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Abstract
The effect of Nd3+ co-doping as sensitizer and deactivator to the Er3+ ion for enhancing ~2.7 μm emissions under 808nm LD pump was first studied in the Er/Nd co-doped PbF2 crystal. The Er0.01Nd0.02Pb0.97F2 crystal has been successfully grown by the Bridgman method, and the crystals’ fluorescence emission properties and energy transfer mechanisms of series crystals were investigated. It can be seen that through the Nd3+ ion, the lower energy level of Er3+: 4I13/2 has been depopulated, and the upper energy level of Er3+: 4I11/2 has been populated at the same time. Simultaneously, the energy transfer efficiency from the Er3+: 4I13/2 level to the Nd3+: 4I15/2 level is 84.06%, and from the Nd3+: 4F3/2 level to the Er3+: 4I11/2 level is 55.81%, respectively, which indicate that the Nd3+ ion is an effective deactivator and sensitizer ion for enhancing the ~2.7 μm emission in Er/Nd: PbF2 crystal. These advantages suggest that the Er/Nd: PbF2 crystal may be a potential material for ~2.7 μm mid-infrared lasers under the 808 nm LD pump.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
For the past several decades, there is a great deal of attention of the laser device at midinfrared wavelength 2.7~3 μm in medical and sensing technologies [1–6]. Similarly, it can be also used as efficient and high-quality laser pump sources for longer-wavelength mid-IR oscillators [6–9].
It is well known that among different rare earth ions, the Er3+ ions can be used as a proper active ion for MIR lasers [10–12,16,17] by the 4I11/2→ 4I13/2 transition. However, owing to the self-saturation transition Er3+: 4I11/2 → 4I13/2 (since the lifetime of the lower level 4I13/2 is longer than that of the upper level 4I11/2), the ~2.7 μm laser operation cannot be obtained efficiently [18,19]. It is known that using sensitizer or deactivator ions, such as Ho3+ [20], Yb3+ [14,21–23], Pr3+ [4,9,13,24,25], or Nd3+ [15,26,27] ions can populate the level of the 4I11/2 level or depopulate the level of the 4I13/2 level to enhance ~2.7 μm emission. Surprisingly, the Nd3+ ions can be used not only as sensitizer but also as deactivator to Er3+ ions [15,26,27]. The simplified energy level diagram illustrated is shown in Fig. 1. Under an 808 nm LD pump, the Er3+ ions are pumped to the excited state(4I9/2), and through the non-radiative relax to 4I11/2 level. At the same time, the Nd3+ ions are also excited to the excited state (2H9/2 + 4F5/2) and relax to Nd3+: 4F3/2. Then some of the Nd3+: 4F3/2 level will go through the ET1 transferring energy to Er3+: 4I11/2, making the ions’ quantity on upper level Er: 4I11/2 populated, which indicate that the Nd3+ ions play sensitizer to Er3+ ions. Subsequently, the 4I11/2 level decays radiatively to 4I13/2 level with ~2.7 μm emission. After that, ions in the 4I13/2 level will go through an ET2 process to the 4I15/2 level of Nd3+, which will depopulate the Er3+: 4I13/2 level, making the possibility of population inversion to enhance the ~2.7 μm emission. Simultaneously, the cross relaxation between Nd3+ and Er3+ ions (CR: Nd: 4I15/2 + Er: 4I13/2 → Nd: 4I9/2 + Er: 4I9/2) would populate the concentration of 4I9/2, which finally will promote ~2.7 μm emission similarly. Above all, the Nd3+ ions can be used as efficient sensitizer and deactivator to Er3+ ions.
