A

utuv84

Piib©

K

1

tiasddbebimoi

t

1d

Spectrochimica Acta Part A 66 (2007) 1052–1057

Enhancement of luminescence properties in Er3+ dopedTeO2–Na2O–PbX (X = O and F) ternary glasses

Kaushal Kumar, S.B. Rai ∗, D.K. RaiLaser and Spectroscopy Laboratory, Department of Physics, Banaras Hindu University, Varanasi 221005, India

Received 3 April 2006; accepted 26 April 2006

bstract

An enhancement of luminescence properties in Er3+ doped ternary glasses is observed on the addition of PbO/PbF2. The infrared to visiblepconversion emission bands are observed at 410, 525, 550 and 658 nm, due to the 2H9/2→ 4I15/2, 2H11/2→ 4I15/2, 4S3/2→ 4I15/2, 4F9/2→ 4I15/2

ransitions respectively, on excitation with 797 nm laser line. A detailed study reveals that the 2H9/2→ 4I15/2 transition arises due to three steppconversion process while other transitions arise due to two step absorption. On excitation with 532 nm radiation, ultraviolet and violet upcon-ersion bands centered at 380, 404, 410 and 475 nm wavelengths are observed along with one photon luminescence bands at 525, 550, 658 and43 nm wavelengths. These bands are found due to the 4G11/2→ 4I15/2, 2P3/2→ 4I13/2, 2H9/2→ 4I15/2, 2P3/2→ 4I11/2, 2H11/2→ 4I15/2, 4S3/2→ 4I15/2,F9/2→ 4I15/2 and 4S3/2→ 4I13/2 transitions, respectively. Though incorporation of PbO and PbF2 both enhances fluorescence intensities however,

bF2 content has an important influence on upconversion luminescence emission. The incorporation of PbF2 enhances the red emission (658 nm)

ntensity by 1.5 times and the violet emission (410 nm) intensity by 2.0 times. A concentration dependence study of fluorescence reveals the rapidncrease in the red (4F9/2→ 4I15/2) emission intensity relative to the green (4S3/2→ 4I15/2) emission with increase in the Er3+ ion concentration. Thisehaviour has been explained in terms of an energy transfer by relaxation between excited ions.

2006 Elsevier B.V. All rights reserved.

ifier

bnamporsimiaSfle

eywords: Upconversion; Excited state absorption; Energy transfer; Glass mod

. Introduction

With the development of efficient semiconductor lasers emit-ing in the near infrared regions, the frequency upconversion ofnfrared-to-visible and UV radiation in rare earth doped materi-ls have been given much attention now a days. There are alsoeveral other important applications of these in areas like opticalata transmission, display devices, sensors [1–5]. The rare earthoped glasses probably have greater impact than those of crystalsecause of its several superior properties over crystals such asasy preparation, large RE3+ doping, large inhomogeneous lineroadening etc [1–25]. The tellurite glass have extensively beennvestigated due to its advantages such as good transparency in

id infra-red region, relatively lower phonon frequency amongstxide glasses, high refractive index and good rare-earth solubil-

ty [6,7].

The host materials used for rare-earth doping play impor-ant role for obtaining highly efficient upconversions. This is

∗ Corresponding author. Tel.: +91 542 230 7308; fax: +91 542 236 889.E-mail address: sbrai49@yahoo.co.in (S.B. Rai).

fl

i(hic

386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2006.04.039

ecause host materials with low phonon energy can reduce theon-radiative loss due to the mutiphonon relaxations and thuschieves strong upconversion luminescence. Efforts have beenade on heavy modified glasses (HMO) owing to their lower

honon energies. Another way to achieve this may be by additionf certain heavy metal oxides like BaO, CaO, PbO and their chlo-ides and fluorides, as network modifiers. They are also founduitable for enhancing the upconversion efficiency and stabil-ty of the TeO2 glass [8–10]. The glasses particularity based on

ixed oxide–fluoride systems combine the properties of both,.e. good optical transparency of fluorides and good chemicalnd thermal properties associated with oxide glasses [11,12].o it is expected that the mixed glasses including tellurite anduoride should bring the interesting properties of the glass. Sev-ral studies have been focused on the properties of RE3+-dopeduoride modified tellurite glasses in recent times [12–15].

