Incorporation of Yb 3+ ions in multicomponent phase-separated fibre glass preforms C.I. Oppo a , R. Corpino a , P.C. Ricci a , M.C. Paul b , S. Das b , M. Pal b , S.K. Bhadra b , S. Yoo c , M.P. Kalita c , A.J. Boyland c , J.K. Sahu c , P. Ghigna d , F. d’Acapito e,⇑ a Department of Physics, University of Cagliari, Cagliari, Italy b Fibre Optics and Photonics Division, Central Glass and Ceramic Research Institute, 196, Raja S.C. Mullick Road, Kolkata 700 032, India c Optoelectronic Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom d Università di Pavia, Dipartimento di Chimica, Viale Taramelli 13, 27100 Pavia, Italy e CNR-IOM-OGG c/o ESRF GILDA CRG, BP220, F-38043 Grenoble, France article info Article history: Received 16 June 2011 Received in revised form 21 September 2011 Accepted 21 September 2011 Available online 21 October 2011 Keywords: Optic fibre Ytterbium Glass-ceramics EXAFS Photo-luminescence abstract The local environment around Yb 3+ ions in silica-based optic fibre preforms has been studied in speci- mens with different composition and submitted to different annealing processes. The formation upon annealing of nano-sized particles of YbPO 4 has been evidenced by X-ray Absorption Spectroscopy whereas optical luminescence present Stark-split states in the anti-Stokes lines, typical of a structurally ordered phase. This evidences the preferential location of Yb in highly coordinated phosphate environ- ment, with remarkable effects of such ordered phase on the optical response of the system. This study confirms and extends analogous observations done in previous literature on Erbium in similar glasses. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction In the development of optical devices based on Rare Earth (RE) ions, the local environment around the RE is of paramount impor- tance for determining the optical properties. Materials based on RE-doped modified silica glass suffer of the limit that RE ions tend to cluster in the matrix with a consequent deterioration of their luminescent properties due to the concentration quenching mech- anism [1]. A way to overcome this has been found by adding small quantities of Al 3+ or P 5+ in the silicate matrix [1,2]. These dopants have the effect of dispersing the REs and avoiding their clustering even if a different dispersion behavior is observed in the two cases. The addition of P leads to a preferential link of these ions with the RE which is absent in Al-doped glasses as reported by pulsed Elec- tron Paramagnetic Resonance–electron spin echo [3]. X-ray Absorption Spectroscopy (XAS) has evidenced a moderate coupling of REs with Al in silicate glasses [4,5] whereas a stronger affinity between P and RE, via formation of locally ordered nuclei of ErPO 4 , has been reported in germano-silicate glasses [6]. In addition, a very recent study [7] evidenced how these nuclei can exhibit lumi- nescence properties typical of RE-doped crystals, in particular pre- senting Stark splitting of the luminescence lines. This spontaneous phenomenon of creation of nanometer-sized crystals in glass is of great interest in glass technology. The delib- erated insertion of REs in nanocrystalline structures offers promis- ing perspectives in terms of resolution and luminescence efficiency [8]. As an example, Yb 3+ :Y 2 O 3 ceramic has been demonstrated to be an efficient gain medium for laser applications [9]. The realization of highly transparent glass ceramic optical waveguides (i.e. glass guides containing ceramic nanoclusters) has been demonstrated to be possible if particular conditions about particle size (<15 nm) and interparticle spacing (same order as particle size), among others, are fulfilled [10]. The present investigation was conducted as an extension of the previous investigations on Er [6,7] demonstrating that the sponta- neous formation of phosphate nuclei is possible also for Yb and that it leads to observable crystal-field effects on the luminescence properties of the material. 2. Experimental details 2.1. Sample preparation Yb-doped yttria-alumina-silica fibre preforms were prepared via the solution doping process along with modified chemical va- pour deposition process (MCVD) followed by a suitable thermal treatment. To generate Yb-doped micro- or nano-crystallites into the core matrix of optical fibre preform, a minor amount of P 2 O 5 0925-3467/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2011.09.010 ⇑ Corresponding author. E-mail address: dacapito@esrf.fr (F. d’Acapito). Optical Materials 34 (2012) 660–664 Contents lists available at SciVerse ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat