Broadband near-infrared photoluminescence and energy transfer in Tm ³⁺/Er³⁺-codoped low phonon energy gallate bismuth lead glasses

Tm³⁺/Er³⁺-codoped gallate bismuth lead (GBL) glasses are prepared and characterized optically. Broadband near-infrared fluorescence spectra composed of Er³⁺ 1.53 μm emission from the ⁴I_13/2 → ⁴I_15/2 transition and Tm³⁺ 1.47 μm emission from the ³H₄ → ³F₄ transition are recorded under 793 nm excitation wavelength. The full-width at half-maximum of the broad emission band depends on the Tm³⁺-Er³⁺ concentration ratio ([Tm]/[Er]), and a maximum bandwidth of 165 nm is achieved when the concentration ratio reaches 4, and the lineshape characteristic of the broadband emission remains the same for a fixed concentration ratio. The energy transfer processes involved are proposed and discussed, and the primary processes responsible for the flat broadband emission can be ascribed to the [Tm³⁺(³H₄) + Tm³⁺(³H₆)] → [Tm³⁺( ³F₄) + Tm³⁺(³F₄)] and [Tm³⁺(³H₄) + Er³⁺(⁴I _15/2)] → [Tm³⁺(³F₄) + Er ³⁺(⁴I_13/2)] processes. An investigation on energy transfer with assistance of matrix phonons shows that the matrix phonons play an important role in bridging the energy gap in the energy transfer processes.