1. Introduction

Mid-IR (MIR) lasers at ~3 μm have attracted much attention in recent years for their wide applications in medical, biological, sensing technologies, as well as being efficient pumping sources for longer wavelength MIR lasers and optical parametric oscillators (OPO) [1–4]. It is well known that Ho³⁺ is a natural candidate for ~3 μm lasers with the transition of ⁵I₆ → ⁵I₇ [5–7]. However, the ~3 μm emission cannot be obtained efficiently because of the population bottleneck effect of the ⁵I₆ → ⁵I₇ transition that makes it a self-terminated transition. In order to achieve intense Ho³⁺: ⁵I₆ → ⁵I₇ MIR emission for practical laser operations, we need (i) appropriate deactivated ions to depopulate the Ho³⁺: ⁵I₇ manifold for population inversion, and (ii) a proper host with low phonon energy to suppress multi-phonon de-excitation.

Fortunately, Pr³⁺ has been explored to be feasible to quench the lower Ho³⁺: ⁵I₇ manifold, while hardly depopulate the upper Ho³⁺: ⁵I₆ manifold, as good results of ~3 μm emissions and lasers achieved in studies of Ho,Pr-codoped fibers and crystals [8–10]. On the other hand, among many alternative hosts, fluoride crystals have several advantages: (i) lower phonon energy to suppress non-radiative relaxations between adjacent energy levels, (ii) longer fluorescence lifetime to improve energy storage, and (iii) lower refractive index to limit nonlinear effects under intense laser pumping. In our previous work [11], a Ho³⁺:LaF₃ crystal with low phonon energy of about 340 cm⁻¹ was reported, and good ~2.9 μm MIR emission performances was achieved, indicating that LaF₃ crystal can be used as a suitable host to MIR solid-state lasers.

To our knowledge, there is still no report on the growth of Ho:Pr:LaF₃ crystal or the ~3 μm emission in this crystal up to now. In this letter, a novel Ho³⁺,Pr³⁺-codoped laser crystal was successfully grown. Pr³⁺ was demonstrated to enhance the Ho³⁺: ⁵I₆→⁵I₇ ~2.86 μm emission by efficient energy transfer from Ho³⁺: ⁵I₇ to Pr³⁺: ³F₂. The absorption spectrum, emission spectrum, fluorescence decay curve and transfer mechanism of Ho:Pr:LaF₃ crystal were all investigated and analyzed for future applications in MIR lasers.

2. Experiments

Commercially available fluoride powders of LaF₃, HoF₃ and PrF₃ with high purity of 99.99% were prepared for crystal growth. The concentrations of Ho and Pr introduced in the raw materials were 1.5 at% and 0.5 at%, respectively. The crystal growth process was similar to our previous work of Ho:LaF₃ [11]. Several samples of 10 × 10 × 1 mm³ were cut and polished to spectral quality for further investigations, as shown in the inset of Fig. 2. The crystal structure identification was undertaken on a D/max2550 X-ray diffraction (XRD) using Cu Ka radiation. The absorption spectrum in the range of 400-2200 nm was recorded by a UV-vis-NIR spectrophotometer (ModelV-570, JASCO). The fluorescence spectrum and fluorescence decay curves of Ho³⁺: ⁵I₇ and Ho³⁺: ⁵I₆ were obtained under excitation of 640 nm with Edinburg Instruments FLS920 and FSP920 spectrophotometers. All the measurements were performed at room temperature.

3. Results and discussions

The XRD pattern of the as-grown Ho:Pr:LaF₃ crystal phase and the standard pattern of pure LaF₃ crystal phase (JCPDS 08-0461), are shown in Fig. 1. It can be seen that the diffraction peaks of the as-grown Ho:Pr:LaF₃ are strong and there is no second phase diffraction peak, indicating that the Ho,Pr-co-doped LaF₃ crystal is well-crystallized and the impurities do not change the essential structure of LaF₃, just like that of the Ho:LaF₃ [11].

 

Fig. 1 XRD pattern of the Ho:Pr:LaF₃ crystal and the PDF database, no. 08-0461.

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The room-temperature absorption spectrum of Ho:Pr:LaF₃ crystal is shown as absorption coefficient in Fig. 2. It is obvious to see many typical absorption bands of Ho³⁺ ions, such as these centered at around 414, 450, 533, 639, and 1150 nm, corresponding to the transitions from Ho³⁺: ⁵I₈ to ⁵G₅, ⁵G₆ + ⁵F₁, ⁵S₂ + ⁵F₄, ⁵F₅, and ⁵I₆, respectively. Although several typical absorption bands of Pr³⁺ ions in the wavelength of 430-490 nm [12] are covered by Ho³⁺ absorption, the bands at around 600 nm and in the range of 1380-1670nm corresponds to the transitions from Pr³⁺: ³H₄ to ¹D₂ and ³F₃ + ³F₄, respectively. It should be noticed that the absorption in the range of 1870-2100 nm shows strong overlap between Ho³⁺: ⁵I₈→⁵I₇ and Pr³⁺: ³H₄→³F₂ + ³H₆, so it is expected that efficient energy transfer will occur between Ho³⁺:⁵I₇ and Pr³⁺:³F₂ + ³H₆.