The motive of the present work is to study the enhancementn luminescence efficiency of Er3+ ions in TeO2–Na2O–PbX

X = O and F) ternary glasses. In our previous work [16], wead synthesized binary glasses TeO2–M2O (M = Li, Na and K)n order to see the effect of alkali modifiers on the fluores-ence/upconversion efficiency of Er3+ ions. Our observations

a Acta Part A 66 (2007) 1052–1057 1053

sdSawr

2

tTNtm

(

w1

i9kstttefilFormmolff

α

wt

3

ifotbgcss

Fs

firfli(

3

soiBrewster’s law, respectively, to evaluate different parametersfor glass samples. Table 1 gives results for the 79TeO2–10Na2CO3–10PbF2–1Er2O3 glass.

K. Kumar et al. / Spectrochimic

how that the fluorescence intensity of Er3+ ions increases withifferent alkali modifiers in the order of K2O < Li2O < Na2O.o, it was concluded that Na2O is the best amongst the otherlkali modifiers. Now, to enhance fluorescence intensity further,e have added PbO/PbF2 in TeO2–Na2O binary glass and the

esults are reported here.

. Experimental

TeO2–Na2O–PbX glasses used in the present work were syn-hesized by the conventional melting and quenching method.he starting materials were analytical grade 99.9% pure TeO2,a2CO3, PbO, PbF2 and Er2O3. The compositions were

aken within the glass forming region and is expressed inol% as

80− x)TeO2+ (20− y)Na2CO3+ yPbO/PbF2+ xEr2O3

here x = 0.5, 1.0, 1.5 and 2.0 mol% and y = 0.0, 4.0, 6.0, 8.0,0.0, 12.0 and 15.0 mol%.

The appropriate batches were thoroughly mixed and meltedn Pt crucible with continuous stirring in an electric furnace at00 ◦C for 20 min and then the melt was poured on a copper plateept at 500 ◦C. The annealing was done in order to minimize thetress in the glasses. The samples were polished carefully to meethe requirement for optical measurements. Emission spectra ofhese glasses were measured using 0.5 m Spex monochroma-or equipped with S-20 photo-multiplier tube as a detector. Twoxcitation wavelengths were used to monitor the upconversion,rst 797 nm line from a Ti:Saphhire laser and second 532 nm

ine from a Nd:YVO4 laser. The IR spectra were recorded on aTIR spectrophotometer (Perkin Elmer, RX 1). For comparisonf fluorescence intensities amongst the glasses with differentare-earth content, spectra were recorded in the same experi-ental conditions and at the same excitation power, keepingodifier and former composition same. Absorption intensity

f the 4S3/2← 4H15/2 transition was measured using halogenamp and monochromator assembly. The absorption coefficientor a particular transition was determined from the followingormula:

(λ) = 2.203D0(λ)/� and D0(λ) = log10I/I0

here � is the sample thickness, I and I0 are the absorbed andhe total incident photon intensity, respectively.

. Results

The ternary glass system TeO2–Na2O–PbO/PbF2 has lim-ted glass forming region, although these glasses have beenound better in efficiencies than the binary glass systems. In ourbservations, the glass transparency decreases as PbO/PbF2 con-ent reaches beyond the 10.0 mol% limit. All the glass sampleselow this limit show good optical transparency and homo-

eneity. The solubility and homogeneity of glass samples werehecked from Er3+ concentration dependent absorption mea-urements for 4S3/2 ← 4I15/2 transition, shown in Fig. 1. Ithows good linearity with the increase in Er3+ concentration

F4

ig. 1. Absorption coefficient vs. Er3+ concentration for the 4S3/2← 4I15/2 tran-ition in TeO2–Na2O–PbO glasses.

rom 0.5 to 2.0 mol%. Some deviation from the linear behaviours found at higher concentrations, which may occur due to satu-ation effect. The effect of the concentration of Er3+ ions on theuorescence intensity is observed and found that the quench-

ng starts when the Er2O3 concentration goes above 1.0 mol%Fig. 2).

.1. Physical properties of glass

The different physical properties of the samples are mea-ured as these parameters are important factor in the productionf laser glass materials. The glass densities and refractivendices were measured by employing the Archimedes and the

ig. 2. The fluorescence intensity vs. Er3+ concentration graph for theS3/2→ 4I15/2 transition in TeO2–Na2O–PbO glasses.

1054 K. Kumar et al. / Spectrochimica Acta Part A 66 (2007) 1052–1057

Table 1Physical properties of the 79TeO2–10Na2CO3–10PbF2–1Er2O3 glass

Physical property Result

M (g) 170.00d (g/cm3) 6.964Rm (cm−3) 10.435N (×1022 ions/cm3) 2.465α (×10−21 cm3) 3.352ε 3.240R (%) 28.570R (A) 64.235F

3

ishffiepotPngipsamflrit

Table 2Wavelength and assignments of upconversion bands and their branching ratiosin the presence of PbO and PbF2 on excitation of 532 nm wavelength

Emission wavelength (nm) βr (for PbO glass) βr (for PbF2 glass)

380 0.002 0.002404–410 0.006 0.009475 0.002 0.001525–550 0.706 0.65468

halrTtofvlmPtwgb

3

utrto TeO and possesses the highest intensity. On the other hand

0

(×1016 cm−2) 1.332

.2. Effect of addition of PbO/PbF2 in glass

The TeO2–M2O (M = Li, Na and K) glasses were preparedn our earlier work [16]. Fluorescence was measured in theseamples and was found that glass with Na2O modifier gives theighest fluorescence. Now to improve the fluorescence intensityurther, we choose this glass and added heavy modifier/glassormer PbO and PbF2 separately. The addition of PbO/PbF2n TeO2–Na2O glass enhances the fluorescence intensity butnhancement in presence of PbF2 is more pronounced than inresence of PbO. The two points are noted: (A) on the additionf PbO/PbF2 in TeO2–Na2O glass, fluorescence intensity fur-her increases and reaches a optimum value at the 10 mol% ofbO/PbF2 content. Further increase in PbO/PbF2 content doesot increase the fluorescence intensity and the glass transparencyoes down; (B) the second point is very interesting. The changen fluorescence intensity at the various emission wavelengths inresence of PbO and PbF2 modified glass samples are not theame. The presence of PbF2 increases the fluorescence intensityt the red (658 nm) and blue (410 nm) wavelengths relativelyore than does at 550 nm, but addition of PbO enhances theuorescence intensity in equal ratio at all wavelengths. The fluo-

escence intensity of violet (410 nm) and red (658 nm) emissionsn PbF2 glass increases by a factor of about 2.0 and 1.5, respec-ively, and is shown in Fig. 3. The fluorescence branching ratios

tvw

Fig. 3. Variation in intensities of the va

58 0.281 0.33243 0.003 0.001

ave been calculated, for both the glass samples containing PbOnd PbF2 separately, to see changes in intensity at different wave-engths and are given in Table 2. The increase in intensity at redegion can be explained by going through the other works [17].he Judd–Ofelt parameters calculated by these authors shows

hat �2 parameter decreases while the �6 parameter increasesn PbF2 substitution in place of PbO and this �6 parameter is theactor for increasing the red emission intensity. The increase iniolet intensity may occur due to increase in lower cutoff wave-ength. On addition of PbF2, the potential (4.03 eV) of F− is

ore than the potential (3.80 eV) of O2−, and bond intensity ofb–F is more than that of Te–O, the polarizability of F− is lower

han that of O2−, and so the excitation energy hν = E + M− ρ,here M the bond energy of anion and cation and ρ is energyained due to polarization of surrounding ions of the absorptionand increases [19].

.3. FTIR spectra

The absorption bands in the IR region have been recordedsing Fourier transform infrared spectrometer. It is observedhat characteristic bands of TeO4 are in 800–350 cm−1 spectralegion. The band at 620 cm−1 in IR spectrum of TeO2 glass is due

2he band at 672 cm−1 is assigned to antisymmetric stretchingibrations of TeO4 group. The intensity of this band decreaseshile the band at 730 cm−1 (antisymmetric stretching vibration

rious bands for different glasses.

