Co-effects of Yb3+ sensitization and Pr3+ deactivation to enhance 2.7 &#x03BC;m mid-infrared emission of Er3+ in CaLaGa3O7 crystal

Yunyun Liu; Zhenyu You; Houping Xia; Yan Wang; Zhaojie Zhu; Jianfu Li; Chaoyang Tu

doi:10.1364/OME.7.002411

1. Introduction

Over the past few decades, mid-infrared (MIR) solid lasers in the 2.7–3 μm wavelength range are considerably attractive because of their strong absorption in vapor, water, and biological tissues. Thus, these lasers are widely utilized in medicine, remote sensing, military countermeasures, and atmosphere pollution monitoring [1–6]. In addition, these lasers can be used as efficient and high-quality pump sources for longer wavelength mid-IR lasers and optical parametric oscillators [7–9]. Erbium ion is an ideal luminescent center for 2.7 μm emission corresponding to the ⁴I_11/2→⁴I_13/2 transition, which has been extensively investigated [10–14]. The mid-infrared lasers of Er³⁺ doped materials are usually obtained under 980 or 808 nm pump since these laser diodes are commercialized and their wavelengths match the intrinsic absorption of Er³⁺.

However, the output power and efficiency of 2.7 µm erbium lasers are not high enough for the application in industry, one main reason is the low absorption efficiency of LD pumping energy, owing to the weak intensity and narrow bandwidth of the absorption band with peak around 980 nm corresponding to ⁴I_11/2→⁴I_15/2 in Er³⁺ activated crystals. In order to solve this problem, Yb³⁺ ion has usually been introduced as the sensitizer to improve the absorption, due to the strong overlap of the luminescence spectrum of Yb³⁺ ions and the absorption spectrum of acceptor Er³⁺ ions [15–18]. The other main reason preventing the development of 2.7 µm laser is the self-terminating “bottleneck” effect, which may result from the fluorescence lifetime of the upper ⁴I_11/2 level is considerably shorter than that of the lower level ⁴I_13/2. To overcome this effect, one method is increasing the concentration of Er³⁺ ions. In theory, the high concentration of Er³⁺ doped systems (at least≥10at%) can help to improve the absorption intensity and line width of Er³⁺ and thus increase the pumping energy; meanwhile, it can also help to suppress the self-saturation problem since high concentration Er³⁺ ions are proposed to induce up-conversion from ⁴I_11/2 and ⁴I_13/2, as well as cross-relaxation from ⁴S_3⁄2, and thus aids to improve 2.7 µm laser in Er³⁺ activated crystals [19]. However, high doping concentration of Er³⁺ may give rise to a decline in the quality of crystal, and then limits the laser output efficiency and beam quality. Another method to conquer the self-termination bottleneck is the co-doping of deactivation ion, which can be effective in depopulation of ⁴I_13/2 level of Er³⁺. Up to now, several rare earth ions can be co-doped into the gallate hosts to depopulate the ⁴I_13/2 state, such as Pr³⁺ in Er³⁺ activated GGG [20], SGGM [16,21], GYSGG [22], and CaGdAlO₄ crystals [23], Eu³⁺ in Er³⁺: SGGM [24] and Er³⁺: LaYSGG crystal [25], Tm³⁺ or Ho³⁺ in Er³⁺: GGG, Er³⁺: SGGM crystal [21,26]. We are now focusing our scientific program on Pr³⁺ ion, because its energy level ³F₄ is adjacent to ⁴I_13/2 level of Er³⁺. Predictably, it is believed that co-doping of both Yb³⁺ and Pr³⁺ with Er³⁺ may turn on the possibility of excellent 2.7 μm lasers performance from Er³⁺ under a conventional 980 nm LD pump.

