Investigation on energy transfer from Er3+ to Nd3+ in tellurite glass

JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008, p. 899

Investigation on energy transfer from Er3+ to Nd3+ in tellurite glass SHEN Xiang ( ), NIE Qiuhua () , XU Tiefeng ( ), DAI Shixun (  ), WANG Xunsi ( ) (Faculty of Information Science and Engineering, Ningbo University, Ningbo 315211, China) Received 5 August 2007; revised 10 January 2008

Abstract: A study of energy transfer of Er3+/Nd3+ codoped tellurite glasses was presented. By Nd3+ co-doping, both the Er3+ green emission corresponding to the Er3+: (4S3/2, 2H11/2)→4I15/2 transitions and the red emission corresponding to the Er3+: 4F9/2→4I15/2 transitions were quenched. The energy transfer mechanism between Er3+ and Nd3+ was discussed based on their energy level characteristics. The interaction parameters, CD-A , for the energy transfer processes from Er3+ to Nd3+ in tellurites glass were calculated. Finally, the resonant transfer Er3+: 4 I9/2→Nd3+: (4F5/2, 2H9/2) was proposed to be the most probable microscopic process to occur in contrast with the other processes. Keywords: energy transfer; tellurite glass; rare earths

Several examples of energy transfer between rare earth (RE) ions are known to exist. Energy absorbed by one RE ion, which will otherwise appear as fluorescence may be lost to the lattice through the intermediary of a second RE ion. In addition, energy gained by one RE ion via transfer from a second may produce an enhancement of its emission[1]. Until now, energy transfer between Er3+ and other RE ions have been investigated extensively. For example, the intensity of Er3+ emission in tellurite, bismuth glasses, and crystals is strengthened by energy transfer from Yb3+ ions[2–4]. Energy transfer from Er3+ to Tm3+ and Ho3+ ions in crystals were reported by Johnson et al.[1]. Ce3+ co-doping improves the 1.5 µm fluorescence characteristic of Er3+ owing to the energy transfer between them[5]. Several studies have reported the energy transfer between Er3+ and Nd3+ in crystals[6–8]. In this study, the interaction among optically exited ions of Er3+-Nd3+ system in tellurite glass was presented.

annealed to room temperature at a rate of 10 °C/h, and were then cut and polished carefully in order to meet the requirements for the optical measurements. The absorption spectra were recorded between 400 and 2000 nm with Perkin-Elmer Lambda 950 UV-VIS-NIR spectrophotometer. The emission spectra were measured with TRIAX550 spectrophotometer on excitation at 800 nm laser diode (LD). The temporal decay shapes of the fluorescence signals for 880 nm and 1.5 µm bands, which were pumped by a moduated 800 nm LD, were stored after averaging 128 times by a Tektronix TDS3052 digital oscilloscope. The lifetimes of 4F3/2 level of Nd3+ and 4I13/2 level of Er3+ were obtained by fitting the exponential function to the experimentally observed fluorescence decay curves. All measurements were taken at room temperature.

1 Experimental

Fig.1 illustrates the absorption spectra of Er3+/Nd3+ codoped tellurite glasses in the visible and near-infrared region, which indicates the major transitions associated to the RE ions present in the host matrix. The absorption of Nd3+ for the 4I9/2→ (4F5/2, 2H9/2) transition overlaps that of Er3+ for the 4 I15/2→4I9/2 transition. Since the absorption of Er3+ at 800 nm is very weak, the absorption of Nd3+ around 800 nm is predominant in this co-doped sample. Another absorption band around 522 nm is attributed to Nd3+: 4I9/2→4G7/2 and Er3+: 4I15/2→2H11/2 transition as shown in Fig.1.

The compositions of glasses employed were (70–x) TeO2-14.5ZnO-15Na2O-0.5Er2O3-xNd2O3 (x=0, 0.2, 0.5, 0.8, 1) and 70TeO2-14.5ZnO-15Na2O-0.5Nd2O3 in mol%. The starting materials were reagent grade TeO2, ZnO, and Na2CO3. About 10 g batches of starting materials were fully mixed and then melted in Pt crucibles at 850 °C in an electric furnace. When the melting was complete, the melt liquid was cast into stainless steel plate. The obtained glasses were

2 Results and discussion

Foundation item: Project supported by the Natural Science Foundation of Zhejiang Province (2006C21082) and the Education Department Project of Zhejiang Province (20061664) Corresponding author: SHEN Xiang (E-mail: [email protected]; Tel.: +86-574-87600947)

