Near-infrared quantum cutting luminescence in Pr3+/Yb3+ doped lead bismuth borate glass

In this paper, thermally stable lead-bismuth-borate glasses were doped with 0.5 mol% of Pr3+ ions at several concentration levels of Yb3+ ions. Structural characterizations were performed via Raman, differential scanning calorimetry, optical absorption and fluorescence spectra. The Judd–Ofelt intensity parameter, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\Omega }_2$$\end{document}Ω2, of Pr3+ doped glass was comparatively higher than those from reported ones, which reflects the increase of co-valency and asymmetry of chemical bonds in the local environment of Pr3+. Near-infrared emission in 900–2200 nm wavelength range was recorded through 443 nm blue laser pumping. Visible to near-IR quantum cutting and concentration quenching mechanisms were discussed to understand the luminescent behaviour. Intense IR emission (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim 1.0\,\upmu {\text {m}})$$\end{document}∼1.0μm) features generated by absorbing one visible photon leads to quantum efficiencies close to 128% in Pr3+/Yb3+ co-doped samples which may improve the solar spectrum absorption and accordingly, increase the efficiency of c-Si solar cells. Emission cross-section, lifetime, figure of merit and gain bandwidth corresponding to Pr3+: \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$^3F_2 \rightarrow ^3H_4$$\end{document}3F2→3H4 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim 2.0\,\upmu$$\end{document}∼2.0μm) were comparatively reported suggesting that the glass with molar composition 0.5Pr3+/0.1Yb3+ might be a potential candidate for \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim 2.0\,\upmu$$\end{document}∼2.0μm laser operation with low pump threshold.

www.nature.com/scientificreports/ excitation of the Pr 3+ : 3 P J levels. Van Wijngaarden et al. 10 theoretically showed first-order resonant mechanism: 3 P 0 → 1 G 4 (Pr 3+ ) to 2 F 5/2 → 2 F 7/2 (Yb 3+ ) or 1 G 4 → 3 H 4 (Pr 3+ ) to 2 F 5/2 → 2 F 7/2 (Yb 3+ ) in LiYF 4 : Pr 3+ /Yb 3+ crystals. Tanabe et al. 11 showed also a first order resonance energy transfer mechanism in Pr 3+ /Yb 3+ co-doped oxyfluoride glass-ceramics. Moreover, Pr 3+ with its rich spectrum of electronic levels involves a large number of optical transitions in the near-and mid-infrared wavelength range, which have potential uses in amplifiers, remote sensing, tissue welding, micro-surgery, environmental trace gas detection and spectroscopic applications. In order to achieve efficient near-and mid-infrared emissions, low phonon energy of the host material is required to reduce probabilities of multiphonon relaxations between the electronic levels of RE 3+ ions. Tellurite 12 , germanate 13 , fluoride 14 and chalcogenide 15 glasses doped with RE ions are all promising systems for near-and mid-infrared lasers as they possess low phonon energy. Bi 2 O 3 and PbO based heavy metal oxide (HMO) glasses also possess excellent IR transmission, low phonon energy, high refractive index and good corrosion resistance compared to other conventional oxide glasses. Generally, the B 2 O 3 network former allows wider range of glass forming with heavy metal oxides (PbO, Bi 2 O 3 and WO 3 ) than silicates, phosphates and tellurites. In this way, we have chosen lead-bismuth-borate glass composition, as host matrix, as they may exhibit excellent properties: high density, high refractive index, nonlinear refractive index, broad transmission window and low phonon energy [16][17][18][19] . They are also stable, moisture resistant, have relatively low melting temperature and high polarizability (small field strength) of Bi 3+ and Pb 2+ cations. Such unique characteristics evince their potential applications in photonics, mechanical sensors, and reflecting windows [20][21][22] .
In this work, lead-bismuth-borate host matrix were doped with Pr 3+ and Yb 3+ ions. While doped lead-borate, bismuth-borate and lead-bismuth-borate glasses have been extensively investigated for nonlinear and magnetooptic applications [23][24][25] , fewer literature was reported concerning the topics of optical spectroscopy and laser applications. This paper investigates the influence of Yb 3+ ions on near infrared quantum cutting luminescence ( ∼ 1.0 & 2.0 µ m) in Pr 3+ /Yb 3+ codoped glasses through 443 nm excitation. Moreover, it conducts investigations about radiative excited states lifetimes and about energy transfer mechanisms between Pr and Yb ions.

