The basic spectroscopic parameters of Ho 3þ -doped fluoroindate glass for emission at 3.9 mm Laercio Gomes a , Vincent Fortin b , Martin Bernier b ,R  eal Vall  ee b , Samuel Poulain c , Marcel Poulain c , Stuart D. Jackson d, * a Center for Lasers and Applications, IPEN/CNEN-SP, P.O. Box 11049, S~ ao Paulo, SP, 05422-970, Brazil b Center for Optics, Photonics and Lasers (COPL), Universite Laval, Quebec, G1V 0A6, Canada c Le Verre Fluore, Campus KerLann, F-35170, Bruz, Brittany, France d MQ Photonics, Department of Engineering, Faculty of Science and Engineering, Macquarie University, North Ryde, 2109, Australia article info Article history: Received 16 June 2016 Received in revised form 1 August 2016 Accepted 22 August 2016 Available online 30 September 2016 Keywords: Spectroscopy of laser materials Fluoride glass Fibre lasers Mid-infrared abstract This report details the first study of the fundamental spectroscopic properties of a new optical material for prospective application as a gain medium for fiber laser emission at 3.9 mm. We have investigated the decay processes that are relevant to the excited states of the 5 I 5 / 5 I 6 transition in singly Ho 3þ -doped InF 3 (fluoroindate) glass using time-resolved fluorescence spectroscopy. The HoF 3 concentration in the glass was 10 mol.%. We excited the 5 I 7 and 5 I 6 energy levels using selective excitation at 1982 nm and 1150 nm, respectively. We have established that a strong energy-transfer upconversion process by way of the well-known dipole-dipole interaction between two holmium ions excited to the 5 I 6 level populate the 5 I 5 (upper laser) level of the 3.9 mm transition. The 5 I 6 and 5 I 5 energy levels emit luminescence with measured peaks located at 2.85 mm and 3.92 mm, respectively and the luminescence efficiencies of these emissions are 78% and 0.2%, respectively. Results from numerical simulations show that for this high Ho 3þ concentration, a population inversion for the 3.9 mm transition is reached only for a short time (t < 100 ms) after direct upper laser level pumping at a wavelength of 889 nm. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Powerful and highly efficient fiber lasers with emission at wavelengths longer than 3 mm are needed for applications in sensing, nonlinear optics, frequency combs, materials processing and optical pumping. Since the first demonstration of longest emission wavelength from a fiber laser at 3.9 mm two decades ago [1,2], the only extension of this pioneering work was the recent demonstration of pulsed emission from a bulk InF 3 sample pumped by Cr 3þ :LiSAF laser emitting at 889 nm [3] and the demonstration of supercontinuum generation using InF 3 -based fibre [4]. Whilst a number of transitions at intermediate wavelengths between this limit and 3 mm have recently produced more than 1 W CW from a fiber laser [5,6], more work is clearly required to address the limited number of fiber lasers emitting at longer wavelengths. All the major demonstrations of mid-infrared emission from fiber lasers have employed fibers made primarily from fluorozirconate glass, limiting the operation at 3.9 mm to cryogenic temperatures. Fluo- roindate glasses were discovered by M. Poulain in 1981 and display a lower phonon energy than ZBLAN which shifts multiphonon absorption edge by approximately 1 mm InF 3 glasses are 15 times more transparent at 3900 nm than ZBLAN glasses. The main com- ponents of those glasses are InF 3 , SrF 2 , BaF 2 and ZnF 2. In this report, we investigate the luminescence and energy level decay properties of Ho 3þ -doped fluoroindate glass. We carry out the basic spectroscopic measurements, compare the results with other previously reported fluoroindate glasses and assess the suitability of this optical material for emission in the mid-infrared. We calculated the luminescence efficiency of these levels after the experimentally determined decay time characteristics were compared with the calculated radiative lifetimes from Judd-Ofelt theory. Multiphonon decay and cross-relaxation resulted in the rapid depopulation of the 5 F 5 and 5 I 5 energy levels to the two lower excited states, namely the 5 I 6 and 5 I 7 levels. A numerical model is used to understand the gain dynamics of the 5 I 5 / 5 I 6 transition in singly Ho 3þ -doped InF 3 fluoroindate glass for comparatively high Ho 3þ concentration. * Corresponding author. E-mail address: stuart.jackson@mq.edu.au (S.D. Jackson). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat http://dx.doi.org/10.1016/j.optmat.2016.08.019 0925-3467/© 2016 Elsevier B.V. All rights reserved. Optical Materials 60 (2016) 618e626