Influences of Mg²⁺ ion on dopant occupancy and upconversion luminescence of Ho³⁺ ion in LiNbO₃ crystal

The effects of a Mg²⁺ ion on the dopant occupancy and upconversion luminescence of a Ho³⁺ ion in LiNbO₃ crystal are reported. The birefringence gradient of the crystal is measured to investigate the optical homogeneity. The X-ray powder diffraction spectrum and the upconversion luminescence are used to investigate defect structure and spectroscopic properties of Mg,Ho:LiNbO₃. Under 808-nm excitation, blue, red, and very intense yellow-green bands are observed. Based on the energy levels of Ho³⁺ in LiNbO₃, and the pump intensity dependence of the observed emission, an excitation scheme is presented. The upconversion emission spectra reveal an enhancement of upconversion intensity when the Mg²⁺ ions are introduced into Ho:LiNbO₃. The main upconversion mechanism is discussed in this work.