Inhomogeneous broadening of optical transitions in Yb:CaYAlO 4 P.O. Petit * , J. Petit, Ph. Goldner, B. Viana LCMCP, UMR-CNRS 7574, ENSCP, 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France Available online 28 June 2007 Abstract Yb:CaGdAlO 4 is a promising laser media for femtosecond impulsions in the infrared, because of a broad emission line around 1 lm. This feature is related to the crystal structure where the doping ion, which substitutes Gd 3+ , is located in a disordered environment where Ca 2+ and Gd 3+ are randomly distributed on a same crystallographic site. Since Gd 3+ ions prevent any EPR measurement to be carried out, CaYAlO 4 , which is isostructural to CaGdAlO 4 , has been studied, in the present work. The origin of the broadening of the optical absorption and emission lines has been studied by low temperature optical spectroscopy on CaYAlO 4 . Two families of sites have been highlighted and are related to the broadening of optical transitions. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction CaGdAlO 4 is a recently developed laser media. It was first doped with Nd 3+ , Er 3+ and Ti 3+ [1–3] in the late nine- ties. The main interest of the material was then to be a so-call disordered matrix resulting in broad absorption and emission properties. The former was favourable to cover the drift of the laser diode when the temperature var- ies. More recently [4,5], in order to develop new infrared ytterbium laser, Yb 3+ :CaGdAlO 4 has been proposed. A quantum defect as low as 0.8% was obtained which repre- sents, to the best of our knowledge, the lowest quantum defect obtained in Yb-doped hosts [5]. In addition, ultra- short impulsions of 47 fs were obtained by taking advantage of the broad emission between 1030 and 1080 nm. [4] In addition, the thermal conductivity of this aluminate host is very high. Along the c-axis, k c equals 6.3 W m À1 K À1 and along a-axis, k a equals 6.9 W m À1 K À1 . [5] These features make this material an outstanding laser as far as the optical and thermo mechanical properties are considered. CaGdAlO 4 has a corundum structure with a quadratic cell. The dimensions of the unit cell are, respectively, a = 3.66 A ˚ and c = 12.01 A ˚ . There is only one site for the Yb 3+ cation which is the site statistically occupied by Ca 2+ and Gd 3+ in the CaGdAlO 4 structure. The site point symmetry is C 4v . Due to the weak difference between the atomic radius values of Ca 2+ and Gd 3+ [6] (1.18 A ˚ and 1.11 A ˚ , respectively), it is indeed clear that a disordered occupation of the sites is expected [1]. We analyzed the emission spectra of Yb 3+ :CaGdAlO 4 in the near infrared range by low temperature selective optical spectroscopy and electron paramagnetic resonance. But as the occurrence of Gd 3+ in CaGdAlO 4 crystal leads to broad and unresolved EPR bands, measurements were per- formed on Yb 3+ :CaYAlO 4 which has the same structure and similar optical properties [7]. Therefore, the interpreta- tion for the disordered cationic distribution presented in this paper is supposed to be comparable in both compounds. 2. Experimental CaCO 3 , Al 2 O 3 and Yb 2 O 3 (3 N purity) and Y 2 O 3 (4 N purity) were used as starting materials for the compound elaboration. Crystals with two ytterbium concentrations (CaY 1Àx Yb x AlO 4 with x equals 0.1% and 1%) were grown by optical floating zone method under air atmo- sphere. The corresponding ytterbium concentrations are 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.05.017 * Corresponding author. E-mail address: pierre-olivier-petit@enscp.fr (P.O. Petit). www.elsevier.com/locate/optmat Available online at www.sciencedirect.com Optical Materials 30 (2008) 1093–1097