CaLaGa₃O₇ (abbr. as CLGO) crystal as a novel host matrix was chosen in this work. The CLGO single crystal belongs to the large Melilite ABC₃O₇ group, which crystallizes in a tetragonal crystal system with the space group of P $\bar{4}$ 2₁m [27]. As it melts congruently with the melting point about 1600 °C [27], large-sized crystals with high optical quality can be obtained by using the Czochralski method. In the crystal structure of CaLaGa₃O₇, Ca²⁺ and La³⁺ ions are statistically distributed in the same cation positions, which results in certain structural disorder [28]. So in Er³⁺ ions doped CLGO crystal, the absorption and emission lines of Er³⁺ ions are strongly inhomogeneously broadened. This would be beneficial to the LD pump and the generations of tunable or ultra-short pulse lasers.

In this paper, we report measurements the spectroscopic performance with focus on 2.7 μm emissions of Er³⁺/Pr³⁺: CaLaGa₃O₇, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals. The effects of Pr³⁺ and Yb³⁺ on the fluorescence properties of Er³⁺ in CaLaGa₃O₇ crystal are discussed carefully. Also, the energy transfer mechanisms of Er³⁺/Yb³⁺/Pr³⁺ triply doped CaLaGa₃O₇ crystal are studied.

2. Experimental details

Er³⁺/Yb³⁺/Pr³⁺: CLGO single crystal was successfully grown by Czochralski method. The initial concentrations of Er³⁺, Yb³⁺ and Pr³⁺ were 10 at.%, 5 at.% and 0.15 at.%, respectively. 10 at.% Er³⁺/0.15 at.% Pr³⁺: CLGO, 10 at.% Er³⁺/5 at.% Yb³⁺: CLGO, 10 at.% Er³⁺: CLGO and 5 at.% Yb³⁺: CLGO single crystals were also grown for spectral comparison. The used chemicals were La₂O₃, Er₂O₃, Yb₂O₃, Pr₂O₃, Ga₂O₃ (4N purity) and CaCO₃ (A.R. grade). The stoichiometric amounts of chemicals were weighed accurately according to the compositional formula. In order to compensate the evaporation losses of Ga₂O₃ in the growing process, excess 1.0 wt.% Ga₂O₃ was added to the starting materials. The weighed chemical powders were mixed, ground thoroughly and pressed into pellets. And then, the pellets were placed in the platinum crucible and sintered at 1200 °C for 48 h to react completely. The sintered tablets were milled again and pressed into tablets, sintered at 1250 °C for 48 h, then repeated the above procedures twice to confirm the finally synthesized polycrystalline compounds by using the X-ray diffraction (Miniflex600) method. And then, the synthesized polycrystalline materials formed by solid-state reaction were placed and melted in an iridium crucible. The crystals were grown with the c-cut orientated CLGO seed and carried out in a DJL-400 furnace (NCIREO, China). The neck pulling rate was set at 1~2.0 mm/h. The pulling rate varied from 0.5 to 1 mm/h and the crystal rotation speed was kept 10~15 rpm. After the growth was completed, the crystal was cooled slowly to room temperature at a rate of 5~35 K/h in order to prevent crystal crack.

The concentrations of Er³⁺, Yb³⁺ or Pr³⁺ ions in all crystals were measured by using an inductively coupled plasma atomic emission spectrometry (ICP-AES, Ultima2, Jobin-Yvon). The absorption spectra was measured using a Perkin-Elmer UV–VIS–NIR Spectrometer (Lambda-900). The emission spectra and fluorescence lifetime were measured using an Edinburgh Instruments FLS920 Spectrometer, by using 980 LD pump source. Several c-cut samples were cut from the as-grown crystals with dimensions of 10.0 × 5.0 × 1.0 mm³. The two 10.0 × 5.0 mm² faces were polished for spectral experiments. The experimental conditions were maintained exactly same for measurement of each group of spectra in order to get the comparable results.

