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Spectrochimica Acta Part A 71 (2008) 508–512

Strong blue emission from Pr3+ ions through energy transfer processfrom Nd3+ to Pr3+ via Yb3+ in tellurite glass

Kaushal Kumar a, S.B. Rai a,∗, Anita Rai a,b

a Laser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, Indiab Department of Chemistry, Govt. College, Gazipur, UP, India

Received 31 July 2007; accepted 13 December 2007

bstract

Energy transfer excited upconversion emission in Nd3+/Pr3+-codped tellurite glass have been studied on pumping with 800 nm wavelength.he upconversion emission bands from Pr3+ ion are observed at the 488, 524, 546, 612, 647, 672, 708 and 723 nm due to the (3P0 + 3P1) → 3H4,P1 → 3H5, 3P0 → 3H5, 3P0 → 3H6, 3P0 → 3F2, 3P1 → 3F3, 3P0 → 3F3 and 3P0 → 3F4 transitions, respectively. The addition of ytterbium ions (Yb3+)n the upconversion emission intensity is also studied and result shows an eight times enhancement in the upconversion intensity at 488 nm from

r3+ ions. The pump power and concentration dependence studies are also made. It is found that Yb3+ ions transfer its excitation energy to Nd3+

rom which it goes to Pr3+. No direct transfer to Pr3+ is seen. This is verified by codoping Nd3+ and Pr3+ into the host.2008 Elsevier B.V. All rights reserved.

ACS: 42.70.Hj; 42.70.Ce

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eywords: Optical properties; Upconversion; ESA; Luminescence; Energy tran

. Introduction

The rare-earth doped tellurite glasses have been the subjectf several spectroscopic investigations, due to their potentialpplications in various areas like optical sensing, telecom-unications, biomedical, biochemical studies, etc. [1–4]. The

haracteristic properties of tellurite host is high refractive index,ong infrared transmission range (0.35–5 �m), higher stabilitygainst divitrification, high non-linear index and lower cut ofhonon frequency (<800 cm−1) amongst other oxide glass for-ers [5]. Tellurium dioxide (TeO2), like vanadium pentaoxide

V2O5), is a conditional glass former and requires a modifiero form the stable glass. In a binary tellurite composition, theffect of the free electron pair is limited by the introductionf the modifier. Addition of alkali oxides in the TeO2 matrixhanges the coordination of Te from a TeO4 trigonal bipyramido TeO3 trigonal pyramid and TeO3+1 polyhedra.

In recent years, increasing interest has been found inpconversion studies in single as well as in multi-ions dopedlasses/crystals. The study of frequency upconversion pro-

∗ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 236 9889.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

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386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2007.12.043

esses is important to understand the mechanisms of interactionetween the RE ions in different hosts. These studies have ofteneads to the discovery of new lasers based on energy transfer. It

ay also help to identify appropriate hosts for efficient upcon-ersion emissions. The multi-ions doped materials give emissionands that are not seen in single ion-doped materials, with simi-ar excitations. The upconversion emissions from Pr3+ ions haveeen observed in glasses codoped with Nd3+/Yb3+. Yb3+ ion hasarge absorption cross-section [6] in the region of 980 nm and theong lifetime of the excited 2F5/2 level acts as energy reservoirn upconversion process. For 800 nm pumping, there is strongbsorption cross-section of Nd3+ ions. An efficient excitationransfer (ET) has been observed between Nd3+ as a donor andr3+, Tm3+, Tb3+ and Sm3+ Ho3+, etc. as acceptors [7–12].

Recently, efficient frequency upconversion emissions fromr3+ ions have been reported in Pr3+/Nd3+ codoped fluoroin-ate glass [13]. These authors have observed the energy transferpconversion in Pr3+ through Nd3+ ions and laser action at blue488 nm) and red (613 and 716 nm) bands in Nd3+:Pr3+ dopedlass fibre on pumping with 796 nm radiation. A trace presence

f Yb3+ in Nd3+/Pr3+ combination has been found to enhancehis intensity further and Gouveia-Neto et al. [14] studied theffect of addition of Yb3+ ions in codoped Pr3+/Nd3+ glass andound fourfold enhancement in blue emission from Pr3+ ions.

