1. Introduction

Owing to the strong absorption of radiation by water and hydroxyapatite at near 3 μm region, mid-IR (MIR) lasers operating around 3 μm have attracted much attention in view of the possible applications in medical and sensing technologies [1–6]. It can also be served as efficient and high quality laser pump sources for longer-wavelength mid-IR oscillators, such as mid-IR optical parametric oscillators (OPO) and THz local oscillators [7–12]. Among various rare earth ions, Dy³⁺ ion is well known to be the important candidate that can provide 3 μm emission due to its ⁶H_13/2→⁶H_15/2 transition. The Dy³⁺ ion possesses a number of pump bands in the near infrared, such as 1.3 μm (⁶H_15/2→⁶H_9/2+⁶F_11/2), and 1.7 μm (⁶H_15/2→⁶H_11/2). However, it is lack of commercially available high-power laser diodes (LD) corresponding to these pump bands of Dy³⁺ ion. Fortunately, codoping of Yb³⁺ as a sensitizer to Dy³⁺ has been used as a feasible alternative to obtain the 3 μm emission. In the Dy³⁺, Yb³⁺ codoped system, the Yb³⁺ ion is excited to the excited state (²F_5/2) under the excitation of a common 970 nm LD pump, then transfer the accumulated excitation energy to Dy³⁺:⁶H_5/2. Up to now, effective energy transformation from Yb³⁺:²F_5/2 to Dy³⁺:⁶H_5/2 has been studied in many luminescent hosts [13–15].

In order to get powerful 3 μm MIR emission from Dy³⁺ ions, the ideal host material is expected to possess low phonon energy suppressing nonradiative relaxation between adjacent energy levels, minimal absorption coefficient in the H₂O absorption band at around 3 μm with no effect to the 3 μm emission, and high radiative emission rates improving laser emission efficiency. Among many alternatives, fluoride crystals have been regarded as natural candidates for such rare-earth-doped optical materials. In our previous work [16–18], we have focused our attentions on a new potential gain medium β–PbF₂ crystal, due to its combination of good thermal properties, moderate mechanical properties, excellent solubility for rare-earth ions, and high transparency in a wide wavelength range. Moreover, PbF₂ crystal has the advantage of being characterized by significantly lower phonon energy, which is beneficial for suppressing multiphonon de-excitation processes. These characteristics render PbF₂ crystal as extremely suitable to be used as a host to mid-IR solid–state lasers.

The spectroscopic properties around 3 μm emission of a Dy³⁺/Yb³⁺ co-doped PbF₂ crystal under a 970 nm LD pump are investigated for the first time in the present Letter. Yb³⁺ ion was demonstrated to be an effective sensitizer for Dy³⁺ ion in PbF₂ crystal. Additionally, the established structure features from the Raman spectra has also been made to demonstrate the crystal’s feasibility for future applications in MIR lasers under a common 970 nm LD pump.

2. Experimental section

The Dy³⁺ (2.0 at. %) single doped, Yb³⁺ (2.0 at. %) single doped, and Dy³⁺(2.0 at. %)/Yb³⁺ (2.0 at. %) co-doped PbF₂ crystals were grown by the Bridgman technique. The PbF₂ (99.99%), DyF₃ (99.99%), and YbF₃ (99.99%) powers have been used as starting material. The constituent fluorides were weighted and thoroughly mixed. Then, it was heated for 5 h at 200 °C to make dehydration treatment. After that, the polycrystalline material was loaded into a 30 mm–diameter platinum crucible for crystal growth. In order to avoid volatilization of the melt, the assembled crucible was sealed immediately. During the growth of crystal, firstly, the melt in the crucible was melted for 10 h in the high-temperature zone with a temperature of 980 °C. Then the growth process was driven by lowering the crucible at a rate of 0.4 mm/h in the gradient zone with a temperature gradient around 35 °C/cm–40 °C/cm. At last, the as grown crystal was cooled to room temperature at the rate of 30 °C/cm–40 °C/h. Samples were then cut and polished with dimension of 10 mm×10 mm×5 mm for further optical measurements. The thickness of the sample was 5 mm.

The inductively coupled plasma-atomic emission spectrometry (ICP-AES) method was used to measure the concentration of Dy³⁺, and Yb³⁺ ions in the crystals. The doping concentrations of Dy and Yb in the co-doped crystal were (2.29±0.01) at. % ((6.59±0.09)×10²⁰ ions/cm³) and (2.02±0.01) at. % ((5.46±0.10)×10²⁰ ions/cm³), respectively. The doping concentrations of the single-doped crystal was (2.05±0.01) at. % ((5.90±0.09)×10²⁰ ions/cm³) of Dy ions, and (2.01±0.01) at. % ((5.43±0.10)×10²⁰ ions/cm³) of Yb ions.