K. Kumar et al. / Spectrochimica Acta Part A 66 (2007) 1052–1057 1055

F1

oPoc

4

4

s52

t5ic

dtpoptflricittel

pateidTt

Fu6

ldrdagebieIa 3/2level with moderate rate, however at higher concentrations ofEr3+, the energy transfer between the two excited ions, one in4I9/2 and other in 4S3/2 level, may also contribute to increase the

ig. 4. FTIR spectrum of the glass at different concentration of PbO: (a) for0 mol% PbO; (b) for 5 mol% PbO; (c) for 0 mol% PbO.

f Te with non-bridging oxygen) grows up with the increase inbO content and provides evidence of a growth in the numberf the TeO3 groups in the glass structure with increase in PbOontent. The FTIR spectrum is shown in Fig. 4.

. Upconversion mechanisms

.1. Excitation with 797 nm radiation

On excitation with 797 nm wavelength, observed upconver-ion fluorescence spectrum shows four bands centered at the 410,25, 550 and 658 nm wavelengths. These bands arise from theH9/2← 4I15/2, 2H11/2→ 4I15/2, 4S3/2→ 4I15/2, 4F9/2→ 4I15/2ransitions, respectively. The intense green emission band at50 nm is visible even at pump power <30 mW. This clearlyndicates an efficient upconversion in this glass host. The fluores-ence at 410 nm appears at relatively high pump power > 80 mW.

To get more insight into the upconversion mechanism, theependence of upconversion intensity was measured as a func-ion of the incident pump power P. In frequency upconversionrocess, the upconversion intensity Iup varies with the nth powerf the pump power P (that is Iup∝Pn), where n is the number ofump photons absorbed per visible photon emitted. Fig. 5 showshe log–log plots of intensity versus pump power for the differentuorescence bands with 797 nm excitation. For the green and theed emission the slope of the curves (1.75 and 1.63, respectively)ndicates that these involve two-photon absorption (Fig. 5). Theorresponding value for the 2H9/2→ 4I15/2 transition at 410 nms 2.7 indicating it to involve three incident photons. The depar-ure of n from integral values is a consequence of many fac-ors, e.g. absorption of the upconverted fluorescence, saturationffects, involvement of non-radiative decays and intermediateevels, etc.

The excitation wavelength that is resonant to the 4I9/2 levelopulates it, which is confirmed by the Stokes emission appear-nce at 803 nm. This level relaxes through multiphonon processo the 4I11/2 and the 4I13/2 levels. At low concentration (wherenergy transfer amongst nearby ions is negligibly small) the ions

n these low lying excited states absorb photons from the inci-ent beam and promoted to higher excited levels 4S3/2/2H11/2.he ions excited to these levels decay to the ground state emit-

ing intense green radiation. Some of the excited ions in the 4S3/2Fi

ig. 5. Dependence of upconversion fluorescence intensity on excitation powernder 797 nm excitation, where the curves denotes: (a) 525 nm; (b) 550 nm; (c)58 nm; (d) 410 nm peaks.

evel decay non-radiatively to the 4F9/2 level from which theyecay radiatively to the ground state giving red fluorescence. Theelative variations in the intensities of green and red emission forifferent Er3+concentrations are shown in Fig. 6. The intensitiest different concentrations are normalized with respect to thereen emission intensity. From the figure it is observed that redmission band shows rapid growth relative to the green emissionand. It is also noted that quenching in the fluorescence intensitys observed at the concentrations >1.0 mol% Er2O3 however redmission is not quenched at the same rate as the green emission.t seems that at low Er3+ concentrations, the 4F9/2 level is prob-bly populated via a non-radiative relaxation through the 4S

ig. 6. Relative change in intensities for the green and red emissions with thencrease of Er3+ concentration.

1056 K. Kumar et al. / Spectrochimica Act

p

E

IitacedietdididTaw

at(l

4

flTtplTTrct

pe4a8b8b

tpgFl2

2

Fig. 7. Red emission slopes at the different Er3+ ion concentration.

