In this work we report the study of energy transfer between Nd3+ and Yb3+ 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 Nd3+→Yb3+ transfer efficiency obtained from the Nd3+ lifetimes in the single doped and codoped samples reaches 73% for the highest Nd3+ concentration. The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd3+→ Yb3+ 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 Yb3+ to Nd3+ is also observed.
©2010 Optical Society of America
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 CaSiO3/Ca3(PO4)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 . Regarding the optical properties of this system, it was found that the lifetimes and emission cross-sections of the 1.06 μm (Nd3+) and 1.5 μm (Er3+) emissions in this glass are equivalent to those of the best commercially used alkaline-silicate glasses . 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 Nd3+ 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 Nd3+ ions in this glass matrix .
Laser action in the infrared region from Yb3+ ions presents several advantages if compared to Nd3+ ions due to the energy level scheme of Yb3+ with only two levels 2F7/2 and 2F5/2. This avoids some problems such as excited-state absorption, cross-relaxation, and upconversion. Moreover, the longer lifetime of Yb3+ 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 Yb3+ ions limits the pump wavelength region around 980 nm. The use of Nd3+ ions as sensitizer allows to use a wide range of excitation wavelengths due to the Nd3+ absorption bands. Efficient energy transfer between Nd3+ and Yb3+ ions has been demonstrated both in glasses and crystals, e.g [4–15].
In this work we report the study of energy transfer between Nd3+ and Yb3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glasses at room temperature by using steady-state and time-resolved laser spectroscopy. The transfer efficiency has been obtained from the lifetimes in the single doped and codoped samples as a function of Nd3+ concentration. The donor decay curves obtained under pulsed excitation have been used to establish the multipolar nature of the Nd3+→Yb3+ transfer process and the energy transfer microparameter.
2. 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 (80CaSiO3 + 20Ca3(PO4)2 in mol%) by pressureless sintering at 1200 °C for 10 h. Nd2O3 and Yb2O3 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 . 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 . The glasses were doped with 0.5, 1, 2, and 3 wt% of Nd2O3 which correspond to 0.53x1020, 1.05x1020, 2.08x1020, and 3.18x1020 Nd3+ ions/cm3 respectively and codoped with 2 wt% of Yb2O3 (1.78x1020 Yb3+ ions/cm3). Single doped samples with 0.5, 1, 2, and 3 wt% of Nd2O3 and a single doped sample with 2 wt% of Yb2O3 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.
3. Results and discussion
3.1 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% Nd2O3 and 2 wt% Yb2O3. The inhomogeneously broadened bands are assigned to the transitions from the 4I9/2 ground state to the excited states of Nd3+ ions and to the 2F7/2→ 2F5/2 optical transition corresponding to Yb3+ ions. The spectra obtained for the other codoped samples are similar, except for the band intensities, which are dependent on the Nd3+ concentration. The integrated absorption coefficient for different absorption bands shows a linear dependence on concentration, which indicates that the relative concentrations of Nd3+ are correct.
The steady-state emission spectra were performed by exciting at 805 nm in the 4I9/2→4F5/2 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 Nd2O3 normalized to the Nd3+ emission at around 880 nm (4F3/2→4I9/2). As can be seen the codoped samples show a broad emission due to the superposition of Nd3+ (4F3/2→4I9/2, 4I11/2) and Yb3+ (2F5/2→2F7/2) emission bands. It can be also observed that the Yb3+ emission increases with Nd3+ concentration. The presence of the Yb3+ (2F5/2→2F7/2) emission clearly indicates the existence of an efficient Nd3+→Yb3+ energy transfer. After excitation in the 4F5/2 level of Nd3+, the 4F3/2 level is populated by fast nonradiative relaxation and the energy is transferred to the 2F5/2 emitting level of Yb3+.
The absorption and emission cross-section spectra of Yb3+ 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 . The form of this equation is,4],
It is worthy to mention that in contrast with other glasses [5–9,11] and crystals , the energy mismatch between the lowest Stark of 4F3/2 (Nd3+) and the highest Stark of 2F5/2 (Yb3+) 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  with an energy mismatch around 300 cm−1 and in strontium barium niobate laser crystals  (636 cm−1). On the other hand, a comparison of the energy gap and the spectral overlap between the Nd3+ emission (4F3/2→4I9/2) and the Yb3+ absorption (2F7/2→2F5/2) for different glasses and crystals showed that as the energy gap decreases the spectral overlap increases . Figure 4 shows the spectral overlap between the Nd3+ emission and Yb3+ absorption in the eutectic glasses. The Nd3+ emission cross-section has been calculated from Eq. (1). This unusual overlap can be related with the variety of quasi-isolated crystal field site distributions of rare-earth ions in this glass matrix . Therefore, these eutectic glasses can be considered as promising hosts for an efficient Nd3+ →Yb3+ energy transfer.
