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Fibers 2014, 2(1), 24-33; doi:10.3390/fib2010024
Published: 8 January 2014
Abstract: Energy transfer mechanisms between Er3+ and Pr3+ in Er3+/Pr3+ codoped germinate glass are investigated in detail. Under 980 nm LD pumping, 2.7 μm fluorescence intensity enhanced greatly. Meanwhile, 1.5 μm lifetime and fluorescence were suppressed deeply due to the efficient energy transfer from Er3+:4I13/2 to Pr3+:3F3,4, which depopulates the 4I13/2 level and promotes the 2.7 μm transition effectively. The obvious change in J-O parameters indicates that Pr3+ influences the local environment of Er3+ significantly. The increased spontaneous radiative probability in Er3+/Pr3+ glass is further evidence for enhanced 4I11/2 → 4I13/2 transition. The Er3+:4I11/2→Pr3+:1G4 process is harmful to the population accumulation on 4I11/2 level, which inhibits the 2.7 μm emission. The microscopic energy transfer coefficient of Er3+:4I13/2→Pr3+:3F3,4 is 42.25 × 10−40 cm6/s, which is 11.5 times larger than that of Er3+:4I11/2→Pr3+:1G4. Both processes prefer to be non-phonon assisted, which is the main reason why Pr3+ is so efficient in Er3+:2.7 μm emission.
Mid-infrared laser, especially ~3 μm laser, has extensive potential application, such as remote sensing and laser microsurgery. The abundant level systems of Er3+, Dy3+ and Ho3+ ions make the production of ~3 μm laser possible [1,2,3]. Therefore, a series of methods were used to enhance the intensity of photoluminescence around 2.7 μm, including codoping with other rare-earth ions, such as Tm3+, Nd3+, Yb3+ and Pr3+ [1,4,5,6,7,8]. Among them, Pr3+ is very effective at sensitizing Er3+:2.7 μm emission. Furthermore, Er3+/Pr3+ codoped tunable CW laser has been obtained in the ZBLAN fiber [9,10]. However, represented by ZBLAN glass, fluoride glass performs with small ΔT (=Txp − Tg, Txp is the crystallization peak temperature) value, poor chemical and thermal stability, and difficulties in glass preparation and fiber drawing. Researchers have done a lot of work in developing new mid-IR glass to overcome the shortcomings of fluoride glasses. Many results has been reported on chalcogenide, fluorophosphate, tellurite, germanate and PbO-Bi2O3-Ga2O3 glass [11,12,13,14,15]. Nonetheless, the chalcogenide glass requires a complex fabrication route, especially in refining raw materials and moisture removal process. Besides, the character of bismuth trioxide glass depends on the melting condition. Among the remains of choices, germinate glass does have proper phonon energy and higher glass transition temperature for resisting laser damage. Thus, the Er3+/Pr3+ codoped germanate glass can have preferable spectroscopic properties as a candidate for 2.7 μm fiber laser material.
Except for Pr3+, no laser output has been reported in other RE3+ sensitized Er3+ glasses. In order to understand the sensitizing mechanisms of Pr3+ to Er3+ further, in this work, we studied the energy transfer (ET) dynamics as well as the macroscopic ET parameters in a Pr3+/Er3+ codoped germanate glass. The energy transfer mechanism in Er3+/Pr3+ codoped glass is focused to demonstrate the sensitizing effect of Pr3+.
Glass was prepared following the molar composition 56GeO2-15PbO-14Na2O-12Ga2O3-3PbF2-Er2O3-0.5Pr2O3, named GPNG glass. 3 mol% PbF2 was introduced to reduce the hydroxyl groups. All the high-purity powders were well-mixed and melted at 1150 °C for 30 min in an electrical furnace. The melting glass was bubbled with high-purity oxygen gas, and then the melts were poured into preheated stainless-steel mold and annealed for 10 h. The homogeneous samples were cut into 20 mm × 20 mm × 1 mm and well polished.
Refractive index was measured by a Specro-Ellipsometer (Woollam W-VASE, error limit ±0.05%). With 1 nm steps, the absorption spectra were recorded in the range of 400–1700 nm with a Perkin-Elmer Lambda 900 UV/VIS/NIR spectrophotometer. Under 980 nm, laser diode (LD)’s pumping, fluorescence spectra were tested by a computer-controlled TRIAX 320 type spectrometer and the FLSP920 fluorescence spectrophotometer (Edinburgh Analytical Instruments Ltd., Livingston, UK). The emission spectra ranged in 2550–2800 nm, 1400–1700 nm, 500–700 nm and the lifetime of Er3+:4I13/2 level was also obtained accordingly. The power of 980 nm LD, the width of the slit to collect signals and the position of the samples were fixed to the same condition in the experiment setup in order to accurately compare the intensity of 2.7 μm emission. All the measurements were carried out at room temperature.
