Crystal growth and spectroscopic studies of novel Yb-doped K 5 Nd(MoO 4 ) 4 single crystals A. Majchrowski a , M. Swirkowicz b , L. Jaroszewicz a , M. Piasecki c , I.V. Kityk d , M.G. Brik e, ⁎ a Institute of Applied Physics, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland b Institute of Electronic Materials Technology, Wolczynska 133, 01-919 Warsaw, Poland c Institute of Physics, J. Dlugosz University, Al. Armii Krajowej 13/15, Częstochowa, Poland d Electrical Engineering Department, Czestochowa Technological University, Al. Armii Krajowej 17, Czestochowa, Poland e Institute of Physics, University of Tartu, Riia 142, Tartu 51014, Estonia abstract article info Article history: Received 4 July 2010 Accepted 28 July 2010 Available online 3 August 2010 Keywords: Rare-earth ions Doped crystals Absorption spectra Birefringence K 5 Nd(MoO 4 ) 4 crystals with different Yb 3+ concentrations were grown using Czochralski technique. Room- temperature absorption spectra were recorded and assigned on the basis of Dieke's diagram for Nd 3+ ion; the standard Judd–Ofelt theory has been used to analyze the spectra. Increase of Yb 3+ concentration leads to variation of the corresponding Judd–Ofelt intensity parameters. Significant contribution of the Yb↔Nd energy transfer into the formation of the Nd 3+ absorption spectra causes the observed changes. After illumination of the crystals with CW Nd:YAG laser changes of the birefringence for the laser line of CW He– Ne laser at 633 nm were studied. Birefringence changes show good correlation with the content of Yb ions. © 2010 Elsevier B.V. All rights reserved. 1. Introduction In the last decades considerable attention has been paid to the development of the crystalline lasers, since they play very important role in modern physics and wide applications to quantum electronic equipment were realized [1,2]. Among other studied crystals, those doped with Yb 3+ ions have been intensively and successfully developed recently [3]. Trivalent ytterbium with its 4f1 13 electron configuration has two 4f electronic terms: the 2 F 7/2 ground state, and the 2 F 5/2 excited term, situated about 10,000 cm -1 above the ground state. These two levels usually are split under the influence of the surrounding crystal field, which allows to achieve the laser generation in a three-level laser system. The lack of concentration quenching, absence of excited state absorption and re-absorption in the visible spectral range are the main advantages of using Yb 3+ ions, when compared to Nd 3+ . In addition, thermal effects are reduced and quantum defects are distinctly smaller [4]. In this article we report on the growth and optical absorption spectra of the Yb-doped K 5 Nd (MoO 4 ) 4 single crystals. We note here that the laser properties of K 5 Nd(MoO 4 ) 4 were studied in Refs. [5,6]; Yb–Nd energy transfer was considered in Ref. [7]. However, no Judd–Ofelt analysis of the Nd 3+ absorption spectra in this host was reported. The aim of our paper is to investigate the influence of ytterbium doping on absorption spectra and explain observed behaviour on the basis of the Judd–Ofelt theory. The prominent peaks in the absorption spectra are caused by the Nd 3+ ions; effective energy transfer between the ytterbium and neodym- ium ions modifies the absorption spectra and makes these crystals promising materials for applications in optical switchers. 2. Crystal growth Potassium neodymium molybdate melts congruently at 795 °C. There are no destroying phase transitions between temperature of crystallization and the room temperature, so the crystals can be grown by means of the Czochralski method [8]. Two-zone resistance furnace with low temperature gradients was used in our experiments. K 5 Nd(MoO 4 ) 4 :Yb single crystals were grown from a platinum crucible in the air. The growth conditions were as follows: rotation — 30 rpm, pulling rate — 1.5 mm/h, and seed orientation — [001]. A low temperature gradient was necessary to avoid cracking of growing crystals; post-growth cooling rate — 20 °C/h. As-grown K 5 Nd(MoO 4 ) 4 were 40–50 mm long and their diameter reached 15 mm. Increasing level of Yb in the melt (up to 6 at.%) led to the growth of crystals that easily cracked. We were unable to obtain K 5 Nd(MoO 4 ) 4 single crystals when the concentration of ytterbium, substituting Nd in the melt, was higher than 6 at.%. A large difference of the ionic radii of Nd 3+ and Yb 3+ (1.1 Å and 0.9 Å, respectively), can be a probable reason for this behaviour due to difficulty in substituting Yb 3+ for Nd 3+ ions. The as- grown crystals of K 5 Nd(MoO 4 ) 4 obtained from melts containing more than 6 at.% of Yb ions were of poor optical quality caused by inclusions and multiple cracking, that is typical for constitutional supercooling Materials Letters 64 (2010) 2363–2365 ⁎ Corresponding author. E-mail address: brik@fi.tartu.ee (M.G. Brik). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.07.076 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet