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

Abstract

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 Ho^3+:5(S₂, F₄) level, though higher concentration of Ho³⁺ ion would decrease the probability of ETU and increase of ESA. In contrast, Ho^3+: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.