3. Results and Discussions
3.1. Absorption Spectra and Infrared Transmittance Spectrum
Figure 1 displays the absorption spectra of GPNG glass singly doped with 1 mol% Er2O3, 0.5 mol% Pr3+ and coped with both Er2O3 and Pr2O3 at room temperature. The labeled transitions for both RE3+ ions correspond to the ground state to the specific higher levels. The absorption bands at 980 nm illustrate that 980 nm LD can excite this glass.
The mid-infrared transmittance spectrum of Er3+/Pr3+ codoped sample is shown in the inset of Figure 1. The transmittance reaches 86% and extends to 6.0 μm in the present glass. A typical absorption band of OH− groups appears at around 3 μm. Due to the overlap with the emission wavelength, the OH− groups are regarded as a catastrophe in mid-infrared laser materials. The content of OH− groups can be represented by absorption coefficient, which is defined by:
3.2. Fluorescence Spectra and Judd-Ofelt Analysis
Figure 2 shows the measured photoluminescence spectra of Er3+ singly doped and Er3+/Pr3+ codoped GPNG glass that correspond to the Er3+:4I13/2→4I15/2 and Er3+:4I11/2→4I13/2 transition, respectively. It is clearly observed that 2.7 μm emission is significantly enhanced. Meanwhile, the 1.5 μm emission is deeply quenched by Pr3+ codoping. In addition, the lifetime of 1.5 μm decrease sharply by Pr3+ codoping. The lifetime of Er3+:1.5 μm in singly doped sample is 3.23 ms, while that of codoped sample too weak to be detected. According to the previous study , this phenomenon suggests that energy transfer occurs between Er3+ and Pr3+ owing to the equally energetic spacing of multiplet-to-multiplet transitions. Pr3+ ions obviously depopulate the Er3+:4I13/2 level in this ET process.
Judd-Ofelt  analysis is widely employed to determine the spontaneous emission transition probabilities, radiative lifetime and branching ratios. The Judd-Ofelt intensity parameters of Er3+ and Er3+/Pr3+ glass were calculated in Table 1. Previous studies indicated that Ω6 is inversely proportional to the covalence of Er-O band. Ω2 represents symmetry around Er3+ ions. In addition, Ω6 is also related to the optical basic of host materials. Table 1 shows that Ω2 of GPNG glass is higher than other systems, while Ω6 is smaller than fluoride and fluorotellurite glass.
|Table 1. Judd-Ofelt intensity parameters in various glasses (unit: 10−20 cm2).|
|Ω2||6.60||13.23||5.14 ± 0.10||2.98||4.23||4.38|
|Ω4||1.75||2.81||1.02 ± 0.08||1.40||1.04||3.05|
|Ω6||0.99||2.26||0.91 ± 0.06||1.04||0.61||1.04|
Theoretically, J-O parameters are influenced by the properties of the host. However, results in this work show that J-O parameters change obviously when Pr3+ added in, and this is because the local environment of Er3+ has been interfered largely by Pr3+, which I nduces the variations on Ω2,4,6. As a result, the spontaneous radiative probability (Arad) increased from 36.69 s−1 to 60.93 s−1. Moreover, the calculated radiative lifetime of 4I11/2 level decreased from 4.55 ms to 2.16 ms (Table 2). Obviously, increasing Arad promotes the 4I11/2→4I13/2 transition as well as the 2.7 μm photoluminescence.
