Green and red fluorescence upconversion in neodymium-doped low phonon antimony glasses

A new K2O–B2O3–Sb2O3 (KBS) glass system having low phonon energy (about 600 cm−1) doped with Nd3+ ions has been developed. Infrared reflection spectroscopic (IRRS) studies have been employed to establish its low phonon energy. Ultraviolet–visible–near infrared (UV–vis–NIR) absorption and photoluminescence upconversion properties with special emphasis on the spectrochemistry of the 15K2O–15B2O3–70Sb2O3 (mol%) glass have been studied doping with different concentrations (0.1–1.0 wt%) of Nd2O3. UV–vis–NIR absorption band positions have been justified with quantitative calculation of nephelauxetic parameter and covalent bonding characteristics of the host. NIR to visible upconversion has been investigated by exciting at 805 nm at room temperature. Two upconverted bands originating from the 4G7/2 → 4I9/2 and 4G7/2 → 4I13/2 transitions are found to be centered at 540 nm (green, medium) and 650 nm (red, strong), respectively. These bands have been explained from the evaluation of the absorption and excitation spectra. The upconversion processes have been explained by the excited state absorption (ESA), energy transfer (ET), cooperative energy transfer (CET) and cross-relaxation (CR) mechanisms involving population of the metastable (storage) energy level (4G7/2) by effects of multiphonon deexcitation and thermal population. It is evident from the IRRS study that the upconversion phenomena are expedited by the low multiphonon relaxation rate in antimony glasses owing to their low phonon energy (602 cm−1, the main and highest intensity Sb–O–Sb stretching band) which is very close to that of fluoride glasses (500–600 cm−1).