Superbroadband near-IR photoluminescence from Pr-doped fluorotellurite glasses

Praseodymium(Pr 3+ )-doped fluorotellurite glasses were synthesized and broadband photoluminescence (PL) covering a wavelength range from 1.30 to 1.67 μm was observed under both 488 and 590 nm wavelength excitations. The broadband PL emission is mainly due to the radiative transition from the manifolds Pr 3+ : 1 D2 to 1 G4. The PL line-shape, band width, and lifetime were modified by the Pr 3+ dopant concentration, and a quantum efficiency as high as 73.7% was achieved with Pr 3+ dopant in a low concentration of 0.05 mol%. The good spectroscopic properties were also predicted by the Judd-Ofelt analysis, which indicates a stronger asymmetry and covalent bonding between the Pr 3+ sites and the matrix lifgand field. The large stimulated emission cross-section, long measured lifetime, and broad emission bandwidth confirm the potential of the Pr 3+ singly doped fluorotellurite glass as broadband luminescence sources for the broadband near-infrared optical amplifications and tunable lasers. ©2012 Optical Society of America OCIS codes: (160.5690) Rare-earth-doped materials; (300.6280) Spectroscopy, fluorescence and luminescence; (230.2285) Fiber devices and optical amplifiers. References and links 1. G. A. Thomas, B. I. Shraiman, P. F. Glodis, and M. J. Stephen, “Towards the clarity limit in optical fibre,” Nature 404(6775), 262–264 (2000). 2. S. Kasap, “Optoelectronics,” in The Optics Encyclopedia, T. Brown, K. Creath, H. Kogelnik, M. A. Kriss, J. Schmit, and M. J. Weber, eds. (Wiley-VCH, Weihein, Germany, 2004), Vol. 4., pp. 2237–2284. 3. See, for example, M. J. F. Digonnet, ed., Rare-Earth-Doped Fiber Lasers and Amplifiers (Second Edition, Revised and Expanded), (Marcel Dekker, New York, 2009), and references therein. 4. H. Takebe, K. Yoshino, T. Murata, K. Morinaga, J. Hector, W. S. Brocklesby, D. W. Hewak, J. Wang, and D. N. Payne, “Spectroscopic properties of Nd and Pr in gallate glasses with low phonon energies,” Appl. Opt. 36(24), 5839–5843 (1997). 5. M. Naftaly, S. Shen, and A. Jha, “Tm-doped tellurite glass for a broadband amplifier at 1.47 μm,” Appl. Opt. 39(27), 4979–4984 (2000). 6. B. Zhou, H. Lin, D. Yang, and E. Y. B. Pun, “Emission of 1.38 μm and gain properties from Ho-doped lowphonon-energy gallate bismuth lead oxide glasses for fiber-optic amplifiers,” Opt. Lett. 35(2), 211–213 (2010). 7. C. Strohhöfer and A. Polman, “Silver as a sensitizer for erbium,” Appl. Phys. Lett. 81(8), 1414–1416 (2002). 8. K. Driesen, V. K. Tikhomirov, C. Görller-Walrand, V. D. Rodriguez, and A. B. Seddon, “Transparent Hodoped nano-glass-ceramics for efficient infrared emission,” Appl. Phys. Lett. 88(7), 073111 (2006). 9. J. Dong, Y. Q. Wei, A. Wonfor, R. V. Penty, I. H. White, J. Lousteau, G. Jose, and A. Jha, “Dual-pumped tellurite fiber amplifier and tunable laser using Er/Ce codoping scheme,” IEEE Photon. Technol. Lett. 23(11), 736–738 (2011). 10. S. Tanabe, “Rare-earth-doped glasses for fiber amplifiers in broadband telecommunication,” C. R. Chim. 5(12), 815–824 (2002). 11. S. Y. Seo, J. H. Shin, B. S. Bae, N. Park, J. J. Penninkhof, and A. Polman, “Erbium-thulium interaction in broadband infrared luminescent silicon-rich silicon oxide,” Appl. Phys. Lett. 82(20), 3445–3447 (2003). 12. L. Huang, A. Jha, S. Shen, and X. Liu, “Broadband emission in Er-Tm codoped tellurite fibre,” Opt. Express 12(11), 2429–2434 (2004). 13. Z. Xiao, R. Serna, C. N. Afonso, and I. Vickridge, “Broadband infrared emission from Er-Tm:Al2O3 thin films,” Appl. Phys. Lett. 87(11), 111103 (2005). #156370 $15.00 USD Received 12 Oct 2011; revised 1 Dec 2011; accepted 5 Dec 2011; published 1 Feb 2012 (C) 2012 OSA 13 February 2012 / Vol. 20, No. 4 / OPTICS EXPRESS 3803 CORE Metadata, citation and similar papers at core.ac.uk Provided by PolyU Institutional Repository

Among the rare earth ions, Pr 3+ shows potential to yield luminescence at specific near-IR wavelength bands within the expanded low loss window considering its rich multiple energy levels. Apart from the 1.3 µm emission (Pr 3+ : 1 G 4 → 3 H 5 ) [10], an intense emission located at 1.6 µm from the Pr 3+ : 3 F 4,3 → 3 H 4 transition was also obtained in selenide glasses [27], which presents a promising candidate for the U-band optical amplification. We recently observed the superbroadband near-IR luminescence from Pr 3+ -singly doped bismuth gallate glasses [28]. However, the relatively stronger intrinsic absorption of the bismuth gallate glass at the short wavelength side (<500 nm) would result in a serious depression of the blue light pump efficiency. To overcome it, development of host matrix with higher transmission at blue wavelength region together with unique ligand field is necessary.