The lifetime of the 4F3/2 level has been obtained in the single doped and codoped samples by exciting at 805 nm, at the center of the 4I9/2→4F5/2 absorption band, and collecting the luminescence at 890 nm (4F3/2→4I9/2 emission). The decays of the 4F3/2 level of Nd3+ 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 Nd3+ ions. The fluorescence lifetime of the 4F3/2 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) .
The lifetimes of the 4F3/2 level are affected by the presence of Yb3+ ions. The decays of the 4F3/2 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 Yb3+ ions. The time dependent behavior of the Nd3+ fluorescence from the codoped samples is shown in Fig. 4(b). The values of the Nd3+ 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.
The Nd3+→Yb3+ energy transfer efficiency has been estimated from the lifetime values in the single doped and codoped samples according to the expression,Figure 5 also shows the transfer efficiency for the codoped samples as a function of donor concentration. The transfer efficiency reaches 73% for the highest Nd3+ concentration.
The presence of nonradiative Nd3+→Yb3+ energy transfer can be demonstrated by the time-dependent behavior of the Nd3+ fluorescence from the codoped samples. Figure 4(b) showed an increasing rate for the Nd3+ 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 Nd3+→Yb3+ 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 ,Figure 6 shows the fit for the sample doped with 3% of Nd2O3 and 2% of Yb2O3. 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 Nd3+→Yb3+energy transfer microparameter is 1.6x10−39 cm6/s. Similar values for the energy transfer microparameter are obtained for the samples codoped with 1 and 2 wt% of Nd2O3, whereas the migration rate increases from 1467 s−1 (1 wt% of Nd2O3) to 4482 s−1 for the sample doped with 3 wt% of Nd2O3. The value obtained for the energy transfer microparameter is similar to the one found in metaphosphate glasses (1.6x10−39 cm6/s) , lower than those found in tellurite (3.8x10−39 cm6/s) , Pb-ultraphosphate (2.4x10−39 cm6/s) , and borate glasses (6x10−39 cm6/s)  and higher than those found in fluorindogallate glasses (0.34x10−39 cm6/s and 0.45x10−39 cm6/s) [8,9].
3.3 Back transfer
In order to investigate the existence of back transfer from Yb3+ to Nd3+ we have performed emission spectra under excitation at 890 nm where only Yb3+ ions absorb. The existence of back transfer can be demonstrated by the presence of the Nd3+ emission. Figure 7 shows the emission spectrum of the codoped sample with 3 wt% of Nd2O3 and 2% Yb2O3 together with the emission spectrum of the Yb3+ single doped glass normalized to the Yb3+ emission. As can be observed, after excitation of Yb3+ ions there is emission around 1064 nm corresponding to the 4F3/2→4I11/2 transition of Nd3+ which indicates the presence of Yb3+→Nd3+ energy transfer at room temperature.
We have also investigated the temporal evolution of Yb3+ emission in the codoped samples obtained by exciting the Nd3+ ions at 805 nm. As an example, Fig. 8 shows the decays of the Yb3+ fluorescence in three codoped samples. The decays show an initial rise time with a rate corresponding to excitation by transfer from Nd3+ ions followed by an exponential decay. The lifetime value of the codoped sample with 0.5% of Nd2O3 is close to the radiative lifetime (0.81 ms); however, as Nd2O3 concentration increases the Yb3+ lifetime decreases from 0.82 ms for the sample doped with 0.5% of Nd2O3 to 0.77 ms for the sample with the highest Nd2O3 concentration. This reduction of the Yb3+ lifetime can be due to the presence of Yb3+→Nd3+ back transfer.
In this work we have demonstrated efficient Nd3+→Yb3+ energy transfer in 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glasses from the emission spectra and the decrease of the Nd3+ fluorescence lifetimes in the presence of Yb3+ ions. In contrast with other glass matrices and crystals, this energy transfer is non-phonon assisted due to the small energy difference between the Nd3+ emission (4F3/2→4I9/2) and the Yb3+ absorption (2F5/2→2F7/2) 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 Nd3+ concentration. The analysis of the donor decay curves is consistent with a dipole-dipole energy transfer mechanism assisted by donor migration. Back transfer from Yb3+ to Nd3+ is also observed in the emission spectra of the codoped samples under excitation of Yb3+ ions and in the small reduction from 0.82 ms to 0.77 ms of Yb3+ ions in the codoped samples. However the efficiency of this process is very low in comparison to the Nd3+→Yb3+ energy transfer.
Finally, the efficient Nd3+→Yb3+ energy transfer obtained for the highest Nd3+ 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 Yb3+ ions under Nd3+ excitation.
This work was supported by the Spanish Government under projects MAT2008-05921, MAT2009-14282-C02-02, and Consolider CSD2007-00013 (SAUUL), and the Basque Country Government (IT-331-07).
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