3. Results and discussion

The concentrations of Er³⁺, Yb³⁺ and Pr³⁺ ions in Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal are measured to be 1.48 × 10²⁰ ions·cm⁻³, 0.44 × 10²⁰ ions·cm⁻³ and 0.26 × 10²⁰ ions·cm⁻³, respectively. The concentrations of Er³⁺ and Pr³⁺ ions in Er³⁺/Pr³⁺: CLGO crystal are 1.22 × 10²⁰ ions·cm⁻³ and 0.23 × 10²⁰ ions·cm⁻³, respectively. The concentrations of Er³⁺ and Yb³⁺ ions in Er³⁺/Yb³⁺: CLGO crystal are measured to be 1.66 × 10²⁰ ions·cm⁻³ and 0.50 × 10²⁰ ions·cm⁻³, respectively. While the Er³⁺ concentration in Er³⁺: CLGO crystal is 1.25 × 10²⁰ ions·cm⁻³ and Yb³⁺ concentration in Yb³⁺: CLGO crystal is 0.31 × 10²⁰ ions·cm⁻³.

The absorption spectra of Er³⁺: CLGO, Er³⁺/Pr³⁺: CLGO, Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals are shown in Fig. 1. It is clear to see that in the range of 350–1600 nm, there are seven typical absorption bands of Er³⁺ ion, centered at 378, 488, 523, 652, 801, 979 and 1535 nm, which correspond to the transitions from ⁴I_15/2 to ⁴G_11/2, ⁴F_7/2, ²H_11/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2, respectively. As compared with the absorption spectrum of Er³⁺: CLGO crystal, there is almost no change in the intensity and location of the characteristic absorption bands of Er³⁺ ions in Er³⁺/Yb³⁺: CLGO crystal. But, there is a much stronger absorption band in the range of 940 to 1020 nm, which is involved both the contributions from Yb³⁺ and Er³⁺ ions, owing to the transition between Yb³⁺: ²F_7∕2→²F_5∕2 and Er³⁺: ⁴I_15∕2→⁴I_11∕2 (inset of Fig. 1). Therefore, Yb³⁺ ion can act as a sensitizer to Er³⁺ ion by nonradiative energy transfer, resulting in an increased absorption efficiency, which is beneficial to enhance the pump efficiency effectively. Furthermore, as seen in the absorption spectrum of Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal (inset of Fig. 1), the absorption band centered at 980 nm is much stronger than that in Er³⁺: CLGO crystal, owing to the existence of Yb³⁺ ions. By contrast, in the range of 940–1020 nm, the absorption of the crystal without Yb³⁺ ion (Er³⁺: CLGO and Er³⁺/Pr³⁺: CLGO) is significantly weak. For Er³⁺/Pr³⁺: CLGO crystal, the characteristic absorption peaks of Pr³⁺ are not very clear since its concentration is extremely low as compared with Er³⁺.

Fig. 1 Absorption spectra of Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals at room temperature.

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The absorption cross–section σ_a can be determined by the following equation:

σ_{a} (λ) = \frac{2.303}{N_{0} \times l} O D (λ)

Where λ is the wavelength, l is the thickness of the crystal, OD is the optical density, and N₀ is the Er³⁺ concentration in the crystal. Thereby, the calculated absorption cross-sections of Er³⁺: CLGO and Er³⁺/Pr³⁺: CLGO crystals are listed in Table 1. Due to the overlap of absorption peak around 980 nm, the cross-section of Er³⁺ in Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals can dot be calculated.

Table 1. Spectroscopic data for Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O₇, Er³⁺/Yb³⁺: CaLaGa₃O₇and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ laser crystals.

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The up-conversion (UC) emission spectra within 500–725 nm of Er³⁺: CLGO, Er³⁺/Pr³⁺: CLGO, Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals are measured under excitation of 980 nm, as shown in Fig. 2. In the spectra of all the crystals, the green and red up-conversion emission bands centered at around 548 nm and 670 nm are observed, corresponding to the transition Er³⁺: ²H_11/2 + ⁴S_3/2→⁴I_15/2 and Er³⁺: ⁴F_9/2→⁴I_15/2 (as shown in Fig. 8), respectively. Compared with Er³⁺: CLGO crystal, the UC emission spectrum of Er³⁺/Yb³⁺: CLGO crystal shows tiny changes in intensity while that of Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals exhibits much weaker intensity. Although the strong UC emission may be used to achieve fluorescent cooling [29,30], it is still a harmful factor for MIR laser output, so the introduce of Pr³⁺ is helpful for the realization of 2.7 µm MIR lasers.