In this work, we report the observation of broadband near-IR luminescence extended from 1.30 to 1.67 µm in Pr 3+ -singly doped novel fluorotellurite glasses for the first time to our best knowledge. Tellurite-based glasses show high refractive index, low phonon energy, good mechanical properties and chemical durability [5,25]. More significantly, near-IR tellurite glass fiber/waveguide amplifiers and lasers have been reported [9,12,29]. Some of fluorides were added to modify the local ligand field between the Pr 3+ and the host matrix. The high transparency at the short wavelength side (<500 nm) enables to improve the pump efficiency under blue excitation sources. The energy transfer processes involved are proposed and discussed, and the optical amplification is also evaluated.

Experimental
Pr 3+ -doped fluorotellurite glass samples were prepared by melting the well-mixed batches of high-purity chemicals with mol% composition of 3LaF 3 -4BaF 2 -4BaCO 3 -9ZnO-80TeO 2 -xPrF 3 (x=0.05, 0.1, 0.3, 0.5, and 1.0) following the procedures described in Ref [25]. For comparison, a sample without Pr 3+ doping was also prepared. The as-prepared glasses were cut and optically polished for optical measurements. The refractive indices of the glass samples were measured using a Metricon 2010 prism coupler and they are 2.068 and 2.013 at wavelengths of 632.8 and 1536 nm. The profile of the indices on wavelength was determined to be ( ) (the wavelength λ is in unit of nm) using Cauchy formula [30]. The Raman spectrum of undoped glass sample was measured by 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 photoluminescence (PL) spectra were recorded using a photomultiplier tube (PMT) and a near-IR PMT detector, respectively. The excitation sources were tuned from a continuous xenon lamp by a monochromator. The photoluminescence excitation (PLE) 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. Figure 1 shows the optical absorption spectrum of the 0.5 mol% Pr 3+ -doped glass sample.  The inset (b) of Fig. 1 shows the Raman spectrum of the undoped host measured using a 488 nm laser excitation. The maximum phonon energy is determined to be ~766 cm −1 by Gaussian fitting, which can be assigned to the stretching vibrations of TeO 3+δ polyhedral, and/or tellurium and non-bridging oxygen atoms in TeO 3 trigonal pyramid units [32,33]. This value is comparable to many other oxide-based tellurite and fluorotellurite glasses [32-36]. The band with the most intense peak at 658 cm −1 is due to the anti-symmetric vibration modes of TeO 4 , and the bands in lower frequency region at 311 and 451 cm −1 can be attributed to the bending vibration bonds of O-Te-O/Te-O-Te linkages of TeO 4 [34]. The Raman spectrum intensity of the peak at 658 cm −1 in comparison to that at ~766 cm −1 in fluorotellurite glass is slightly lower than those of the oxide-based tellurite glasses, this may be ascribed to the breakage effect of F ions on Te-O bonds and/or formation of more TeO 3+δ /Te(O,F) 3+δ groups [34][35][36]. This indicates that fluorides play a different role as network modifiers in comparison to the oxides.

