1. Introduction

Over the past few decades, mid-infrared (MIR) solid lasers in the 2.7-3μm wavelength range have attracted much attention for possible applications in medical surgery, dentistry, remote atmospheric sensing and light detection because of their strong absorption in vapor, water, and biological tissues [1]. Also, these lasers can be used as efficient and high-quality pump sources for an optical parametric oscillator (OPO) to achieve mid-infrared laser radiation with sufficient intensity in 3-12μm wavelength range [2–4].

The lanthanide element Er³⁺ ion can be used for 2.7μm mid-infrared emission, which corresponds to the transition from ⁴I_11/2 to ⁴I_13/2 level. The mid-infrared lasers of Er³⁺ doped materials are usually under 980 or 808 nm pump since these laser diodes are commercialized and their wavelengths match the intrinsic absorption of Er³⁺. However, a detrimental self-termination bottleneck effect is possible for this transition, owing to the much longer lifetime of the ⁴I_13/2 level than that of the ⁴I_11/2 level [5]. To overcome this effect, previous researches have shown that increasing Er³⁺ concentration (>30 at. %) 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 2.7μm laser in Er³⁺ activated crystals [6]. However, the doping of too high concentration of Er³⁺ may bring about a degeneration of the optical-quality and thermal properties of the crystal, limiting the laser output efficiency and beam quality. Another method to conquer the self-termination bottleneck is used as deactivation ions (such as Tm³⁺, Ho³⁺, Pr³⁺, Dy³⁺, Nd³⁺, Eu³⁺) to be co-doped into the host materials to the depopulated the lower level ⁴I_13/2, and the effective 2.7μm emissions are achieved [7–15]. In this work, we find Tb³⁺ ion as a novel deactivator to quench the lower level of Er³⁺: ⁴I_13/2, because Tb³⁺ ion energy level ⁷F₀ is adjacent to the ⁴I_13/2 level of Er³⁺. As the energy level scheme illustrated in Fig. 1 shows, after the ions in the Er³⁺: ⁴I_11/2 level decay radiatively to Er³⁺: ⁴I_13/2 level with around 2.7μm emission, the ions in the Er³⁺: ⁴I_13/2 level will undergo an effective energy transfer (ET) process to Tb³⁺: ⁷F₀ level. Therefore, Er³⁺, Tb³⁺ co-doped crystal is worth studying as a mid-infrared lasers candidate.

 

Fig. 1 Simplified energy level diagram of Er³⁺ and Tb³⁺ co-doped system. ESA: excited state absorption, ETU: energy transfer up-conversion, ET: energy transfer.

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Besides, laser host materials are helpful to achieve the great fluorescence characteristics and practical applications. It is especially important for matrix materials that need high thermal conductivity and low phonon energy (shown in Table 1), because of the narrow energy level gap between ⁴I_11/2 and ⁴I_13/2. For laser materials operating at 2.7μm of Er³⁺, these two parameters become even more important due to most of the population in ⁴I_11/2 relaxing to ⁴I_13/2 by multi-phonon relaxation, producing lots of heat. The low phonon energy conduces to the decrease of the multi-phonon relaxation rate for Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition, and therefore decreasing the populations on ⁴I_13/2 state [22–24]. Among many alternatives, PbF₂ crystal, a new potential gain medium, has been regarded as natural candidates for such rare-earth-doped optical materials due to its combination of high thermal conductivity, moderate mechanical properties, stable chemical properties, excellent solubility for rare-earth ions, and high transparency in a wide wavelength range, as well as significantly lower phonon energy (257 cm⁻¹) [25–27]. Herein, the PbF₂ single crystal is chosen as a matrix for Er³⁺ and Tb³⁺ ions to investigate the mid-infrared optical spectra.

Table 1. Phonon energy of different laser host materials.

