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Abstract
Broadband mid-infrared emissions are obtained from Dy3+/Er3+ co-doped tellurite glasses under 808 or 980 nm excitation. The maximum effective emission bandwidths of mid-infrared emission is 92.45 nm in Dy3+/Er3+ co-doped tellurite glass pumped by 808 nm, while it can reach 209.00 nm pumped by 980 nm. The effects of different laser excitations on the energy transfer mechanism between Dy3+ and Er3+ ions have been investigated in tellurite glasses. Under 808 nm excitation, the energy transfer efficiency from Er3+:4I13/2 to Dy3+:6H11/2 level is 73.1% and the energy transfer coefficient from Er3+:4I11/2 to Dy3+:6H5/2 level and from Er3+:4I13/2 to Dy3+:6H11/2 level are 6.89 × 10−38 and 0.01 × 10−38 cm6/s, respectively. Under 980 nm excitation, the energy transfer efficiency from Er3+:4I13/2 to Dy3+:6H11/2 level can reach as high as 80%. Moreover, the maximum emission cross-section of 2500-3100 nm broadband emission when pumped by 808 nm is 1.90 × 1020 cm2 at 2765 nm, while it can reach as high as 4.99 × 1020 cm2 at 2724 nm pumped by 980 nm. Thus, the 980 nm excitation is more efficient for Dy3+/Er3+ co-doped tellurite glass to realize low-threshold and high gain applications at broadband mid-infrared laser.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recently, 2-3 μm fiber lasers have been extensively investigated because of their important potential applications, including remote sensing, laser surgery and environmental trace-gas detections [1–3]. It is worth noting that various molecules have strong rovibrational absorption lines and many atmospheric transparence windows are located in this mid-infrared region [4,5]. Dy3+ is a suitable candidate for the applications due to the Dy3+:6H13/2→6H15/2 transition at 2.8 μm emission. However, it is difficult to obtain 2.8 μm emission efficiently by Dy3+ due to the lack of absorption bands for the commercial pump source. Codoping of Tm3+ or Nd3+ as a sensitizer to achieve the mid-infrared emission has been reported as a feasible program [6–9]. Besides, the mid-infrared emission (2.7-2.8 μm) can be also obtained in Tm3+/Er3+ [10] or Er3+/Pr3+ [11] co-doped glasses. But few investigations have been reported on mid-infrared emission in Dy3+/Er3+ co-doped glasses. The Dy3+/Er3+ co-doped system is particularly interesting since Er3+ is also an attractive laser ion among 2-3 μm emission due to the Er3+:4I11/2→4I13/2 transition.
In order to achieve strong mid-infrared emissions and practical gain, an appropriate host glass is indispensable. Recently research claimed to get great mid-infrared emissions from tellurite glass [12,13], germanate glass [14,15], fluorophosphate glass [16], bismuthate glass [17] and silicate glass [18]. Rare-earth doped tellurite glass can be a promising glass for optical applications on account of its low phonon energy (The maximum of present article prepared host glass is measured as 750 cm−1) and high rare earth ions solubility. In this work, tellurite glass (TeO2-WO3-ZnO-La2O3) is used as matrix, Dy3+ as doping, and Er3+ as co-doping ions. The Er3+:4I11/2 level may play an important role in the 6H11/2 lower laser level of Dy3+, which is beneficial for 2.8 μm emission. Since the 2.7 μm emission of Er3+ is very close to the 2.8 μm emission of Dy3+, a large luminous zone around 2.75 μm in Dy3+/Er3+ co-doped tellurite glass can be obtained by commercial pump source. In addition, since Er3+ has a strong absorption band at both 808 and 980 nm, it is essential to consider and compare the fluorescence properties of Dy3+/Er3+ co-doped tellurite glass under 808 or 980 nm LD excitation.
