Intensity of optical transitions of Er 3+ in Yb 3 Al 5 O 12 crystal M. Kaczkan a , M. Borowska a , K. Kołodziejak b , T. Łukasiewicz b , M. Malinowski a,b, * a Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland b Institute of Electronic Materials Technology, Wo ´ lczyn ´ ska 133, 01-919 Warsaw, Poland Available online 26 March 2007 Abstract An analysis of the optical transitions in Er 3+ ions located in Yb 3 Al 5 O 12 (YbAG) crystal is given. The room temperature absorption spectrum from the UV to IR wavelength region have been measured and discussed. The intensity parameters of Er 3+ in investigated host were evaluated using the Judd–Ofelt theory to be X 2 = 0.27 · 10 20 , X 4 = 0.90 · 10 20 , X 6 = 1.16 · 10 20 cm 2 and compared with the values for the YAG:Er 3+ and other erbium doped laser materials. Based on the obtained results the radiative transition probability, branching ratios and radiative lifetimes of the excited states of Er 3+ in YbAG were calculated. Ó 2007 Elsevier B.V. All rights reserved. PACS: 78.20.e; 78.40.q 1. Introduction Because of the energy level structure erbium-doped solid-state laser materials are attractive as both infra-red and a short wavelength laser materials. Recently, an infra-red wavelengths pumped green laser have been dem- onstrated in Er 3+ activated LiYF 4 , KYF 4 and Y 3 Al 5 O 12 [1–3]. In 1992 Allain et al. [4] demonstrated continuous wave (cw) green upconversion lasing in Er 3+ doped fluoro- zirconate glass fiber pumped by infra-red radiation. Very recently, the Er 3+ :ZBLAN fiber upconversion laser has been examined in detail by Kaczmarek et al. [5] and obser- vations of photon avalanche in this erbium system were reported [6,7]. Recent investigations showed a potential for generating at shorter wavelengths in the blue and violet range in Er 3+ fluoride hosts [8,9]. As the absorption of erbium ions in the near IR range ( 4 I 11/2 level is pumped around 980 nm and 4 I 9/2 is pumped around 810 nm) is impractically low, the ytterbium (Yb 3+ ) ions are often used as sensitizers to increase the efficiency of pumping. Ytterbium offers the advantage of high absorp- tion cross-section and a good spectral overlap of its emis- sion with the erbium 4 I 11/2 absorption leading to an efficient energy transfer from ytterbium to erbium. This two-step pumping process effectively increases efficiency of the upconversion. The pump energy is transferred non- radiatively from Yb 3+ ions to Er 3+ ions, which then emit around 550 lm [10,11]. The main goal of the studies reported here is to investi- gate the intensity of the optical transitions of Er 3+ in YbAG host. This crystal is very promising material in which every erbium ion has ytterbium ions as the nearest neighbors, so the effect of the Yb–Er interaction on the behavior of Er 3+ ion is expected to be strong. The knowl- edge about spectroscopic properties of YbAG:Er 3+ system is limited only to one paper of Pacheco and Di Bartolo [12]. 2. Results and discussion 2.1. Experimental Single crystals of YbAG doped by 1.5 at.% of Er 3+ have been investigated. The sample was grown by the Czochral- ski technique in the Institute of Electronic Materials Tech- nology (ITME) in Warsaw. The starting materials, Yb 2 O 3 (99.999%), Al 2 O 3 (99.999%) and Er 2 O 3 (99.999%), were 0925-3467/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.optmat.2007.02.014 * Corresponding author. Address: Institute of Microelectronics and Optoelectronics, Koszykowa 75, 00-662 Warsaw, Poland. E-mail address: m.malinowski@elka.pw.edu.pl (M. Malinowski). www.elsevier.com/locate/optmat Available online at www.sciencedirect.com Optical Materials 30 (2008) 703–706