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To our knowledge, there is still no report on the growth of Er³⁺/Tb³⁺: PbF₂ crystal or the 2.7μm MIR emission in this crystal up to now. In this work, Er³⁺ doped and Er³⁺/Tb³⁺ co-doped PbF₂ single crystals were grown successfully using the Bridgman method. Its absorption spectra, J-O parameters, near-infrared, up-conversion, and mid-infrared fluorescence spectra, as well as the fluorescence decay curves, were measured at room temperature. To analyze the energy transfer between the Er³⁺ ion and deactivators Tb³⁺, the energy transfer mechanisms are discussed and the energy transfer coefficients are determined under a common 980 nm laser diode (LD). Tb³⁺ ion was demonstrated to be an efficient deactivated ion for Er³⁺ ion to greatly facilitate the Er³⁺: ⁴I_11/2 → ⁴I_13/2 by effective ET from Er³⁺: ⁴I_13/2 level to Tb³⁺: ⁷F₀. The spectroscopy investigation of 2.7μm emission has been made for potential high-power laser output in practical operation.

2. Experimental details

In the Er³⁺/Tb³⁺ co-doped system, if the concentration of Tb³⁺ is too small, the energy transfer efficiency (Er³⁺: ⁴I_13/2 → Tb³⁺: ⁷F₀) would be greatly reduced, resulting in significant decrease in the emission intensity of 2.7μm. On the contrary, if the concentration of Tb³⁺ is too large, the concentration quenching would appear, which would result in the fluorescence quenching. In this work, we used a middle-ground approach to select the concentration of Er³⁺ and Tb³⁺. The Er³⁺/Tb³⁺ co-doped PbF₂ single crystal was grown by the Bridgman method. The initial concentrations of Er³⁺, Tb³⁺ were 1 at. % and 0.5 at. %, respectively. 1 at. % Er³⁺: PbF₂ was also grown for spectral comparison. Reagent grade PbF₂ (99.99%), ErF₃ (99.99%), and TbF₃ (99.99%) powders were used as raw materials. The constituent fluorides were weighted and thoroughly mixed. The temperature gradient across the solid-liquid interface was 35°C- 40°C/cm, and the seeding temperature was about 960°C- 980°C. The growth process was executed by lowering the crucible at a rate of 0.4-0.5mm/h. The detailed crystals growth process was similar to our previous work [25].

The sample was then cut and polished on both sides for further optical measurements. The inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to measure the concentrations of Er³⁺ and Tb³⁺ ions in the as-grown crystals. Crystal structure identification was undertaken on a D/max2550 X-ray diffraction (XRD) using Cu K_α radiation. The absorption spectrums of the as-grown crystals in the wavelength of 300-2600nm were recorded by a UV-vis-NIR spectrophotometer (UV-3150, Shimadzu, Japan). The fluorescence spectra in the range of 510nm to 700nm, 1460nm to 1660nm, 2550nm to 2950nm and fluorescence decay profiles of the two crystals were acquired by Edinburgh Instruments FLS920 and FSP920 spectrophotometers with a laser diode (LD) as the pump source (excited at 980 nm), and an optical parametric oscillator pulse laser. All the measurements were done at room temperature.

3. Results and discussions

The doping concentrations of Er³⁺ and Tb³⁺ in the Er³⁺/Tb³⁺: PbF₂ crystal were measured to be 1.15 at. % (2.33 × 10²⁰ ions/cm³) and 0.55 at. % (1.12 × 10²⁰ ions/cm³), respectively. The doping concentration of Er³⁺ in the Er³⁺: PbF₂ crystal was 1.09 at. % (2.21 × 10²⁰ ions/cm³). According to the effective segregation coefficient k_eff = c₁/c₂, where c₁ and c₂ are the respective concentrations of the ions in the crystal and raw materials, k_eff values of Er³⁺ and Tb³⁺ in PbF₂ crystal were determined to be 1.15 and 1.1, respectively, and k_eff value of Er³⁺ in PbF₂ crystal was determined to be 1.09. In PbF₂ crystal, both Er³⁺ and Tb³⁺ ions usually occupy the lattice position of Pb²⁺. Meanwhile, the charge compensation is attained by the presence of interstitial fluorine ion (F_i¯) [28]. Figure 2 shows the XRD patterns of the as-grown crystals powder, and it is well consistent with the standard JCPDF file [No. 06-0251] for PbF₂ crystal which means that there is no phase transformation even after Er³⁺ and Tb³⁺ are doped into the PbF₂ crystal.

 

Fig. 2 XRD patterns of the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ single crystals.