The present article reports the mid-infrared fluorescence properties of Dy3+/Er3+ co-doped tellurite glasses under different excitations. A detailed investigation of the energy transfer mechanism and ion-ion interactions between Dy3+ and Er3+ under 808 or 980 nm excitation are presented. The absorption spectra and radiative lifetime of exited states are measured and discussed. Moreover, the quantitative analysis of energy transfer process between Dy3+ and Er3+ based on the Förster and Dexter theory is helpful to improve the technological applications of the co-doped glasses. In addition, the analysis of the gain curves of 2.7 and 2.8 μm emissions is conducted to understand optical properties of present co-doped tellurite glasses. This work may provide a feasible program for the design of broadband mid-infrared laser material.
2. Experimental
2.1 Glass preparation
The investigated glasses have the following compositions (mol %): (1) 65TeO2-20WO3-5ZnO-(9.5-x)La2O3-0.5Dy2O3-xEr2O3 (x = 0, 0.5, 1, 1.5, 2), (2) 65TeO2-20WO3-5ZnO-9.5La2O3-0.5Er2O3, where x is given in mol%. High purity TeO2 (99.99%), WO3 (99%), ZnO (99%), La2O3 (99.99%) are used, and the samples are prepared by a melting quenching technique. Accurately weighed batches of 25 g are placed in a crucible and melted at 850 °C for 45 min in air atmosphere. Afterwards, the melts are quickly poured on preheated stainless steel plates and annealed for 3 h near glass transition temperature, then they are cooled slowly inside a furnace to room temperature. Finally, the annealed samples are cut and polished to a thickness of 2 mm for optical property measurements.
2.2 Spectroscopic measurements
The absorption spectra are measured by a Perkin-Elmer Lambda 950 UV/VIS/NIR spectrophotometer in 400-2200 nm wavelength range. The fluorescence spectra (1400-and 2500-3100 nm) are measured with a FLS980 spectrometer (Edinburgh Instruments, UK) upon 808 and 980 nm LD excitation with a maximum power of 2 W. In order to accurately compare the intensity of mid-infrared emissions, the position and power of the 808/980 nm excitation and the width (0.1 nm) of the slit to collect the luminescence signal were fixed to the same conditions. Additionally, samples are set at the same place in the experimental setup. Fluorescence lifetimes of the Er3+:4I13/2 (1.55 μm) level are recorded with light pulses of the 808/980 nm LD and analyzed using a digital 100 MHz oscilloscope from Tektronix (TDS 3012C). All fluorescence spectra and decay times are measured at 300 K.
3. Results
3.1 Absorption and J-O intensity parameters
Figure 1 shows the absorption spectra of tellurite glasses singly doped with 0.5 mol % Dy3+, 0.5 mol % Er3+, and co-doped with 0.5 mol % Dy3+/0.5 mol % Er3+ in the wavelength region of 400~2000 nm. Absorption bands for Dy3+/Er3+ doped samples corresponding to transition from their ground state to higher levels are labeled. The shape and peak positions of each transition for Dy3+/Er3+ doped samples are similar to those in other glasses [19, 20]. It is worth noting that the obvious absorption band around 808 and 980 nm are observed in the Dy3+/Er3+ co-doped sample, which indicates that the glasses can be excited by commercially available 808 or 980 nm LD. Besides, the absorption coefficient around 808 nm is obviously increased in co-doped samples due to the Dy3+:6H15/2→6F5/2 and Er3+:4I15/2→4I9/2 transitions.