Results and discussion
Raman spectra of Pr 3+ /Yb 3+ doped glasses excited by 633 nm laser and Gaussian fittings of the spectrum, are shown in Fig. 1a. There are ten fitted peaks located at ν ∼ 121, 145, 184, 250, 392, 955, 1112, 1631, 1744 and 1847 cm −1 , respectively. The most intense band at frequency ∼ 200 cm −1 is associated to the vibration of heavy metal ions i.e., vibration involving motion of Bi 3+ cations that are incorporated in the glass network as [BiO 3 ] pyramidal and [BiO 6 ] octahedral units 26,27 . The 250 cm −1 can be attributed to the vibrations of Bi-O bonds 27  Thermal property of the prepared glasses with addition of Yb 3+ ions are analysed by DSC measurements and are shown in Fig. 1b along with glass transition temperature values. T g is an important parameter to testify the glass thermal stability to resist thermal damage at high pump-power laser intensities for a laser glass. It can be seen that the increase of T g with increase of Yb 3+ ions from 451 to 487 • C , indicates the enhancement of rigidity of the glass. This property could be beneficial for stabilization of the glass structure. Density is also a property that reveals the underlying structure of the glass. The density (d) increase of the glasses (see inset data in Fig. 1b) is interrelated with the increase in the packing degree of the glass structure with the increase of Yb 3+ concentration. Figure 2a displays the spectra of absorption of Pr 3+ doped and Pr 3+ /Yb 3+ co-doped glasses. The absorption bands of Pr 3+ which correspond to transitions from the ground state 3 H 4 to the excited levels are labeled www.nature.com/scientificreports/ in Fig. 2a. The absorption band around 1011 nm of Pr 3+ is weak. Adding Yb 3+ as co-dopant to Pr 3+ leads to enhancement of the absorption around the range, 875-1065 nm, as well as, the presence of the strong Ytterbium absorption cross-section at 980 nm related to 2 F 7/2 → 2 F 5/2 transition, as labeled in Fig. 2a. This band intensity increases with Yb 3+ content as shown in Fig. 2b and a linear variation of the absorption coefficient is verified in Fig. 2c, which is an indicative of Yb 3+ ions solubility in the glass network, revealed by the linear fit ( R 2 = 0.99).
On the other hand, the absorption of Pr 3+ in the blue-violet wavelength region is effective to absorb photons, which are not efficiently absorbed by the solar cells. Therefore, co-doping of Pr 3+ and Yb 3+ ions are not only applicable to solar cells but also applicable to near-infrared amplifiers due to their unique spectral characteristics. Judd-Ofelt (J-O) theory is commonly applied to RE doped glasses to testify the spectroscopic and laser properties such as radiative transition probabilities, radiative lifetime, branching ratios of certain emitting levels of RE ions based on absorption spectrum. Detailed theoretical and calculation method have been well described in previous publications 17, [34][35][36][37] . Thus, only results for the Lead bismuth borate Pr 3+ doped glass will be presented. The obtained Judd-Ofelt intensity parameters, ( = 2, 4 and 6) for several host glasses containing Pr 3+ ions are reported in Table 1. As it is known, the 2 is related to the covalency of RE ions and ligand anions, which reflects the asymmetry of local environment around the RE ions. The covalency of Pr-O bond, in the studied glass, is stronger than those of zinc-bismuth-borate 29 , lead-phosphate 30 , oxyfluoride 31 , fluorotellurite 32 and silicate 33 glasses, pointing therefore to stronger asymmetry around the RE ion. 4 and 6 are associated to the bulk properties like rigidity and viscosity of hosts.
