm excitation.A reduced intensity ratio of UC emission at 380 nm to the NIR emission at 760 nm observed in Ho：LiNbO3 was attributed to the efficient cross relaxation process of5G4+5I8→5F4/5S2+5I6.The studies on the ultraviolet-visible-near infrared （UV-Vis-NIR） absorption spectra indicate that Ho3+ ions may enter into Nb sites in LiNbO3 crystals doped with 0.3 mol% and 0.5 mol% Ho3+ ions.

Key Words ：Ho：LiNbO3 crystal； Upconversion； Ultraviolet and violet

1. Introduction

The upconversion （UC） emission arising from the activated rare-earth （RE） ions is important for practical applications such as solar cells， UC laser， biological imaging， temperature sensors and DNA detection [1-4].In particular， the solid-state lasers working in the ultraviolet （UV） spectral range have attributed much attention since the UV lasers have great potential applications of optical data storage， color displays， environmental monitoring， etc.[5-7]. Therefore， many efforts have been expanded to obtain the efficient UC-UV emission. G. Y. Chen[8] reported that the blue and violet UC emissions of Er3+ at 409 nm （2H9/2→4I15/2） and 390 nm （4G11/2→4I15/2） can be observed in Yb3+/Er3+：Y2O3 nanocrystals under 980 nm excitation. Since high-power and cost-effective compact green solid state laser is now readily available， Ho3+ ions doped host materials will be an ideal candidate for the UV solid-state lasers. Ho3+ ion has several high-lying metastable levels that can give rise to transitions at various wavelengths. The 3D3→5IJ （J=6 and 8） and 5Gk→5I8 （k=3，4，5 and 6） transitions of Ho3+ ions can emit ultraviolet and violet UC emissions.

LiNbO3 crystal doped with rare-earth ion （RE：LiNbO3） has attracted tremendous research interest since it combines the nonlinear optical properties of LiNbO3 with the amplifying and lasing characteristics of rare-earth ion[1-3]. Moreover， the multi-functional LiNbO3 crystal， which has great chemical and physical stability， good mechanical and photoelectric properties， creates sufficient conditions for opening up new perspectives to the studies of integration and tiny devices [9-10]. Most studies on Ho3+ doped LiNbO3 were focused on visible region under NIR laser excitation. For example， R. Wang et al. reported green and red UC emissions of Ho3+ and Yb3+ co-doped LiNbO3 under 980 nm laser excitation[11]. However， UC emission of Ho：LiNbO3 in UV and violet region has rarely been reported.

In this work， high quality single-crystalline Ho：LiNbO3 were grown by Czochralski method. The ultraviolet- visible-near infrared （UV-Vis-NIR） absorption spectra were measured， and the mechanism for the shift of absorption edges was discussed. Under 550 nm excitation， the UC emissions in the UV and violet region were investigated， and the corresponding UC mechanism was proposed.

2. Experiment

Congruent LiNbO3 crystals （[Li]/[Nb]=0.946） doped Ho3+ ions were grown by Czochralski technique along the c-axis. The concentrations of Ho3+ions were changed from 0.1， 0.3 to 0.5 mol%. The purity of all raw materials （Ho2O3， Li2CO3 and Nb2O5） is 99.99%. The mixtures， which were produced by mixing all raw materials for 24 h， were placed into a platinum crucible and sintered at 750 ℃ for 180 minutes in a furnace to remove CO2. Then， the mixtures were melted at about 1240 ℃ for 60 minutes to get the polycrystal. During the growth process， a seed crystal was dipped into the molten polycrystal， and the Ho：LiNbO3 single crystal was pulled from the melt by controlling the temperature gradients， adjusting the rotation speed of the seed crystal to 15.0-15.5 r/min， and pulling the seed crystal at a rate of 1.0-1.2 mm/h. As-grown crystals exhibited a yellow tint. They were cut into c-oriented plates with a thickness of 2.0 mm. These c-plates were polished to optical grade on both sides for optical characterization. Hereafter， these crystals are named as Ho-0.1， Ho-0.3 and Ho-0.5， respectively.