According to the absorption spectra, the intensity parameters Ωt (t = 2, 4, 6) can be calculated by Judd-Ofelt theory. The line strength of a transition between an initial |aJ> and final |bJ'> manifold is expressed by the equation [21,22],
where <aJ'‖U(t)‖bJ'> is the reduced matrix elements of unit tensor operators for corresponding transition. It is precisely because of the line strength of the absorption band between an initial manifold |aJ> and final manifold |bJ'>, SJJ' is approximated with the experimental values of the integrated absorbance by the following expression [23],where k(λ) is the absorption coefficient, λm is the mean wavelength of the absorption band confirmed as the center of gravity, e is the elementary charge, h is Planck’s constant and c is the speed of light. According to the formula (1) and (2), the intensity parameters Ωt (t = 2, 4, 6) of Dy3+ and Er3+ are calculated and shown in Table 1. The parameter Ω2 is usually related to the covalence of chemical bonds and ligand anions with the asymmetry of local environment formed by the RE3+. Calculated Ω2 of the Dy3+ and Er3+ in individual samples are apparently higher than the reported fluorite glass [24,25], but lower than those of germanate glass [26,27] and phosphate glass [28,29]. In general, the lower of Ω2 the ligand chemical bonds are more ionic and the ion site becomes more centrosymmetric. It is reported that the value of 1/Ω6 is proportional to ionicity between the ligand field and doped rared earth ions [30, 31]. As is shown in Table 1, the 1/Ω6 value of Er3+ is large, which indicates that the covalency is low and ionic characteristic is high between Er3+ ions in the prepared glass.
Table 1. The J-O intensity parameters of the Dy3+/Er3+ doped tellurite glass.
3.2 Emission spectra of Dy3+/Er3+ doped tellurite glasses
Figure 2 displays the near-infrared fluorescence spectra in Dy3+/Er3+ doped tellurite glasses pumped by (a) 808 nm LD and (b) 980 nm LD, respectively. The emission peak centered at 1545 nm is obviously observed due to Er3+:4I13/2→4I15/2 transition. It is found that the fluorescence intensity reduces gradually with the increment of Er2O3 concentration, which indicating the efficient energy transfer between Er3+ and Dy3+ ions. Compared with the fluorescence spectra in Dy3+/Er3+ co-doped tellurite glasses pumped by 808 nm LD in Fig. 2(a), the fluorescence intensity reduces more significant in Dy3+/Er3+ co-doped tellurite glasses pumped by 980 nm LD in Fig. 2(b).
Fig. 2 Near-infrared fluorescence spectra in Dy3+/Er3+ doped tellurite glasses pumped by (a) 808 nm LD and (b) 980 nm LD.
The mid-infrared fluorescence spectra in Dy3+/Er3+ doped tellurite glasses pumped by (a) 808 nm LD and (b) 980 nm LD are shown in Fig. 3. Under 808 nm LD excitation, the emission peaks near 2.75 μm can be found in Dy3+/Er3+ co-doped system, which are attributed to Er3+:4I11/2→4I13/2 transition and Dy3+:6H13/2→6H15/2 transition. As shown in Fig. 3(a), the maximum effective emission bandwidth in 0.5Dy3+/2Er3+ co-doped sample is larger than that in Er3+ singly-doped sample, which indicates an efficient energy transfer from Er3+ to Dy3+ at an excitation of 808 nm LD. Figure 3(b) shows the fluorescence spectra in Dy3+/Er3+ doped tellurite glasses pumped by 980 nm LD. It is shown that the changes of mid-infrared emissions in Dy3+/Er3+ doped samples are similar to that in Fig. 3(a). When the Er2O3 concentration is 1.5 mol %, the mid-infrared emissions reach the maximum value. The decreased mid-infrared emissions are owing to the concentration quenching. Compared with the Dy3+/Er3+ co-doped system pumped by 808 nm LD in Fig. 3(a), the emitting region is larger in Fig. 3(b). The results indicate that the broadband mid-infrared emissions can be obtained pumped by both 808 and 980 nm LD in Dy3+/Er3+ co-doped system.
Fig. 3 Mid-infrared fluorescence spectra in Dy3+/Er3+ doped tellurite glasses pumped by (a) 808 nm LD and (b) 980 nm LD.