The most intense absorption peak in the blue region is the result of the excitation of Pr 3+ ions from ground state, 3 H 4 to excited states, 3 P j ( 1 I 6 ) (j = 0, 1 and 2), which contribute to NIR and visible emissions (see Fig. 3a, b). In the present work, upon 443 nm excitation, the Pr 3+ ions are excited to 3 P 2 level and non-radiative decay to 3 P 0 and 1 D 2 levels. Then, radiatively decay to lower levels of Pr 3+ ions, exhibiting NIR and visible emissions of Pr 3+ ions (see Fig. 3d). Subsequently, emission corresponding to the transition of Yb 3+ : 2 F 5/2 → 2 F 7/2 emission occurs in Pr 3+ /Yb 3+ co-doped glasses. In order to feed the ions to Yb 3+ : 2 F 5/2 level, there are two possible resonant energy transfer processes involved, ET1: Fig. 3d) [38][39][40] . Most of previous studies point out that the ET1 is more efficient than that of the cooperative energy transfer (ET2).
In the present work, the visible emission intensity decreases with increase of Yb 3+ ions concentration for a fixed concentration of Pr 3+ at 0.5 mol%. Generally, energy transfer in a pair of ions occurs due to resonant energy levels between donor and acceptor ions, or through phonon assistance. Also, the average distance between Pr and Yb ions is greatly influenced by the concentration of Yb 3+ . Therefore, based on previous assertions, the decrease of visible emission in co-doped systems is likely to be carried out by the CR mechanisms (ii & iii, see Fig. 3d) when the average distance between Pr-Yb is shorter than a critical distance for an efficient energy transfer www.nature.com/scientificreports/ ( ∼ 10 Å) 44 . The remarkable decrease of visible emission intensity maxima of 1 D 2 → 3 H 4 ( ∼ 604 nm) with respect to 3 P 0 → 3 H 6 emission ( ∼ 612 nm), is due to the competition between the above mentioned CR mechanisms. According to literature 45 , the expected experimental lifetime for 1 D 2 → 3 H 4 emission is equal to the 1 D 2 → 1 G 4 ( ∼ 1480 nm) emission lifetime values (see Table 2). Moreover, the energy transfer efficiency (η ET ) between Pr 3+ -Yb 3+46-48 and total quantum efficiency (η QE ) of ions excited to 3 P J levels are important parameters and can be expressed as follows, where, τ (Pr.xYb) and τ Pr (11.7 µ s) are the average lifetimes with and without Yb 3+ ions, respectively, and η Pr is set to be 1 38,39 . Table 2 reports the energy transfer efficiencies for the 1 D 2 → 1 G 4 transition. The η ET is increased from 1 % to 28 %, and η QE is increased from 100 to 128 % with increasing Yb 3+ ions. The η QE is an indicative of the ratio increase of emitted photons compared to the absorbed photons in function of the Yb 3+ concentration. Figure 3a also shows NIR luminescence around 2.0 µ m in 1850 2200 nm wavelength region, attributed to the Pr 3+ : 3 F 2 → 3 H 4 transition upon 443 nm excitation. We assume that the 3 F 2 and 3 H 6 multiplets are populated from 1 G 4 levels. As can be seen in Fig. 2a, the 3 H 4 → 1 G 4 absorption band has low intensity and quite low absorption cross-section which indicates that populating 1 G 4 level by direct excitation is not efficient. Therefore, sensitizing Pr 3+ with Yb 3+ is more efficient to populate 1 G 4 level via the emission of Yb 3+ : 2 F 5/2 → 2 F 7/2 which nicely overlaps the absorption of Pr 3+ 3 H 4 → 1 G 4 , indicating that the resonant energy transfer may occur as, ( 2 F 5/2 (Yb 3+ ) : 3 H 4 (Pr3+)) → ( 2 F 7/2 (Yb 3+ ) : 1 G 4 (Pr 3+ )). Considering the efficient QC in between Pr 3+ and Yb 3+ with increasing Yb 3+ ions, and resonant transfer of energy, the emission of 1 G 4 level should vanish. However, we could not neglect the back transfer of energy from Yb 3+ which could become more and more efficient inducing an increase of 1 G 4 population. In singly Pr 3+ doped glass, we could not detect emission from 1 G 4 level, but the Pr 3+ /Yb 3+ co-doped glass exhibit an emission around 1182 nm which corresponds to Pr 