Fig. 2 Up-conversion emission spectra of Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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Figure 3 shows the near-infrared (NIR) emission spectra of four crystals within the range of 1475–1600 nm excited by 980 nm. As seen from it, the intensity of the 1535 nm emission corresponding to Er³⁺: ⁴I_13/2→⁴I_15/2 in Er³⁺/Yb³⁺: CLGO crystal is very similar with that of Er³⁺ single-doped CLGO crystal. So the sole introduction of Yb³⁺ into Er³⁺: CLGO crystal is not very helpful to the inhibition of NIR emissions. While the addition of Pr³⁺ can significantly inhibit the harmful NIR emission as shown in the spectra of Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals.

Fig. 3 Near-infrared emission spectra of Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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Figure 4 demonstrates the mid-infrared emission (MIR) spectra in the range of 2500–3000 nm excited by an optical parametric oscillator (OPO) laser at 980 nm. The broad MIR emission band centered at around 2700 nm can be assigned to the ⁴I_11/2→⁴I_13/2 transition of Er³⁺. As it shows that there is almost no difference in the emission spectrum shape and peak position of the four crystals. However, Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal exhibits the strongest MIR emission and its intensity is over 7 times than that in Er³⁺: CLGO crystal. The enhanced MIR emission is also obtained in Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺: CLGO crystals. As an important parameter affecting the potential 2.7 µm laser performance, the emission cross-sections are calculated by the F-L equation [31]:

σ_{e m} = \frac{β λ^{5}}{8 π c n^{2} τ_{r}} \frac{I (λ)}{\int λ I (λ) d λ}

Where

I (λ) / \int λ I (λ) d λ

is the normalized line shape function of the experimental emission spectrum, β is the fluorescence branching ratio, c is the speed of light, n is the refractive index and τ_r is the radiative lifetime. The obtained emission cross-sections of all the crystals are also listed in Table 1.

Fig. 4 Mid-infrared emission spectra of Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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The decay curves of Er³⁺: ⁴I_11/2 and ⁴I_13/2 levels in Er³⁺/Pr³⁺: CLGO, Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals are shown in Fig. 5 and Fig. 6, respectively. For Er³⁺ ions, the monitoring wavelengths are 1535 and 2700 nm, respectively. The decay curves of ⁴I_11/2 and ⁴I_13/2 levels of Er³⁺ in all crystals show multi-exponential behavior, which can be obtained by fitting with

I (t) = A + B_{1} e x p (t / τ_{1}) + B_{2} e x p (t / τ_{2}) + \cdot \cdot \cdot + B_{n} e x p (t / τ_{n})

Then, the fluorescence lifetimes are calculated by

τ = \frac{B_{1} τ_{1}^{2} + B_{2} τ_{2}^{2} + \cdot \cdot \cdot + B_{n} τ_{n}^{2}}{B_{1} τ_{1}^{} + B_{2} τ_{2} + \cdot \cdot \cdot + B_{n} τ_{n}}

The fluorescence lifetimes were fitted and presented in Table 1, too. It is noticed that the fluorescence lifetime of ⁴I_13/2 state is longer than that of ⁴I_11/2 state in the four crystals. As compared with Er³⁺: CLGO, the fluorescence lifetimes of ⁴I_11/2 and ⁴I_13/2 levels of Er³⁺ only change slightly in Er³⁺/Yb³⁺: CLGO crystal. While, the fluorescence lifetimes of ⁴I_11/2 and ⁴I_13/2 levels of Er³⁺ both decrease in Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals, which is mainly due to the energy transfer via Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ and Er³⁺: ⁴I_13/2→Pr³⁺: ³F₄. And it is obvious that the declining degree of the lifetime of ⁴I_13/2 level is much larger than that of ⁴I_11/2 level. Based on the fluorescence lifetimes, the energy transfer efficiency from Er³⁺ to Pr³⁺ in Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals can be obtained from the following equation [20,23]:

