Abstract
The visible and near-infrared (NIR) up-conversion luminescence (UCL) of a concentration series of fluoride phosphors SrF₂:Ho (C_Ho = 1.3, 3.5, 4.8, 7.4, 8.9, 10.8, 13.0, 13.6 mol.%) upon excitation of the ⁵I₇ level of Ho³⁺ ions are presented. The dependences of the UCL intensity corresponding to the ⁵F₃→⁵I₈, ⁵S₂(⁵F₄)→⁵I₈, ⁵F₅→⁵I₈, ⁵I₄→⁵I₈, ⁵F₃→⁵I₆, ⁵I₅→⁵I₈, ⁵F₅→⁵I₇, ⁵I₆→⁵I₈ transitions on the concentration of Ho³⁺ ions were revealed. The UCL energy yield, color coordinates, and correlated color temperature (CCT) values were estimated. The maximum values of UCL energy yields of 0.03% (380–780 nm) and 0.09% (380–1100 nm) were registered for SrF₂:Ho (3.5 mol.%) sample. All studied fluoride phosphors are characterized by a red glow of UCL. This glow corresponds to CCT of 6711–7474 K upon incident power density of 28 W/cm². The UCL mechanisms of SrF₂:Ho phosphors were proposed. The SrF₂:Ho phosphors can be used as laser beam viewers of the two-micron radiation of thulium and holmium commercial lasers.