Contents lists available at ScienceDirect Ceramics International journal homepage: www.elsevier.com/locate/ceramint Short communication Enhancement mechanisms of Tm 3+ -codoping on 2 μm emission in Ho 3+ doped fluoroindate glasses under 888 nm laser excitation Ruicong Wang a , Haiyan Zhao a , Meng Zhang a , Jiquan Zhang a , Shijie Jia a , Jun Zhang b,c , Hangyu Peng b , Gilberto Brambilla d , Shunbin Wang a,** , Pengfei Wang a,e,* a Key Laboratory of In-fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin, 150001, China b State Key Laboratory of Luminescence and Application, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun, 130033, China c University of Chinese Academy of Sciences, Beijing, 100049, China d Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, United Kingdom e Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Optoelectronic Engineering, Shenzhen University, Shenzhen, 518060, China ARTICLE INFO Keywords: Fluoroindate glass Mid-infrared Luminescence Ho 3+ /Tm 3+ co-doped ABSTRACT Ho 3+ doped and Ho 3+ /Tm 3+ co-doped fluoroindate glass samples were prepared and their emission properties were compared. Under 888 nm laser excitation, the emission at 2 μm of Ho 3+ ions with co-doping 2 mol% Tm 3+ ions had a 2.9-fold improvement compared with that of Ho 3+ doped. The absorption and emission spectra, and energy level lifetime of Ho 3+ /Tm 3+ co-doped glass samples were measured to analyze the energy transfer processes and enhancement mechanisms. The luminescence intensity at 2 μm can be greatly increased due to the bidirectional energy transfer between Tm 3+ and Ho 3+ ions. 1. Introduction In the past decades, 2 μm lasers have attracted wide research at- tention because of their potential applications in medical, military field, eye-safe laser radar, laser medicine surgery, monitoring of atmospheric pollutants, remote sensing, nonlinear frequency conversion, atmo- sphere transmission and high-resolution spectroscopy of low-pressure gases [1–5]. Compared with other types of lasers, fiber lasers have many advantages such as small size, simple structure, good beam quality and strong heat dissipation capability [6,7]. Tm 3+ doped fiber lasers in the 2 μm band have been widely investigated in silica fibers, mostly because Tm 3+ ions can be pumped by commercial 808 nm laser diodes (LD) using the double cladding fiber structure [8–10]. Ho 3+ ions have a wider 2 μm band emission, which can provide laser output in the 2.1 μm band spectrum [11,12]. Normally, Tm 3+ ions act as common sensitizing ions when co-doping with Ho 3+ ions, because the energy level of Tm 3+ : 3 F 4 matches the Ho 3+ : 5 I 7 and the energy transfer pro- cess from Tm 3+ : 3 F 4 to Ho 3+ : 5 I 7 is efficient [13–15]. Ho 3+ ions can provide a 2 μm emission through the transition Ho 3+ : 5 I 7 → 5 I 8 . Under 808 nm pumping, Tm 3+ ions can be excited to the 3 H 4 level followed by a cross-relaxation process ( 3 H 4 + 3 H 6 → 2 3 F 4 ), which dramatically enhances the quantum efficiency of the 2 μm emission. Silicate glass matrices have many advantages, such as good chemical stability, high thermal stability, easy thermal processing, high transmittance in the UV–Visible region and low preparation cost [13,14]. In 1965, Johnson first realized Ho 3+ doped laser operation [15], and since the absorption band of Ho 3+ does not overlap with the emission band of the commonly commercially used high-power pump sources, Tm 3+ ions are usually used as the sensitized ions of Ho 3+ ions. In 2002, Taniguchi et al. re- ported a 1970 nm laser from a 270 cm long Tm 3+ /Ho 3+ co-doped silica fiber laser with 450 mW maximum output power [16]. In 2007, Ho 3+ / Tm 3+ co-doped fiber lasers can achieve 83 W laser output power at 2 μm with slope efficiencies of 42% under 793 nm laser diode excitation [17]. Over the past 20 years, the maximum output power available from a diode-pumped silicate fiber laser emitting at around 2 μm has continuously increased to 1000 W [18]. However, the solubility of rare earth ions in the silicate glass matrix is low, and the higher con- centrations of rare earth ions are prone to agglomeration, which causes fluorescence quenching. Silica-based fiber has large phonon energy (~1000 cm -1 ), which is no longer suitable for fiber lasers with 2.2 μm https://doi.org/10.1016/j.ceramint.2019.11.108 Received 28 September 2019; Received in revised form 28 October 2019; Accepted 13 November 2019 * Corresponding author. Key Laboratory of In-fiber Integrated Optics of Ministry of Education, College of Science, Harbin Engineering University, Harbin, 150001, China. ** Corresponding author. E-mail addresses: shunbinwang@hrbeu.edu.cn (S. Wang), pengfei.wang@tudublin.ie (P. Wang). Ceramics International xxx (xxxx) xxx–xxx 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Please cite this article as: Ruicong Wang, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.11.108