Fig. 1 Simplified energy level diagram of Er3+ and Nd3+ co-doped system. ET1: energy transfer from Nd3+: 4F3/2 level to Er3+: 4I11/2 level; ET2: energy transfer from Er3+: 4I13/2 level to Nd3+: 4I15/2 level; CR: cross relaxion between Nd3+ and Er3+(Nd: 4I15/2 + Er: 4I13/2 → Nd: 4I9/2 + Er: 4I9/2)
The fluoride hosts with rare-earth-doped are always been studied, because large proportion of their maximum phonon frequency are 400–560 cm−1, which is about two times lower than that of the oxygen containing compounds [28–33]. Compared to the oxide crystals, the fluoride crystals have several advantages such as lower phonon energy, lower refractive index, and longer fluorescence lifetime [34–37]. In this work, we chose β-PbF2 crystal from the different fluoride single crystals for efficient laser operation. The reasons are as follows: (i) lower maximum phonon energy (257 cm−1), propitious to inhibit multiphoton deexcitation processes; (ii) higher transparency between a wide wavelength range (0.25 μm-15 μm); (iii) good conductivity for heat, excellent solubility for rare-earth ions, and proper mechanical properties; (iv) a fluorite type structure of the cubic space group (FM‾3m), probably to grow into the form of large-size transparent single crystal [38–40]. All the statements demonstrate that the β-PbF2 crystal is a suitable host for mid-IR solid–state lasers. At present, the combination of Nd3+ and Er3+ has been widely used in tellurite glasses, fluorotellurite glass, fluorophosphate glass or other glass materials [15,26,27]. However, to the best of our knowledge, it has hardly practiced in the fluoride crystal hosts.
In this work, the Er/Nd co-doped PbF2 crystal has been successfully grown. The spectral properties around ~2.7 μm emission of the Er/Nd co-doped PbF2 crystal under an 808 nm LD pump are firstly observed. Nd3+ ions can be demonstrated to be an effective sensitizer and deactivator to Er3+ ions in PbF2 crystal. Moreover, the optical properties of Er/Nd co-doped PbF2 crystal are also been studied and they could demonstrate the fluoride crystal’s feasibility for future applications in MIR lasers under a common 808 nm LD pump.
2. Experiments section
2.1 Material synthesis
The Er single-doped, Nd single-doped, and Er/Nd co-doped PbF2 crystal were grown by the Bridgman method. The PbF2 (99.99%), ErF3 (99.99%), and NdF3 (99.99%) have been used as initial material. The melt was homogenized in a platinum crucible in the high temperature zone at 960°C for 8 h. The growth process was pushed by depressing the crucible at a rate of 0.5 mm/h. The solid–liquid interface was located in the gradient zone. The temperature gradient across the solid–liquid interface was around 35 °C/cm–40 °C/cm. After the growth has been finished, the furnace was cooled to room temperature at the rate of 30 °C/h–40 °C/h. The two ions usually occupy the lattice position of Pb2+, which brings about the excess of positive charge. At the same time, to maintain the electrical neutrality of the system, charge compensation is attained by the presence of interstitial fluorine ion (Fi¯).
The concentrations of Er3+ and Nd3+ ions were measured by the inductively coupled plasma atomic emission spectrometry analysis. The doping concentrations of Er3+ and Nd3+ in a co-doped crystal were measured to be 1.08 at. % (2.18 × 1020 ions/cm3) and 1.01 at. % (2.04 × 1020 ions/cm3), respectively. The doping concentrations in single-doped crystals were 1.04 at. % (2.13 × 1020 ions/cm3) of Er3+, and 1.02 at. % (2.06 × 1020 ions/cm3) of Nd3+. Simultaneously, the ionic radii of Er3+ ions and Nd3+ ions are 88.1 Å and 99.5 Å, which are smaller than that of Pb2+ ions (120 Å). So, the distribution coefficients of Er3+ and Nd3+ ions are all larger than 1. While the ionic radii of Er3+ is smaller than Nd3+, which is more likely to be doped in PbF2 crystal, and the distribution coefficient of Er3+ is larger than that of Nd3+.