X-ray diffraction （XRD） patterns were recorded on a Rigaku D-Max 2200PC diffractometer with Cu-Kα radiation （λ=0.15418 nm）. The UV-Vis-NIR absorption spectra of Ho：LiNbO3 crystals were obtained by a Perkin-Elmer Lamba 900 spectrophotometer. A combined fluorescence lifetime and steady state spectrometer FLSP920 by EDINBURGH INSTRUMENTS LTD. were used to measure the spectra of UC and NIR emissions.

3. Result and discussion

Fig.1 shows the XRD patterns of Ho：LiNbO3 crystals with different concentrations of Ho3+ ions （0.1 0.3 and 0.5 mol% ） as well as the standard LiNbO3 card （JCPDS# 20-0631）. It can be seen that all the peaks of Ho：LiNbO3 crystals can be indexed to the standard XRD pattern of LiNbO3， indicating that the grown crystals are single phase. Moreover， the fact that no impurities or secondary phases are observed in these crystals implies clearly that the doped Ho3+ ions occupy the normal Li+ or Nb5+ sites but the interstitial sites of LiNbO3 crystal lattice.

Fig.2 （a） displays the UC emission spectra of Ho-0.1， Ho-0.3 and Ho-0.5 crystals under the excitation of 550 nm. As shown in Fig. 2 （a）， the blue UC emission has a luminescence peak at 452 nm that corresponds to the 5G6→5I8 of Ho3+ ion. UV and violet UC emissions centered at 415 nm， 399 nm， 380 nm， 341 nm and 300 nm are ascribed to the transitions of 5G5→5I8， 3D3→5I6， 5G4→5I8， 5G3→5I8 and 3D3→5I8， respectively. It can be seen that the intensities of these UC emissions decrease gradually with the increasing concentrations of Ho3+ ions. Fig. 2 （b） shows the NIR emission spectra ranged from 600 nm to 800 nm under 550 nm excitation. It can be seen that there are a weak red luminescence peak centered at 660 nm and a strong NIR emission at 760 nm. The red peak corresponds to the 5F5→5I8 transition， and the NIR emission is attributed to the 5S2/5F4→5I7 transitions. The intensities of the red and NIR emissions increase with the increasing concentration of Ho3+ ions. In general， for the increased concentration of Ho3+ ions， there should be an enhanced value for the fluorescence intensity， which contradicts the experimental results shown in Fig. 2 （a）. As well known， the fluorescence intensity is strongly dependent on the local environment， the dopant concentration and the distribution of active ions in a host material. The behavior that the intensities of UC emissions are regardless of the concentration of Ho3+ ions suggests that the cross relaxation （CR） processes play an important role in Ho：LiNbO3 crystals. Fig. 2 （c） shows the intensity ratios of UC emission at 380 nm to the NIR emission at 760 nm as a function of Ho3+ ions concentration. It can be seen that the intensity ratio of 380 nm to 760 nm （I380/I760） decreases with the increasing concentration of Ho3+ ions.

Fig.3 displays the energy level diagrams of Ho3+ ion as well as the proposed UC mechanism under 550 nm excitation. The two-photon process for populating the 3F2 state of Ho3+ ion is similar to that in Ho3+- doped Y2O3 ceramic [12] and can be described as follows： Ho3+ ions in the 5I8 ground state absorb the 550 nm photons and are excited to the 5F4/5S2 states through ground state absorption （GSA： 5I8 + a photon→5F4/5S2） process. Then， by absorbing another 550 nm photon， Ho3+ ions in 5F4/5S2 state are populated to the 3F2 state by excited state absorption （ESA1： 5F4/5S2 + a photon→3F2）. In general， the rate of nonradiative relaxation process （Wnr） can be determined by the following expression [13]：