η_{T} = 1 - τ_{D A} / τ_{A}

where τ_DA is the lifetime of Er³⁺ in the existence of Pr³⁺, and τ_A is the lifetime of Er³⁺ in the absence of Pr³⁺ [23]. For Er³⁺/Pr³⁺: CLGO crystal, the energy transfer efficiencies of Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ and Er³⁺: ⁴I_13/2→Pr³⁺: ³F₄ are calculated to be 11.7% and 85.0%, respectively. And those of Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ and Er³⁺: ⁴I_13/2→Pr³⁺: ³F₄ in the Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal are 18.2% and 86.7%, respectively. That means the lifetime of the Er³⁺: ⁴I_13/2 level decreases much quicker than that of Er³⁺: ⁴I_11/2 level in Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals. It proves that the co-dopant Pr³⁺ is helpful to suppress the self-termination effect, and thus the accumulated population in the ⁴I_13/2 level can relax rapidly enough to maintain the required population inversion to achieve 2.7 μm laser output. The results of the energy transfer efficiencies of Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals are comparable with that in several crystals, as shown in Table 2.

Fig. 5 Decay curves of Er³⁺: ⁴I_11/2 level in Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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Fig. 6 Decay curves of Er³⁺: ⁴I_13/2 level in Er³⁺: CaLaGa₃O₇, Er³⁺/Pr³⁺: CaLaGa₃O_7, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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Table 2. Comparison of the lifetimes changes and energy transfer efficiencies in several crystals.

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To further explore the sensitization effect of Yb³⁺ in CLGO crystals, the decay curves of Yb³⁺: ²F_5/2 energy levels in Yb³⁺: CLGO, Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals are measured and shown in Fig. 7. For Yb³⁺: CLGO crystal, the decay curve is single exponential, while the decay curve of Yb³⁺: ²F_5/2 in Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals show multi-exponential behavior. The fitted fluorescence lifetimes of all crystals are also listed in Table 1. The energy transfer efficiency from Yb³⁺ ions to Er³⁺ ions can also be obtained from Eq. (5), where τ_DA is the lifetime of Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals, respectively, τ_D is the lifetime of Yb³⁺ ions in Yb³⁺ ions in Er³⁺/Yb³⁺: CLGO crystal. For Er³⁺/Yb³⁺: CLGO crystal, the energy transfer efficiency is 46.8%, while Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal, the fitted fluorescence lifetime is shorter than that in Er³⁺/Yb³⁺: CLGO crystal and the corresponding energy transfer efficiency is calculated to be 71%. In conclusion, Yb³⁺ is a practicable sensitizer to enhance the 2.7 μm emission in Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals, that is, Yb³⁺ ions can transfer pumping energy to Er³⁺ ions effectively and thus enhance the absorption efficiency of Er³⁺ ions obviously. These results indicate that the co-dopant Yb³⁺ and Pr³⁺ ions are beneficial in achieving 2.7 μm laser in Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal.

Fig. 7 Decay curves of Yb³⁺: ²F_5/2 level in Yb³⁺: CaLaGa₃O₇, Er³⁺/Yb³⁺: CaLaGa₃O₇ and Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystals.

