We report the first demonstration of superbroadband emission extending from 1.30 to 1.68 μm in praseodymium(Pr3+)-erbium(Er3+) codoped fluorotellurite glasses under 488 nm excitation. This superbroad near-infrared emission is contributed by the Pr3+: 1D2→1G4 and Er3+: 4I13/2→4I15/2 transitions which lead to emissions located at 1.48 and 1.53 μm, respectively. The quenching of the Pr3+ emission resulted from the cross relaxation [1D2, 3H4]→[1G4, 3F3,4] was effectively compensated by the codoping of Er3+. The results suggest that, other than the heavy-metal and transition-metal elements of active bismuth (Bi), nickel (Ni), chromium (Cr), etc., Pr3+-Er3+ codoped system is a promising alternative for the broadband near-infrared emission covering the expanded low-loss window.
©2012 Optical Society of America
Development of superbroadband near-infrared luminescence sources for broadband optical amplifiers and tunable lasers covering entirely the expanded low-loss telecommunication window (~1.2-1.7 μm) attracts considerable attention, in particular, after the progress made in the production of hydroxyl (OH–)-free silica fibers (dry optical fibers) [1,2]. Previous investigations were focused on the bismuth (Bi) [3–6], and then an extension to other heavy metal (HM) and transition metal (TM) ions, such as nickel (Ni), chromium (Cr), lead (Pb), etc. [7–9]. However, the bandwidth and peak wavelength of the broadband emissions from HM/TM ions depend sensitively on the host matrix and the excitation wavelength, and also the luminescence origin for some of them requires further studies . To date, there has been little work reports on the superbroadband luminescence from rare earth (RE) doped systems, although they play crucial roles as optical amplifiers and laser sources in telecommunication systems . Typical RE emissions/amplifications cover only separate C-, L-, S-, E-, and O-bands. Thus, novel host matrix, dual-pump configurations, nanostructures, and REs codoping schemes have been investigated to further improve the bandwidth and the quantum efficiency . Recently, broadband emissions located at 1.20 and 1.47 μm were observed in thulium (Tm3+)-doped glasses [12,13], and a broader emission band from 1.0 to 1.7 μm was obtained using Tm-Bi codoping scheme by taking advantage of the Bi 1.3 μm emission .
Praseodymium (Pr3+) shows promising to achieve some novel near-infrared emissions due to the rich multiple energy levels. Apart from the well-known 1.3 μm emission (1G4→3H5 transition) , an emission around 1.6 μm from the 3F3,4→3H4 transition was also observed in Pr3+-doped selenide glass . We recently observed superbroadband near-infrared luminescence in Pr3+-doped bismuth gallate glass . However, the low transmission of bismuth gallate glass in the blue region resulted in a depression of the pump efficiency using blue light, at which Pr3+ possesses intense absorption bands. In addition, the Pr3+ near-infrared emission, especially at the longer wavelength side, was seriously quenched by the cross relaxation [1D2, 3H4]→[1G4, 3F3,4] because of the intense ground-state absorption 3F3,4←3H4 which is overlapped with the Pr3+ emission.
In the present work, we propose Pr3+-Er3+ codoping scheme to achieve the superbroadband near-infrared emission for the first time to our best knowledge. Efficient Er3+ 1.53 μm emissions/amplifications have already been demonstrated under 488 nm excitation (into the absorption band Er3+: 4F7/2←4I15/2) . Fluorotellurite glasses were selected as host because of their broad transmission window, good mechanical properties and chemical durability, and optical amplification and laser operation have been achieved in tellurite glass fibers .
Glass samples were prepared by melting well-mixed high purity materials of lanthanum fluoride (LaF3), barium fluoride (BaF2), barium carbonate (BaCO3), zinc oxide (ZnO), and tellurium oxide (TeO2) following a standard melt-quenching procedure as described in . Praseodymium fluoride (PrF3) and erbium fluoride (ErF3) were used as RE dopants with doping levels of xPrF3-yErF3 mol% where x = 0, 0.1, 0.2, 0.3; y = 0.1, and x = 0.3; y = 0, 0.05. LaF3 and BaF3 were added to modify the ligand field between the RE sites and the matrix. The as-prepared glasses were cut and optically polished for optical measurements.
