Fluorescence and upconversion spectral studies of Ho³⁺ in alkali bismuth gallate glasses

The absorption and emission spectra of Ho³⁺ in alkali bismuth gallate glass matrices have been investigated in the visible and near-infrared. Strong fluorescence in the green (λ ∼549 nm), arising from the ⁵S₂ → ⁵I₈ transition of the ion, has been observed when Ho-doped potassium bismuth gallate glass is optically-excited in the red (∼646 nm). The measured quadratic dependence of the relative 549 nm intensity on pump power demonstrates that the upconversion mechanism is the sequential absorption of two photons. For a pump power of 285 mW at 646 nm, the upconversion efficiency is measured to be 5.5% which is among the highest values reported for glassy or crystalline matrices.