4. Discussions
4.1 Energy transfer mechanism between Dy3+ and Er3+ when pumped by 808 nm LD
According to the near-infrared and mid-infrared florescence spectra in Dy3+/Er3+ doped tellurite glasses, the energy level diagram and possible mechanism for the emission bands are presented in Fig. 4. The Er3+ ions are initially excited from the 4I15/2 ground state to 4I9/2 level through ground-state absorption (GSA1) at an excitation of 808 nm LD. On one hand, the Er3+:4I9/2 level transfers its energy to the Dy3+:6F5/2 level via Er3+:4I9/2 + Dy3+:6H15/2→Er3+:4I15/2 + Dy3+:6F5/2 (ET1) transition. On the other hand, the ions on the 4I9/2 level can relax to the lower 4I11/2 level through multi-phonon relaxation (MPR). In addition, one of the energy of the 4I9/2 level is depopulated by the well-known cross-relaxation (CR) process via Er3+:4I15/2 + 4I9/2→4I13/2 + 4I13/2 transition. Then, the 4I11/2 level transfers a part of its energy to the Dy3+:6H5/2 level via Er3+:4I11/2 + Dy3+:6H15/2→Er3+:4I15/2 + Dy3+:6H5/2 (ET2) transition. Meanwhile, the 4I11/2 level radiates its energy to the lower 4I13/2 level producing 2.7 μm emission via 4I11/2→4I13/2 + 2.7 μm transition. Once the 4I13/2 level is populated, the 4I13/2 level will transfers a part of its energy to the adjacent Dy3+:6H11/2 level via Er3+:4I13/2 + Dy3+:6H15/2→Er3+:4I15/2 + Dy3+:6H11/2 (ET3) transition. The residual ions on the 4I13/2 level radiate to the 4I15/2 ground state via 4I13/2→4I15/2 + 1.55 μm transition.
Fig. 4 Energy level diagram and energy transfer mechanism among Dy3+ and Er3+ when pumped by 808 nm LD.
The ions on the Dy3+:6H15/2 ground state can also be pumped to a higher 6F5/2 level through ground-state absorption (GSA2) at an excitation of commercial 808 nm LD. The ions on the 6H11/2 level can relax to the lower 6H13/2 level through multi-phonon relaxation (MPR). Once the 6H13/2 level is populated, the Dy3+ ions will return to 6H15/2 ground state producing 2.8 μm emission via radiative transition of 6H13/2→6H15/2 + 2.8 μm. So the energy transfer process between Dy3+ and Er3+ can be summed up in the following upon 808 nm excitation:
4.2 Energy transfer mechanism between Dy3+ and Er3+ when pumped by 980 nm LD
Figure 5 displays the energy level diagram and possible mechanism for the emission bands by using 980 nm pumping scheme. The ions on the Er3+:4I15/2 level are excited to the 4I11/2 level by ground-state absorption (GSA) when pumped by available 980 nm LD. The 4I11/2 level transfers its energy to Dy3+:6H5/2 level due to small energy gap between these two energy levels (ET1). Afterwards, the ions on 4I11/2 level radiate to the lower 4I13/2 level and resulting in 2.7 μm emission. The 4I13/2 level transfers a part of its energy to the adjacent Dy3+:6H11/2 level via an ET2 process. The residual ions on the 4I13/2 level radiate to the 4I15/2 ground state and resulting in 1.55 μm emission. The ions on the Dy3+:6H11/2 level can relax to the lower 6H13/2 level through multi-phonon relaxation (MPR). Once the 6H13/2 level is populated, the ions on 6H13/2 level will radiate to the 6H15/2 ground state and resulting in 2.8 μm emission. So the energy transfer process between Dy3+ and Er3+ can be summed up in the following upon 980 nm excitation:
Fig. 5 Energy level diagram and energy transfer mechanism among Dy3+ and Er3+ when pumped by 980 nm LD.
