The use of Tb3+ co-doping for the enhancement of Er3+: 4I11/2 → 4I13/2 mid-infrared emissions can conquer the self-termination bottleneck and provide possible applications in medical surgery, dentistry, remote atmospheric sensing, light detection, and the optical parametric oscillator. The effect of Tb3+ co-doping on the fluorescence emission properties and mutual energy transfer mechanisms were investigated. It was found that Tb3+ greatly increased Er3+ 2.7μm emission by depopulating the Er3+: 4I13/2 level while having little influence on the Er3+: 4I11/2 level, leading to a greater population inversion. The energy transfer efficiency from Er3+: 4I13/2 to Tb3+: 7F0 is as high as 90.27%, and the Er3+: 4I11/2 → 4I13/2 fluorescence lifetime ratio (τ(4I11/2)/ τ(4I13/2)) of the Er3+/Tb3+: PbF2 crystal was calculated to be 422.79%, indicating that Tb3+ ion is an excellent deactivator with which the self-termination bottleneck effect was effectively suppressed. All of these factors imply that Er3+/Tb3+: PbF2 crystal may be a promising material for 2.7μm laser applications.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
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 . 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 4I11/2 to 4I13/2 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 4I13/2 level than that of the 4I11/2 level . 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 4I11/2 and 4I13/2, as well as cross-relaxation from 4S3/2, and thus aids to improve 2.7μm laser in Er3+ activated crystals . 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 4I13/2, 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+: 4I13/2, because Tb3+ ion energy level 7F0 is adjacent to the 4I13/2 level of Er3+. As the energy level scheme illustrated in Fig. 1 shows, after the ions in the Er3+: 4I11/2 level decay radiatively to Er3+: 4I13/2 level with around 2.7μm emission, the ions in the Er3+: 4I13/2 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.
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 4I11/2 and 4I13/2. For laser materials operating at 2.7μm of Er3+, these two parameters become even more important due to most of the population in 4I11/2 relaxing to 4I13/2 by multi-phonon relaxation, producing lots of heat. The low phonon energy conduces to the decrease of the multi-phonon relaxation rate for Er3+: 4I11/2 → 4I13/2 transition, and therefore decreasing the populations on 4I13/2 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.
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+: 4I11/2 → 4I13/2 by effective ET from Er3+: 4I13/2 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+: 4I13/2 → 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 .
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 Kα 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 keff = c1/c2, where c1 and c2 are the respective concentrations of the ions in the crystal and raw materials, keff values of Er3+ and Tb3+ in PbF2 crystal were determined to be 1.15 and 1.1, respectively, and keff 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 (Fi¯) . 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.
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 (4I15/2) to 4G9/2, 4G11/2, 2H9/2, 4F5/2 + 4F3/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, 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 4I11/2 by radiation at around 980nm, which can avoid various non-radiative losses and thermal loading .
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 4G11/2 + 4G9/2, 2H9/2, 4F5/2 + 4F3/2, 4F7/2, 4S3/2 + 2H11/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2 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 . 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.
Once the intensity parameters Ω2, Ω4, Ω6 of Er3+ are received, the spontaneous emission probabilities (A), radiative lifetime (τrad), 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+: 4I11/2 → 4I13/2 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%) , and Er3+: CaLaGa3O7 (13.3%) . 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+: 4I11/2 → 4I13/2 transition, which is as high as 16.3%. The larger fluorescence branching ratio β of Er3+: 4I11/2 → 4I13/2 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.
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 4I13/2→4I15/2 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+: 4I13/2 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.
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+: (2H11/2, 4S3/2) → 4I15/2, and the red up-conversion emission band centered at 669 nm can be assigned to the transition of Er3+: 4F9/2 → 4I15/2 (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 4I13/2 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.
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+: 4I11/2 → 4I13/2 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+: 4I11/2 → 4I13/2 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 :
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 τr 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 4I11/2 → 4I13/2 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 4I11/2 level by 980 nm pumping, then decayed to 4I13/2 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+: 4I13/2 level will decay radiatively to Er3+: 4I15/2 with 1.5μm emission or undergo an energy transfer process to Tb3+: 7F0 due to the energy of Er3+: 4I13/2 is slightly higher than Tb3+: 7F0. Meanwhile, a part of the Er3+ ions in the 4I13/2 level transfer to the Er3+: 4F9/2 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 4I11/2 level of Er3+ transfer to the Er3+: 4F7/2 level with an ESA2 process by absorbing 980 nm pump and then relaxes nonradiative transition to 2H11/2 and 4S3/2 level. Consequently, the green emission can be obtained by the Er3+: 2H11/2 → 4I15/2 and Er3+: 4S3/2 → 4I15/2 transitions. When Tb3+ were introduced, the ions in 4I13/2 level of Er3+ are drastically depopulated by ET2 process (Er3+: 4I13/2 → 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+: 4I13/2 + 4I13/2 → 4I15/2 + 4I9/2) will be beneficial to the realization of 2.7μm lasers due to the fast-multi-phonon decay from the Er3+: 4I9/2 level to Er3+: 4I11/2 level . 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+: 4I13/2 and 4I11/2 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 τm of the 4I11/2 and 4I13/2 manifold in Er3+/Tb3+: PbF2 crystal is 5.881ms and 1.391ms, respectively. In Er3+: PbF2 crystal, the fluorescence decay curve of the Er3+: 4I13/2 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+: 4I11/2 in Er3+/Tb3+: PbF2 crystal decreases a little from 6.029ms to 5.881ms due to the energy transfer Er3+: 4I11/2 → Tb3+: 7F0 process, the Er3+/Tb3+: PbF2 crystal exists a quicker attenuation of the lower level lifetime compared with the 4I13/2 lifetimes of the Er3+: PbF2 crystal (4I13/2: 14.291ms), which is mainly attributed to the energy transfer from the Er3+: 4I13/2 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+: 4I13/2 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: ηET = 1-τEr/Tb/τEr, where τEr/Tb and τEr 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+: 4I13/2 than 4I11/2 level in the Er3+/Tb3+: PbF2 compared with the Er3+: PbF2 crystal. Furthermore, based on the measured results above, the Er3+: 4I11/2 to Er3+: 4I13/2 fluorescence lifetime ratio (τ(4I11/2)/ τ(4I13/2)) 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.
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+: 4I13/2. 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+: 4I11/2 → 4I13/2 transition. It was demonstrated that the energy transfer efficiency from Er3+: 4I13/2 to Tb3+: 7F0 is as high as 90.27%, and the Er3+: 4I11/2 → 4I13/2 fluorescence lifetime ratio (τ(4I11/2)/ τ(4I13/2)) of the Er3+/Tb3+: PbF2 crystal was calculated to be as high as 422.79%, benefiting the possible population inversion for Er3+: 4I11/2 → 4I13/2. 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.
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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