3') SGI)DFWRU\3UR ZZZILQHSULQWFQ

900

The upconversion fluorescence spectrum of Er3+ (0.5mol%) in tellurite glass at room temperature (295 K) is presented in Fig.2, showing three peaks centered at about 533, 547, and 656 nm when excited by 800 nm LD. Fig.3

Fig.1 Absorption spectra of 0.5mol% Er3+-doped tellurite glasses co-doped with: (1) 0 and (2) 0.5mol% Nd3+

Fig.2 Upconversion spectrum of Er3+ in tellurite glass under 800 nm excitation

Fig.3 Measured integrated intensities of green and red luminescences in Er3+/Nd3+ co-doped glass under 800 nm excitation

JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008

indicates the variation of Er3+ emissions (2H11/2→4I15/2, 4 S3/2→4I15/2, and 4F9/2→4I15/2) with varying Nd3+ concentration at room temperature. As seen in Fig.3, the intensity of green emission rapidly decreases with increasing Nd2O3concentration. The red emission intensity rapidly increases at first, and then slightly decreases with further addition of Nd2O3 content. The possible upconversion mechanism for the green-red emissions upon excitation at 800 nm LD shown in Fig.4 can be written as follows: (1) ground state absorption (GSA): Er3+ (4I15/2)+a photon→Er3+ (4I9/2), (2) excited state absorption (ESA): Er3+ (4I13/2)+a photon→Er3+ (2H11/2), and Er3+ (4I11/2)+a photon→Er3+ (4F3/2), (3) energy transfer (ET): Er3+ (4I13/2)+Er3+ (4I11/2)→Er3+(4F9/2) +Er3+ (4I15/2), and Er3+ (4I11/2)+Er3+ (4I11/2)→Er3+ (4F7/2)+Er3+ (4I15/2). According to the energy matching, the possible energy transfer (ET) from Er3+ to Nd3+ can be attributed to the following processes[6]: (1) Er3+ (4I9/2)+ Nd3+ (4I9/2)→Er3+ (4I15/2)+Nd3+ (4F5/2, 2H9/2); (2) phonon-assisted ET: Er3+ (4I11/2)+Nd3+ (4I9/2)→Er3+ (4I13/2)+Nd3+ (4I13/2); (3) phonon-assisted ET: Er3+ (4I13/2)+Nd3+ (4I9/2)→ Er3+ (4I9/2)+Nd3+ (4I15/2). These ET processes, illustrated in Fig.4, reduce the population and the lifetime of Er3+ (4I11/2) and Er3+ (4I13/2) levels; in fact, the lifetime for Er3+ (4I13/2) level decreases from 3.16 ms to 360 µs with increase of Nd2O3 content. Since the green-red emissions were based on the long-lived Er3+ (4I11/2), and Er3+ (4I13/2) levels[3], the intensities of green-red emissions were reduced as aforementioned. The reason why the green emission decreases faster than the red emission may be attributed to the fact that the efficiency of ET process (2) from Er3+ to Nd3+ was higher than that of process (3). In this article, erbium is designed as donor (D), while the ion responsible for the depopulation of donor levels will be called acceptor (A)——which is the role of Nd3+ ions. A method, reported by Tarelho et al.[9], can be used to calcu-

Fig.4 Schematic diagram of the energy levels of Er3+ and Nd3+ ions in tellurite glasses, and energy transfer processes

3') SGI)DFWRU\3UR ZZZILQHSULQWFQ

SHEN X et al., Investigation on energy transfer from Er3+ to Nd3+ in tellurite glass

late the important microscopic parameters of interaction between RE ions (i.e., critical radius (RC) and transfer constant (CD-D or CD-A (cm6/s)) based on the actual theory of phonon-assisted energy transfer. The transfer constant (cm6/s) is defined as follows: 6

CD-D =

RC

(1)

τD

where, τD is the total lifetime of the donor state without the presence of the acceptor; and RC is the critical radius of interaction, calculated using the overlap integral method based on the calculation of the emission (donor) and the absorption (acceptor) cross-section superposition. In the case of phonon-assisted (nonresonant) energy transfer, RC can be calculated using the extended overlap integral method. The expressions are given in Eq.(2) for resonant RC =

low

6cτ D

6

4

gD 2

up

(2π ) n g D

∫σ

D emis

A

(λ )σ abs (λ ) d λ

and Eq.(3) for phonon-assisted low 6cτ D g D ∞ 6 D + A RC = σ emis (λN )σ abs (λ ) d λ × 4 2 up ∑ ∫ (2π ) n g D N = 0