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from Pr3+ ions must occur due to the energy transfer fromNd3+ to Pr3+ ions. The concentrations of both the rare earthions in the sample have been optimized to get the maximumfluorescence intensity from the Pr3+ ions. Beyond this con-

K. Kumar et al. / Spectrochim

ere Yb3+ acts as a bridging ion to transfer effectively energyrom Nd3+ to Pr3+.

The present study reports the observation of upconversionmissions from Pr3+ ions due to efficient energy transfer fromd3+ to Pr3+ in presence/absence of Yb3+ ions in tellurite glassn 800 nm excitation. Efforts have been made to understand andsolate the most possible energy transfer routes.

. Experimental

The samples used in present investigations have been pre-ared by the conventional melt quenching method with theollowing compositions:

80 − x)TeO2 + 20Li2CO3 + xNd2O3 (1)

here x = 0.25, 0.5, 0.7, 1.0, 1.25 and 1.5.

80 − x − y)TeO2 + 20Li2CO3 + xPr2O3 + yNd2O3 (2)

here x and y varies from 0.00 to 1.5 mol%

79.3 − z)TeO2 + 20Li2CO3 + 0.20Pr2O3 + 0.50Nd2O3

+ zYb2O3 (3)

here z = 0.0–3.5 mol%The method used in glass preparation is described in Ref.

15]. Upconversion measurements were made by excitinghe samples with 800 nm wavelength from Ti:Sapphire laserumped by a Nd:YVO4 laser. Fluorescence was dispersed using0.5 m monochromator and detected by Hamamatsu R928 pho-

omultiplier tube. The emission decay was recorded by choppinghe pump beam and viewing the decay profile using a 150 MHzigital storage oscilloscope.

. Results and discussion

In order to observe the energy transfer between the Nd3+,r3+ via Yb3+ ions, we studied spectra of: (a) singly doped glass,b) doubly doped glass and (c) triply doped glass samples. Ourndings are presented schematically below.

.1. Excitation of single Nd3+ ions-doped glass

We tried to observe the upconversion fluorescence emissionrom singly doped Nd3+ glass at different ion concentrations.n excitation with 800 nm radiation, samples with differentd3+ concentrations fluoresce with different dominant colourshich are visible even with bare eye. When the Nd3+ ion

oncentration is below 0.5 mol%, emission looks blue in colour.ithin a concentration range of 0.5–1.0 mol% it gives green-

ellow appearance and when concentration is above 1.0 mol

it shines yellowish-orange. Quenching in emission intensity

s observed for the Nd3+ ion concentration above 1.0 mol%.he observed emission bands with their assignments are listedelow:

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ta Part A 71 (2008) 508–512 509

30 nm 2P1/2 → 4I9/2

82 nm 2P1/2 → 4I11/2

25 nm 2P1/2 → 4I13/2

35 nm 4G7/2 → 4I9/2

80 nm 2P1/2 → 4I15/2

00 nm 4G7/2 → 4I11/2 and 4G5/2 + 2G7/2 → 4I9/2

64 nm 4G7/2 → 4I13/2

66 nm 4G7/2 → 4I15/2

Along with the above emission bands, two stokes emis-ion bands are also observed at 814 and 874 nm due to theF3/2 → 4I9/2 and 4F5/2 → 4I9/2 transitions, respectively (Fig. 1).

detail of this is already discussed in our earlier paper [16].o fluorescence from Pr3+ or Yb3+ ions is seen individually by00 nm excitation.

.2. Excitation of double ions-doped glasses

(i) Pr3+ + Yb3+ ions: The Yb3+ + Pr3+ double-doped telluriteglass does not show any fluorescence emission on exci-tation with 800 nm radiation. This is due to the fact thatneither Yb3+ nor Pr3+ absorbs this wavelength.