Compared to 808 nm pumping scheme, the 4I11/2 level is pumped directly by 980 nm LD, which avoids the multi-phonon relaxation from the 4I9/2 level. Thus, 2.7 μm emission can be obtained efficiently via Er3+:4I11/2→4I13/2 transition in Er3+ single-doped sample when pumped by 980 nm LD. In conclusion, 2.8 μm emission can be obtained efficiently pumped by both 808 nm and 980 nm LD via radiative transition of Dy3+:6H13/2→6H15/2. Therefore, both the 808 nm and 980 nm pumping scheme are suitable to get broadband mid-infrared emissions from the Dy3+/Er3+ co-doped tellurite glasses.
4.3 Emission radiative properties and energy transfer micro-parameters analysis
In order to make sure about the pumping source impacts on the fluorescence intensity of Dy3+/Er3+ co-doped tellurite glasses, the maximum fluorescence intensity of 0.5Dy2O3-xEr2O3 (x = 0, 0.5, 1, 1.5 and 2) co-doped tellurite samples pumped by 808 nm and 980 nm LD are shown in Fig. 6(a). Although Er3+ ions have absorption at both 808 and 980 nm, the results show that ions on the Er3+:4I15/2 level can be more easily excited to higher energy state pumped by 980 nm LD. The maximum value of the fluorescence intensity excited by 808 nm occurs at concentrations of 0.5 mol% Dy2O3 co-doped with 2 mol% Er2O3. And the maximum value of the fluorescence intensity excited by 980 nm occurs at concentrations of 0.5 mol% Dy2O3 co-doped with 1.5 mol% Er2O3.
Fig. 6 (a) Maximum fluorescence intensity of 0.5Dy2O3-xEr2O3 (x = 0, 0.5, 1, 1.5 and 2) co-doped tellurite samples when pumped by 808 and 980 nm LD. (b) Stimulated emission cross-sections of the 2500-3100 nm broadband emissions from Dy3+/Er3+ co-doped tellurite glasses excited by 808 and 980 nm, respectively. (c) Calculated emission (solid line) and absorption (dashed line) cross-sections corresponding to 1.55, 2.7 and 2.8 μm emissions pumped by 808 nm LD. (d) Calculated emission (solid line) and absorption (dashed line) cross-sections corresponding to 1.55 and 2.7 μm emissions pumped by 980 nm LD.
The emission and absorption cross-section are calculated to evaluate lasing ability of luminescent center and analyze the energy transfer process between the doped Dy3+ ions and Er3+ ions. According to the Füchtbauer-Ladenburg theory [32], the emission cross-section σem can be obtained by following expression,
where λ is the wavelength; Arad is spontaneous transition probability; I(λ) is the emission spectrum; n and c are the refractive index and light speed value respectively. It is shown from Fig. 6(b) that the maximum emission cross-section (σem, wave center at 2765 nm) is 1.90 × 1020 cm2 for 0.5Dy3+/2Er3+ co-doped sample when pumped by 808 nm LD. And the maximum emission cross-section (σem, wave center at 2724 nm) is 4.99 × 1020 cm2 for 0.5Dy3+/1.5Er3+ co-doped sample when pumped by 980 nm LD. It is found that the maximum emission cross-section excited by 980 nm is about 2.5 times as large as which excited by 808 nm. As a result, such large emission cross-sections of the 2500-3100nm broadband emissions provide favorable conditions for efficient mid-infrared laser output.To further understand the mid-infrared radiative performance, the effective emission bandwidth (Δλeff) can be represented by [33],
where Imax is the peak intensity of fluorescence spectrum. The calculated maximum effective emission bandwidths (Δλeff) of mid-infrared emission when respectively excited by 808 and 980 nm are 92.45 and 209.00 nm. As is shown in Fig. 3, it is found that the Dy-Er co-doped samples have a larger Δλeff when excited by 808 or 980 nm, which indicates that the fluorescence areas have been expanded in Dy-Er co-doped samples. Furthermore, the maximum gain bandwidth (σem × Δλeff) of mid-infrared emission excited by 808 and 980 nm are 17.6 × 10−26 and 104.29 × 10−26 cm3, respectively. The result indicates that the 980 nm excitation is more suitable for Dy3+/Er3+ co-doped tellurite glass to achieve low-threshold and high gain applications in the mid-infrared region.The absorption cross-section σabs can be calculated by the McCumber theory [34] as following expression,
where Zup and Zlow are corresponding to partition functions of upper and lower multiplets, respectively. λ, λp, k and T are wavelength of the transition, SF centre, Boltzmann’s constant and the temperature (T = 300K), respectively. E0 is determined as the energy difference between the lowest stark levels of the ground and excited states. According to the formula (3) and (5), the emission and absorption cross-sections are calculated in Fig. 6(c) and 6(d). It is discovered that the sample corresponding to Er3+:4I11/2→4I13/2 transition has a larger stimulated emission cross-section. As is shown in Fig. 6(c), the emission cross-section (σem, wave center at 2815 nm) is 0.69 × 1020 cm2 for 0.5Dy3+ singly-doped sample pumped by 808 nm LD, which is larger than that in ZBLAN glass (0.46 × 1020 cm2) [35] and fluoride glass (0.04 × 1020 cm2) [7], but is smaller than that in bismuth-germanate glass (0.78 × 1020 cm2) [36]. Thus, a larger emission cross-section of Dy3+:6H13/2→6H15/2 transition is conducive for Dy-Er co-doped system to realize broadband mid-infrared emissions. Besides, the pump source has a little effect on the emission cross-section corresponding to Er3+:4I13/2→4I15/2 transition.To clarify the mechanism of the broadband mid-infrared emission, it is essential to understand the energy transfer processes of Er3+:4I11/2→Dy3+:6H5/2 and Er3+:4I13/2→Dy3+:6H11/2 transitions in detail. According to Förster and Dexter theory [37, 38], the energy transfer properties between Dy3+/Er3+ and Er3+/Er3+ when pumped by 808 nm LD are calculated and listed in Table 2, together with the critical radius of the interaction RC, the joined phonon numbers (N) hidden within the energy transfer process and contribution ratios (%) to the total probable rate. As is shown in Table 2, the energy transfer coefficient of Er3+:4I11/2→Dy3+:6H5/2 (ET2) is as high as 6.89 × 10−38 cm6/s, while that is only 0.01 × 10−38 cm6/s in the energy transfer of Er3+:4I13/2→Dy3+:6H11/2 (ET3). Combining with the energy transfer mechanism among Dy3+ and Er3+ in Fig. 4, it is found that the energy transfer of ET2 is more likely to occur than that of ET3. It is worth noting that more than three phonons are needed to compensate the energy mismatch in the energy transfer ET3, while only one phonon is needed to compensate this mismatch. Usually, the more phonons needed in the energy transfer process, the less the energy transfer efficiency is [39]. Thus, the energy transfer process ET2 from Er3+:4I11/2 to Dy3+:6H5/2 level is more efficient than the energy transfer process ET3 from Er3+:4I13/2 to Dy3+:6H11/2 level. Moreover, the energy transfer process between Er3+:4I11/2 and Er3+:4I13/2 level without participation of phonons is a kind of resonant, which leads to the energy transfer coefficient of Er3+:4I11/2→Er3+:4I13/2 is very large. The large energy transfer coefficient between Er3+:4I11/2 and Er3+:4I13/2 level is beneficial for producing 2.7 μm emission, which can be observed in Fig. 3(a).
Table 2. The energy transfer micro-parameters between Dy3+/Er3+ when pumped by 808 nm LD.