Results and discussion
A Judd-Ofelt analysis can be performed to predict the spontaneous transition properties of Pr 3+ in host matrix [37,38]. In the present work, absorption bands ( 3 P 2 , 3 P 1 + 1 I 6 , 3 P 0 , 1 D 2 , 1 G 4 , 3 F 4 , 3 F 3 , 3 F 2 , and 3 H 6 ,) were used for the Judd-Ofelt calculation, and the overlapped bands were de-convoluted using Gaussian fitting. The reduced matrix elements of the tensor operators used in the calculation of the electric dipole line strengths (S ed ) can be considered independent of the host materials and the values of them were quoted from Ref [39]. The phenomenological Judd-Ofelt intensity parameters t Ω (t=2,4,6) were calculated by a leastsquares fitting of the electric dipole oscillator strengths from the experiment (P exp ) and calculation (P cal ). The root-mean-square deviation rms δ between the P exp and P cal was calculated by   Figure 2(a) shows the near-IR PL spectra of Pr 3+ -doped samples under 488 nm excitation. Broadband emissions covering 1.30-1.67 µm wavelength region are observed for the samples with lower Pr 3+ dopant concentration. With the increasing of Pr 3+ , the PL intensity decreases after an initial increase, and the emission peak extends to the short wavelength side whilst the emission band at the long wavelength side declines. Figure 2(b) shows the full-width at halfmaximum (FWHM) of the emission bands. It can be seen that the FWHM increases with the increasing of Pr 3+ and reaches a maximum FWHM bandwidth of 196 nm when Pr 3+ concentration is equal to 1.0 mol%. This broadband emission can be assigned to the Pr 3+ : 1 D 2 → 1 G 4 transition, the unique ligand of the Pr 3+ sites in the fluorotellurite matrix enables the observation of broadband emission from this transition. The emission band at the shorter wavelength side might be also contributed by the Pr 3+ : 1 G 4 → 3 H 5 transition [23]. In deed, an intense emission at around 1.33 µm corresponding to this transition is observed under 996 nm wavelength excitation [see Fig. 2(c)]. The PL intensity decline at higher Pr 3+ doping level, in particular, at the longer wavelength side can be ascribed to the cross relaxation processes [ 1 D 2 , 3 H 4 ]→[ 1 G 4 , 3 F 4,3 ], considering the spectral overlap of the emission band ( 1 D 2 → 1 G 4 ) and the absorption band ( 3 F 4,3 ← 3 H 4 ) as shown in Fig. 2(c). The closer Pr 3+ -Pr 3+ separation at higher Pr 3+ doping level makes this process occur easily, leading to a depleting of the emission level Pr 3+ : 1 D 2 [28]. To optimize the excitation wavelengths, the PLE spectra were measured at 1480 nm and are shown in Fig. 3(a). It can be seen that, apart from the excitation at around 590 nm, broad excitation bands extending from 440 to 490 nm also have contributions to the near-IR emission. The excitation band corresponds to the manifolds ( 3 P 0,1,2 , 1 I 6 ), on which the energy can decay rapidly to the next lower emission level 1 D 2 mainly through the cross relaxation [ 3 P 0 , 3 H 4 ]→[ 1 D 2 , ( 3 H 6 , 3 F 2 )]; the radiative decay can be ignored because the 3 P 0 → 1 D 2 transition rate is very low according to the Judd-Ofelt result. It can be also found that this excitation band exhibits similar change compared with the near-IR PL spectra as the Pr 3+ concentration increases whilst the excitation band ( 3 P 0,1,2 , 1 I 6 ) gives a relatively rapider decrease in comparison to the 590 nm ( 1 D 2 ) excitation band [see Fig. 3(a)]. The excitation band around 590 nm shows advantage, in particular, at higher Pr 3+ dopant concentration, is primarily due to the resonant excitation route by which the emission level Pr 3+ : 1 D 2 is populated directly. By comparing the PLE spectra with the absorption spectrum [see Fig. 3(b) and the inset], it can be found that the absorptions in 440-490 nm wavelength region with respect to the 590 nm band is much more intense than that in the PLE spectra. This result indicates that there are some other dacays for the energy residing in the ( 3 P 0,1,2 , 1 I 6 ) manifolds. To further understand this phenomenon, the visible PL spectra under 445 nm excitation were recorded and are shown in Fig. 4. It should be mentioned that there is no obvious change in the near-IR emission line-shapes measured under 445 nm compared with those measured under 488 nm wavelength excitation. Intense visible emissions peaking around 490, 528, 611, and 643 nm are observed, and they were assigned to the Pr 3+ : 3 P 0 → 3 H 4 , 3 P 0 → 3 H 5 , 3 P 0 → 3 F 3 , and 3 P 0 → 3 F 3 transitions, respectively. This observation is in aggrement with the Judd-Ofelt calculation result that these transtitions posses larger branch ratios than those transitions terminating to other levels. The recorded visible PL spectra show similar altering tendency as the broadband near-IR emission; they decrease after an initial increase with the further increment of Pr 3+ in concentration. The mechanism responsible for this can be attributed to the cross relaxation process [ 3 P 0 , 3 H 4 ]→[ 1 D 2 , ( 3 H 6 , 3 F 2 )], higher Pr 3+ dopant concentration enables an easy occurrence of this process, leading to a population of the broadband emission level 1 D 2 . The less population of Pr 3+ : 3 P 0 results in a decreasing change of the visible emissions.