⎛ P+ P− P+ ⎞ ⎜ ∑ ( N −k ) k k ⎟ ⎝ k=0 ⎠

(2)

901

emission cross-sections at –1532 nm for the optical transition involving 4I15/2 and 4I13/2 states of Er3+. Using the same method, one can obtain the absorption and emission crosssections at –980 nm and –800 nm for the excited states 4I11/2 and 4I9/2, respectively. Using these data in Eq.(2), the critical radii of resonant (Er-Er) energy transfer and respective CD-D were calculated. The obtained values are given in Table 2. Fig.6 shows the spectral cross-section superposition between 4I9/2→4I15/2 emission band of Er3+ and the fundamental absorption of Nd3+: 4I9/2→ (4F5/2, 2H9/2). Fig.7 exhibits the spectral cross-section superposition between the one-phonon emission sideband of 4I13/2→4I15/2 zero phonon emission of Er3+ and the absorption of Nd3+: 4I9/2→4I15/2. Fig.8 shows the Table 1 Lifetime measured for excited states of Er3+ in tellurite glass Level

τD/ms

4

I13/2

3.19

4

I11/2

0.37

4

I9/2

0.28

(3)

N

where, n is the refractive index of the medium, c is the speed of light, and σAabs(λ) and σDemis(λ) are the absorption (acceptor) and emission (donor) cross-section spectra. gDlow and gDup are the degeneracies of donor (D) states, respectively, from the lower and upper levels involved in the process. N is the total number of phonons involved in the process, (N-k) is the number of phonons emitted (or created) by the donor, and k is the number of phonons absorbed (or annihilated) by the acceptor. λ+N denotes the wavelength translation of the emission cross-section spectrum by E=[Nћω]–1, owing to the multiphonon emission by the donor. λ+N can be obtained using 1 + λN = (4) 1 λ − N • ω0 P+(N-k) is the probability of multiphonon emission by the donor state. P–k is the probability of the acceptor to absorb k phonons and P+k is the probability of the acceptor to emit k phonons in the process. The electron-phonon coupling constant S0 has been estimated to be ~0.31[9] and the mean phonon energy that couples with the phonon sideband is ћω–750 cm–1 for Er3+ in tellurite glass[5]. The lifetimes measured at room temperature for excited states of Er3+ and the results are listed in Table 1. The absorption and emission cross-sections were obtained based on the McCumber theory, which was well described earlier[10]. As an example, Fig.5 shows the absorption and

Fig.5 Spectral cross-section superposition between fundamental 4 I15/2→4I13/2 absorption and 4I13/2→4I15/2 emission of Er3+ at ~1532 nm

Fig.6 Spectral cross-section superposition between 4I9/2→4I15/2 emission of Er3+ and the fundamental absorption of Nd3+: 4I9/2→ (4F5/2, 2H9/2)

3') SGI)DFWRU\3UR ZZZILQHSULQWFQ

902

JOURNAL OF RARE EARTHS, Vol. 26, No. 6, Dec. 2008

Fig.7 Spectral cross-section superposition between one-phonon emission sideband of 4I13/2→4I15/2 zero phonon emission of Er3+ and absorption of Nd3+: 4I9/2→4I15/2

spectral cross-section superposition between 4I11/2→4I13/2 emission of Er3+ and the fundamental absorption of Nd3+: 4 I9/2→4I13/2. Table 3 lists the microscopic parameters obtained using these data. Comparing the obtained CD-A values for the energy transfer from Er3+ to Nd3+ in tellurite glasses at room temperature, it can be found that the resonant transfer Er3+: 4I9/2→Nd3+: (4F5/2, 2H9/2) is the most probable microscopic process to occur in contrast with the other processes. Our calculated critical radii (1) RC=1.53 nm CD-D=40.×10–40 cm6/s for Er3+: 4 I13/2→Er3+:4I13/2 transfer, and (2) RC=0.85 nm and CD-A= 1.18×10–40 cm6/s for Er3+: 4I13/2→Nd3+: 4I15/2 are similar to the values in Refs.[11,12] and Ref.[8], respectively.

3 Conclusion Er3+ and Nd3+ co-doped tellurite glasses were prepared and investigated. The intensity of green emission rapidly decreased with increasing Nd2O3-concentration, and the red emission intensity rapidly increased at first, and then slight decreased with further addition of Nd2O3 content. This phenomenon was attributed to energy transfer from Er3+ to Nd3+. Discussion of energy transfer mechanism and calculation of interaction parameters between these ions showed that resonant transfer Er3+: 4I9/2→Nd3+: (4F5/2, 2H9/2) was the most probable microscopic process to occur in contrast with others.