The refractive index of the glass samples was measured using a Metricon 2010 prism coupler. The Raman spectrum of undoped glass sample was measured using a HORIBA Jobin Yvon HR800 Raman spectrometer with a 488 nm laser excitation source. The absorption spectra were recorded using a Perkin Elmer UV-VIS-NIR Lambda 19 double beam spectrophotometer. The visible and infrared emission spectra were recorded using an Edinburgh Instruments FLSP920 spectrofluorometer. The wavelengths of excitation sources were tuned from a continuous xenon lamp by a monochromator. The excitation spectra were recorded using the same setup with a continuous wavelength xenon lamp as the excitation source. The emission decay curves were recorded using the same setup with a flash xenon lamp as the excitation source. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 1 shows the absorption spectra of Pr3+-, Er3+-singly doped and Pr3+-Er3+ codoped samples. All the absorption bands observed are due to the electronic transitions from the ground-state to the respective excited states as indicated in Fig. 1. The transparency at the blue wavelength region is much higher than bismuth gallate glass, which would allow efficient pumping with blue light sources. It is interesting to observe that both Pr3+ and Er3+ possess absorptions around 488 nm [see inset (a) of Fig. 1]; they correspond to the Pr3+: 3P0←3H4 and Er3+: 4F7/2←4I15/2 transitions, respectively. This resonant energy-level matching makes it possible to achieve the superbroadband emission in the Pr3+-Er3+ codoping scheme by pumping with a single-wavelength light source. This is in agreement with the excitation spectra, as shown in inset (b) of Fig. 1.
Using the absorption spectra, Judd-Ofelt analysis was performed [18,19]. The values of intensity parameters Ωt (t = 2, 4, 6) are calculated to be (3.57, 6.60, 5.18) × 10−20 cm2, and (5.62, 1.12, 1.78) × 10−20 cm2 for Pr3+- and Er3+-singly doped samples, respectively, using a least-squares fitting of the experimental and theoretical electric dipole oscillator strengths. The larger Ω2 value of Pr3+ in fluorotellurite than those other fluoride contained oxide glasses indicates a stronger asymmetry and covalent environment of Pr3+ in fluorotellurite glass [20,21]. A similar result is also obtained for Er3+-doping, in which the value of Ω2 is larger than those ZnF2 contained tellurite glasses . The spontaneous transition properties of Pr3+ and Er3+ are listed in Table 1 . The spontaneous transition probability of Pr3+: 1D2→1G4 is 880.1 s–1 (with branch ratio of 11.1%), which is comparable to that of Pr3+ in bismuth gallate glass . Regarding the Er3+: 4I13/2→4I15/2, the spontaneous transition probability (454.1 s–1) is larger than that in ZnF2 contained and oxide-based tellurite glasses [22,23].
Figure 2 compares the near-infrared emissions of Pr3+-Er3+ codoped samples under 488 nm excitation for different Pr3+ concentrations. Compared with the narrower Er3+ 1.53 μm emission band (Er3+: 4I13/2→4I15/2 transition), the emission in Pr3+-Er3+ codoped samples shows an obvious extension and enhancement in the short wavelength side, resulting in a superbroad emission band at 1.3-1.68 μm range. The broad emission at short wavelength region is contributed by the Pr3+: 1D2→1G4 transition, which leads to a broad near-infrared emission band under blue excitation. This is in agreement with the Pr3+-doped bismuth gallate glass . Figure 2 inset shows the normalized emissions of the Pr3+-Er3+ codoped samples with different Er3+ concentrations. The composite near-infrared emission at the longer wavelength side is enhanced by incorporation of Er3+. The emission observed at 1.23 μm is due to the Er3+: 4S3/2→4I11/2 transition, which shows a relative increasing trend with the increase of Pr3+.
Figure 3(a) shows the decay curves of Pr3+: 1D2 measured at a monitoring wavelength of 1.42 μm. This wavelength is significantly away from the Er3+ 1.53 μm emission band, and hence the effect of the Er3+ 1.53 μm emission on the decay measurement can be ignored. Since the decay curves recorded deviate slightly from the single exponential function, the lifetime is then determined by , where is the decay as a function of time t. The mean lifetime decreases from 55.5, 49.1, to 47.8 μs after the presence of Er3+ and further increase from 0.05 to 0.10 mol%. This can be attributed to the cross relaxation [Pr3+(1D2), Er3+(4I15/2)]→[Pr3+(1G4), Er3+(4I13/2)], by which some of the energy in Pr3+:1D2 were transferred to neighboring Er3+ in the ground state due to the broad spectral overlap between them [see inset of Fig. 3(a)], resulting in a depleting of the Pr3+:1D2. Another possible ET from Pr3+:1D2 to Er3+:4F9/2 can be ignored considering the large energy gap between them (~1490 cm–1) and the relatively low phonon energy of the host (the maximum phonon energy is ~770 cm–1 according to the Raman spectrum). Indeed no emission from relevant Er3+ levels such as 4F9/2 is observed in the Pr3+-Er3+ codoped sample when excited at 594 nm (into the Pr3+:1D2 absorption band). The overall efficiency can be evaluated by the quantum efficiency ηQE of the emitting level Pr3+:1D2 which is calculated by ηQE = τm/τr, where τr is the radiative lifetime obtained by the Judd-Ofelt calculation . The value of ηQE, which is 44.0% in the (0.3 mol%)Pr3+-singly doped sample, shows a slight decrease after the codoping of Er3+ but remains above 38.0% in the (0.3 mol%)Pr3+-(0.1 mol%)Er3+ codoped sample.