（1）

where C and α are constants， ΔE is the energy gap between the relaxing and the lower state， and ?ω is the highest phonon energy in the material. The value of represents the number of phonons （P） required to bridge ΔE. According to the above equation， Wnr is inversely proportional to P （）. According to the energy diagram level of Ho3+ ion， △E between the 3F2 and 3D3 state is 2966 cm-1. In the LiNbO3 crystal， the phonon energy of LiNbO3 of about 880 cm-1 makes nonradiative relaxation of 3F2 state efficient， where less than four phonons are required to bridge △E between the 3F2 and 3D3 state[14]. Therefore， Ho3+ ions at the 3F2 state could decay to the 3D3 state by nonradiative relaxation process. The UV emissions at 300 nm and 399 nm are produced via radiative relaxation processes of 3D3→5I8 and 3D3→5I6， respectively. The emission centered at 341 nm arises from the radiative transition process of 5G3→5I8， in which the 5G3 state is populated through step-by-step nonradiative relaxation processes of the 3D3 state. Owing to the long-lived intermediate 5F4/5S2 excited state of Ho3+ ion， the following possible cross relaxation （CR1： 5F4/5S2 + 5I8→5I4 + 5I7） processes may occur. Consequently， by absorbing a 550 nm photon， the Ho3+ ions in the 5I7 state are excited to the 5G5 state through the ESA2 （5I7 + a photon→5G5） process. The UC emission at 415 nm is produced by the radiative transition process of 5G5→5I8. According to Eq. （1）， only three phonons of LiNbO3 are required to bridge the energy gap of 1793 cm-1 between 5G5 and 5G6 states， the 5G5 state depopulates nonradiatively to the 5G6 state. The radiative transition process Ho3+ ions from the 5G6 to 5I8 state produces the 452 nm UC emission. It has been proposed that there is CR3 （5F4/5S2 + 5I8→5I6 + 5I6） for the long-lived intermediate 5F4/5S2 excited state of Ho3+ ion. On the other hand， the CR1 process increases the population of 5I4 state and leads to the occurrence of CR2 （5I4 + 5I7→5I6 + 5I6） process. Obviously， the increasing population of the 5I6 state， arising from the CR2 and CR3 processes， results in the ESA3 （5I6 + a photon→5G4） process. And then the radiative transition process of 5G4 → 5I8 contributes to the 380 nm emission.

As for the NIR emission under 550 nm excitation， the NIR emission at 760 nm is produced by radiative relaxation processes of 5S2/5F4→5I7. The 5F5 state， which is populated through nonradiative transition of the 5S2/5F4 states， results in the 660 nm emission. According to Fig. 2 （b）， the intensity of 660 nm NIR emission is much weaker than that of 760 nm NIR emission， indicating that there is an inefficient nonradiative transition process from the 5S2/5F4 to the 5F5 state. Moreover， the strong NIR emission at 760 nm indicates that the 5I7 state could be populated radiatively by the 5S2/5F4 states rather than nonradiative transition from the 5S2/5F4 state [15].

It can be seen from Fig. 2 （c） that the decreased intensity ratio of UC to NIR emission （I380/I760） means a decreasing population of the 5G4 state and in turn a higher population in the 5F4/5S2 states. This is may be due to the efficient CR4 （5G4 + 5I8→5F4/5S2 + 5I6） process. It is well known that the CR process can readily occur when the distance between ions is small enough. The rate of CR process is inversely proportional to the distance between two neighboring ions. Therefore， as the concentration of Ho3+ ion increases， the distance between two neighboring Ho3+ ions becomes short. The shorten distance between two Ho3+ ions in Ho-0.5 crystal leads to a more efficient CR4 （5G4 + 5I8→5F4/5S2 + 5I6） process and results in a reduced intensity ratio of UC to NIR emission （I380/I760）， in agreement with the experimental results shown in Fig. 2 （c）.

