Frequency upconversion mechanism in Ho3+/Yb3+-codoped TeO2–TiO2–La2O3 glasses

Frequency upconversion from Ho3+/Yb3+-codoped glass or crystal under Yb3+ sensitization is a known phenomenon. However, inconsistencies are prevalent in the understanding of double energy transfer mechanisms for Ho3+/Yb3+-codoped systems. In this context, rate equations are proposed for Ho3+/Yb3+-codoped low-phonon TeO2–TiO2–La2O3 glass under Yb3+ sensitization with continuous and pulsed excitations. The proposed rate equations are validated with experimental results to elucidate the mechanisms responsible for populating 5(S2, F4) and 5F5 energy levels of Ho3+ ion. The solutions of rate equations with experimental results are substantiating the occurrence of both excited state absorption (ESA) and energy transfer upconversion (ETU) mechanisms in populating Ho3+:5(S2, F4) level, though higher concentration of Ho3+ ion would decrease the probability of ETU and increase of ESA. In contrast, Ho3+:5F5 level has been populated via ETU only. Numerical solutions to the rate equations are also proposed to elucidate the mechanics for populating 5(S2, F4) and 5F5 levels of Ho3+ ion. The proposed rate equation for pulsed excitation explains the characteristics of respective decay curves, which are further used to quantify energy transfer coefficient (W02) as (1.77 ± 0.12) × 10− 17cm3 s−1 for Ho3+/Yb3+-codoped TTL glass host.