2.2 Measurements
The following formulas: kRE = Cs/C0 can be used to calculate the distribution coefficient of Er3+ ions (kEr) and Nd3+ ions (kNd), where Cs is the doped ions’ concentration in the as-grown crystals and Co is the doped ions’ concentration in the melt. The kEr of Er3+ ions in the Er: PbF2 crystal and Er/Nd: PbF2 crystal can be calculated to be 1.09 and 1.08, respectively. Meantime, the kNd of Nd3+ ions in the Nd: PbF2 crystal and Er/Nd: PbF2 crystal can be calculated to be 1.02 and 1.01.
The absorption spectra of Er: PbF2 and Er/Nd: PbF2 crystals in the range of 400–1700 nm ware measured by the JASCO V-570UV/VIS spectrophotometer. The fluorescence spectra in the wavelength of 500-700, 1450-1650, and 2600–3000 nm and the fluorescence decay curves of the Er3+: 4I11/2, 4I13/2 level and Nd3+: 4F3/2 level for the single-doped and co-doped crystals were measured by the Edinburgh Instruments FLS920 and FSP920 spectrophotometers under an 808 nm pump. All measurements were done at room temperature.
3. Experiments result and discussion
3.1 Absorption spectra and Judd-Ofelt theory analyses
3.1.1 The absorption spectra and absorption cross section
The Er: PbF2 and Er/Nd: PbF2 crystals’ visible near infrared absorption spectra in the range of 400-1700 nm are illustrated in Fig. 2. The transitions from the 4I15/2 to 2H9/2, 4F3/2 + 4F5/2, 4F7/2, 2H11/2, 2S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2 of Er3+ are corresponding to the absorption band centered at 406, 460, 487, 521, 541, 650, 803, 975, and 1532 nm, respectively. Meantime, the transitions from the 4I9/2 to 2K13/2 + 4G7/2 + 4G9/2, 4G9/2+2G7/2, 4F9/2, 4F7/2 + 2S3/2, 4F5/2 + 2H9/2 and 4F3/2 of Nd3+, respectively centered at around 522, 576, 651, 741, 799 and 863 nm. In the range of 768-834 nm. There is one strong absorption band located around 808nm with a full width at half-maximum (FWHM) about 20 nm, due to the Er3+: 4I15/2→4I11/2 and Nd3+: 4I9/2→4F7/2 + 2S3/2 transition, which appropriately correspond to the emitting wavelength of high-power AlGaAs laser diodes (LD).
The cross section σabs can be calculated by the following equation:
where the α is the absorption coefficient of 808 nm in Er: PbF2 and Er/Nd: PbF2 crystals; and the c is the concentration of doping ions. The absorption cross section σabs of Er: PbF2 and Er/Nd: PbF2 crystals are 0.346 × 10−20 cm2 and 0.629 × 10−20 cm2, respectively.3.1.2 The Judd-Ofelt theory analyses
Based on the absorption spectra, more spectroscopic parameters relative to the optical properties of Er3+ can be obtained by the Judd-Ofelt (J-O) theory [41,42]. The RMS error deviation of intensity parameters was 0.182, which confirms the reliability of calculations and the effectiveness of the J–O theory for predicting the optical properties of Er3+.
The experimental and calculated line strengths are shown in Table 1., and the intensity parameters Ω2,4,6 of Er3+ are shown in Table 2., respectively. It’s well known that the higher Ω2 shows a higher covalency and lower symmetry [26,43,44]. It is obviously that the Ω2 of the Er/Nd co-doped PbF2 crystal is higher than that of Er single doped PbF2 crystal, which reveals that the introduction of Nd3+ ions will lower the symmetrical characteristic surrounding Er3+ ions. Furthermore, we also use the Ω2,4,6 to calculate the radiative transition probability (A), and fluorescence branching ratio (β) of the upper levels (4I13/2, 4I11/2) in the Er/Nd co-doped PbF2 crystal, shown in Table 3. Via the higher fluorescence branching ratio of the 4I11/2 → 4I13/2 transition of Er/Nd: PbF2 crystal (15.9%) than that of Er single doped crystal (12.1%, 11.3%, 13.3%). We can infer that the Nd3+ ions’ co-doping has positive effect on the Er3+: ~2.7 μm fluorescence emission in Er/Nd: PbF2 crystal.