To understand the multi-phonon assisted energy transfer from Er3+:4I13/2 to Dy3+:6H11/2 level more intuitively and clearly, emission cross-sections assisted by m (m = 0, 1, 2, 3) phonons for the Er3+:4I13/2→4I15/2 transition and absorption cross-sections for the Dy3+:6H15/2→6H13/2 transition are described in Fig. 7. The emission cross section with the participation can be calculated by [40],
where S0 is the Huang-Rhys factor, which is 0.31 for rare earth ions. It is found that there is almost no spectral overlapping of Er3+:4I13/2→Dy3+:6H11/2 transition with the participation of 0, 1, 2 phonons. However, a spectral overlapping can be obtained until the matrix absorbs three phonons. Hence, the participation of overmuch phonons disperse the energy transfer possibility of Er3+:4I13/2→Dy3+:6H11/2 transition.Fig. 7 Emission cross-sections assisted by m (m = 0, 1, 2, 3) phonons for the Er3+:4I13/2→4I15/2 transition and absorption cross-sections for the Dy3+:6H15/2→6H13/2 transition.
4.4 Fluorescence lifetime and energy transfer efficiency analysis
The decay curves of 1.55 μm emissions in Er3+ singly and Dy3+/Er3+ co-doped tellurite glasses when pumped by 808 and 980 nm LD are measured and potted in Fig. 8. To further illuminate the energy transfer process of Er3+:4I13/2→Dy3+:6H11/2, the energy transfer efficiency can be estimated from the measured lifetime of 1.55 μm emissions by the following equation [41],
where τEr/Dy and τEr are the lifetimes of tellurite glasses co-doped with Dy3+/Er3+ and singly doped with Er3+, respectively. It is found that the lifetime of the 1.55 μm emissions reduce evidently with the introduction of Dy3+ ions, which can verify the existence of Er3+:4I13/2→Dy3+:6H11/2 transition. Figure 8(a) and 8(b) show that the energy transfer efficiency η of Er3+:4I13/2→Dy3+:6H11/2 pumped by 808 nm and 980 nm are 73.1% and 80%, respectively. The results indicate that the energy transfer between Dy3+:6H11/2 and Er3+:4I13/2 level is more effective pumped by 980 nm. Hence, such high energy transfer efficiency pumped by 808 nm and 980 nm are beneficial for broadband mid-infrared emissions.Fig. 8 (a) The decay curve at 1.55 μm in Er3+ singly and Dy3+/Er3+ co-doped tellurite glasses pumped by 808 nm LD. (b) the decay curve at 1.55 μm in Er3+ singly and Dy3+/Er3+ co-doped tellurite glasses pumped by 980 nm LD. (c) Measured lifetimes of the 4I13/2 level as a function of the Er3+ concentration. (d) Quenching rates (1/τ-1/τr) of Er3+:4I13/2→4I15/2 transition as a function of the product of Dy3+ and Er3+ concentration (NDy × NEr).
To further analyze and compare the interaction mechanism from the Er3+:4I13/2 to Dy3+:6H11/2 level when the samples are excited by 808 or 980 nm, the quenching rates (1/τ-1/τr) of Er3+:4I13/2→4I15/2 transition as a function of the product of Dy3+ and Er3+ concentration are shown in Fig. 8(d). Based on the lifetimes of the 4I13/2 level as a function of Er3+ concentration in Fig. 8(c), the relation between the quenching rate and the product of Dy3+ and Er3+ concentration (NDy × NEr) can be described by the following equation [42],
where τr is the intrinsic radiative lifetime, K is the constant of donor-donor and donor-acceptor energy transfer, NA and ND are the concentrations of acceptor and donor, respectively.It is obtained that the value of K when pumped by 808 and 980 nm are 55.7 × 10−40 and 102.1 × 10−40, respectively. The result indicates that a dipole-dipole energy transfer from Er3+:4I13/2 to Er3+:4I15/2 level is stronger when the samples are pumped by 980 nm. Meanwhile, the energy transfer efficiency from Er3+:4I13/2 to Dy3+:6H11/2 level is also higher when pumped by 980 nm. The above results show that the 980 nm excitation is more efficient for Dy3+/Er3+ co-doped tellurite glasses to obtain broadband mid-infrared emissions. The behavior is consistent with the mid-infrared fluorescence spectra in Dy3+/Er3+ co-doped tellurite glasses.
