Characterization of Yb:YAG ceramics as laser media Daniele Alderighi a,⇑ , Angela Pirri a , Guido Toci a , Matteo Vannini a , Laura Esposito b , Anna Luisa Costa b , Andreana Piancastelli b , Marina Serantoni b a IFAC-CNR Istituto di Fisica Applicata ‘‘Carrara”, Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy b ISTEC-CNR, Istituto di Scienza e Tecnologia dei Materiali Ceramici, Via Granarolo 64, 48018 Faenza (RA), Italy article info Article history: Received 28 May 2010 Received in revised form 24 August 2010 Accepted 27 August 2010 Available online 24 September 2010 Keywords: Laser ceramics Solid-state laser Yb Ytterbium Laser crystal YAG abstract We report the preparation and characterization of 9.8 at.% Yb 3+ doped YAG polycrystalline ceramics for laser applications. Reactive sintering of commercial powders in a clean atmosphere and under high vac- uum has been used to achieve the YAG phase. The selected experimental conditions for the powder treat- ment, shaping and sintering are described and their influence on the optical quality of the obtained samples has been discussed. Microstructural, optical and laser characterization of the ceramics have been performed. In particular the influence of the pre-sintering (calcination) and sintering cycles has been investigated by laser characterization allowing to find unexpected loss mechanisms that cannot be revealed by standard optical characterization. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Ytterbium activated crystals have received an increasing inter- est in the last years as efficient active media for solid-state lasers. The simple laser energy level scheme of the active ion Yb 3+ consists of only two manifolds, 2 F 5/2 and 2 F 7/2 , resulting in a small quantum defect and allowing an efficient optical pumping by commercial diode sources. Moreover the Ytterbium activated materials show a broad emis- sion spectrum, enabling a wide range of tunability and/or the gen- eration of short pulses in the fs regime and a long life-time of the upper manifold (1 ms) that allows for efficient energy storage. All these characteristics resulted in a very wide spectrum of inter- esting applications. At the state of the art, the best performance of solid-state lasers in the near infrared based on single crystal or glasses have been obtained by using Yb-doped fiber lasers. With these devices CW power levels exceeding 1 kW [1,2] have been demonstrated. On the other hand, fiberlasers undergo fundamental limitations for the operation in high energy, high peak power and ns duration laser pulses regimes that are required in many indus- trial and scientific applications. In fact the generation of few nano- second pulses is limited to energies of few mJ [3]. High peak power and high energy pulses generation is therefore exclusive domain of lasers based on bulk crystalline and ceramic materials. In particular, with respect to the monocrystalline coun- terparts the ceramics allow lower costs of fabrication, larger sizes up to several cm and higher flexibility in the engineering of the dop- ant distribution (such as dopant gradients) opening the possibility of co-doping and co-sintering techniques. These techniques can be usefully exploited to improve the management of the thermal load and stresses in lasing materials, which are among the main limiting factors for the achievement of high average power levels. In order to enable laser operation a ceramic material must exhi- bit a very low concentration of defects as secondary phases, grain boundaries and residual pores [4,5]. The strict control of the stoichi- ometry is mandatory to avoid the formation of secondary phases, whereas residual pores need to be below 150 ppm [6,7]. In order to fulfil these requirements, powders need to be nanometric or at least sub-micrometric in size and extremely pure. On the other hand, nanometric powders tend to aggregate during the shaping phase leading to poor, not homogeneous packing and to formation of residual pores during sintering [8]. Very fine powders are also dif- ficult to handle and tend to absorb water on the surface. Finally, the powder manipulation easily introduces impurities. All these features have to be fully controlled in order to avoid formation of scattering centers which decrease transparency of the material [9–14]. 2. Experimental procedure 2.1. The ceramic process Characteristics of the selected commercial powders are re- ported in Table 1, and the morphology is shown in Fig. 1. The 0925-3467/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2010.08.022 ⇑ Corresponding author. Tel.: +39 055 522 5318; fax: +39 055 522 5305. E-mail address: D.Alderighi@ifac.cnr.it (D. Alderighi). Optical Materials 33 (2010) 205–210 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat