Efficient Nd 3 + → Yb 3 + energy transfer in 0 . 8 CaSiO 3-0 . 2 Ca 3 ( PO 4 ) 2 eutectic glass

In this work we report the study of energy transfer between Nd and Yb 3+ ions in glasses with the 0.8CaSiO3-0.2Ca3(PO4)2 eutectic composition at room temperature by using steady-state and time-resolved laser spectroscopy. The Nd →Yb transfer efficiency obtained from the Nd 3+ lifetimes in the single doped and codoped samples reaches 73% for the highest Nd 3+ concentration. The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd → Yb 3+ transfer process and the energy transfer microparameter. The nonradiative energy transfer is consistent with an electric dipole-dipole interaction mechanism assisted by energy migration among donors. Back transfer from Yb 3+ to Nd 3+ is also observed. ©2010 Optical Society of America OCIS codes: (160.5690) Rare earth doped materials; (300.6280) Spectroscopy; fluorescence and luminescence. References and links 1. J. Llorca, and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). 2. J. A. Pardo, J. I. Peña, R. I. Merino, R. Cases, A. Larrea, and V. M. Orera, “Spectroscopic properties of Er and Nd doped glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition,” J. Non-Cryst. Solids 298(1), 23–31 (2002). 3. R. Balda, J. Fernández, I. Iparraguirre, J. Azkargorta, S. García-Revilla, J. I. Peña, R. I. Merino, and V. M. Orera, “Broadband laser tunability of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glass,” Opt. Express 17(6), 4382– 4387 (2009). 4. M. J. Weber, “Optical properties of Yb and Nd -Yb energy transfer in YAlO3,” Phys. Rev. B 4(9), 3153– 3159 (1971). 5. C. Lurin, C. Parent, G. Le Flem, and P. Hagenmuller, “Energy transfer in a Nd-Yb borate glass,” J. Phys. Chem. Solids 46(9), 1083–1092 (1985). 6. C. Parent, C. Lurin, G. Le Flem, and P. Hagenmuller, “Nd→Yb energy transfer in glasses with composition close to LiLnP14O12 metaphosphate (Ln=La, nd, Yb),” J. Lumin. 36(1), 49–55 (1986). 7. W. Ryba-Romanowski, S. Golab, L. Cichosz, and B. Jezowska-Trzebiatowska, “Influence of temperature and acceptor concentration on energy transfer from Nd toYb and from Yb to Er in tellurite glass,” J. NonCryst. Solids 105(3), 295–302 (1988). 8. F. Batalioto, D. F. Sousa, M. J. V. Bell, R. Lebullenger, A. C. Hernandes, and L. A. O. Nunes, “Optical measurements of Nd/Yb codoped fluorindogallate glasses,” J. Non-Cryst. Solids 273(1-3), 233–238 (2000). 9. D. F. de Sousa, F. Batalioto, M. J. V. Bell, S. L. Oliveira, and L. A. O. Nunes, “Spectroscopy of Nd and Yb codoped fluoroindogallate glasses,” J. Appl. Phys. 90(7), 3308–3313 (2001). 10. D. Jaque, M. O. Ramirez, L. E. Bausá, J. García-Solé, E. Cavalli, A. Speghini, and M. Bettinelli, “Nd→Yb energy transfer in the YAl3(BO3)4 nonlinear laser crystal,” Phys. Rev. B 68(3), 035118 (2003). 11. F. Liégard, J. L. Doualan, R. Moncorgé, and M. Bettinelli, “Nd→Yb energy transfer in a codoped metaphosphate glass as a model for Yb laser operation around 980 nm,” Appl. Phys. B 80(8), 985–991 (2005). 12. R. Balda, J. Fernández, I. Iparraguirre, and M. Al-Saleh, “Spectroscopic study of Nd/Yb in disordered potassium bismuth molybdate laser crystals,” Opt. Mater. 28(11), 1247–1252 (2006). #128191 $15.00 USD Received 10 May 2010; revised 8 Jun 2010; accepted 9 Jun 2010; published 11 Jun 2010 (C) 2010 OSA 7 June 2010 / Vol. 18, No. 13 / OPTICS EXPRESS 13842 13. U. Caldiño, D. Jaque, E. Martín-Rodríguez, M. O. Ramírez, J. García Solé, A. Speghini, and M. Bettinelli, “Nd/Yb resonant energy transfer in the ferroelectric Sr0.6 Ba0.4 Nb2O6 laser crystal,” Phys. Rev. B 77(7), 075121 (2008). 14. Z. Jia, A. Arcangeli, X. Tao, J. Zhang, C. Dong, M. Jiang, L. Bonelli, and M. Tonelli, “Efficient Nd→Yb energy transfer in Nd,Yb:Gd3Ga5O12 multicenter garnet crystal,” J. Appl. Phys. 105, 083113 (2009). 15. A. Lupei, V. Lupei, A. Ikesue, and C. Gheorghe, “Spectroscopic and energy transfer investigation of Nd/Yb in Y2O3 transparent ceramics,” J. Opt. Soc. Am. B 27(5), 1002–1010 (2010). 16. M. J. Weber, D. C. Ziegler, and C. A. Angell, “Tailoring stimulated emission cross sections of Nd laser glass: Observation of large cross sections for BiCl3 glasses,” J. Appl. Phys. 53(6), 4344–4350 (1982). 17. A. I. Burshtein, “Hopping mechanism of energy transfer,” Sov. Phys. JETP 35, 882–885 (1972).


