Photon-avalanche upconversion of Ho3+ in NaYF4 and enhancement of violet, blue and green emissions induced by sensitization of Tm3+

Ho3+ singly doped and Ho3+/Tm3+ co-doped hexagonal NaYF4 powders have been synthesized by a solid-state reaction method. Under excitation of 671 nm diode laser, upconverted blue, green and red emission bands are observed in Ho3+ singly doped sample. Temporal evolution and excitation power dependent behavior for the green emission are explored, indicating that a photon-avalanche mechanism is responsible for the upconversion processes in Ho3+ singly doped hexagonal NaYF4 sample. With the introduction of Tm3+, the intensities of both blue and green emissions of Ho3+ are efficiently enhanced, which are attributed to two energy transfer processes from Tm3+ to Ho3+, i.e., 3F4 (Tm3+) + 5I8(Ho3+) → 3H6 (Tm3+) + 5I7(Ho3+) and 1G4 (Tm3+) + 5I8(Ho3+) → 3H6 (Tm3+) + 5F3(Ho3+). The result offers a new sensitization approach to enhance the upconversion efficiency of Ho3+ under 671 nm excitation.