Table 1. Barycenter wavelengths, and measured and calculated line strengths of Er/Nd: PbF2 crystal.
Table 2. Judd–Ofelt parameters, calculated branching ratio, and lifetime of Er: PbF2 and Er/Nd: PbF2 crystals.
Table 3. Line strengths, branching ratios, and transition probabilities in Er/Nd: PbF2 crystal.
Table 4. displays the intensity parameters Ω2,4,6 of Er3+ in various materials. It is known that the Ω6 reflects the stiffness and viscosity of the matrix, depending on the overlapping parts of the 4f and 5f tracks. The Ω4/Ω6 is an important spectral quality factor to predict laser efficiency in laser active materials. Simultaneously, the larger of the Ω4/Ω6, the better optical properties the material has [26,43,44]. It is clearly that the Ω4/Ω6 of Er/Nd: PbF2 crystal is obviously higher than that of the other materials, which can be predicted that the Er/Nd: PbF2 crystal has excellent optical properties.
Table 4. Judd–Ofelt parameters of Er3+ in various materials.
3.2 Fluorescence spectra and emission cross section
To further more clear of the mechanism that the Nd3+ ions playing on the Er3+ ions, the emission spectra of Er: PbF2 and Er/Nd: PbF2 crystals at the wavelength range around 2600-3000 nm, 1450-1650 nm, and 500-700 nm under an 808 nm pump are shown in Fig. 3(a)., Fig. 3(b). and Fig. 3(c)..
Fig. 3 Emission spectra and emission cross sections of Er: PbF2 and Er/Nd: PbF2 crystals; (a): Emission spectra at the wavelength range around 2600-3000 nm. (b): Emission spectra at the wavelength range around 1450-1650 nm. (c): Emission spectra at the wavelength range around 500-700 nm. (d): Emission cross section at the wavelength range around 2550–3000 nm.
Figure 3(a). shows the emission spectra assigned to the Er3+ transition of 4I11/2 → 4I13/2. When co-doped with Nd3+ ions, the transition has been visibly enhanced, about twice higher as it used to be, which is supposed to be attributed by the higher fluorescence branching ratio of the 4I11/2 → 4I13/2 transition in the Er/Nd: PbF2 crystal. Furthermore, the FWHM of the emission spectrum is about 109 nm around 2750 nm, broadly and smoothly. These results show that the Nd3+ ions are an excellent deactivator and sensitizer to enhance ~2.7 μm emission in the Er/Nd doped PbF2 crystal, which reveals the crystal’s probability for a short pulse generation and wide tuning range, simultaneously.
Figure 3(b). shows that the emission spectra corresponding to the radiative transitions between 4I13/2 and 4I15/2 energy levels. It is obviously that the emission intensity at 1545 nm of the Er: PbF2 crystal is higher than that of Er/Nd: PbF2 crystal owing to the ET2 between Er3+ and Nd3+ ions. When activated to different energy level by an 808 nm pump, ions on the Er3+: 4I13/2 level will decay radiatively to Er3+: 4I15/2 level with ~1.53 μm emission. However, with the Nd3+ ions co-doping, the 4I13/2 energy level will through an energy transfer process to Nd3+: 4I15/2 due to the near energy between the Er3+: 4I13/2 level and the Nd3+: 4I15/2 level. Simultaneously, a cross relaxion between Nd3+ and Er3+ (Nd: 4I15/2 + Er: 4I13/2 → Nd: 4I9/2 + Er: 4I9/2) will also take effect, which would depopulate the lower level of Er3+: 4I13/2 and weaken the transition between 4I13/2 and 4I15/2.