 

Fig. 2 Room temperature absorption coefficient of the Ho:Pr:LaF₃ crystal.

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Thanks to the development of high-power Tm-doped silica fiber lasers at ~1.9 μm, absorption band at around 1928 nm adapts well for such in-band pumping [13,14]. On the other hand, the absorption around 1150 nm is also strong enough to match well with Yb-doped fiber lasers [15] or the diode lasers based on highly strained InGaAs quantum wells operating in the wavelength range beyond 1100 nm [16,17].

The fluorescence spectrum of the Ho:Pr:LaF₃ crystal is shown in Fig. 3. There are three main emission bands at around 2823, 2857 and 2876 nm assigned to the Ho³⁺: ⁵I₆ → ⁵I₇ transition. By comparing, it is evident that the emission intensities around 2857 and 2876 nm of Ho:Pr:LaF₃ is a little stronger than that of the Ho:LaF₃. In addition, the right inset of Fig. 3 shows that the ~2 μm emission (Ho³⁺: ⁵I₇ → ⁵I₈) of Ho:Pr:LaF₃ is much weaker than that of Ho:LaF₃. It indicates that co-doping Pr weakens the ~2 μm emission while benefits the ~2.8 μm emission. Thus, output of ~2.86 μm lasers can be realized on the Ho:Pr:LaF₃ crystal.

 

Fig. 3 Fluorescence spectra of the as-grown Ho:Pr:LaF₃ crystal compared with that of the Ho: LaF₃ crystal.

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Moreover, the emission cross sections can be calculated by the Fuchtbauer–Ladenburg equation [18], and the results are shown as the left inset of Fig. 3. The maximum emission cross section of Ho:Pr:LaF₃ is 1.31 × 10⁻²⁰ cm² at 2875 nm, and that of the Ho: LaF₃ is 1.12 × 10⁻²⁰ cm².

The fluorescence decay curves of Ho³⁺: ⁵I₆ and ⁵I₇ manifolds for the Ho:Pr:LaF₃ crystal are shown in Fig. 4. The experiment curves both exhibit single exponential decaying behavior, and the lifetimes are fitted to be 7.46 and 7.58 ms for Ho³⁺: ⁵I₆ and ⁵I₇, respectively. In our previous work [11], the lifetimes of Ho³⁺: ⁵I₆ and ⁵I₇ manifolds in the Ho:LaF₃ crystal are measured to be 10.37 and 25.81 ms, respectively. By comparing, it can be seen that the lifetime of upper level ⁵I₆ in Ho:Pr:LaF₃ crystal decreases a little from 10.37 ms to 7.46 ms, whereas that of lower level ⁵I₇ in Ho:Pr:LaF₃ crystal decreases heavily from 25.81 ms to 7.58 ms. The decrease of lifetimes of Ho³⁺: ⁵I₆ and ⁵I₇ levels in the Ho:Pr:LaF₃ crystal are respectively attributed to the energy transfers within Ho³⁺: ⁵I₆ → Pr³⁺: ³F₄ (ET₁) and Ho³⁺: ⁵I₇ → Pr³⁺: ³F₂ (ET₂), as shown in the right of Fig. 4.

 

Fig. 4 Fluorescence decay curves of Ho: ⁵I₆ and ⁵I₇ manifolds in the Ho:Pr:LaF₃ crystal and schematic of energy-level and energy-transfer of Ho³⁺ and Pr³⁺.

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Meanwhile, the efficiency of the energy transfer within Ho³⁺ and Pr³⁺ ions can be estimated by: $\eta =1-\raisebox{1ex}{${\tau }_{HO-\mathrm{Pr}}$}\!\left/ \!\raisebox{-1ex}{${\tau }_{HO}$}\right.$, where ${\tau }_{HO-\mathrm{Pr}}$ and ${\tau }_{HO}$ are the lifetimes of Ho³⁺ in the Ho:Pr:LaF₃ and Ho: LaF₃ crystals, respectively. It is calculated to be 28% and 71% for ET₁ and ET₂, respectively. As the energy transfer efficiency of ET₂ is much greater than that of ET₁, the lifetime of Ho³⁺: ⁵I₇ level decreases more than that of Ho³⁺: ⁵I₆ level in the Ho:Pr:LaF₃ crystal compared with the Ho:LaF₃ crystal. Hence, the self-termination problem is almost restrained because of co-doping with Pr³⁺ ions, which can facilitate ~2.86 μm laser operations for the Ho:Pr:LaF₃ crystal.

4. Conclusion

In conclusion, enhanced emission at ~2.86 μm was observed in a novel Ho³⁺:Pr³⁺:LaF₃ crystal for the first time. Compared with Ho³⁺ singly-doped LaF₃ crystal, the Ho³⁺,Pr³⁺-codoped LaF₃ crystal possessed stronger emission intensity at ~2.86 μm. It was also found that the energy transfer efficiency of Ho³⁺: ⁵I₇ → Pr³⁺: ³F₂ is much greater than that of Ho³⁺: ⁵I₆ → Pr³⁺: ³F₄, which almost restrained the self-termination problem in this Ho³⁺:Pr³⁺:LaF₃ crystal. All the results indicate that co-doping Pr³⁺ with Ho³⁺ in LaF₃ crystal plays an important role in ~2.86 μm emissions.