Blue–green cooperative upconverted luminescence and radiative energy transfer in Yb 3 þ -doped tungsten tellurite glass P. Babu a , I.R. Martín b , G. Venkataiah c , V. Venkatramu d , V. Lavín b , C.K. Jayasankar c,n a Department of Physics, Government Degree College, Satyavedu 517 588, India b Departamento de Physica, MALTA Consolider Team, Instituto Universitario de Materiales y Nanotecnología, and Instituto Universitario de Estudios Avanzados en Atómica, Molecular y Fotónica, Universidad de La Laguna, 38200 San Cristobal de La Laguna, Santa Cruz de Tenerife, Spain c Department of Physics, Sri Venkateswara University, Tirupati 517 502, India d Department of Physics, Yogi Vemana University, Kadapa 516 003, India article info Article history: Received 6 June 2015 Received in revised form 23 August 2015 Accepted 26 August 2015 Available online 10 September 2015 Keywords: Tellurite glass Yb 3 þ ions Cooperative luminescence Near-infrared emission Energy transfer abstract Ytterbium-doped tungsten tellurite glasses have been prepared and studied their cooperative upcon- verted luminescence and radiative energy transfer properties. In a 3.0 mol% Yb 2 O 3 -doped glass, near- infrared emission band is peaked at around 977 nm with a full width at half maximum of around 15 nm. This glass emits blue–green upconverted emission under 980 nm excitation due to cooperative processes involving two interacting Yb 3 þ ions. The upconverted emission band is centered at around 502 nm with a bandwidth of around 45 nm. Power dependence of cooperative emission intensity and the temporal evolutions of the near-infrared and blue–green emissions confirm the presence of cooperative lumi- nescence. Photoluminescence and lifetimes have been measured by moving the laser excitation from one edge of the sample and found that radiative energy transfer is predominant in 3.0 mol% of Yb 2 O 3 -doped glass. The absorption coefficient obtained from absorption spectrum is in good agreement with that obtained by fitting the curve of luminescence intensity versus distance from the edge of the sample. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Tellurite based glasses are of scientific and technical interest due to their unique properties such as relatively low phonon energy, high refractive index, good infrared transmittivity, low glass transition and melting temperatures and good thermal and chemical stabilities [1]. On the other side, Ytterbium (Yb 3 þ )-doped materials have potential technological applications since they may generate tunable lasers in the infrared region from 920 to 1060 nm and visible blue–green emission at about 500 nm by means of cooperative effects [2]. There are only two manifolds in the Yb 3 þ energy level scheme, the 2 F 7/2 ground state and the 2 F 5/2 excited state multiplets. Thus the lack of intermediate levels and the large separation between the excited and the ground state manifolds significantly reduces nonradiative decays. Cooperative luminescence (CL) is a process in which two interacting Yb 3 þ ions in the 2 F 5/2 excited state decay simulta- neously to the ground state, emitting one photon at twice the energy of single ion transition. It is a special type of electronic transition occuring in a spectral region where the individual ions do not have absorption or emission transitions. It was first observed by Nakazawa and Shionoya in 1970 for Yb 3 þ in YbPO 4 crystalline powder [3]. Since then there were many studies on CL of Yb 3 þ ions in crystals [4–6], glasses [2,7,8], glass-ceramics [9–11] and phosphors [12–14]. The CL depends on phonon energy of the host, Yb 3 þ ions inter-ionic distance, edge of the transmission window in the visible region and laser power employed for exci- tation. The blue–green CL around 500 nm of Yb 3 þ ions has a considerable advantage over the similar emission of Tm 3 þ ions for 3-D display technology as the Yb 3 þ ions require only one laser pumping beam in the near-infrared (NIR) region [11]. In addition, the CL finds potential applications in scintillators, optical bist- ability, planar lasers for optical devices in telecommunications and as structural probe in solids [2]. Radiative energy transfer or luminescence self trapping, first observed for Yb 3 þ ions in glasses [15] and theoretically studied by Auzel et al. [16], is a process in which photons that are sponta- neously emitted from the metastable level are trapped by reab- sorption by other rare earth ions in the ground state. These excited state ions then relax by spontaneously emitting more photons which are again reabsorbed and the entire process is repeated. This leads to increase in fluorescence lifetime, as measured over the volume of the sample, relative to the lifetime of a single Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2015.08.052 0022-2313/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: ckjaya@yahoo.com (C.K. Jayasankar). Journal of Luminescence 169 (2016) 233–237