1. 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 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 [1]. 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 [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 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 [3].

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 ²F_7/2 and ²F_5/2. 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 [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 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 ⁴I_9/2 ground state to the excited states of Nd³⁺ ions and to the ²F_7/2→ ²F_5/2 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.

 

Fig. 1 Room temperature absorption spectrum of a codoped sample with 3 wt% of Nd₂O₃ and 2 wt% of Yb₂O₃.

Download Full Size | PDF

The steady-state emission spectra were performed by exciting at 805 nm in the ⁴I_9/2→⁴F_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 Nd₂O₃ normalized to the Nd³⁺ emission at around 880 nm (⁴F_3/2→⁴I_9/2). As can be seen the codoped samples show a broad emission due to the superposition of Nd³⁺ (⁴F_3/2→⁴I_9/2, ⁴I_11/2) and Yb³⁺ (²F_5/2→²F_7/2) emission bands. It can be also observed that the Yb³⁺ emission increases with Nd³⁺ concentration. The presence of the Yb³⁺ (²F_5/2→²F_7/2) emission clearly indicates the existence of an efficient Nd³⁺→Yb³⁺ energy transfer. After excitation in the ⁴F_5/2 level of Nd³⁺, the ⁴F_3/2 level is populated by fast nonradiative relaxation and the energy is transferred to the ²F_5/2 emitting level of Yb³⁺.

 

Fig. 2 Room temperature emission spectra of Nd³⁺ and Yb³⁺ in the codoped samples together with the emission spectrum of Nd³⁺ ions in a single doped glass. The spectra are normalized to the 880 nm emission of Nd³⁺.

Download Full Size | PDF

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 [16]. The form of this equation is,

 

Fig. 3 Absorption and emission cross section of Yb³⁺ in the single doped sample.

Download Full Size | PDF

It is worthy to mention that in contrast with other glasses [5–9,11] and crystals [10], the energy mismatch between the lowest Stark of ⁴F_3/2 (Nd³⁺) and the highest Stark of ²F_5/2 (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 [12] with an energy mismatch around 300 cm⁻¹ and in strontium barium niobate laser crystals [13] (636 cm⁻¹). On the other hand, a comparison of the energy gap and the spectral overlap between the Nd³⁺ emission (⁴F_3/2→⁴I_9/2) and the Yb³⁺ absorption (²F_7/2→²F_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 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 [3]. Therefore, these eutectic glasses can be considered as promising hosts for an efficient Nd³⁺ →Yb³⁺ energy transfer.

 

Fig. 4 Spectral overlap between Nd³⁺ emission and Yb³⁺ absorption in the single doped samples.

Download Full Size | PDF

3.2 Lifetimes

The lifetime of the ⁴F_3/2 level has been obtained in the single doped and codoped samples by exciting at 805 nm, at the center of the ⁴I_9/2→⁴F_5/2 absorption band, and collecting the luminescence at 890 nm (⁴F_3/2→⁴I_9/2 emission). The decays of the ⁴F_3/2 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 ⁴F_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 ⁴F_3/2 level are affected by the presence of Yb³⁺ ions. The decays of the ⁴F_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 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.

 

Fig. 5 Lifetimes of the ⁴F_3/2→⁴I_9/2 emission for the single doped samples (black) and codoped samples (pink) and Nd³⁺-Yb³⁺ energy transfer efficiency (blue) as a function of Nd³⁺ concentration.

Download Full Size | PDF

 

Fig. 4 Logarithmic plot of the fluorescence decays of the ⁴F_3/2→⁴I_9/2 emission as a function of Nd³⁺ concentration in (a) single doped and (b) codoped samples.

Download Full Size | PDF

The Nd³⁺→Yb³⁺ energy transfer efficiency has been estimated from the lifetime values in the single doped and codoped samples according to the expression,

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 [17],

 

Fig. 6 Experimental emission decay curve of level ⁴F_3/2 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).

Download Full Size | PDF

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 ⁴F_3/2→⁴I_11/2 transition of Nd³⁺ which indicates the presence of Yb³⁺→Nd³⁺ energy transfer at room temperature.

 

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 Yb³⁺ emission.

Download Full Size | PDF

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.

 

Fig. 8 Logarithmic plot of the fluorescence decays of the ²F_5/2→²F_7/2 emission of Yb³⁺ ions in the codoped samples for three different Nd³⁺ concentrations.

Download Full Size | PDF

4. Conclusions

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 (⁴F_3/2→⁴I_9/2) and the Yb³⁺ absorption (²F_5/2→²F_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 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.