1. Time resolved fluorescence and energy transfer analysis of Nd3+-Yb3+-Er3+ triply-doped Ba-Al-metaphosphate glasses for an eye safe emission (1.54 µm)

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1. Time resolved fluorescence and energy transfer analysis of Nd3+-Yb3+-Er3+ triply-doped Ba-Al-metaphosphate glasses for an eye safe emission (1.54 µm)

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  * Corresponding author : Tel.: +91-33 2473 3469; Fax: +91-33 2473 0957 Email : glasslab42@hotmail.com (K. Annapurna) Time resolved spectra and energy transfer analysis of Nd 3+ -Yb 3+ -Er 3+  triply-doped Ba-Al-metaphosphate glasses for an eye safe emission (1.54 µµµµ m)  Atul D. Sontakke, Kaushik Biswas, Ashis K. Mandal, K. Annapurna* Glass Technology Laboratory, Central Glass and Ceramic Research Institute (Council of Scientific and Industrial Research) 196, Raja S.C. Mullick Road, Kolkata 700032, India Abstract  This paper reports on the development and systematic analysis of energy transfer mechanisms in Nd 3+ -Yb 3+ -Er 3+  co-doped new series of barium-alumino-metaphosphate glasses. The time resolved fluorescence spectra of Nd 3+  in triply doped Ba-Al-metaphosaphte glasses have revealed that, Yb 3+  ions could function as quite efficient bridge for an energy transfer between Nd 3+  and Er 3+  ions. As a result, a fourfold emission enhancement at 1.54 µ m of Er 3+ ions has been achieved through an excitation of 4 F 5/2  level of Nd 3+  at 806 nm for the glass having 3 mol% Yb 3+  with an energy transfer efficiency reaching up to 94%. Decay of donor (Nd 3+ )   ion fluorescence has been analyzed based on theoretical models such as Inokuti-Hirayama, Burshtein (migration) and Yokota-Tanimoto (diffusion) and corresponding energy transfer parameters have been discussed. Primarily, electrostatic dipole-dipole (s ~ 6) interactions are found to be responsible for the occurrence of energy transfer process in theses glasses.  Key words:  Metaphosphate glasses, energy transfer, fluorescence, sensitized Er 3+  emission.   2 Introduction Sensitized emission occurrence from the lanthanide ions (Ln 3+ ) has attracted significant attention and importance. The energy transfer efficiency among donor-acceptor ions depends on overlapping of donor’s emission with that of activator’s absorption and their inter-ionic distances. Hence, heavily doped solid-state gain media including laser crystals, glasses and rare earth doped fibers have recently been considered more relevant for energy transfer luminescence studies. A great deal of work has been carried out in exploring the energy transfer amongst lanthanides (RE 3+   →  RE 3+ ) as well as transition metal ions with lanthanides (TM →  RE 3+ ) ions in the visible (VIS) region [1-3]. The main interest of those works has been focused on rare earth ions like Eu 3+ , Tb 3+ , which could be sensitized by UV absorbing Ce 3+ , Gd 3+  ions for their potential use in lighting, display and dosimetric applications [4, 5]. With the emergence of laser diodes, the interest has been extended towards the NIR emitting ions like Pr 3+ , Nd 3+ , Tm 3+ , Ho 3+ , Er 3+ and Yb 3+  etc. NIR emission plays an important role in optical communications, biomedical applications and lasers [6, 7] such as Nd 3+  based laser systems, which are well known for their high power applications [8]. However, for certain singly doped ions, like Yb 3+  based lasers, various technical problems arise for high power operation; since the energy difference between excitation and emission is very small due to its two level configurations [9]. Some attempts have been made earlier in finding a suitable glass host so as to bring in a wider separation from the Stark components of lower energy states of Yb 3+ resulting in with a quasi-three level system [10]. Yet another good solution to overcome this difficulty is to sensitize it with Nd 3+  ion so as to achieve an efficient lasing at 980 nm from Yb 3+  on exciting the Nd 3+  ion with easily available high power 800nm   3laser diode [11, 12]. Beside its lasing performance, Yb 3+  serves as good sensitizer for several lanthanides (Pr 3+ , Ho 3+ , Tm 3+ , Er 3+  etc.) because of its high absorption and emission cross-section in combination with higher allowed doping concentrations. Its sensitization is based on either resonant energy transfer (RET) or phonon-assisted energy transfer (PAET) for improved NIR emission and energy transfer upconversion (ETU) for visible emissions from activator ions [13, 14]. Among these, Er 3+  possesses special interest due to its NIR emission at eye safe wavelength (1.53 µ m). In addition, it has been recognized as one of the most efficient rare earth ions to be used in optical communication and range finder applications since its emission coincides with the third communication/atmospheric