opulation of 4F9/2 level. This process is expressed as

r3+(4S3/2) + Er3+(4I9/2) → Er3+(4F9/2) + Er3+(4F9/2)

t means that an ion in 4S3/2 level decays to 4F9/2 level andts excess energy is taken up by an ion in 4I9/2 state to raisehe latter to the 4F9/2 state. With inhomogeneous broadeningnd Stark splitting of the levels in question, there is suffi-ient energy coincidence between the two states of the abovequation. The inter-ionic distance and experimental evidence ofecrease in the slope between the rare-earth concentration andncident photon intensity for the red emission also favours thenergy transfer process. The inter ionic distance between thewo Er3+ ions is found∼20 A in the case of 0.5 mol% rare-earthoping and ∼12 A in the case of 2.0 mol% rare-earth dopingn 79TeO2–10Na2O–10PbF2 glass. At the 0.5 mol% rare-earthoping, ions are far enough to avoid ET and so ESA is the dom-nant process at this concentration. The nonlinearity in slopes at

ifferent Er3+ concentrations may also indicate the ET process.he slopes for the red emission at different Er3+ concenrationre shown in Fig. 7. The violet emission also increases rapidlyith the Er3+ concentration, but since this emission appears even

l8cE

Fig. 8. Fluorescence spectrum of 1.0 mol% Er3+ doped

a Part A 66 (2007) 1052–1057

t low (0.5 mol%) Er3+ ion concentrations, it is concluded thathe 2H9/2 level is populated through excited state absorptionabsorption of an incident photon by an excited ion) in the 4F9/2evel.

.2. Excitation with 532 nm radiation

The 532 nm radiation as excitation source leads to additionaluorescence bands at 380, 404, 475 and 843 nm wavelengths.he emission bands in the region 404–410 nm are probably due

o overlapping of emission from two different levels. The smallereak at 404 nm is due to the 2P3/2→ 4I13/2 transition and thearger one at 410 nm is ascribed to the 2H9/2→ 4I15/2 transition.he band at 475 nm is assigned to the 2P3/2→ 4I11/2 transition.he band due to 2P3/2→ 4I15/2 transition expected at 320 nm

egion which has smaller transition probability is not observedlearly except a broad feature around 320 nm. This may be dueo the limitations of our apparatus and absorption by the glass.

The log–log plots of the fluorescence intensity versus theump power for the various observed bands indicate two-photonxcitation for the ultraviolet (380 nm) and the violet (404 and10 nm) bands and one photon excitation for the green (525nd 550 nm) and the red (658 nm) bands. The bands at 475 and43 nm are too weak so that their power dependence could note studied. Considerations of energy levels of Er3+ suggest that43 nm peak is due to a one-photon process whereas 475 nmand involves two incident photons.

The excitation energy (18,796 cm−1) is ∼500 cm−1 greaterhan the energy of 4S3/2 level, so this level is populated throughhonon relaxation. The radiative decay of the 4S3/2 level toround level gives strong fluorescence at 550 nm wavelength.or emission at 380, 404 and 475 nm wavelengths, the 4S3/2

evel absorbs second incident photon and excited to high lyingD5/2 level. The 2D5/2 level first relax non-radiatevely to theP3/2 and 4G11/2 levels and then radiatively to the ground and

ow lying excited levels. It is observed that emission at 410 and43 nm wavelengths are viewed simultaneously at the loweroncentrations of Er3+ ions. But, when the concentration ofr3+ ions become high, the emission intensities are reduced

TeO2–Na2O–PbO glass under 532 nm excitation.

K. Kumar et al. / Spectrochimica Act

Ft

gll(ttD4pftntutisl

5

fl

omewafl

A

atS

R

[[

[

[[[

[[[[

[

[[

ig. 9. Simplified energy level diagram of Er3+ ion and possible upconversionransition pathways for 532 nm excitation.

reatly and at 2.0 mol% Er3+ ions, emission at both the wave-engths disappears. The reason for this behaviour is that the 4S3/2evel populates to the 4I13/2 level through radiative emission4S3/2→ 4I13/2) and that the 4I13/2 level has much longer life-ime so population builds up in this level sufficiently. This levelhen absorbs second incident photon and excites the 2H9/2 level.ecay of the 2H9/2 level to the ground level gives emission at10 nm. But, when Er3+ concentration gets high the 4S3/2 levelreferably transfer its energy to 4F9/2 level through energy trans-er process rather than decay to the 4I13/2 level. As a result ofhis no second photon absorption take place and the result iso emission from the 2H9/2 level, which is an observed fact. Athe lower Er3+ concentrations (<1.5 mol%), the 4F9/2 level pop-lates through non-radiative relaxation of 4S3/2 level but abovehis concentration the energy transfer to this level is also anmportant contribution. The observed fluorescence spectrum ishown in Fig. 8 and the various pathways are shown in the energyevel diagram (Fig. 9).