Introduction
Eutectic structures are a paradigm of composite materials with a fine microstructure whose characteristics are controlled by the solidification conditions.Rapid solidification of some eutectic systems opens up the possibility of fabricating glass.The favorable conditions of eutectic mixtures to produce glasses with a low number of components are also remarkable from the point of view of their photonic applications.A good optical quality glass can be produced by fast directional solidification of the CaSiO 3 /Ca 3 (PO 4 ) 2 binary eutectic system.This eutectic presents two non-conventional and interesting properties: firstly, the degenerated lamellar structure of the system favors the biological transformation of the tricalcium phosphate phase into hydroxiapatite, giving rise to a biological material with a microstructure similar to that of human bone.Secondly, it is possible to form a eutectic glass of this composition with excellent optical properties [1].Regarding the optical properties of this system, it was found that the lifetimes and emission cross-sections of the 1.06 µm (Nd 3+ ) and 1.5 µm (Er 3+ ) emissions in this glass are equivalent to those of the best commercially used alkaline-silicate glasses [2].More recently, we have demonstrated laser emission under pulsed pumping which shows a behavior close to a Q-switch operation.Wavelength-resolved pump excitation of Nd 3+ ions in this glass allows for a broad band tunability (10 nm) of the laser emission which is related with the variety of quasi-isolated crystal field site distributions of Nd 3+ ions in this glass matrix [3].
Laser action in the infrared region from Yb 3+ ions presents several advantages if compared to Nd 3+ ions due to the energy level scheme of Yb 3+ with only two levels 2 F 7/2 and 2 F 5/2 .This avoids some problems such as excited-state absorption, cross-relaxation, and upconversion.Moreover, the longer lifetime of Yb 3+ allows greater energy-storage efficiency with diode laser pumped schemes and broader absorption and emission bands which is promising for the generation of shorter light pulses.However, the simple energy level scheme of Yb 3+ ions limits the pump wavelength region around 980 nm.The use of Nd 3+ ions as sensitizer allows to use a wide range of excitation wavelengths due to the Nd 3+ absorption bands.Efficient energy transfer between Nd 3+ and Yb 3+ ions has been demonstrated both in glasses and crystals, e.g [4][5][6][7][8][9][10][11][12][13][14][15].
In this work we report the study of energy transfer between Nd 3+ and Yb 3+ ions in 0.8CaSiO 3 -0.2Ca 3 (PO 4 ) 2 eutectic glasses at room temperature by using steady-state and timeresolved laser spectroscopy.The transfer efficiency has been obtained from the lifetimes in the single doped and codoped samples as a function of Nd 3+ concentration.The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd 3+ →Yb 3+ transfer process and the energy transfer microparameter.