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The above spectral results can be interpreted by the energy transfer diagrams in Fig. 8. When Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal is pumped by 980 nm LD, populations on Yb³⁺: ²F_7/2 level and populations on Er³⁺: ⁴I_15/2 state are excited to Yb³⁺: ²F_5/2 and Er³⁺: ⁴I_11/2 levels by the ground state absorption. Then the populations on Yb³⁺: ²F_7/2 state relax to Er³⁺: ⁴I_11/2 through the energy transfer process Yb³⁺: ²F_5/2→Er³⁺: ⁴I_11/2 (ET1) and thus increases the populations on Er³⁺: ⁴I_11/2 level, which leads to an enhanced 2.7 μm emission corresponding to ⁴I_11/2→⁴I_13/2, as observed in Fig. 4. Meanwhile, the weakened 1.5 μm emission corresponding to ⁴I_13/2→⁴I_15/2 in Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal is due to the presence of rapid energy transfer Er³⁺: ⁴I_13/2→Pr³⁺: ³F₄ (ET3), which deactivates the population on the Er³⁺: ⁴I_13/2 state effectively. Furthermore, due to the enhanced 2.7 μm emission and the energy transfer progress Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ (ET2), the populations on the Er³⁺: ⁴I_11/2 state decrease, and also the cross-relaxation ⁴I_11/2 + ⁴I_11/2→⁴F_7/2 + ⁴I_15/2 (CR) from the Er³⁺: ⁴I_11/2 state makes the populations on ⁴I_11/2 state decrease. Consequently, the red and green UC emissions are weakened, as demonstrated in Fig. 2. So the enhanced 2.7 μm emission is a result of comprehensive effect of the above progresses.

Fig. 8 The energy transfer diagram among Yb³⁺, Er³⁺ and Pr³⁺ ions in Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystal.

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The energy transfer microscopic parameter derived from Dexter’s theory has been widely utilized to investigate the energy transfer process among rare earth ions, which can be evaluated by the calculations of the absorption and emission cross sections of rare earth ions. The dipole-dipole interaction among Er³⁺ ions has been proved by I-H model [32]. According to Förster [33] and Dexter [34], the microscopic energy transfer probability between donor (D) and acceptor (A) ions can be denoted as [35,36]

W_{D - A} = \frac{C_{D - A}}{R^{6}}

where R is the distance between donor and acceptor. The C_D-A is the energy transfer constant that can be expressed as follows [36]

C_{D - A} = \frac{R_{C}^{6}}{τ_{D}}

where R_C is the critical radius of the interaction and τ_D is the intrinsic lifetime of the donor excited level. When phonons participate in the considered process, the energy transfer coefficient (C_D-A) can be determined by the following equation [36]

C_{D - A} = \frac{6 c g_{l o w}^{D}}{{(2 π)}^{4} n^{2} g_{u p}^{D}} \sum_{m = 0}^{\infty} e^{- (2 \bar{n} + 1)}^{S_{o}} \frac{S_{o}^{m}}{m!} {(\bar{n} + 1)}^{m} \int σ_{e m}^{D} (λ_{m}^{+}) σ_{a b s}^{A} λ d λ

where c is the light speed, n is the refractive index,

g_{l o w}^{D}

low and

g_{u p}^{D}

up are the degeneracy of the lower and upper levels of the donor, respectively.

ℏ ω_{o}

is the maximum phonon energy,

\bar{n} = 1 / (e^{ℏ ω_{0} / K T} - 1)

is the average occupancy of the phonon mode at the temperature of T. m is the number of the phonons participating in the energy transfer. S₀ is the Huang–Rhys factor and

λ_{m}^{+} = 1 / (1 / λ - m ℏ ω_{0})

is the wavelength with m phonon creation.

To calculate the energy transfer microparameters of Yb³⁺ and Er³⁺ ions in Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal, we determined the absorption and emission cross sections of Yb³⁺ at 980 nm, Er³⁺ at 980 nm and Er³⁺ at 1535 nm, respectively, as displayed in Fig. 9. Then the energy transfer microparameters between them were obtained and shown in Table 3. It is found that the energy transfer microparameter from Yb³⁺: ²F_5/2 level to Er³⁺: ⁴I_11/2 state is as high as 7.98 × 10⁻⁴⁰ cm⁶ s⁻¹, suggesting that efficient energy transfer between them can be achieved in this crystal. Furthermore, zero phonon is necessary to assist energy transfer from Er³⁺ to the adjacent Er³⁺ ions, Yb³⁺ to Yb³⁺ ions and Yb³⁺ to Er³⁺ ions, which is primarily due to the overlap between absorption and emission cross section and therefore energy transfer happens. Besides, the energy transfer microscopic parameter for ⁴I_13/2 level is more than twenty times larger than that of ⁴I_11/2 level, indicating that ⁴I_13/2 level has more opportunity to transfer its ions to the same level nearby as compared to ⁴I_11/2 level. Thus, the population inversion for 2.7 μm emission is readily realized for Er³⁺ in Er³⁺/Yb³⁺/Pr³⁺: CLGO and efficient mid-infrared radiation can be determined.