(ii) Nd3+ + Yb3+ ions: A glass-containing Nd3+ and Yb3+

together show all the transitions of Nd3+ with enhancedemission intensity.

iii) Nd3+ + Pr3+ ions: An excitation with 800 nm radiation ofNd3+ + Pr3+-doped glass emit fluorescence from Pr3+ aswell as from Nd3+ ions. The 800 nm radiation energymatches to energy of the 4F5/2 level of Nd3+ion. The exci-tation to the Pr3+ ions with the same wavelength does notshow any fluorescence in visible region because the exci-tation energy is ∼4200 cm−1 shorter to the energy of 1D2level. However in presence of Nd3+/Pr3+ both, fluorescencefrom Pr3+ is also seen (see Fig. 2). These emission bands

ig. 1. Upconversion spectrum of Nd3+ doped glass samples at three differentoncentrations of Nd3+ ions.

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1. In the double-doped (Nd3+/Pr3+) sample, the pump photons

ig. 2. Upconversion emission spectrum of single(Nd3+), double(Nd3+ + Pr3+)nd triple(Nd3+/Pr3+/Yb3+)-doped tellurite glass.

centration of Nd3+ ions quenching in the emission intensitystarts takes place.

Fig. 3 shows fluorescence quenching in single andouble(Nd/Pr)-doped samples. Graph A represents the quench-ng of Nd3+ ions and graph B represents the quenching of Pr3+

ons in presence of Nd3+ ions. It is noted that quenching ofuorescence intensity in doubly doped sample occurs at lessoncentration value than that of Nd3+ alone. In the presence ofr3+ ions, excited Nd3+ ions transfers a part of energy to thenexcited Pr3+ ions and the emissions are observed from 3P1nd 3P0 levels of Pr3+ to various low lying levels. In Fig. 2, one

nds a typical upconversion emission spectrum of radiationmanating from the Nd3+/Pr3+ codoped sample. The spectrumxhibit following emission bands from Pr3+ ions:

ig. 3. Graph showing concentration quenching in single (Nd3+)-doped samplend double (Nd3+ + Pr3+)-doped samples. F

ta Part A 71 (2008) 508–512

ransition Wavelength (nm)

3P0 + 3P1) → 3H4 470–488P1 → 3H5 524P0 → 3H5 546P0 → 3H6 612P0 → 3F2 647P1 → 3F3 672P0 → 3F3 708P0 → 3F4 723

.3. Excitation of triple Nd3+ + Pr3+ + Yb3+ ions dopedlass

Addition of Yb3+ ions in the glass-containing Pr3+ and Nd3+

esults the same bands however it produces eight times enhance-ent in the upconversion emission intensity of the bands from

he Pr3+ ions (see Fig. 2). Fig. 4 shows the variation of upcon-ersion intensity of 488 nm emission with the concentration ofb3+ ions at a fixed concentration of Pr3+and Nd3+ ions and the

ntensity is maximum around 1.5 mol% concentration of Yb3+

ons. A decrease in intensity is observed for Yb3+ content above.5 mol% indicating the back energy transfers to Nd3+ ions.

. Upconversion mechanism

In order to understand the actual process/processes responsi-le for the upconversion emission from the Pr3+ ions, extensivetudies have been made for glasses having different concentra-ions of rare earth ions. The power dependence has also beenerformed. The following energy transfer channels are found toe possible:

first populate the 4F5/2, 2H9/2 levels of Nd3+ ions. The lifetimeof 4F5/2 level is ∼10−7 s. So the Nd3+ ions rapidly relax tometastable level 4F3/2. Absorption of another incident photon

ig. 4. Variation in 3P0 → 3H4 emission intensity with Yb3+ concentration.