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Figure 3 shows the absorption spectra of the Er³⁺ singly doped and Er³⁺/Tb³⁺ co-doped PbF₂ single crystals in the wavelength region 300-2600 nm. As seen in Er³⁺: PbF₂ crystal, eleven characteristic absorption bands of Er³⁺ centered at the wavelengths of 363, 377, 406, 449, 486, 521, 541, 650, 801, 974, 1508 (1532) nm have been revealed, which are assigned to the transitions from the ground state (⁴I_15/2) to ⁴G_9/2, ⁴G_11/2, ²H_9/2, ⁴F_5/2 + ⁴F_3/2, ⁴F_7/2, ²H_11/2, ⁴S_3/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2 and ⁴I_13/2, respectively. While in Er³⁺/Tb³⁺: PbF₂ crystal, besides the characteristic absorption bands of Er³⁺, we also observe the absorption bands of Tb³⁺: the main peaks centered at 1868, 1980, 2250 nm are assigned to the transitions from Tb³⁺: ⁷F₆ to ⁷F₀ + ⁷F₁ + ⁷F₂ + ⁷F₃. In the range of 920-1040 nm, the highest absorption of 0.65 cm⁻¹ has been obtained at 25.5 nm with a full width at half maximum (FWHM) of 974 nm, which is suitable for being pumped by the commercial 980 nm InGaAs laser diodes. The Er³⁺ ions can be directly pumped into upper laser level ⁴I_11/2 by radiation at around 980nm, which can avoid various non-radiative losses and thermal loading [29].

 

Fig. 3 Absorption spectra of Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ single crystals in the range of 300-2600nm.

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The Judd-Ofelt (J-O) theory is the most effective method in the analysis of the spectroscopic properties of the rare-earth ions doped in crystals or glasses [30,31]. Nine absorption bands corresponding to the manifolds ⁴G_11/2 + ⁴G_9/2, ²H_9/2, ⁴F_5/2 + ⁴F_3/2, ⁴F_7/2, ⁴S_3/2 + ²H_11/2, ⁴F_9/2, ⁴I_9/2, ⁴I_11/2, and ⁴I_13/2 in the room temperature absorption spectra (shown in Fig. 3) were chosen to determine the J-O intensity parameters Ω₂, Ω₄, Ω₆ for the corresponding Er³⁺(4f¹¹) transitions in Er³⁺/Tb³⁺: PbF₂. Many researchers have applied the J-O analysis to determine the important spectroscopic and laser parameters [32–39]. Table 2 shows the experimental and calculated line strengths. The intensity parameters Ω₂, Ω₄, Ω₆ of Er³⁺ (shown in Table 3) was also calculated by the room-temperature absorption spectrum based on the Judd-Ofelt theory. It is well known that Ω₂ is affected by the symmetry of the rare-earth ions site. The value of Ω₂ drops with the improved symmetry [40]. The larger Ω₂ of Er³⁺ in our Er³⁺/Tb³⁺: PbF₂ crystal indicates that the co-doping of Tb³⁺ ions would bring about a lower symmetry surrounding Er³⁺ ions.

Table 2. Barycenter wavelengths, measured and calculated line strengths of Er³⁺/Tb³⁺: PbF₂ crystal.

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Table 3. Judd-Ofelt parameters Ω_2,4,6, calculated branching ratio, and lifetime of Er³⁺/Tb³⁺: PbF₂, Er³⁺: PbF₂, Er³⁺: NaYF₄, and Er³⁺: CaLaGa₃O₇ crystals. (τ_m is measured radiative lifetime)

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Once the intensity parameters Ω₂, Ω₄, Ω₆ of Er³⁺ are received, the spontaneous emission probabilities (A), radiative lifetime (τ_rad), and the fluorescence branching ratio (β) of different upper levels for the Er³⁺/Tb³⁺ co-doped PbF₂ crystal can be calculated, and the results are shown in Table 4. It is clear to see that the fluorescence branching ratio β of Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition in the Er³⁺: PbF₂ crystal is as high as 14.5%, which is larger than that of the Er³⁺ doped other crystals, such as Er³⁺: NaYF₄ (8%) [41], and Er³⁺: CaLaGa₃O₇ (13.3%) [42]. Moreover, compared with the Er³⁺ single-doped PbF₂ crystal, the Er³⁺/Tb³⁺ co-doped PbF₂ crystal possesses a larger fluorescence branching ratio β of Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition, which is as high as 16.3%. The larger fluorescence branching ratio β of Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition in the Er³⁺: PbF₂ indicates that the PbF₂ single crystal is an excellent matrix to investigate the mid-infrared optical spectra. Meanwhile, the co-doping of Tb³⁺ ions makes the Er³⁺/Tb³⁺: PbF₂ crystal more easily induces the 2.7μm fluorescence emission and facilitates laser operation.