Fig. 9 Absorption and emission cross sections of Yb³⁺ at 980 nm and Er³⁺ at 980 and 1535 nm in Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ crystal.

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Table 3. Energy transfer microparameters of Yb³⁺ and Er³⁺ in Er³⁺/Yb³⁺/Pr³⁺: CaLaGa₃O₇ laser crystal.

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4. Conclusion

In conclusion, the optical properties of Er³⁺/Pr³⁺: CLGO, Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals were studied. The relevant absorption and emission cross-sections as well as the fluorescence lifetimes were calculated and compared. As a result, the strongest mid-infrared emissions as well as the weakest up-conversion and near-infrared emissions were shown in Er³⁺/Yb³⁺/Pr³⁺: CLGO as compared with Er³⁺: CLGO, Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺: CLGO crystals. Studies also showed that Yb³⁺ ions acted as a sensitizer due to the energy transfer via Yb³⁺: ²F_5/2→Er³⁺: ⁴I_11/2, which increased the absorption cross-section with peak at 980 nm in Er³⁺/Yb³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals. At the same time, with the existence of Pr³⁺ ions, the 1.5 μm emission is weakened due to the two energy transfer routes Er³⁺: ⁴I_11/2→Pr³⁺: ¹G₄ and Er³⁺: ⁴I_13/2→Pr³⁺: ³F₄ in Er³⁺/Pr³⁺: CLGO and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystals. Furthermore, the fluorescence lifetimes of Er³⁺: ⁴I_11/2 and ⁴I_13/2 levels both decreased in Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal. But the decrease extent of the lifetime of lower level ⁴I_13/2 is much larger than that of upper level ⁴I_11/2. The lifetime of the ⁴I_13/2 state decreases from 8.41 ms in Er³⁺: CLGO crystal to 1.12 ms in Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal by over 7 times, while that of ⁴I_11/2 level just falls from 0.77 ms to 0.63 ms, which indicating that the self-saturation for the Er³⁺ 2.7 μm laser is inhibited to a great degree. Moreover, energy transfer microparameters between Yb³⁺ and Er³⁺ were also calculated and analyzed based on Dexter's model. These results indicate that the doping of Yb³⁺ and Pr³⁺ is helpful to achieve an enhanced 2.7 μm emission and Er³⁺/Yb³⁺/Pr³⁺: CLGO crystal could be considered as an excellent candidate for mid-infrared lasers.

Funding

National Natural Science Foundation of China (Grant No. 51472240, 61675204 and 11304313); The National Key Research and Development Program of China (Grant No. 2016YFB0701002); the Strategic Priority Research Programs of the Chinese Academy of Science (Grant No. XDB20010200); State Key Laboratory of Structure Chemistry (Grant No.20160012, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science); Science and Technology Plan Cooperation Project of Fujian Province (Grant No. 2015I0007); Natural Science Foundation of Fujian Province (Grant No. 2015J05134, 2016J01274).

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Crystals	σ_a @980 (10⁻²¹ cm²)	⁴I_11/2, σ_e (10⁻¹⁹ cm²)	Er³⁺: ⁴I_11/2	Er³⁺: ⁴I_13/2	Yb³⁺: ²F_5/2 τ_f (ms)
Crystals	σ_a @980 (10⁻²¹ cm²)	⁴I_11/2, σ_e (10⁻¹⁹ cm²)	τ_m (ms) η_t (ET2)	τ_f (ms) η_t (ET3)	Yb³⁺: ²F_5/2 τ_f (ms)
Er³⁺: CLGO	1.91	1.79	0.77 ~	8.41 ~	~
Er³⁺/Pr³⁺: CLGO	1.84	2.28	0.68 11.7%	1.26 85.0%	~
Er³⁺/Yb³⁺: CLGO	~	1.88	0.79 ~	7.96 ~	0.33
Er³⁺/Yb³⁺/Pr³⁺: CLGO	~	2.38	0.63 18.2%	1.12 86.7%	0.18

