Modification of the spontaneous emission lifetime of Tb 3+ in a binary glass G. Manoj Kumar a, * , D. Narayana Rao b a Advanced Center of Research in High Energy Materials, University of Hyderabad, Hyderabad 500 046, India b School of Physics, University of Hyderabad, Hyderabad 500 046, India article info Article history: Available online 28 November 2008 PACS: 42.65.Pc 32.70.Cs 42.70.Ce Keywords: Glasses Lifetimes Local field effects abstract Spontaneous emission (SE) lifetimes of Tb 3+ are modified in a binary glass – PbO–B 2 O 3 , by carefully con- trolling the refractive index. The results are analyzed in the framework of local field in view of the real and virtual cavity models. The analysis of lifetimes as a function of real refractive index yielded a free space lifetime of 5.68 ms. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Rare earth doped materials in general and glasses in particular have been a subject of many investigations. Tb 3+ doped materials are of particular interest because of their use in varied fields of photonics such as display applications [1]. The long spontaneous emission (SE) lifetimes and narrow spectral widths of the green emission and high quantum efficiency make Tb 3+ based molecules an ideal choice for biological probes [2]. In display applications, Tb 3+ is used for its green emission. The ability to control the SE life- times is vital in any of these applications. The SE lifetimes can be controlled precisely by manipulating the refractive index of the host medium [3–5]. Historically, the subject of spontaneous emis- sion has fascinated many researchers and SE has played a very important role in establishing the concepts of the modern quan- tum theory. Einstein demonstrated that an excited atom must un- dergo SE if the radiation and the matter are to achieve thermal equilibrium [6]. The SE lifetimes depend on the environment through radiative and nonradiative processes. The radiative rate of the emitting ion depends on the refractive index of the material in which the ion is embedded. Rare earths are an ideal choice to conduct these kinds of studies as the intra 4f–4f transitions which are responsible for the strong fluorescence are well shielded from the surroundings. Also, the ls–ms SE lifetimes make it easy for the experiment. Much of the earlier work on the subject of modi- fication of lifetimes vis-à-vis the refractive index of the host, has been on Eu 3+ – in thin films [7,8], Europium complexes in solutions [9,10], Eu 3+ in liquids [11], gases [12] and solids [13,14]. In a recent study, the lifetimes of 4 I 13/2 of Er 3+ doped in silica–hafnia planar waveguides have been studied [15]. The lowest of the excited state to the highest of the ground state energy difference is about 12,500 cm 1 for Eu 3+ whereas for Tb 3+ it is 15,000 cm 1 . The high- est phonon energy of the PbO–B 2 O 3 glass is around 1300 cm 1 . This implies that the probability of nonradiative decay for Tb 3+ to glass is smaller compared to Eu 3+ , which makes it more attractive ion for this kind of experiment. The study of the SE lifetimes of Tb 3+ is not only interesting from a fundamental view point but also for its po- tential application. In this communication, we report the study of the modification of spontaneous emission lifetimes of Tb 3+ in a bin- ary glass matrix. The usual local field treatment of the lifetimes of an emitter embedded in a dielectric is based on the assumption of a cavity around the atom. Two distinct models have been proposed in liter- ature, the real and the virtual cavities. The virtual cavity is filled with the same dielectric as that of the surroundings and in the real cavity it is empty. The SE lifetimes for the real and virtual cavities for a transparent dielectric is given by [16,17] sðnÞ¼ 1 n 3n 2 2n 2 þ 1   2 sð0Þ ð1Þ sðnÞ¼ 1 n n 2 þ 2 3   2 sð0Þ ð2Þ 0925-3467/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2008.10.010 * Corresponding author. Tel.: +91 40 23134402; fax: +91 40 23012800. E-mail address: manojsp@uohyd.ernet.in (G.M. Kumar). Optical Materials 31 (2009) 1343–1345 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat