IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 24, NO. 5, SEPTEMBER/OCTOBER 2018 1600713 Highly Efficient, Compact Tm 3+ :RE 2 O 3 (RE = Y, Lu, Sc) Sesquioxide Lasers Based on Thermal Guiding Pavel Loiko , Philipp Koopmann, Xavier Mateos , Josep Maria Serres, Venkatesan Jambunathan , Antonio Lucianetti, Tomas Mocek, Magdalena Aguil´ o, Francesc D´ ıaz, Uwe Griebner, Valentin Petrov , and Christian Kr¨ ankel Abstract—Cubic sesquioxides, RE 2 O 3 , where RE = Y, Lu or Sc, are attractive host crystals for thulium (Tm 3+ ) doping. A com- parison of the spectroscopic properties of Tm 3+ :RE 2 O 3 crystals in terms of transition cross sections and cross-relaxation (CR) ef- ficiency required for efficient upconversion pumping is presented. Thermo-optic properties of Tm 3+ :RE 2 O 3 crystals (thermal lens- ing, fractional heat loading, and thermo-optic coefficients) are de- scribed. The positive thermal lens, broadband emission, and ef- ficient CR of the Tm:RE 2 O 3 crystals enable the development of compact, highly efficient and power-scalable lasers operating above 2 μm, based on thermal guiding. Nowadays, Tm:Lu 2 O 3 microchip lasers are capable of generating nearly 5 W of output power at ∼2.06 μm with a slope efficiency η of 67% and in a rod geometry— up to 47.5 W with η of 59%. For multiwatt output at even longer wavelengths around 2.15 μm, Tm:Sc 2 O 3 is an interesting candidate. Index Terms—Solid-state lasers, sesquioxide crystals, thulium doping, thermal lensing. Manuscript received October 18, 2017; revised December 19, 2017; accepted December 29, 2017. Date of publication January 5, 2018; date of current version January 25, 2018. This work was supported in part by the Spanish Government under Projects MAT2016-75716-C2-1-R (AEI/FEDER, UE) and TEC 2014-55948-R, and in part by the Generalitat de Catalunya under Project 2014SGR1358. The work of F. D´ ıaz was supported by the ICREA Academia Award 2010ICREA-02 (for excellence in research). The work of P. Loiko was supported by the Government of the Russian Federation under Grant 074-U01 through ITMO Postdoctoral Fellowship Scheme. (Corresponding author: Pavel Loiko.) P. Loiko is with the ITMO University, Saint-Petersburg 197101, Russia (e-mail: kinetic@tut.by). P. Koopmann is with the Institut f¨ ur Laser-Physik, Universit¨ at Hamburg, Hamburg D-22761, Germany (e-mail: pkoopman@physnet.uni-hamburg.de). X. Mateos, J. M. Serres, M. Aguil´ o, and F. D´ ıaz are with the F´ ısica i Cristal·lografia de Materials i Nanomaterials, Enginyeria de Materials i Mi- cro/Nano Sistemes, Departament de Qu´ ımica F´ ısica i In` organica, Universitat Rovira i Virgili, Tarragona E-43007, Spain (e-mail: xavier.mateos@urv.cat; jserrese@gmail.com; magdalena.aguilo@urv.cat; f.diaz@urv.cat). V. Jambunathan, A. Lucianetti, and T. Mocek are with the HiLASE Centre, Institute of Physics, Czech Academy of Sciences, Doln´ ı Bˇ reˇ zany 25241, Czech Republic (e-mail: jambunath@fzu.cz; lucianetti@fzu.cz; mocek@fzu.cz). U. Griebner and V. Petrov are with the Max-Born-Institute for Nonlin- ear Optics and Short Pulse Spectroscopy, Berlin D-12489, Germany (e-mail: griebner@mbi-berlin.de; petrov@mbi-berlin.de). C. Kr¨ ankel is with the Zentrum f¨ ur Lasermaterialien - Kristalle, Leibniz- Institut f ¨ ur Kristallz¨ uchtung, Berlin 12489, Germany, and is also with the Institut f¨ ur Laser-Physik, Universit¨ at Hamburg, Hamburg D-22761, Germany (e-mail: christian.kraenkel@ikz-berlin.de). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSTQE.2018.2789886 I. INTRODUCTION R ARE-EARTH (RE) sesquioxides, RE 2 O 3 , represent a well-known family of crystals. Depending on the RE element and the growth conditions, there exist several struc- tural modifications: hexagonal (A-type, for RE = La ... Nd), monoclinic (B-type, for RE = Sm, Eu, Gd) or cubic (C-type, for RE = Sm ... Lu, Sc, and Y). In particular the last one, or the bixbyite structure (the mineral bixbyite, (Mn,Fe) 2 O 3 , possesses the space group Ia ¯ 3) [1], is attractive for laser applications [2]. The cubic crystals of Lu 2 O 3 (Lutetia), Sc 2 O 3 (Scandia), and Y 2 O 3 (Yttria) have been recognized as excellent host matrices for doping with laser-active RE 3+ ions, such as Yb 3+ [3], [4], Tm 3+ [5], [6], Ho 3+ [7], [8], Nd 3+ [9] or Er 3+ [10]. These crystals will be referred in this paper as “cubic sesquioxides.” It should be noted that there also exist cubic “mixed” sesquioxide crystals. Any solid solution of the above 3 obviously will exhibit cubic structure, e.g., (Lu,Sc) 2 O 3 [11]. The interest in RE 3+ -doped cubic sesquioxides is mostly due to their superior thermal properties and spectroscopic fea- tures. The RE 2 O 3 crystals possess high thermal conductivity (12.8 W/mK for undoped Lu 2 O 3 , higher than that of YAG) with weaker dependence on the RE 3+ doping level [6], [12], and weak thermal expansion [13], [14]. These crystals have high melting temperature (2450 °C for Lu 2 O 3 [15]), under- lying good mechanical and chemical stability, and wide band- gaps. The cubic sesquioxides exhibit a broad transparency range (0.22–8 μm) [2] and moderate maximum phonon frequency for oxide materials [16]. There are two crystallographic sites in the bixbyite structure that can be occupied by the RE 3+ dopant ions (C 2 and C 3i symmetry, 24d and 8b Wyckoff positions, respectively) [17] but the optical properties are largely determined by doping ions on C 2 -sites [2]. This is because for the C 3i site, the electric-dipole transitions are forbidden due to inversion symmetry. For both sites, the RE 3+ ions have a VI-fold O 2- coordination. This coor- dination is lower than e.g., for RE 3+ in YAG and features short RE 3+ -O 2- distances leading to a strong crystal field and cor- respondingly broad absorption and emission spectral bands [2]. The latter enables very broad laser wavelength tuning ranges [18], [19] and the generation of ultrashort pulses in the mode- locked (ML) regime [20]–[24]. Due to the good thermal proper- ties of the host, power-scaling of bulk and thin-disk continuous- wave (CW) [12], [18] and ML [25]–[28] sesquioxide lasers is feasible. 1077-260X © 2018 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information.