To further understand the phenomena observed, it is necessary to consider other energy transfer (ET) processes involved between Pr3+ and Er3+. To investigate the ET process from Pr3+:3P0 to Er3+:4F7/2, we measured the Pr3+: 3P0→3H4 emission as well as its decay curves at monitoring wavelength of 495 nm under 488 nm excitation, and they are shown in Fig. 3(b) and inset. The lifetime shows a decrease for the Pr3+-doped samples after the addition of Er3+. This is in accordance with the decrease of 526, 611, and 643 nm emissions which originate from the common level Pr3+:3P0, confirming the occurrence of the ET process Pr3+(3P0)→ Er3+(4F7/2). The ET from Er3+:4S3/2 to Pr3+:1D2 can be taken no consideration because the energy mismatch between them is as large as ~1580 cm–1. This is also confirmed by the emission spectrum measured under excitation of 525 nm [see Fig. 3(c)] at which the excitation energy is absorbed only by Er3+(2H11/2). In Fig. 3(c), the Er3+ green emissions from Er3+: (2H11/2,4S11/2)→4I15/2 transition are clearly observed while no Pr3+ relevant emission is observed. The Er3+:4I11/2 and Pr3+:1G4 are quasi-resonant levels, and the ET process Er3+(4I11/2)→Pr3+(1G4) occurred, which is confirmed by the observation of the Pr3+ 1.3 μm emission in Pr3+-Er3+ codoped sample under 980 nm excitation, while there is no 1.3 μm emission observed in Pr3+-singly doped sample under the same excitation. This ET was also observed in Er3+-Pr3+ codoped ZBLAN glasses [24,25], and the 1.3 μm emission/amplification was obtained in Pr3+-Yb3+ codoped fiber using a 980 nm laser diode pump . As for the ET from the resonant levels of Er3+:4I13/2 to Pr3+:3F4,3, it is confirmed by the significant decrease of the Er3+ 1.53 μm emission after the addition of Pr3+, as shown in Fig. 3(d). By depleting the terminal level of Er3+: 4I11/2→4I13/2 transition through this resonant ET process, enhanced gain at 2.7-2.8 μm range was obtained in Er3+-Pr3+ codoped ZBLAN fiber . The relative increase of the Er3+ 1.23 μm emission compared with of 1.53 μm [see Fig. 2] can be ascribed to the improved population inversion between the upper and lower levels besides the decrease of 1.53 μm emission itself due to the ET3 process. Under 488 nm excitation, the emitting level Er3+:4S3/2 is populated via the multi-phonon relaxations from Er3+:4F7/2 followed the ET1 process and meanwhile the terminal level depleted by the ET from it to Pr3+:1G4. The both contribute to an increase of the population inversion and thereafter the relatively enhanced 1.23 μm emission. All the ET processes involved are schematically illustrated in Fig. 4 .
The stimulated emission cross-section σem for the Pr3+: 1D2→1G4 transition was determined to be 0.90 × 10−20 cm2 by scaling the emission spectrum through the Füchtbauer-Ladenburg approach  because this transition occurs between the two excited states. Theσem of Pr3+ emission in fluorotellurite is larger than in bismuth gallate glass (0.70 × 10−20 cm2)  and of Ni2+ in glass-ceramics (0.63 × 10−20 cm2) , although smaller than the active Bi ions in germanate glass (1.59 × 10−20 cm2) . The σem for the Er3+: 4I13/2→4I15/2 transition was derived to be 1.06 × 10−20 cm2 using the McCumber relationship . It is larger than those in TeZnNa (0.85 × 10−20 cm2)  as well as many other types of oxides such as germanate (0.58 × 10−20 cm2), silicate (0.73 × 10−20 cm2), and phosphate (0.76 × 10−20 cm2) .
Superbroadband near-infrared emission covering the expanded low-loss telecommunication window was achieved in Pr3+-Er3+ codoped fluorotellurite glasses under 488 nm excitation. Er3+ is demonstrated to be a good candidate to compensate the quenching of Pr3+ near-infrared emission resulted from the cross relaxation process [1D2, 3H4]→[1G4, 3F3,4]. The results confirm that Pr3+-Er3+ codoped fluorotellurite glass is promising for the superbroadband amplified spontaneous emission sources, optical amplification, and tunable lasers applications. Further investigations and experiments are underway.
The authors thank support from The Hong Kong Polytechnic University (grants G-YH91, G-YJ20, and A-PK72) and from the RGC of Hong Kong SAR, China (project CityU 119708).
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