Experimental details
Ceramic precursor rods, 3 mm in diameter and 50-100 mm in length, were prepared from the powder mixture of wollastonite (CS)-tricalcium phosphate (TPC) with the eutectic composition (80CaSiO 3 + 20Ca 3 (PO 4 ) 2 in mol%) by pressureless sintering at 1200 °C for 10 h.Nd 2 O 3 and Yb 2 O 3 were added to the precursors to obtain the doped and codoped samples.Glass rods were then produced from the precursors by the laser floating zone method [2].This inverted glass with a high content of CaO modifier presents a highly transparent optical window from 0.35 to 4 µm and is not hygroscopic.Its refractive index is 1.65 [2].The glasses were doped with 0.5, 1, 2, and 3 wt% of Nd 2 O 3 which correspond to 0.53x10 20 , 1.05x10 20 , 2.08x10 20 , and 3.18x10 20 Nd 3+ ions/cm 3 respectively and codoped with 2 wt% of Yb 2 O 3 (1.78x10 20Yb 3+ ions/cm 3 ).Single doped samples with 0.5, 1, 2, and 3 wt% of Nd 2 O 3 and a single doped sample with 2 wt% of Yb 2 O 3 were also prepared.
The room temperature absorption spectra in the 300-2500 nm spectral range were recorded by using a Cary 5 spectrophotometer.The steady-state emission measurements were made by using a Ti-Sapphire ring laser (0.4 cm −1 linewidth) in the 780-920 nm range.The fluorescence was analyzed with a 0.22 m SPEX monochromator, and the signal was detected by a Hamamatsu R7102 photomultiplier and finally amplified by a standard lock-in technique.Lifetime measurements were performed by exciting the samples with a Ti-sapphire laser, pumped by a pulsed frequency doubled Nd:YAG laser (9 ns pulse width), and detecting the emission with a Hamamatsu R7102 photomultiplier.Data were processed by a Tektronix oscilloscope.

Absorption and emission spectra
The room temperature absorption spectra were obtained for all samples in the 300-2500 nm range with a Cary 5 spectrophotometer.As an example, Fig. 1 shows the absorption spectrum of the codoped glass with 3 wt% Nd 2 O 3 and 2 wt% Yb 2 O 3 .The inhomogeneously broadened bands are assigned to the transitions from the 4 I 9/2 ground state to the excited states of Nd 3+ ions and to the 2 F 7/2 → 2 F 5/2 optical transition corresponding to Yb 3+ ions.The spectra obtained for the other codoped samples are similar, except for the band intensities, which are dependent on the Nd 3+ concentration.The integrated absorption coefficient for different absorption bands shows a linear dependence on concentration, which indicates that the relative concentrations of Nd 3+ are correct.The steady-state emission spectra were performed by exciting at 805 nm in the 4 I 9/2 → 4 F 5/2 absorption band.For all samples the spectra are characterized by inhomogeneously broadened bands.Figure 2 shows the emission spectra for all codoped samples together with the emission spectrum of the single doped glass doped with 3 wt% of Nd 2 O 3 normalized to the Nd 3+ emission at around 880 nm ( 4 F 3/2 → 4 I 9/2 ).As can be seen the codoped samples show a broad emission due to the superposition of Nd 3+ ( 4 F 3/2 → 4 I 9/2 , 4 I 11/2 ) and Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) emission bands.It can be also observed that the Yb 3+ emission increases with Nd 3+ concentration.The presence of the Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) emission clearly indicates the existence of an efficient Nd 3+ →Yb 3+ energy transfer.After excitation in the 4 F 5/2 level of Nd 3+ , the 4  The absorption and emission cross-section spectra of Yb 3+ ions in the single doped glass are shown in Fig. 3.The absorption cross-section was calculated from the absorption spectrum whereas the emission cross section was calculated by using the Fuchtbauer-Landeburg (F-L) equation [16].The form of this equation is, ( ) ( ) where λ p is the peak fluorescence wavelength, β is the branching ratio for the transition, n is the refractive index of the host matrix, c the velocity of light, τ R the radiative lifetime of the emitting level, and I(λ) the emission intensity.The radiative lifetime can be calculated from the expression [4], ( ) were g f , and g i are the degeneracies of the initial ( 2 F 5/2 ) and final ( 2 F 7/2 ) states, λ 0 is the mean wavelength of the 2 F 5/2 → 2 F 7/2 electronic transition, n is the refractive index, N is the Yb 3+ concentration, and α is the absorption coefficient of the It is worthy to mention that in contrast with other glasses [5][6][7][8][9]11] and crystals [10], the energy mismatch between the lowest Stark of 4 F 3/2 (Nd 3+ ) and the highest Stark of 2 F 5/2 (Yb 3+ ) levels is only about 260 cm −1 .This short energy gap indicates that no thermal assistance is needed to have an efficient energy transfer.A similar situation was found in molibdate crystals [12] with an energy mismatch around 300 cm −1 and in strontium barium niobate laser crystals [13] (636 cm −1 ). O the other hand, a comparison of the energy gap and the spectral overlap between the Nd 3+ emission ( 4 F 3/2 → 4 I 9/2 ) and the Yb 3+ absorption ( 2 F 7/2 → 2 F 5/2 ) for different glasses and crystals showed that as the energy gap decreases the spectral overlap increases [13].Figure 4 shows the spectral overlap between the Nd 3+ emission and Yb 3+ absorption in the eutectic glasses.The Nd 3+ emission cross-section has been calculated from Eq. ( 1). Tis unusual overlap can be related with the variety of quasi-isolated crystal field site distributions of rare-earth ions in this glass matrix [3].Therefore, these eutectic glasses can be considered as promising hosts for an efficient Nd 3+ →Yb 3+ energy transfer.