Fig.4 shows the UV-Vis-NIR absorption spectra of Ho： LiNbO3 crystals. The absorption bands of Ho：LiNbO3 located at 365 nm， 389 nm， 421 nm， 458 nm， 487 nm， 543 nm， 649 nm， 655 nm and 1147 nm are attributed to the transitions from the 5I8 ground state to the 3H6， 5G4， 5G5， 5G6， 5F3， 5F4， 5F5 and 5I6 states of Ho3+ ion， respectively. The inset is the partial enlarged ?gure， which shows the absorption edge positions of Ho-0.1， Ho-0.3 and In-0.5 crystals. The absorption edges of Ho-0.1， Ho-0.3 and Ho-0.5 crystals are 331 nm， 324 nm and 323 nm， respectively. Compared with the absorption edge at 322 nm of pure LiNbO3 crystal， 0.1 mol% Ho3+ ions doping leads to redshift. The absorption edges of Ho-0.3 and Ho-0.5 crystals shift to the blue relative to Ho-0.1. As LiNbO3 is octahedral ferroelectrics， the fundamental absorption edge of the LiNbO3 crystal can be determined by the valence electron transition energy which is from 2p orbital of O2- to 4d orbital of Nb5+[16]. The position of the absorption edge is affected by the valence electronic state of O2-. The blueshift of the absorption edge is attributed to the increasing valence electron transition energy which is caused by the lower polarization ability of the doped ion than that of the replaced ion. Otherwise， the absorption edge shifts toward the red band. According to Li vacancy model [17]， the lithium-deficient in congruent LiNbO3 crystal （[Li]/[Nb]=0.946） yields the intrinsic defect antisite NbLi4+ （the Nb5+ ions locate in Li sites）. When Ho3+ ions enter into the LiNbO3 crystal， they first repel the NbLi4+ and occupy the normal Li sites at the same time. Since the polarization ability of the Ho3+ ion is larger than that of the Li+， the deformation of the electron cloud increases. The behavior that the polarization ability of Ho3+ ion is larger than that of Li+ ion leads to the redshift of absorption edges， in agreement with experimental results shown in Fig. 4. As the concentration of Ho3+ ions increases， Ho3+ ions may replace the Nb5+ ions and occupy the normal Nb sites. Therefore， the redshift of the absorption edges in Ho-0.3 （324 nm） and Ho-0.5 （323 nm） crystals relative to pure LiNbO3 （322 nm） implies that the Ho3+ ions may enter Li and Nb sites because the polarization ability of Ho3+ ion is larger than that of Li+ and Nb5+. The blueshift of the absorption edges in Ho-0.3 and Ho-0.5 relative to Ho-0.1 crystal may be attributed to the fact that the difference of the polarization ability between Ho3+ and Li+ ions is larger than that between Ho3+ and Nb5+ ions.

4. Conclusion

Congruent Ho：LiNbO3 crystals doped with 0.1， 0.3 and 0.5 mol% Ho3+ ions are grown by Czochralski technique. The increased concentrations of Ho3+ ions enhance the ultraviolet and violet UC emissions. The ultraviolet and violet UC emissions at 300 nm， 341 nm and 399 nm are populated by the GSA and ESA processes under 550 nm excitation. The 380 nm UC emission and 415 nm/452 nm UC emissions mainly arise from the CR process of 5F4/5S2 + 5I8 → 5I4 + 5I7 and CR of 5F4/5S2 + 5I8 → 5I6 + 5I6/5I4 + 5I7 → 5I6 + 5I6， respectively. The decreased intensity ratio of 380 nm UC emission to 760 NIR emission is attributed to the efficient CR process of 5G4 + 5I8 → 5F4/5S2 + 5I6. Studies on the UV-vis-NIR absorption spectra show that the blueshift of absorption edges is observed in Ho-0.3 and Ho-0.5 crystals， which leads to the shorten distance between the two neighboring Ho3+ ions.

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