Table 4. Calculated radiative transition rates, branching ratios and calculated radiative lifetimes for different transition levels of Er³⁺/Tb³⁺: PbF₂ crystal.

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The near-infrared emission spectra of Er³⁺ singly doped and Er³⁺/Tb³⁺ co-doped PbF₂ crystals within the range of 1460-1660 nm excited by 980 nm are shown in Fig. 4. The emission band centered on about 1546 nm corresponds to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺. It is obvious that the near-infrared intensity of the Er³⁺/Tb³⁺ co-doped PbF₂ sample is quite weaker (about 1/2) than that of the Er³⁺ doped PbF₂ sample due to the energy transfer progress between Er³⁺ and Tb³⁺. As we know, the near-infrared emission, as a competitive emission, is a negative factor for mid-infrared laser output. The 1.55μm emission quenching phenomenon indicates that the depopulation of Er³⁺: ⁴I_13/2 level by Tb³⁺ in Er³⁺/Tb³⁺: PbF₂ crystal is very effective, thus the sole introduction of Tb³⁺ into Er³⁺: PbF₂ crystal is beneficial to the realization of 2.7μm lasers.

 

Fig. 4 The near-infrared emission spectra of the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals.

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Figure 5 displays the up-conversion emission spectra of Er³⁺ singly doped and Er³⁺/Tb³⁺ co-doped PbF₂ crystals in the range of 510-700 nm via excitation of the 980 nm LD (at the pump power of 1.05 W). In the spectra of these two crystals, the green up-conversion emission band centered at 549 nm corresponds to the transition of Er³⁺: (²H_11/2, ⁴S_3/2) → ⁴I_15/2, and the red up-conversion emission band centered at 669 nm can be assigned to the transition of Er³⁺: ⁴F_9/2 → ⁴I_15/2 (as shown in Fig. 1). When Tb³⁺ were introduced, the intensity of the up-conversion emission in the Er³⁺/Tb³⁺: PbF₂ crystal decreases greatly due to the fact that the energy transfer process ET2 is more efficient, and ions in the ⁴I_13/2 level are largely depopulated, which restrains the process of excited-state absorption (ESA1). Although the strong up-conversion emission may have many applications such as achieving fluorescent cooling [43–45], it is still not beneficial to the realization of 2.7μm lasers. The illustration in Fig. 5 shows the dependence of the up-conversion fluorescence intensity around 549 and 669 nm on the pumping intensities in the Er³⁺/Tb³⁺: PbF₂ crystal. It is clear to see that the green and red up-conversion fluorescence intensity is positively correlated with the incident pump power, and the green up-conversion fluorescence grows faster than the red up-conversion fluorescence. The results show that the addition of Tb³⁺ can significantly inhibit the visible up-conversion fluorescence, helping for the realization of 2.7μm mid-infrared lasers.

 

Fig. 5 The up-conversion emission spectra of the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals. The inset shows the dependence of the up-conversion fluorescence intensity for the Er³⁺/Tb³⁺: PbF₂ crystal.

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The mid-infrared emission spectra of the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals within the range of 2550-2950 nm pumped by 980nm are shown in Fig. 6. The observed strongest emission bands around 2660 nm and 2745 nm are assigned to Er³⁺: ⁴I_11/2 → ⁴I_13/2 in Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals. By comparing, it is found that the emission intensity of the Er³⁺/Tb³⁺: PbF₂ crystal is almost 1.4 times that of the Er³⁺: PbF₂ crystal, which indicates that the introduction of Tb³⁺ ions can enhance the 2.7μm fluorescence intensity of the Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition effectively. As an important parameter affecting the potential 2.7μm photoluminescence of Er³⁺, the corresponding emission cross sections are subsequently calculated by Fuchtbauer-Ladenburg equation [46]:

 

Fig. 6 The mid-infrared emission spectra of the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals.