References: Fig.8 Spectral cross-section superposition between 4I11/2→4I13/2 emission of Er3+ and fundamental absorption of Nd3+: 4 I9/2→4I13/2 Table 2 Critical radii (RC) and CD-D(cm6/s) constants of the Er→Er resonant energy transfers Energy migration (resonant)

CD-D/(10–40 cm6/s)

RC/nm

4

I13/2 I13/2

40.2

1.53

4

I11/24I11/2

7.83

0.81

4

4

3.24

0.67

4

I9/2 I9/2

Table 3 Critical radii (RC) and CD-A(cm6/s) constants of the Er→Nd energy transfer Energy transfer (DA) Er3+: 4I9/2Nd3+: (4F5/2, 2H9/2) Er3+: 4I11/2Nd3+: 4I13/2 Er3+: 4I13/2Nd3+: 4I15/2

N(# phonons) (% phonon-assist) 0 (100%) 0 (100%) 0,1 (75.8%, 24.2%)

CD-A

RC/

6

(cm /s)

nm

87.14×10–40

1.16

0.83×10–40

0.56

1.18×10–40

0.85

[1] Johnson L F,. Van Uitert L G, Ruvin J J, Thomas R A. Energy transfer from Er3+ to Tm3+ and Ho3+ ions in crystals. Phys. Rev., 1964, 133: 494. [2] Xu Tiefeng, Li Guangpo, Nie Qiuhua, Shen Xiang. Study of upconversion fluorescence property of novel Er3+/Yb3+ co-doped tellurite glasses. Spectrochimica Acta Part A, 2006, 64: 560. [3] Nie Q H, Jiang C, Wang X S, Xu T F, Li H Q. Frequency upconversion properties of Er3+/Yb3+-codoped lead-germaniumbismuth oxide glasses. Mater. Res. Bull., 2006, 41: 1496. [4] Hinojosa S, Meneses-Nava M A, Barbosa-Garcia O, DiazTorres L A, Santoyo M A, Mosino J F. Energy back transfer, migration and energy transfer (Yb-to-Er and Er-to-Yb) processes in Yb, Er:YAG. J. Lumin., 2003, 102-103: 694. [5] Yang Jianhu, Zhang Liyan, Wen Lei, Dai Shixun, Hu Lili, Jiang Zhonghong. Comparative investigation on energy transfer mechanisms between Er3+ and Ce3+ (Eu3+, Tb3+) in tellurite glasses. Chem. Phys. Lett., 2004, 384: 295. [6] Shi W Q, Bass M, Birnbaum M. Investigation of the interactions between dissimilar ions in (Er, Nd): Y3Al5O12. J. Opt. Soc, Am. B : Opt. Phys., 1989, 6: 23. [7] Barbosa-Garcia O, McFarlance Ross A, Birnbaum Milto, Armando Diaz-Torres Luis. Neodymium- to-erbium nonradiative energy transfer and fast initial fluorescence decay of the 4F3/2

3') SGI)DFWRU\3UR ZZZILQHSULQWFQ

SHEN X et al., Investigation on energy transfer from Er3+ to Nd3+ in tellurite glass state of neodymium in garnet crystals. J. Opt. Soc. Am. B: Opt. Phys., 1997, 14: 2731. [8] Jagosich F H, Gomes L, Tarelho L V G, Courrol L C, Ranieri I M. Deactivation effects of the lowest excited states of Er3+ and Ho3+ introduced by Nd3+ ions in LiYF4 crystals. J. Appl. Phys., 2002, 91: 624. [9] Tarelho L V G, Gomes L, Ranieri I M. Determination of microscopic parameters for nonresonant energy-transfer processes in rare-earth-doped crystals. Phys. Review B: Condens. Matter, 1997, 56: 14344.

903

[10] McCumber D E. Theory of phonon-terminated optical masers. Phys. Rev. A: At. Mol. Opt. Phys., 1964, 134: A299. [11] Dai Shixun, Yu Chunlei, Zhou Gang, Zhang Junjie, Wang Guonian, Hu Lili. Concentration quenching in erbium-doped tellurite glasses. J. Lumin., 2006, 117: 39. [12] Xu S Q, Yang Z M, Dai X S, Yang Jianhu, Hu Lili, Yang Zhonghong. Spectroscopic properties and judd-ofelt theory analysis of Er3+-doped heavy metal oxyfluoride silicate glass. J. Rare Earths, 2004, 22: 375.

3') SGI)DFWRU\3UR ZZZILQHSULQWFQ