It can be observed in Fig. 4 that the blue wavelength emission gives a deeper decrease than the green and red emissions although they originate from the common level 3 P 0 . This means that some other quenching mechanism occurred on the Pr 3+ : 3 P 0 → 3 H 4 emission. Considering the spectral overlap between the high-energy emission side and the low-energy absorption side of Pr 3+ : 3 P 0 ↔ 3 H 4 , some energy in the higher sub-levels of Pr 3+ : 3 P 0 can be absorbed by a nearby Pr 3+ in the ground state when they are in a close separation, resulting in a depleting of the higher energy sub-levels of Pr 3+ : 3 P 0 . This is confirmed by the changes involved in the emission peak wavelength that presents a red-shift from 485 to 495 nm in this process, as shown in Fig. 4 inset (a). In addition, the transition from the 3 P 1 to 3 H 4 may contribute to this blue emission considering the observation of a shoulder around 470 nm which decreases with the increasing of Pr 3+ concentration and nearly to be completely quenched when it reaches 1.0 mol%. This can be attributed to the process [ 3 P 1 -3 H 4 ]:[ 3 H 4 -3 P 0 ], taking into account that Pr 3+ : 3 P 0 shows intense absorption around 470 nm [see Fig. 4 inset (a)]. It should be also noted that the shoulder at the short wavelength side of the 613 nm emission, which has been demonstrated due to the hypersensitive Pr 3+ : 3 P 1 → 3 F 3 transition [31], shows a similar decrease tendency as the shoulder around 470 nm. This can be ascribed to the cross relaxation [ 3 P 1 , 3 H 4 ]→[ 3 H 6 , 1 D 2 ], by which some of Pr 3+ in 3 P 1 non-radiatively decay to 3 H 6 with the energy being absorbed by Pr 3+ ions in the ground state. The matched energy difference, as shown in inset (b) of Fig. 4, enables an easy occurrence of this process especially at higher Pr 3+ doping level. The above explanations are also confirmed by the lifetime changes of the 3 P 0 and 1 D 2 states; the both decrease as the increment of Pr 3+ in concentration, as shown in Figs. 5(a) and 5(b) and the insets of them. The lifetime values are determined using is the dependence of decay curve on the time t [30], since the decay curves show a slight deviation from the single exponential function. All the aforementioned energy transfer processes together with the excitation routes are schematically illustrated in Fig. 6.   To further evaluate the gain performance of this glass system, it is of significance to determine the stimulated emission cross-section σ em . Considering that the Pr 3+ near-IR emission occurs between two excited states, the profiles of σ em can be described by the equation . The production σ em τ m is obtained to be 8.31 × 10 −26 cm 2 s, which is proportional to the amplification gain and inversed laser oscillation threshold, suggesting that this host matrix is promissing for the gain media as broadband amplifiers and tunable lasers.

Conclusions
In conclusion, broadband NIR emissions in the wavelength region of 1.30-1.67 µm were achieved from Pr 3+ -singly doped fluorotellurite glasses. The bandwidth (FWHM) was improved by the increase of Pr 3+ concentration although it resulted in a quenching of the emission at higher dopant concentration. The spontaneous transition properties analyzed using the Judd-Ofelt theory indicate a stronger covalent bonding between the Pr 3+ sites and the matrix lifgand field. The results suggest that the Pr 3+ -doped fluorotellurite glass system is promising for broadband near-IR luminescence sources and optical amplifiers/lasers operating at the expanded low-loss telecommunication transmission window. Further investigations and experiments are underway.