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where I(λ)/∫λI(λ)dλ is the normalized line shape function of the experimental emission spectrum, β is the fluorescence branching ratio, c is the velocity of light in vacuum, n is the refractive index and τ_r is the radiative lifetime. The maximum emission cross section of the Er³⁺/Tb³⁺: PbF₂ is 0.64 × 10⁻²⁰ cm² at 2745 nm, which is larger than that of Er³⁺ single doped PbF₂ crystal (0.49 × 10⁻²⁰ cm²). This enhanced fluorescence emission cross section may benefit from the superiority of the PbF₂ matrix as well as the higher fluorescence branching ratio β of ⁴I_11/2 → ⁴I_13/2 transition (from 14.5% to 16.3%) with the co-doped of Tb³⁺ ions.

Mechanism of Er³⁺-Tb³⁺ energy transfer in the PbF₂ crystal is demonstrated in Fig. 1. Initially, the Er³⁺ ions were excited into a ⁴I_11/2 level by 980 nm pumping, then decayed to ⁴I_13/2 level by multi-phonon relaxation or 2.7μm radiation. However, the 2.7μm emission in the Er³⁺ singly doped crystal is very weak due to self-termination bottleneck. After that, ions in the Er³⁺: ⁴I_13/2 level will decay radiatively to Er³⁺: ⁴I_15/2 with 1.5μm emission or undergo an energy transfer process to Tb³⁺: ⁷F₀ due to the energy of Er³⁺: ⁴I_13/2 is slightly higher than Tb³⁺: ⁷F₀. Meanwhile, a part of the Er³⁺ ions in the ⁴I_13/2 level transfer to the Er³⁺: ⁴F_9/2 level with an ESA1 process by absorbing 980 nm pump and then radiatively relaxing to the ground state with a 669 nm red emission. On the other hand, some ions in the ⁴I_11/2 level of Er³⁺ transfer to the Er³⁺: ⁴F_7/2 level with an ESA2 process by absorbing 980 nm pump and then relaxes nonradiative transition to ²H_11/2 and ⁴S_3/2 level. Consequently, the green emission can be obtained by the Er³⁺: ²H_11/2 → ⁴I_15/2 and Er³⁺: ⁴S_3/2 → ⁴I_15/2 transitions. When Tb³⁺ were introduced, the ions in ⁴I_13/2 level of Er³⁺ are drastically depopulated by ET2 process (Er³⁺: ⁴I_13/2 → Tb³⁺: ⁷F₀), which can explain the phenomena of less 1.5μm emission and up-conversion emission in the Er³⁺/Tb³⁺: PbF₂ crystal. In addition, the ETU process (Er³⁺: ⁴I_13/2 + ⁴I_13/2 → ⁴I_15/2 + ⁴I_9/2) will be beneficial to the realization of 2.7μm lasers due to the fast-multi-phonon decay from the Er³⁺: ⁴I_9/2 level to Er³⁺: ⁴I_11/2 level [47]. All in all, it is beneficial to suppress effectively the self-termination problem and reduce the 2.7μm laser threshold.