|Table 2. Calculations of Arad, branching ratio (β) and radiative lifetime in Er3+ and Er3+/Pr3+ samples.|
|Arad||β||τ (ms)||Arad||β||τ (ms)|
3.3. Energy Transfer Mechanisms between Er3+and Pr3+
Previous study  has demonstrated possible mechanisms based on multiplets of Er3+ and Pr3+ ions in Figure 3. Pumped by 980 nm LD, ground state ions are excited to 4I11/2 level (ground state absorption, GSA). The involved energy transfer processes based on the 4I11/2 level are as follows: Er3+: 4I11/2 + a photon→Er3+: 4F7/2 (ESA1), Er3+: 4I11/2 + 4I11/2→Er3+:4I15/2 + 4F7/2 (ETU1), 4I11/2→4I13/2 transition with 2.7 μm emission, and energy transfer from Er3+: 4I11/2 level to Pr3+: 1G4 level (ET1). After ETU1 and some nonradiative transitions among 4F7/2, 2H11/2, 4S3/2 and 4F9/2, the above-mentioned levels are populated. These excited ions contribute to upconversion spectra for green and red emission. Based on lower level 4I13/2, 4I13/2 + 4I13/2→4I15/2 + 4I9/2 (ETU2) and nonradiative relaxation from 4I9/2 level to 4I11/2 level populate the 4I11/2. Therefore, the ETU2 process is beneficial to 2.7 μm emission. Moreover, ET2 from Er3+:4I13/2 to Pr3+:3F3,4 depletes the Er3+: 4I13/2 level. It is observed that the intensity of red emission is higher than that of green emission in Figure 4. The upper level of red emission (4F9/2) is mainly populated by ESA2: 4I13/2 + a photon→4F9/2. 4I13/2 level is significantly depleted, which is valuable to 2.7 μm emission.
As it mentioned above, codoping with Pr3+ can enhance the intensity of 2.7 μm emission through energy transfer processes. To profoundly investigate this system, Forster and Dexter’s [22,23] method is used to quantitatively analyze the energy transfer microscopic parameters between Er3+ and Pr3+. The energy transfer probability rate between donor (D) Er3+ and acceptor (A) Pr3+ can be expressed as :
SDA(0,0,E) is the overlap between donor emission and acceptor absorption with zero phonon participation. S0D and S0A are the Huang-Rhys factors in germanate glass. In the resonance and quasi resonance energy transfer process, the overlap SDA is calculated by Miyakawa and Dexter :
In the case of m-phonon emission by the donor and no phonon absorption by the acceptor, the integral overlap is:
If we just focus on m-phonon creation process, the probability rate of energy transfer can be obtained by:
The calculation results by Equations (2)–(10) are listed in Table 3. It also shows the contribution percentage of different phonon numbers to the probability rate involved in the energy transfer. As a result, the transfer coefficient of ET1 Er3+: 4I11/2→Pr3+: 1G4 is 3.67 × 10−40 cm6/s while that of ET2: Er3+: 4I13/2 → Pr3+: 3F3, 4 is 42.25 × 10−40 cm6/s. The coefficient of ET2 is 11.5 times larger than that of ET1; therefore, codoping with Pr3+ ions efficiently depletes the lower level of 2.7 μm.
Results also show that non-phonon assisted energy transfer is predominant in the ET process, indicating that energy match is the main reason why Pr3+ is very effective in sensitizing Er3+:2.7 μm emission.
|Table 3. Energy transfer microscopic parameter of Er3+ and Pr3+ in germanate glass.|
|Energy Transfer||N (% Phonon Assisted)||Transfer Coefficient (10−40 cm6/s)|
|Er3+: 4I11/2→Pr3+: 1G4||0||1||2||3||3.67|
|Er3+: 4I13/2→Pr3+: 3F3, 4||0||1||2||42.25|
In summary, Er3+/Pr3+ codoped GPNG glass has much enhanced 2.7 μm emission and greatly decreased 1.5 μm fluorescence due to the efficient energy transfer from Er3+: 4I13/2 to Pr3+: 3F3,4, which depopulates the 4I13/2 level and promotes the 4I11/2→4I13/2 transition effectively. Fluorescence lifetime decreases with Pr3+ introduction, proving the population evacuation on 4I13/2. Interestingly, J-O parameters change obviously, indicating that Pr3+ influences the local environment of Er3+ significantly. The increased spontaneous radiative probability in Er3+/Pr3+ glass is further evidence for enhanced 4I11/2→4I13/2 transition. Pr3+ can also suppress the upconversion luminescence, and this will increase the quantum efficiency of the system. ET process of Er3+: 4I11/2→Pr3+: 1G4 is harmful to the population accumulation on 4I11/2 level, which inhibits the 2.7 μm emission. The calculation of energy transfer microscopic indicates that the transfer coefficient of Er3+: 4I13/2 to Pr3+: 3F3,4 is 42.25 × 10−40 cm6/s, which is 11.5 times larger than that of Er3+: 4I11/2→Pr3+: 1G4, and both processes are preferred to be non-phonon assisted, which is the main reason why Pr3+ is so efficient in Er3+:2.7 μm emission. This result provides another proof to illustrate the energy transfer mechanisms between Pr3+ and Er3+.
This research was supported by the Chinese National Natural Science Foundation (No. 51172252, 61177083 and 51372235).
Conflicts of Interest
The authors declare no conflict of interest.
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