Journal of Luminescence 109 (2004) 129–133 Temperature dependence on the cross-relaxation rates in Tm 3þ doped strontium fluorapatite Lester J. Richardson a , Carl E. Bonner Jr. a, *, Jasper Lewis a , George B. Loutts a , Waldo J. Rodriguez b , Brian M. Walsh b a Center for Materials Research, Norfolk State University, 700 Park Avenue, Norfolk, VA 23504, USA b NASA Langley Research Center, Hampton, VA 23681, USA Received 18 April 2003; received in revised form 31 December 2003; accepted 3 February 2004 Abstract The cross-relaxation dynamics of Tm 3þ -doped strontium fluorapatites (S-FAP) has been examined using time- resolved fluorescence spectroscopy. The temperature dependence of the fluorescence lifetime crystal suggests a dipole– dipole interaction between the Tm 3þ ions and the energy is transported by an incoherent hopping mechanism. The temperature dependence of the fluorescence lifetime suggests this incoherent hopping mechanism is used to bridge the energy mismatch between emission from the donor and the energy required for absorption by the acceptors. Using the energy transfer model of Agranovich, the cross-relaxation rate was determined from the emission decay dynamics of the 3 H 4 - 3 H 6 transition. The observed cross-relaxation rate ranged from 11 s 1 at 10 K to 81 s 1 at 200 K for 0:21 at% Tm 3þ in S-FAP an energy barrier of 44:3 cm 1 : The model suggest that rates of 10 6 ion hops s 1 for 1 at% Tm 3þ in S-FAP could be achieved, rendering this material a potentially efficient laser medium. r 2004 Elsevier B.V. All rights reserved. PACS: 33.20; 42.70.Hj; 78.40.q Keywords: Absorption spectra; Emission spectra; Laser materials; Thulium ions; Fluorapatites; Time-resolved emission The introduction of doubly resonant ions such as Tm 3þ and Pr 3þ into crystals such as YAG and YLF has lead to a number of different laser crystals that take full advantage of the ease at which the materials achieve population inversion through a process known as cross-relaxation [1–7]. Cross-relaxation is an energy transfer process that occurs when one excited state ion transfers some of its energy to a nearby ion leaving both ions in some excited state [8–11]. The process has been partially described by the Dexter energy transfer theory [12], though a more complete description has been provided by Inokuti et al. [13]. The rate of cross-relaxation depends on two conditions. The first condition is the ratio of donors to acceptors. An excess of acceptors results in direct energy transfer from donor to acceptor. With an excess of donors, energy is transferred primarily by migration or hopping between donors until an acceptor is found. The second condition affecting the rate of cross-relaxation is the temperature of the crystalline environment. Cross-relaxation is a phonon-assisted process. Phonons can provide the ARTICLE IN PRESS *Corresponding author. Tel.: +1-757-823-2097. E-mail address: cebonner@nsu.edu (C.E. Bonner Jr.). 0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.02.002