To further explore the energy interaction mechanism, the fluorescence decay curves of Er³⁺: ⁴I_13/2 and ⁴I_11/2 multiplets for the Er³⁺: PbF₂ and Er³⁺/Tb³⁺: PbF₂ crystals were measured at 1546 and 2745 nm excited by 980 nm LD pump, respectively, as shown in Fig. 7. By single-exponential fitting, the fluorescence lifetime τ_m of the ⁴I_11/2 and ⁴I_13/2 manifold in Er³⁺/Tb³⁺: PbF₂ crystal is 5.881ms and 1.391ms, respectively. In Er³⁺: PbF₂ crystal, the fluorescence decay curve of the Er³⁺: ⁴I_13/2 was slightly deviation from a single-exponential, which is due to overlapping emission contributions from different Er³⁺ sites and the existence of energy transfer between Er³⁺ ions at the current doping concentration (e.g. ETU process shown in Fig. 1) [48,49]. Despite the lifetime of Er³⁺: ⁴I_11/2 in Er³⁺/Tb³⁺: PbF₂ crystal decreases a little from 6.029ms to 5.881ms due to the energy transfer Er³⁺: ⁴I_11/2 → Tb³⁺: ⁷F₀ process, the Er³⁺/Tb³⁺: PbF₂ crystal exists a quicker attenuation of the lower level lifetime compared with the ⁴I_13/2 lifetimes of the Er³⁺: PbF₂ crystal (⁴I_13/2: 14.291ms), which is mainly attributed to the energy transfer from the Er³⁺: ⁴I_13/2 level to Tb³⁺: ⁷F₀ level, as shown in Fig. 1. It is confirming that Tb³⁺ ions can be used as effective deactivation ions to depopulate the Er³⁺: ⁴I_13/2 level for enhancing the 2.7μm emission. What’s more, the energy transfer efficiency of ET process can be estimated from the measured lifetime of the 2.7μm and 1.5μm emissions by the following equation: η_ET = 1-τ_Er/Tb/τ_Er, where τ_Er/Tb and τ_Er are the Er³⁺ lifetimes monitored with and without Tb³⁺ ions, respectively. According to the equation and lifetimes, the efficiencies of the energy transfer Er³⁺ to Tb³⁺ in ET1 and ET2 are evaluated as 2.45% and 90.27%, respectively. The energy transfer rate of ET2 is more prominent than that of ET1, which can explain the significantly quicker lifetime shortening of Er³⁺: ⁴I_13/2 than ⁴I_11/2 level in the Er³⁺/Tb³⁺: PbF₂ compared with the Er³⁺: PbF₂ crystal. Furthermore, based on the measured results above, the Er³⁺: ⁴I_11/2 to Er³⁺: ⁴I_13/2 fluorescence lifetime ratio (τ(⁴I_11/2)/ τ(⁴I_13/2)) of the Er³⁺/Tb³⁺: PbF₂ crystal was calculated to be as high as 422.79%, which is almost ten times that of the Er³⁺: PbF₂ crystal (42.19%). Therefore, under 980 nm pumping scheme, co-doping of Tb³⁺ with Er³⁺ can turn on the possibility of excellent 2.7μm lasers, while demanding lower the pump intensities. As a deactivated ion, Tb³⁺ can further improve the energy transfer efficiency by changing its doping concentration. However, if the concentration of Tb³⁺ is too large, the concentration quenching would appear, which would result in the fluorescence quenching. Therefore, it is important to explore the appropriate doping ratio. This improvement can be made in future research work.

 

Fig. 7 (a)(b) Fluorescence decay curves of the Er³⁺: ⁴I_11/2 and ⁴I_13/2 energy levels of Er³⁺: PbF₂ crystal. (c)(d) Fluorescence decay curves of the Er³⁺: ⁴I_11/2 and ⁴I_13/2 energy levels of Er³⁺/Tb³⁺: PbF₂ crystal.

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4. Conclusion

In conclusion, Er³⁺ singly doped and Er³⁺/Tb³⁺ co-doped PbF₂ single crystals were successfully grown by using the Bridgman method. The efficient emission at 2.7μm was observed in the Er³⁺/Tb³⁺ co-doped PbF₂ crystal under the excitation of a common 980 nm LD. The strongly reduced up-conversion and near-infrared emissions, as well as the enhanced mid-infrared emission, were obtained in the Er³⁺/Tb³⁺: PbF₂ crystal at the same time, indicating that the introduced Tb³⁺ efficiently depopulates the lower laser level of Er³⁺: ⁴I_13/2. Compared with the Er³⁺: PbF₂ crystal, the Er³⁺/Tb³⁺ crystal has higher fluorescence branching ratio (16.3%), and higher emission cross section (0.64 × 10⁻²⁰ cm²) corresponding to the stimulated emission of Er³⁺: ⁴I_11/2 → ⁴I_13/2 transition. It was demonstrated that the energy transfer efficiency from Er³⁺: ⁴I_13/2 to Tb³⁺: ⁷F₀ is as high as 90.27%, and the Er³⁺: ⁴I_11/2 → ⁴I_13/2 fluorescence lifetime ratio (τ(⁴I_11/2)/ τ(⁴I_13/2)) of the Er³⁺/Tb³⁺: PbF₂ crystal was calculated to be as high as 422.79%, benefiting the possible population inversion for Er³⁺: ⁴I_11/2 → ⁴I_13/2. These results indicate that Tb³⁺ co-doping is beneficial in achieving 2.7μm laser in Er³⁺/Tb³⁺ crystal, and this crystal can be acted as a promising material for 2.7μm laser applications.