Crystals		⁴I_11/2			⁴I_13/2		Ref.
Crystals		τ_m (ms)	η_t		τ_m (ms)	η _t	Ref.
CaGdAlO₄	Er³⁺ singly doped	0.45		~	0.982	~		[23]
	Er³⁺/Pr³⁺ co-doped	0.0848		81%	0.0743	92%
SrGdGa₃O₇	Er³⁺ singly doped	0.62		~	10.81	~		[21]
	Er³⁺/Pr³⁺ co-doped	0.44 17		7%	1.10	90.5%
	Er³⁺/Yb³⁺/Pr³⁺ co-doped	0.44 2		.0%	1.02	90.6%		[16]
SrLaGa₃O₇	Er³⁺ singly doped	0.71		~	9.74	~		[4]
	Er³⁺/Yb³⁺/Pr³⁺ co-doped	0.51		8.2%	1.78	81.7%		[18]

Energy transfer
Er³⁺: ⁴I_11/2→Er³⁺:⁴I_11/2	0 99.98	1 0.02	1.24
Er³⁺: ⁴I_13/2→Er³⁺:⁴I_13/2	0 100	1 0	26.82
Yb³⁺:²F_5/2→Yb³⁺ :²F_5/2	0 100	1 0	25.70
Yb³⁺:²F_5/2→Er³⁺:⁴I_11/2	0 100	1 0	7.98

Crystals	σ_a @980 (10⁻²¹ cm²)	⁴I_11/2, σ_e (10⁻¹⁹ cm²)	Er³⁺: ⁴I_11/2	Er³⁺: ⁴I_13/2	Yb³⁺: ²F_5/2 τ_f (ms)
Crystals	σ_a @980 (10⁻²¹ cm²)	⁴I_11/2, σ_e (10⁻¹⁹ cm²)	τ_m (ms) η_t (ET2)	τ_f (ms) η_t (ET3)	Yb³⁺: ²F_5/2 τ_f (ms)
Er³⁺: CLGO	1.91	1.79	0.77 ~	8.41 ~	~
Er³⁺/Pr³⁺: CLGO	1.84	2.28	0.68 11.7%	1.26 85.0%	~
Er³⁺/Yb³⁺: CLGO	~	1.88	0.79 ~	7.96 ~	0.33
Er³⁺/Yb³⁺/Pr³⁺: CLGO	~	2.38	0.63 18.2%	1.12 86.7%	0.18

Crystals		⁴I_11/2			⁴I_13/2		Ref.
Crystals		τ_m (ms)	η_t		τ_m (ms)	η _t	Ref.
CaGdAlO₄	Er³⁺ singly doped	0.45		~	0.982	~		[23]
	Er³⁺/Pr³⁺ co-doped	0.0848		81%	0.0743	92%
SrGdGa₃O₇	Er³⁺ singly doped	0.62		~	10.81	~		[21]
	Er³⁺/Pr³⁺ co-doped	0.44 17		7%	1.10	90.5%
	Er³⁺/Yb³⁺/Pr³⁺ co-doped	0.44 2		.0%	1.02	90.6%		[16]
SrLaGa₃O₇	Er³⁺ singly doped	0.71		~	9.74	~		[4]
	Er³⁺/Yb³⁺/Pr³⁺ co-doped	0.51		8.2%	1.78	81.7%		[18]

Co-effects of Yb³⁺ sensitization and Pr³⁺ deactivation to enhance 2.7 μm mid-infrared emission of Er³⁺ in CaLaGa₃O₇ crystal

Abstract

1. Introduction

2. Experimental details

3. Results and discussion

4. Conclusion

Funding

References and links

Cited By

Figures (9)

Tables (3)

Equations (8)

Optical Materials Express