Crystal structure identification was under taken on a D/max2550 X-ray diffraction (XRD) using Cu K_α radiation. The IR transmittance and absorption spectrums of Dy³⁺/Yb³⁺:PbF₂ crystal were measured by Nicolet 6700 FTIR spectrometer. The Raman spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal was measured by inVia microscopes Raman spectrometer. The absorption spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal in the wavelength of 400–2400 nm was measured by JASCO V-570 UV/VIS spectrophotometer. The fluorescence spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal in the range of 2600–3400 nm was recorded by Edinburgh Instruments FLS920 under excitation of 970 nm. The fluorescence decay curves of Dy³⁺/Yb³⁺:PbF₂ crystal at the fluorescence peak of 2890 nm was recorded under pulse excitation of 970 nm. The decay curves of the Yb³⁺–doped and Dy³⁺/Yb³⁺-codoped PbF₂ crystals at the fluorescence peak of 1030 nm were measured under excitation of 970 nm. All the measurements were taken at room temperature.

3. Experimental results and discussion

The XRD patterns of the Dy³⁺/Yb³⁺:PbF₂ crystal and the standard pattern of PbF₂ (JCPDS 06-0251), are shown in Fig. 1. The diffraction peaks are strong and no second phase diffraction peak is found, indicating that the impurities do not change the essential structure of β–PbF₂.

 

Fig. 1 (a) XRD pattern of Dy³⁺/Yb³⁺:PbF₂ crystal and (b) XRD pattern of PbF₂ crystal (JCPDS 06-0251).

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In order to study the phonon energy of the Dy³⁺/Yb³⁺:PbF₂ crystal, the Raman spectrum was measured and analyzed. The result is shown in Fig. 2(a). As we can see, there are two strong peaks centered at 130 cm⁻¹ and 251 cm⁻¹, which are attributed to the triply degenerate Raman active that the two fluorine lattices against each other while the lead ion lattice is at rest [19]. Therefore, the maximum phonon energy of the Dy³⁺/Yb³⁺:PbF₂ crystal can be speculated as low as 251 cm⁻¹. This low phonon energy of the Dy³⁺/Yb³⁺:PbF₂ crystal is beneficial for suppressing multiphonon de-excitation processes, and improving the ∼3 μm emission efficiency.

 

Fig. 2 (a) The Raman spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal; (b) The MIR absorption spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal. The inset shows the MIR transmittance spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal.

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The IR absorption spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal between 3–15 μm is shown in Fig. 2(b). As it can be seen, there are no absorption bands at the wavelength of 3–9 μm, indicating that the Dy³⁺/Yb³⁺:PbF₂ crystal may be a promising material for IR laser applications. The IR transmittance spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal is also shown in the inset of Fig. 2(b). It is clear to see that the Dy³⁺/Yb³⁺:PbF₂ crystal shows good IR transmittance around 3 μm and extends up to 9 μm (about (85±2)%). The (15±2)% loss includes to the Fresnel reflections, dispersion, and absorption of the crystal. There is a dip in transmission near at 3 μm, which correspond to the transitions of Dy³⁺ ions starting from the ⁶H_15/2 ground state to higher level ⁶H_13/2. As we know, the absorption band region from 3 to 4 μm originated from the stretching vibration of free OH⁻¹ groups will participate in the energy transfer of rare–earth ions and reduce the intensity of emission [20]. The content of OH⁻¹ groups in Dy³⁺/Yb³⁺:PbF₂ crystal can be expressed by the absorption coefficient of OH⁻¹ vibration band at around 3 μm, which can be giving by α_OH⁻ =−ln(T/T₀)/l, Where T and T₀ are the transmitted and incident intensities, respectively, l is the thickness of the measured sample. The absorption coefficient at 3 μm is as low as (0.024±0.008) cm⁻¹, which might have no effect on the 3 μm emission. In other words, the good IR transmission property proves that the Dy³⁺/Yb³⁺:PbF₂ crystal is a promising candidate for IR laser materials.