Lifetimes
The lifetime of the 4 F 3/2 level has been obtained in the single doped and codoped samples by exciting at 805 nm, at the center of the 4 I 9/2 → 4 F 5/2 absorption band, and collecting the luminescence at 890 nm ( 4 F 3/2 → 4 I 9/2 emission).The decays of the 4 F 3/2 level of Nd 3+ in the single doped glasses were found to be exponential for all concentrations.As an example, Fig.

4(a)
shows the decays for the samples doped with 0.5% and 3%.As concentration increases, they remain single exponential but a decrease from 258 to 248 µs is observed when the concentration increases from 0.5 to 3% which indicates the presence of nonradiative energy transfer processes.This behaviour could be associated to a rapid energy diffusion among Nd 3+ ions.The fluorescence lifetime of the 4 F 3/2 level for a sample doped with 0.07% at low temperature (10K) measured under laser excitation at 805 nm, is 330 µs, which is close to the calculated radiative lifetime (340 µs) [2].The lifetimes of the 4 F 3/2 level are affected by the presence of Yb 3+ ions.The decays of the 4 F 3/2 level in the codoped samples exhibit a non-exponential behavior and a shortening of the lifetime if compared with the single doped samples, because of the additional relaxation probability by nonradiative energy transfer to Yb 3+ ions.The time dependent behavior of the Nd 3+ fluorescence from the codoped samples is shown in Fig. 4(b).The values of the Nd 3+ emission lifetimes, monitored at 890 nm as a function of concentration are shown in Fig. 5, which also includes the lifetime of single doped samples for comparison.Nd 3+ -Yb 3+ Fig. 5. Lifetimes of the 4 F3/2→ 4 I9/2 emission for the single doped samples (black) and codoped samples (pink) and Nd 3+ -Yb 3+ energy transfer efficiency (blue) as a function of Nd 3+ concentration.
The presence of nonradiative Nd 3+ →Yb 3+ energy transfer can be demonstrated by the time-dependent behavior of the Nd 3+ fluorescence from the codoped samples.Figure 4(b) showed an increasing rate for the Nd 3+ decays in the codoped glasses due to the additional relaxation probabilities and a nonexponential behavior.These decays have been analyzed to determine the mechanism responsible for the Nd 3+ →Yb 3+ energy transfer, by considering the existence of energy migration among donors.The best agreement between experimental data and theoretical fit occurs with the expression corresponding to the Burshtein model [17], where τ 0 is the intrinsic lifetime of donor ions, γ characterizes the direct Nd 3+ →Yb 3+ energy transfer, and W represents the migration parameter.In the case of dipole-dipole interaction, γ is given by the expression, , where N Yb is the acceptors concentration and C DA is the energy transfer microparameter.Figure 6 shows the fit for the sample doped with 3% of Nd 2 O 3 and 2% of Yb 2 O 3 .The inset shows the same decays but in a semilogarithmic plot.These results indicate that the electronic mechanism of energy transfer is a dipole-dipole interaction.From the fitting in Fig. 6, the value obtained for the Nd 3+ →Yb 3+ energy transfer microparameter is 1.6x10 −39 cm 6 /s.Similar values for the energy transfer microparameter are obtained for the samples codoped with 1 and 2 wt% of Nd 2 O 3, whereas the migration rate increases from 1467 s −1 (1 wt% of Nd 2 O 3 ) to 4482 s −1 for the sample doped with 3 wt% of Nd 2 O 3 .The value obtained for the energy transfer microparameter is similar to the one found in metaphosphate glasses (1.6x10 −39 cm 6 /s) [6], lower than those found in tellurite (3.8x10 −39 cm 6 /s) [7], Pb-ultraphosphate (2.4x10 −39 cm 6 /s) [7], and borate glasses (6x10 −39 cm 6 /s) [5] and higher than those found in fluorindogallate glasses (0.34x10 −39 cm 6 /s and 0.45x10 −39 cm 6 /s) [8,9].

