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TROSCOPY OF CONDENSED MATTER

Upconversion of Infrared Radiation in Er3+-Doped Alkaline-Earth Fluorides E. A. Radzhabova, * and R. Yu. Shendrika a Vinogradov

Institute of Geochemistry, Siberian Branch, Russian Academy of Sciences, Irkutsk, 664033 Russia *e-mail: [email protected] Received January 22, 2020; revised July 30, 2020; accepted August 3, 2020

Abstract—Upconversion in MeF2–ErF3 (Me is Ca, Sr, or Ba) crystals with ErF3 concentrations from 0.01 to 10 mol % excited into the IR absorption bands of Er3+ by laser diodes at wavelengths of 808, 978, and 1542 nm is studied. In the case of excitation at λ = 1542 nm, the dominant band in the MeF2–Er emission spectrum corresponds to the 4I11/2–4I15/2 transition at 990 nm, while the contribution of visible bands is less than 0.1. The upconversion luminescence energy yield of MeF2–10% ErF3 crystals excited by 1542-nm radiation with a power density of about 1 W/сm2 varies from 14 to 26%. Keywords: upconversion, alkaline-earth fluorides, infrared radiation DOI: 10.1134/S0030400X20110211

INTRODUCTION The search for effective luminophores converting near-IR radiation with a wavelength of 1–2 μm into light with a wavelength shorter than 1 μm remains a topical problem. Er3+ ions in crystals are characterized by a strong absorption in the range of 1.4–1.6 μm, which opens the possibility of increasing the efficiency of silicon solar cells due to recording radiation with photon energies smaller than the silicon band gap [1]. Er3+ ions have an energy level diagram suitable for upconversion and are often used in pair with Yb3+ ions in many crystalline and glassy matrices [2]. The green and red luminescence bands of Er3+ in BaF2–5% ErF3 crystals excited by IR light were observed for the first time by Ovsyankin and Feofilov [3–5] and were associated with two- (~980 nm) and three-photon (λ > 1200 nm) absorption. The intensity of the green and red bands in CdF2–ErF3 cubically increased with increasing intensity of exciting light [6, 7]. The green luminescence intensity of BaF2–Er3+ increased as a square of erbium concentration in the range of 0.5– 8% in the case of excitation in the range of 0.9– 1.2 μm, which was interpreted as successive absorption of two photons by the erbium ion [4]. In addition to the visible luminescence bands, laser excitation at 1.5 μm leads to luminescence in the region of 990 nm (CdF2–Er [6], CaF2–Er [8]) with intensities exceeding the intensity of the visible bands. Because of this, it was proposed to use some fluoride materials for increasing the efficiency of silicon photoconverters [1, 8, 9].

The aim of our investigations was to study the mechanisms and measure the energy yield of upconversion in alkaline-earth fluoride crystals MeF2–Er upon excitation into the IR absorption bands of Er3+ ions. EXPERIMENTAL The MeF2 (Me is Ca, Sr, or Ba) crystals were grown by the Stockbarger method in a six-compartment graphite crucible in vacuum [10]. Several percent of CdF2 was added to the batch to purify it from oxygen impurities. We have grown alkaline-earth fluo