The absorption spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal in the range of 400–2400 nm was shown in Fig. 3. There are six absorption bands centered at around 453, 805, 905, 1094, 1275, and 1697 nm, which correspond to the transitions starting from the ⁶H_15/2 ground state to higher levels ⁴F_9/2, ⁶F_5/2, ⁶F_7/2, ⁶H_7/2+⁶F_9/2, ⁶F_11/2+⁶H_9/2, and ⁶H_11/2, respectively. In the range of 860 nm to 1030 nm, the highest absorption of (2.3±0.1) cm⁻¹ has been observed centered at 974 nm with a full band width at half-maximum (FWHM) of (16±1) nm corresponding to the Yb³⁺ optical transition ²F_7/2→²F_5/2, and turns out to be particularly suitable for commercialized 970 nm LD pumping. The strong absorption at around 970 nm indicates that importing of Yb³⁺ ions is expected to provide an efficient excitation channel. The inset of Fig. 3 shows the simplified energy level diagram of the Dy³⁺/Yb³⁺ codoped system. When the crystal is pumped by a 970 nm laser diode, ions of Yb³⁺:²F_7/2 state is excited to Yb³⁺:²F_5/2. Then transferring the energy to the ⁶H_5/2 level of Dy³⁺ ions, and mainly nonradiatively to the upper laser level ⁶H_13/2, makes the possibility of Dy³⁺:⁶H_13/2→⁶H_15/2 transition with ∼ 3 μm emission.

 

Fig. 3 Absorption spectrum of Dy³⁺/Yb³⁺:PbF₂ crystal in the range of 400–2400 nm. The inset shows the energy level scheme of Yb³⁺, Dy³⁺ system. NT: nonradiative transition, ET: energy transfer.

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According to the absorption spectrum (Fig. 3), the J-O intensity parameters Ω_2,4,6 of Dy³⁺ from the Judd-Ofelt theory [21,22] were calculated (shown in Table 1), from which spontaneous emission probabilities (A), and radiative lifetime (τ_R) of the transition of Dy³⁺: ⁶H_13/2→⁶H_15/2 can be also calculated, and also shown in Table 2. The parameter Ω₂ reflects the symmetry of the local environment at the Dy³⁺ site, and dropping with the improved symmetry [23]. As shown in Table 1, the Ω₂ of the Dy³⁺/Yb³⁺:PbF₂ crystal is higher than that of Dy³⁺:PbF₂ crystal, indicating that the codoping of Yb³⁺ ions would bring about a lower symmetry surrounding Dy³⁺ ions in PbF₂ crystal. The spontaneous emission probabilities (A) and the radiative lifetime (τ_R) of the transition of Dy³⁺:⁶H_13/2→⁶H_15/2 in Dy³⁺/Yb³⁺:PbF₂ crystal is as high as (61.9±0.2) s⁻¹, and (16.2±0.1) ms, respectively, which are comparable with those of Dy³⁺:PbF₂ crystal: (61.1±0.2) s⁻¹ and (16.4±0.1) ms, while much higher than those of other crystals. It indicates that the introduction of Yb³⁺ ions has little impact on the ∼3 μm fluorescence emission efficiency of Dy³⁺.

Table 1. Judd-Ofelt parameters Ω_2,4,6 of different Dy³⁺ doped materials.

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Table 2. Spontaneous emission probabilities (A) of ⁶H_13/2→⁶H_15/2, lifetime (τ_R) of ⁶H_13/2 level, and Phonon Energy of different Dy³⁺ doped materials.

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Figure 4 shows the emission spectra of Dy³⁺/Yb³⁺:PbF₂ crystal at around 3 μm originated from the Dy³⁺:⁶H_13/2→⁶H_15/2 transition. It is clear to see that the Dy³⁺/Yb³⁺:PbF₂ crystal shows a broad and smooth emission spectrum from 2600 nm to 3400 nm with FWHM of (196±2) nm, and strong emission at around 3 μm with a peak intensity of 2890 nm. The corresponding stimulated emission cross-section can be calculated according to the Fuchtbauer-Ladenburg theory [27]

 

Fig. 4 Emission spectra of Dy³⁺/Yb³⁺:PbF₂ crystal in the wavelength of 2600–3400 nm. The inset shows the fluorescence decay curve of the ⁶H_13/2 mainfold in Dy³⁺/Yb³⁺:PbF₂.