K. Kumar et al. / Spectrochimica Acta Part A 71 (2008) 508–512 511

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in 4F3/2 excites the ion to 2P1/2 level through ESA. The ionsin 2P1/2 level/levels lower to it (populated by the nonradiativerelaxation process) transfer energy to the Pr3+ ions excitingit in 3P2/3P1 level. Due to this the upconversion intensityarising due to the 2P1/2 → 4Ij transitions are reduced. It isfurther reduced appreciably with the increase in concentra-tion of Pr3+ ions. It indicates a direct involvement of 2P1/2level in energy transfer process. The 3P2 level has greaterabsorption cross-section than the 3P1 or 3P0 level and hencepossesses greater transfer probability [16]. The energy mis-match between the 2P1/2 level of Nd3+ and 3P2 level of Pr3+

being within KT, a strong coupling between them is expected.Thus the mechanism is

2P1/2(Nd) + 3H4(Pr) → 4I9/2(Nd) + 3P2(Pr)

. In the triple-doped (Nd3+ + Pr3+ + Yb3+) samples Yb3+ ionsserve double role, as an accepter and as a donor that bridgebetween Nd3+ and Pr3+ ions. The Yb3+ ions are excited to2F5/2 level by energy transfer from 4F3/2 level of Nd3+ ion.The emission at 880 nm due to the 4F3/2 → 4I9/2 transitiondisappears in the samples doped with Yb3+ ions which sup-port the above process. The excited Yb3+ ion in 2F5/2 levelmay loose its excitation energy through two different ways:(a) the Yb3+ ions transfer its excitation energy to the 1G4level of Pr3+ ion which is close to 2F5/2 level of Yb3+ ionand (b) it may loose energy through the cooperative way. Inprocess (a), the excited Yb3+ ion is in resonance with 1G4level of Pr3+ ion. The two ions in 1G4 level could share their

energy in a way that one ion goes to 3P0 level and the otherto ground state. But this way of energy transfer in samplescontaining Pr3+ and Yb3+ ions is neither observed in our casenor by earlier workers [14,17]. So, the upconversion emis-

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n (Nd–Yb–Yb–Pr) energy transfer system.

sion in triple doped glass due to this process is least probable.The emission at 488 nm from Yb3+ ions due to cooperativeenergy transfer is well-established phenomenon [18]. Theexcited Yb3+ ions cooperatively excites 2D3/2, 4G11/2 levelsof Nd3+ i.e. three ion participation (Yb–Yb–Nd) due to theirenergy match. The excited Nd3+ ion in 4G11/2, 2D3/2 levelstransfer their energy to 3Pj of Pr3+ i.e. four ion participation(Yb–Yb–Nd–Pr). The whole process is

4F3/2(Nd) + 2F7/2(Yb) → 4I9/2(Nd) + 2F5/2(Yb)

2 2F5/2(Yb) + 4I9/2(Nd)

→ 22F7/2(Yb) + 4G11/2,2D3/2(Nd)

and 4G11/2,2D3/2(Nd) + 3H4(Pr) → 4I9/2(Nd) + 3Pj(Pr)

Since this channel is very efficient, the 3Pj level will be pop-ulated highly resulting intense emission from this level. Theexcitation mechanism is shown in Fig. 5.

The dependence of the upconversion emission intensity ofP0 → 3H4 transition upon the excitation power was also exam-ned and result show that the intensity of 488 nm emission isescribed by I488 � P1.67. The slope value n ∼ 1.67 (Fig. 6) isess than 2 and suggests involvement of different excitationays.Time evolution of the 3P0 level has also been studied in triply

oped sample and the decay curve shows large initial risetime∼1.5 ms). This large risetime is due to the involvement of Yb3+

ons in upconversion emission. The lifetime of 3P0 level is foundo be ∼23 �s.

512 K. Kumar et al. / Spectrochimica Ac

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ig. 6. Log–log plot of the upconversion emission intensity (488 nm) as a func-ion of the excitation power.

. Conclusions

The energy transfer upconversion emission from Pr3+ haseen studied in presence of Nd3+ in tellurite glass on excitationith 800 nm radiation. Emission from the 3P0 level of Pr3+ ion

s observed at 488, 524, 546, 612, 647, 672, 708 and 723 nm.

he addition of ytterbium ions has been found to enhance ther3+ upconversion emission several times. The Yb3+ serves asridging ion in transferring energy effectively from Nd3+ to Pr3+.he possible mechanisms for energy transfer are discussed.

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ta Part A 71 (2008) 508–512

cknowledgements

Authors are grateful to CSIR and UGC, New Delhi for finan-ial assistance.

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