3+ : 1 G 4 → 3 H 4 transition (see inset of Fig. 3a). Therefore, we believe that the back transfer energy greatly contribute for the observed 2.0 µ m emission in co-doped samples. Unfortunately, the emission intensity of Pr 3+ ( 3 F 2 → 3 H 4 ) might transfer energy to OH − groups. The observed fluorescence decay curves for the 3 F 2 excited level are well fitted with single exponential function (Fig. 3c), indicating that there is no significant nonlinear energy transfer between Pr 3+ ions other than transfer of energy to OH quenching centers. This is confirmed by the decrease lifetime of the excited 3 F 2 level and the measured lifetime written as follows 49 , where A rad is the radiative decay rate, which is equal to the reciprocal of the decay rate in the absence of OH groups (1/τ 0 ). W mpr is the multiphonon decay rate, and W OH is the energy transfer rate between Pr 3+ and OH − . The gain performance at 1952 nm of the optimized glass can be evaluated through determination of the stimulated emission cross-section 12 , where p is the emission peak wavelength, c is velocity of light, n is refractive index, eff is the effective linewidth and τ m is the measured lifetime. Table 3 presents important spectroscopic parameters of Pr 3+ doped for several glasses. The σ e near 2.0 µm is one order (10 −19 ) higher than those of RE 3+ doped tellurite 50 , germanate 13 , and silicate 51 glasses. As it is known, materials which present large stimulated emission cross-section exhibit low threshold and high gain laser operation. In our case, the σ e and figure of merit ( σ e × τ m ) are relatively higher than the reported glasses in Table 3, suggesting that 0.5Pr 3+ /0.1Yb 3+ codoped PbO − Bi 2 O 3 − B 2 O 3 glass is a promising material for NIR broadband amplifiers. The wavelength dependent gain cross-section can be obtained as a function of population inversion and is written as 12 , G( ) = P σ e ( )-(1-P)σ a ( ), where P is the population inversion of Pr 3+ ions, absorption and emission cross-sections σ a ( ) & σ e ( ) derived from Beer-Lambert and  Figure 4 shows the gain cross-section of Pr 3+ : 3 F 2 → 3 H 4 transition as a function of population inversion (0-1) varied with an increment of 0.1. It can be seen that the gain becomes positive at P = 0.3 in the range of 2075-2300 nm, which means a low pump threshold of Pr 3+ : 3 F 2 → 3 H 4 laser operation. Also, the observed positive gain band becomes longer with the increase of P which is a characteristic of a quasi-three level system 51 .

Conclusions
In were used as raw materials, from which nominal batches of 10 g were prepared and mixed in an agate mortar. Then, the mixture was melted in porcelain crucible at 1050 • C in air for 1 h 30 min and melt was poured into stainless steel moulds. The obtained glass samples were cut and polished for optical characterization. Differential Scanning Calorimetry was performed with NETZSCH DSC 404F3 with heating rate of 10 • C/ min in order to determine the glass transition (T g ) and crystallization temperature (T x ) of the glass samples. Raman spectra were recorded with a Renishaw inVia spectrometer coupled with a Leica DM2700 microscope with 633 nm laser excitation. Optical absorption spectra of the glass were recorded on UV-2500 (SHIMADZU) and NIR (BRUKER MPA-Multi Purpose Analyzer) spectrophotometers. Luminescence measurements were performed on Florolog3-iHR HORIBA fluorescence spectrometer upon 443 nm excitation. Density of the glass samples were estimated with distilled water as immersion liquid by Archimedes' method. All the measurements were conducted at room temperature. Tellurite (Ho/Tm) 50 9.33 × 10 −20 3.29 × 10 −3 3.07 × 10 −23 Germanate (Yb/Tm) 13 6.90 × 10 −20 1.04 × 10 −3 0.72 × 10 −23 Silicate (Tm) 51 3.60 × 10 −20 7.91 × 10 −3 2.84 × 10 −23 www.nature.com/scientificreports/

Data availability
The data sets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.