Figure 3(c). shows the emission spectra fitting to radiative transitions between 4S3/2 / 2H11/2, 4F7/2 and 4I15/2 energy levels, respectively. After the first-level excitation between the ground state, 4I15/2, and the excited level 4I9/2 under an 808 nm pump of Er3+ ions. A non-radiative transition is acted between the 4I9/2 and 4I11/2 level, later the 4I11/2 level is pumped to the 4F7/2 level (shown in Fig. 1.). Then, the 4F7/2 (Er3+) state decays nonradiatively to the 4S3/2/2H11/2 and 4F9/2 levels, which is corresponding to the green emission and red emission ~550 nm from the 4S3/2→4I15/2, the 2H11/2→4I15/2 transitions, and the 4F9/2→4I15/2 transition, respectively (shown in Fig. 1.). It is clearly that the transitions with Nd3+ ions co-doping has been largely weakened, which will enhance the transition of Er3+: 4I11/2 → 4I13/2. And the ET2 (Er3+: 4I13/2→Nd3+: 4I15/2) will cause the energy of the 4I13/2 level of Er ions reduced. Further proves that the Nd3+ ions play effect as deactivator on Er3+ ions in PbF2 crystal.
The corresponding emission cross sections of Er3+: PbF2 and Er/Nd: PbF2 crystals are subsequently calculated by the Fuchtbauer-Ladenburg equation [47]:
where β is the fluorescence branching ratio, I (λ) is the fluorescence intensity, is the normalized line shape function of the experimental emission spectrum, c is the speed of light, n is the refractive index, and τR is the radiative lifetime. The maximum emission cross section at 2750 nm of the Er/Nd: PbF2 was 0.55 × 10−20 cm2, which is higher than that of the Er: PbF2 (0.43 × 10−20 cm2). Simultaneously, the emission cross sections of Er3+: PbF2 and Er3+/Nd3+: PbF2 crystals in the wavelength of 2550–3000 nm are shown in Fig. 3(d)..3.3 The fluorescence lifetime
3.3.1 The fluorescence lifetime of Nd3+:4F3/2
To further attest the energy interaction mechanism, the time-resolved decays of the Nd3+: 4F3/2 in Nd: PbF2 and Er/Nd: PbF2 crystals, shown in Fig. 4. And the time-resolved decays of the Er3+: 4I11/2, and Er3+: 4I13/2 in the Er: PbF2 and Er/Nd: PbF2 crystals were measured, shown in Fig. 5.
Fig. 5 Fluorescence decay curves of Er: PbF2 and Er/Nd: PbF2 crystals for the 4I11/2 and 4I13/2 mainfold.
Figure 4. shows that the measured lifetime of Nd3+:4F3/2 level in Er/Nd: PbF2 crystal is 0.19 ms, which is almost 55.81% shorter than that of Nd: PbF2 crystal (0.43 ms), indicating that the Nd3+ ions can play sensitizer to increase the Er3+: 4I11/2 level for enhancing the ~2.7 μm emission. Simultaneously, the efficiency of the energy transfer within Er3+ and Nd3+ ions can be estimated by ηET1 = 1-τEr/Nd/τNd, whee τEr/Nd and τNd are the lifetimes of Nd3+ in the Er/Nd: PbF2 and Nd: PbF2 crystals, respectively. It can be seen that the energy transfer efficiency from Nd3+: 4F3/2 level to Er3+: 4I11/2 level was calculated to be 55.81%, which shows that the Nd3+ ions can effectively sensitize the upper level of Er3+: 4I11/2.
3.3.2 The fluorescence lifetime of Er3+: 4I11/2 and Er3+: 4I13/2
Meantime, Fig. 5. shows that the measured lifetime of Er3+: 4I11/2 level in Er/Nd: PbF2 crystal is 5.26 ms, which is only 10.69% shorter than that of Er: PbF2 crystal (5.89 ms); at the same time, the measured lifetime of the lower laser level of Er3+: 4I13/2 in the Er/Nd: PbF2 crystal is 2.00 ms, which is 84.60% shorter compared to that of the Er: PbF2 crystal (12.99 ms). The following results confirm that the Nd3+ ions can largely depopulate the lower level of Er3+: 4I13/2 while have little effect on the upper level of Er3+: 4I11/2, which can be used as an effective deactivator to the Er3+ ions for enhancing the ~2.7 μm emission.
