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Effective Upconversion in Alkaline Earth Fluorides Activated by Yb3+–Ho3+ Ions
| Content Provider | Semantic Scholar |
|---|---|
| Author | Radzhabova, E. A. Shendrika, Yu. |
| Copyright Year | 2019 |
| Abstract | A study is performed of optical spectra (absorption, excitation, luminescence, energy yield) associated with the upconversion of infrared excitation of 980 nm into visible radiation in crystals of alkaline– earth f luorides CaF2, SrF2, and BaF2, co-doped with YbF3 (0.01–10 mol %) and HoF3 (0.01–0.3 mol %). In the 0.1–10 W cm−2 range of excitation power densities, the intensity of the upconversion bands at 542, 650, and 752 nm grows as the square of the power. A quadratic dependence of the intensity of upconversion on the concentration of Yb3+ (0.03–3 mol %) and a weak dependence on the Ho3+ concentration (0.01–0.3 mol %) with a maximum at 0.1–0.15 mol % are observed. It is assumed that the upconversion is due to consecutive energy transfer from two closely spaced excited ytterbium ions to the holmium ion. DOI: 10.3103/S1062873819030195 INTRODUCTION The search for effective phosphors that convert near infrared radiation into visible light remains of great relevance [1]. The growing field of upconversion phosphor application, including protective labels on securities and banknotes, biological labels [2], optical thermometry [3], solar energy [4] and new laser media [5], demands that we seek more effective materials. Of the known mechanisms of upconversion, the one most effective is the mechanism of sensitizied energy transfer upconversion [1, 6]. For example, at a low lighting density of 0.4 W cm−2 and an excitation wavelength of 1500 nm, the internal yield of energy in the integrated upconversion spectrum of NaYF4–25% Er is 12% [4]. At the same time, the greatest intensity of upconversion lies in the band around 1000 nm; this can be used to increase the efficiency of silicon solar cells [4]. High efficiency in the upconversion of 750 nm emissions into 550 nm emissions (8.1% at a high pumping power of 2.2 × 103 W cm−2) was achieved in CaF2–1% Ho crystals and could be used to develop an upconversion laser [5]. Due to its large absorption cross section around 980 nm and the effective transfer of the energy of excitation to activators, the Yb3+ ion is an effective sensitizer for Ho3+, Tm3+, and Er3+ ions [1, 7]. The upconversion of a pair of lanthanide impurities (sensitizer Yb and activator Ho3+) has been studied in a variety of fluorides and oxides [7–10]. This pair seems to be the one most promising for converting 900–1000 nm radiation into visible light. The reverse transfer of excitation from Ho3+ ions (sensitizers) to Yb3+ ions (activators) to improve the efficiency of solar cells was studied in [11]. Although the basic mechanisms of upconversion are known [1, 13], details of the energy transfer mechanisms and the structure of defects require additional research. In this work, we studied efficient upconversion in CaF2 , SrF2, and BaF2 crystals activated by YbF3–HoF3 with concentrations in the range of 0.01 to 10 mol %. EXPERIMENTAL MeF2 (Me: Ca, Sr, Ba) crystals were grown according to Stoсkbarger inside three-barreled graphite crucibles in a vacuum [12]. Several percents of CdF2 was added to the initial material to purify it of oxygen impurities. Crystals of alkaline-earth f luorides were grown, into which, in addition to YbF3 impurity (concentration of 0.01–10%), a second lanthanide was added. A small proportion of Yb3+ ions (10%) were converted to the bivalent form in some crystals as they grew. Anti-Stokes luminescence was excited by a 980 nm semiconductor laser with a measured power of 69 mW. Of our investigated Yb–RE lanthanide pairs (RE: Pr, Nd, Sm, Dy, Ho, Er, Tm), only crystals activated by Yb–Ho, Yb–Er, and Er had effective visible upconversion luminescence. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://medphysics-irk.ru/publication/pdf-r/radzhabov-2019-upconversion-en.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