. Conclusions

Er3+ doped ternary tellurite glasses are found to give betteruorescence efficiency than binary tellurite glasses. The addition

[

[

[

a Part A 66 (2007) 1052–1057 1057

f PbO and PbF2 both modifiers separately shows enhance-ent in fluorescence intensity but effect of PbF2 is more on the

mission wavelengths at 658 and 410 nm. Excitation of samplesith 797 and 532 nm results in many fluorescence emissions

t different wavelengths. The green emission has the maximumuorescence efficiency.

cknowledgements

Authors are grateful to CSIR and DST New Delhi for financialssistance. One of the authors (Mr. Kaushal Kumar) would like tohank M/S Laser Science Bombay and The Laser Spectroscopyociety of India for a Scholarship.

eferences

[1] D. Lande, S.S. Orlov, A. Akelta, L. Hesselink, R.R. Neurgaonkar, Opt. Lett.22 (1997) 1722.

[2] E. Downing, L. Hesselink, J. Ralston, R. Macfarlane, Science 273 (1996)1185.

[3] P. Xie, T.R. Gosnell, Opt. Lett. 20 (1995) 1014.[4] S.O. Man, E.Y.B. Pun, P.S. Chung, Appl. Phys. Lett. 77 (2000) 483.[5] L.H. Huang, X.R. Liu, W. Xu, B.J. Chen, J.L. Lin, J. Appl. Phys. 90 (2001)

5550.[6] M. Tsuda, K. Sousa, H. Inoue, S. Inoue, A. Makishima, J. Appl. Phys. 85

(1999) 29.[7] P.V. dos Santos, E.A. Gouveia, M.T. de Araujo, A.S. Liouveia-Neto,

A.S.B. Sombra, J.A. Medeiros Neto, Appl. Phys. Lett. 74 (1999)3607.

[8] D. Wang, W. Zhang, M. Yin, Opt. Mater. 27 (2004) 605.[9] Z. Pan, S.H. Morgan, A. Loper, V. King, B.H. Long, W.E. Collins, J. Appl.

Phys. 77 (1995) 4688.10] R. Reisfeld, Y. Eckstein, J. Chem. Phys. 63 (1975) 4001.11] T. Murata, Y. Moriyama, K. Morinaga, Sci. Technol. Adv. Mater. 1 (2000)

139.12] D. Chen, Y. Wang, Y. Yu, E. Ma, F. Bao, Z. Hu, Y. Cheng, Mater. Chem.

Phys. 95 (2006) 264.13] Y. Kishi, S. Tanabe, J. Alloys Compd. 408 (2006) 842.14] S. Tanabe, K. Hirao, N. Soga, J. Non-Cryst. Solids 122 (1990) 79.15] J. Yang, N. Dai, S. Dai, L. Wen, L. Hu, Z. Jiang, Chem. Phys. Lett. 376

(2003) 671.16] K. Kumar, S.B. Rai, D.K. Rai, Solid State Commun., communicated.17] L. Zhang, H. Sun, S. Xu, K. Li, L. Hu, J. Lumin. 117 (2006) 46.18] J.S. Wang, E.M. Vogel, E. Snitzer, Opt. Matter. 3 (1993) 187.19] S. Xu, G. Wang, J. Zhang, S. Dai, L. Hu, Z. Jiang, J. Non-Cryst. Solids 336

(2004) 230.20] M. Pollnau, D.R. Gamelin, S.R. Luthi, H.U. Gudel, M.P. Hehlen, Phys.

Rev. B 61 (2001) 3357.21] M.P. Hehlen, G. Frei, H.U. Gudel, Phy. Rev. B 60 (1994) 16265.22] N. Jaba, A. Kanoun, H. Mejri, A. Selmi, S. Alaya, H. Maaref, J. Phys.

Condens. Matter 12 (2000) 4523.

23] A.K. Singh, S.B. Rai, D.K. Rai, Solid State Commun. 136 (2005)

346.24] G.S. Maciel, C.B. de Araujo, Y. Messaddeq, M.A. Aegerter, Phys. Rev. B

55 (1997) 6335.25] A. Gharavi, G.L. Mcpherson, Appl. Phys. Lett. 61 (22) (1992) 2635.