As sensitized and deactivated ions, Nd3+ can further improve the energy transfer efficiency by changing its doping concentration. However, if the concentration of Nd3+ is too large, the concentration quenching of Nd3+ ions would appear, which would result in the fluorescence quenching. Therefore, it is important to explore the appropriate doping ratio. This work only clarifies the mechanism between Er and Nd ions in PbF2 crystal, and the improvement would be made in future research work.
3.4 Efficiency of the energy transfer and supplement
The efficiency of the energy transfer within Er3+ and Nd3+ ions can be estimated by: ηET2 = 1-τEr/Nd/τEr, where τEr/Nd and τEr are the lifetimes of Er3+ in the Er/Nd: PbF2 and Er: PbF2 crystals, respectively. And the energy transfer efficiency from Er3+: 4I13/2 level to Nd3+: 4F3/2 level was calculated to be 84.60%, which indicate the Nd3+ ions’ effective deactivation effect to the lower level of Er3+: 4I13/2.
Moreover, the lifetime ratio τ(4I11/2)/ τ(4I11/2) of the upper-laser level 4I11/2 to lower-laser level 4I13/2 in the Er/Nd: PbF2 crystal was worked to be 263.0%, over eight times higher than that of the Er: PbF2 crystal (45.3%). All above results show that the self-termination problem is almost solved by the Nd3+ ions co-doping, largely enhancing the ~2.7 μm fluorescence emission in the Er/Nd: PbF2 crystal.
In this work, we choose 1 at. % Er doping concentration in PbF2 crystal is just to clarify the mechanism between Er and Nd ions, and the optimum concentration of Er ions is important to explore. The 808 nm pump intensity we used in this work is 1.05W. While increasing the pump intensity, the fluorescence intensity gradually enhanced because of the increasing transferring energy between pump and the ions. However, with the intensity continuously strengthen, the energy transfer efficiency will fall gradually, and the fluorescence intensity will maintain a stable value, more energy from pump consumed by the crystal. The energy transfer reverse will also restrain the Nd ions’ sensitization effect when the efficiency from Nd ions to Er ions is higher than the efficiency from Er ions to Nd ions, which will restrain the 2.7 μm emission [48,49]. Similarly, the relative experiments and the output energy or power levels of PbF2 crystal will be made in the future work.
4. Conclusion
To sum up, Er: PbF2 and Er/Nd: PbF2 crystals were firstly successfully grown by the Bridgman method in this work. Co-doping with the Nd3+ ions, the Er/Nd: PbF2 crystal has a higher fluorescence branching ratio (15.9%), and stronger fluorescence emission intensity corresponding to the ~2.7 μm emission from the Er3+: 4I11/2 - 4I13/2 transition. The Nd3+ ions can efficiently depopulate the lower level of Er3+: 4I13/2. Simultaneously, the energy transfer efficiency from Er3+: 4I13/2 level to Nd3+: 4F3/2 level was calculated to be 84.60%, and the energy transfer efficiency from Nd3+: 4F3/2 level to Er3+: 4I11/2 level was calculated to be 55.81%. All the results evidently indicate that the Nd3+ ions can play sensitizer and deactivator to Er3+ ions for enhancing the ~2.7 μm emission. Ultimately, it can be concluded that the Er/Nd: PbF2 crystal can be used as a potential material for ~2.7 μm mid-infrared lasers under an 808 nm LD pump.
Funding
The National Key Research and Development Program of China (2017YFB1104500); National Natural Science Foundation of China (NSFC) (51702124, 51872307, 61735005, 61475067, 61605062); Guangdong Project of Science and Technology Grants (2016B090917002, 2016B090926004); Guangdong Project of Featured Innovation Grants (2017KTSCX012); Guangzhou Union Project of Science and Technology Grants (201604040006); The Fundamental Research Funds for the Central Universities (11617329).
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