Mechanisms and concentration dependence of Tm3+ blue and Er3+ green up-conversion in codoped glasses by red-laser pumping

Tm3+-Er3+ codoped tellurite glasses were pumped at 650 nm and the up-conversion characteristics of Tm3+ and Er3+ luminescences were investigated. By Er3+ codoping, the Tm3+-460 nm up-conversion luminescence due to the D-1(2) --> F-3(4) transition was quenched and the Tm3+-480 nm up-conversion luminescence due to the (1)G(4) --> H-3(6) transition was selectively sensitized, due to which the intensity became more than 20 times larger than in the absence of Er3+ . Both Tm3+-480 nm and Er3+-550 nm up-conversion luminescence intensities have a maximum in the range where the ratio of Er3+- to Tm3+-content is about two. The result was discussed in the context of the energy transfer efficiency calculated from the lifetime of the Er3+:I-4(13/2) level, the competition between the excited state absorption efficiency of Tm3+:(1)G(4) <-- F-3(4) and the ground state absorption of Er3+:F-4(9/2) <-- I-4(15/2) estimated from the Judd-Ofelt analyses, and the concentration quenching of the Er3+:S-4(3/2) level, mainly due to the cross relaxation. It is concluded that the small optimum ratio of the donor- to acceptor-content (congruent to 2) should be ascribed to the specific mechanisms in the present sensitized up-conversion.