4.5 Gain coefficient of 2.7 μm and 2.8 μm emissions
The gain spectra of the energy level determine the shape and amplification of the signal gain spectra when the Dy3+/Er3+ doped tellurite glass as the laser gain medium. Since the 2.7 and 2.8 μm emissions are derived from Er3+:4I11/2→4I13/2 and Dy3+:6H13/2→6H15/2 transitions, respectively, gain coefficient should take the absorption of lower multiplet and the number of particles on two levels into consideration. Thus, on the basis of the emission and absorption cross-sections of Er3+ and Dy3+ in Fig. 6(c), the net gain can be calculated by the following expression [43],
where Nup and Nlow are the number of particles on upper and lower levels, respectively. If all the electrons of ions are concentrated in the upper and lower levels, the gain equation can be simplified as following expression,where N is the total concentration of doped ions, P is the population inversion given by the ratio between the population of upper level and the total concentration. The calculated gain coefficient of Er3+:4I11/2→4I13/2 and Dy3+:6H13/2→6H15/2 transitions for the population inversion values P ranging from 0 to 1 in interval of 0.1 of tellurite glasses are shown in Fig. 9. It is noted that the maximum gain coefficient of Er3+:4I11/2→4I13/2 transition is much higher than that of Dy3+:6H13/2→6H15/2 transition, which means the system has a lower pumping threshold at 2.7 μm emission for laser. Besides, the maximum gain coefficient of Er3+:4I11/2→4I13/2 and Dy3+:6H13/2→6H15/2 transitions corresponding to the wavelength are 2727 and 2842 nm, respectively. And the value G(λ) of Er3+:4I11/2→4I13/2 and Dy3+:6H13/2→6H15/2 transitions become positive number when the P is 0.4. In addition, it is found that the gain band extends to longer wavelength with the increase of the value of P, which is a typical feature of a quasi-three level system.Fig. 9 Gain coefficient with the population inversion values P ranging from 0 to 1 in interval of 0.1 for (a) Er3+:4I11/2→4I13/2 and (b) Dy3+:6H13/2→6H15/2 transitions when pumped by 808 nm.
5. Conclusion
In conclusion, 1.55 μm, 2.7 μm and 2.8 μm fluorescence spectra are measured in Dy3+/Er3+ co-doped tellurite glasses when pumped by 808 nm and 980 nm LD. The energy transfer processes from Er3+ to Dy3+ have been discussed and analyzed in detail at the excitation of 808 nm or 980 nm LD. Under 808 nm excitation, the energy transfer efficiency from Er3+:4I13/2 to Dy3+:6H11/2 level is 73.1% and the energy transfer coefficient (CDA) of Er3+:4I11/2→Dy3+:6H5/2 and Er3+:4I13/2→Dy3+:6H11/2 are 6.89 × 10−38 and 0.01 × 10−38 cm6/s, respectively. Under 980 nm excitation, the energy transfer efficiency from Er3+:4I13/2 to Dy3+:6H11/2 level can reach as high as 80%. Besides, a large emission cross-section of the 2500nm-3100nm broadband emission can be obtained when pumped by 808 nm or 980 nm LD. This work may provide a promising laser material for broadband mid-infrared emission. The results indicate that the 980 nm excitation is more efficient for Dy3+/Er3+ co-doped tellurite glass to realize low-threshold and high gain applications at broadband mid-infrared laser.
Funding
National Natural Science Foundation of China (NSFC) (Nos. 61405182, 61370049, 51372235, 51401197, 61775205, 51472225); Zhejiang Provincial Natural Science Foundation of China (Nos. LY15E020009, LR14E020003 and LY15F050007.
References and links
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