Back transfer
In order to investigate the existence of back transfer from Yb 3+ to Nd 3+ we have performed emission spectra under excitation at 890 nm where only Yb 3+ ions absorb.The existence of back transfer can be demonstrated by the presence of the Nd 3+ emission.Figure 7 shows the emission spectrum of the codoped sample with 3 wt% of Nd  ms for the sample doped with 0.5% of Nd 2 O 3 to 0.77 ms for the sample with the highest Nd 2 O 3 concentration.This reduction of the Yb 3+ lifetime can be due to the presence of Yb 3+ →Nd 3+ back transfer.

Conclusions
In this work we have demonstrated efficient Nd 3+ →Yb 3+ energy transfer in 0.8CaSiO 3 -0.2Ca 3 (PO 4 ) 2 eutectic glasses from the emission spectra and the decrease of the Nd 3+ fluorescence lifetimes in the presence of Yb 3+ ions.In contrast with other glass matrices and crystals, this energy transfer is non-phonon assisted due to the small energy difference between the Nd 3+ emission ( 4 F 3/2 → 4 I 9/2 ) and the Yb 3+ absorption ( 2 F 5/2 → 2 F 7/2 ) bands which is around 260 cm −1 and the important spectral overlap between these bands.The transfer efficiency, which has been studied at room temperature, as a function of donor concentration reaches 73% for the highest Nd 3+ concentration.The analysis of the donor decay curves is consistent with a dipole-dipole energy transfer mechanism assisted by donor migration.Back transfer from Yb 3+ to Nd 3+ is also observed in the emission spectra of the codoped samples under excitation of Yb 3+ ions and in the small reduction from 0.82 ms to 0.77 ms of Yb 3+ ions in the codoped samples.However the efficiency of this process is very low in comparison to the Nd 3+ →Yb 3+ energy transfer.
Finally, the efficient Nd 3+ →Yb 3+ energy transfer obtained for the highest Nd 3+ concentration together with the excellent optical properties of these eutectic glasses suggest that these glasses can be promising materials for the generation of laser action from Yb 3+ ions under Nd 3+ excitation.

Fig. 4 .I.
Fig. 4. Logarithmic plot of the fluorescence decays of the 4 F3/2→ 4 I9/2 emission as a function of Nd 3+ concentration in (a) single doped and (b) codoped samples.The Nd 3+ →Yb 3+ energy transfer efficiency has been estimated from the lifetime values in the single doped and codoped samples according to the expression,

Fig. 6 .
Fig. 6.Experimental emission decay curve of level 4 F3/2 for the codoped sample with 3 wt% of Nd2O3 and 2 wt% of Yb2O3 at room temperature and the calculated fit with Eq. (4) (solid line).

Fig. 7 .
Fig. 7. Room temperature emission spectra obtained for the codoped sample with 3 wt% of Nd2O3 and 2 wt% of Yb2O3 and the single doped glass with 2 wt% Yb2O3.The spectra are normalized to the Yb 3+ emission.
F 3/2level is populated by fast nonradiative relaxation and the energy is transferred to the 2 F 5/2 emitting level of Yb 3+ .Fig.2.Room temperature emission spectra of Nd 3+ and Yb 3+ in the codoped samples together with the emission spectrum of Nd 3+ ions in a single doped glass.The spectra are normalized to the 880 nm emission of Nd 3+ .