window (1.525 – 1.565 µ m) [15]. For Nd 3+ -Yb 3+  or Yb 3+ -Er 3+  systems, the energy transfer is more or less resonant since the emission of sensitizer overlaps with absorption of activator and such systems have been widely studied. However, very few reports are available on Nd 3+  sensitization to Er 3+  ions [16]. Moreover, in this system it was found that the spectral energy mismatch of 1150 cm -1  does exist between donor (Nd 3+ ) and acceptor (Er 3+ ) and hence phonon assisted energy transfer may be responsible for this process. Since Yb 3+  ions can act as activators for Nd 3+  and very efficient sensitizers for Er 3+ , it can be expected to achieve improved energy transfer from Nd 3+  to Er 3+  in the presence of Yb 3+  as bridging ions. An attempt has been made to examine the Nd 3+   →  Er 3+  energy transfer with the presence and absence of Yb 3+  ions in alkali free Barium-alumino-metaphosphate glass host. Metaphosphate glasses are advantageous over silicate and other host glasses for their high rare earth ion doping concentration without considerable fluorescence   4quenching along with possessing low melting temperature and several favorable spectroscopic properties including high emission cross-sections and longer fluorescence lifetime of active ions and hence have been used in the development of high power solid-state lasers [17]. Among them, sodium phosphate, barium phosphate, alumino-metaphosphate and potassium-alumino-metaphosphate glasses have been studied widely [18-21]. In addition to these several other metaphosphate glasses including Zn- or Pb-metaphosphate have also been investigated for their optical and spectroscopic properties in the presence of active ions [22]. However, to our knowledge, there are no reports so far on alkali free barium-alumino-metaphosphate glass system in the literature. Hence, in the present work our main objective is to prepare a new series of alkali free barium-alumino-metaphosphate glasses containing mono, bi and tri rare earth ions (Nd 3+ -Yb 3+ -Er 3+ ) and to examine the intermediate Yb 3+  ion influence on the sensitization efficiency of Nd 3+  for NIR emission from Er 3+  ions. The energy transfer mechanism between Nd 3+  and Er 3+  ions in the presence and absence of Yb 3+  has been systematically analyzed from the measurement of photoluminescence spectra and the time resolved decay profiles of these new series of optical glass systems by employing theoretical models. Experimental Study  The glasses in the following chemical compositions (in mole %) were developed by employing melt quenching method: 1. BAP-Nd : 11.60Al 2 O 3 -20.73BaO-55.54P 2 O 5 -6.72SiO 2 -3.86B 2 O 3 -0.5Nb 2 O 3 -1.05Nd 2 O 3     52. BAP-Er : 11.60Al 2 O 3 -20.73BaO-55.54P 2 O 5 -6.72SiO 2 -3.86B 2 O 3 -0.5Nb 2 O 3 -1.05Er 2 O 3  3. BAP-NdEr : 11.47Al 2 O 3 -20.51BaO-54.95P 2 O 5 -6.64SiO 2 -3.83B 2 O 3 -0.5Nb 2 O 3 - 1.05Nd 2 O 3 -1.05Er 2 O 3  4. BAP-NdYb : 11.48Al 2 O 3 -20.52BaO-54.97P 2 O 3 -6.65SiO 2 -3.83B 2 O 3 -0.5Nb 2 O 3 - 1.05Nd 2 O 3 -1.0Yb 2 O 3  5. BAP-Yb05 : 11.42Al 2 O 3 -20.41BaO-54.66P 2 O 3 -6.61SiO 2 -3.80B 2 O 3 -0.5Nb 2 O 3 - 1.05Nd 2 O 3 -1.05Er 2 O 3 -0.5Yb 2 O 3  6. BAP-Yb10 : 11.36Al 2 O 3 -20.30BaO-54.39P 2 O 3 -6.58SiO 2 -3.78B 2 O 3 -0.49Nb 2 O 3 - 1.05Nd 2 O 3 -1.05Er 2 O 3 -1.0Yb 2 O 3  7. BAP-Yb20 : 11.24Al 2 O 3 -20.10BaO-53.81P 2 O 3 -6.51SiO 2 -3.75B 2 O 3 -0.49Nb 2 O 3 - 1.05Nd 2 O 3 -1.05Er 2 O 3 -2.0Yb 2 O 3  8. BAP-Yb30 : 11.12Al 2 O 3 -19.88BaO-53.27P 2 O 3 -6.44SiO 2 -3.71B 2 O 3 -0.48Nb 2 O 3 - 1.05Nd 2 O 3 -1.05Er 2 O 3 -3.0Yb 2 O 3  Reagent grade metaphosphate chemicals such as Ba(PO 3 ) 2  and Al(PO 3 ) 3  and high purity rare earth oxides, Nd 2 O 3 , Er 2 O 3  and Yb 2 O 3  with purity 99.99% from Alpha-Aesar were used as raw materials for glass preparation. Special precautions were taken in controlling the hydroxyl ion (OH - ) contents in prepared glasses by sintering the batches and maintaining the relative atmospheric humidity (RH) below 40%. Thus thoroughly mixed chemical batches were sintered at 350°C for 6 h to reduce the surface absorbed moisture and to make pre-reacted batch. Each sintered batch was then melted at 1350°C in silica crucibles for 1 h with intermittent stirrings to get homogeneity and later casted them onto preheated graphite molds. The glass samples thus obtained were kept for annealing at 550°C to relieve thermal stresses and cooled slowly to room temperature in a precise temperature controlled annealing furnace. Such annealed glasses were cut and polished in the form of plates in the dimensions of 15 x 20 x 2 mm 3  for optical characterizations. The densities of all glasses were measured by employing the Archimedes buoyancy principle using water as buoyancy liquid. Refractive indices of glasses were
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