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The fluorescence decay curve for the ⁶H_13/2 multiplet of Dy³⁺ ions in Dy³⁺/Yb³⁺:PbF₂ crystal under pulse excitation of 970 nm is shown in the inset of Fig. 4. The fluorescence decay curve exhibit single-exponential nature. The measured lifetime of the ⁶H_13/2 multiplet is as long as (15.4±0.2) ms. A long upper laser level lifetime is beneficial in achieving population inversion and increasing energy storage, which will make Dy³⁺/Yb³⁺:PbF₂ crystal desirable for laser operation around 3.0 μm. The quantum efficiency η of the fluorescent level can be calculated by [28]: η =τ_meas/τ_R, where τ_meas and τ_R refer to the measured and calculated radiative lifetime, respectively. Therefore, the value of η was calculated to be (95.0±0.3)%. This high quantum efficiency is a result of the low phonon energy of the PbF₂ crystal host.

The ability of a donor (Yb³⁺) to transfer its energy to an acceptor (Dy³⁺) is an important factor to evaluate the donor (Yb³⁺) as a sensitizer. To further investigate the energy interaction mechanism, the time-resolved decays of the Yb³⁺:²F_5/2 level for the Dy³⁺/Yb³⁺:PbF₂ crystal and Yb³⁺:PbF₂ crystal were measured, and shown in Fig. 5. The measured lifetime is (52±4) μs for the Dy³⁺/Yb³⁺:PbF₂ crystal, which is much shorter than that of Yb³⁺:PbF₂ crystal ((2.3±0.2) ms). This shortening of the measured lifetime confirms that Yb³⁺ ions are able to transfer energy to the Dy³⁺ ions for 3 μm emission in PbF₂ crystal. The energy transfer (ET) efficiency can be estimated by the following equation: η_ET =1−τ_Yb−Dy/τ_Yb, where τ_Yb−Dy and τ_Yb are the Yb³⁺ lifetimes monitored at 1030 nm, with and without Yb³⁺ ions, respectively. Therefore, the value of η_ET was calculated to be (97.7±0.3)%, indicating that the Yb³⁺ ion can efficiently transfer energy to Dy³⁺ ion, and confirming that the Dy³⁺/Yb³⁺:PbF₂ crystal would be propitious to be pumped by commercialized InGaAs LD.

 

Fig. 5 Fluorescence decay curves of the Yb³⁺:²F_5/2 energy level of Yb³⁺:PbF₂ and Dy³⁺/Yb³⁺:PbF₂ crystals.

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The effective gain cross section G(λ) is given by the following equation: G(λ)=Pσ_em−(1−P)σ_abs, where P is the population inversion, corresponding to the concentration ratio of Dy³⁺ in the ⁶H_13/2 and ⁶H_15/2 levels, σ_abs and σ_em are the absorption cross-section, and emission cross-section, respectively. The calculated gain cross-sections as a function of wavelength with different P (P=0.3, 0.4, 0.5, 0.6, 0.7, and 0.9, respectively.) is shown in Fig. 6. It is obvious to see that the gain cross-section becomes positive from 2900 nm once the population inversion level reaches value of 40%, indicating that a low pumping threshold is achieved for the Dy³⁺:⁶H_13/2→⁶H_15/2 laser operation.

 

Fig. 6 Gain cross-section spectra of Dy³⁺:⁶H_13/2→⁶H_15/2 in Dy³⁺/Yb³⁺:PbF₂ crystal.

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4. Conclusion

In conclusion, Dy³⁺/Yb³⁺ co-doped PbF₂ crystal was successfully prepared, and an intense 3 μm emission was obtained under the excitation of a commercialized 970 nm LD. It is found that the Dy³⁺/Yb³⁺:PbF₂ crystal possesses long fluorescence lifetime (15.4±0.2) ms, high quantum efficiency (95.0±0.3)%, and large emission cross section (1.37±0.11)×10⁻²⁰ cm² corresponding to the stimulated emission of Dy³⁺:⁶H_13/2→⁶H_15/2 transition. It was demonstrated that the energy transfer efficiency from Yb³⁺ to Dy³⁺ is as high as (97.7±0.3)%, confirms that the Yb³⁺ ion can be used as an effective sensitizer for Dy³⁺ ion. It was also demonstrated that the Dy³⁺/Yb³⁺:PbF₂ crystal possesses low phonon energy (251 cm⁻¹), and good IR transmission, which is one of the main reasons for the intense 3 μm emission. It suggests that the Dy³⁺/Yb³⁺:PbF₂ crystal is a promising material for 3 μm laser applications under being pumped by a conventional 970 nm LD.