Luminescent Studies on Germanate Glasses Doped with Europium Ions for Photonic Applications.

Glass and ceramic materials doped with rare earth (RE) ions have gained wide interest in photonics as active materials for lasers, optical amplifiers, and luminescent sensors. The emission properties of RE-doped glasses depend on their chemical composition, but they can also be tailored by modifying the surrounding active ions. Typically, this is achieved through heat treatment (including continuous-wave and pulsed lasers) after establishing the ordering mechanisms in the particular glass-RE system. Within the known systems, silicate glasses predominate, while much less work relates to materials with lower energy phonons, which allow more efficient radiation sources to be constructed for photonic applications. In the present work, the luminescent and structural properties of germanate glasses modified with phosphate oxide doped with Eu ions were investigated. Europium dopant was used as a "spectroscopic probe" in order to analyze the luminescence spectra, which characterizes the changes in the local site symmetries of Eu ions. Based on the spectroscopic results, a strong influence of PO content was observed on the excitation and luminescence spectra. The luminescence study of the most intense D→F (electric dipole) transition revealed that the increase in the PO content leads to the linewidth reduction (from 15 nm to 10 nm) and the blue shift (~2 nm) of the emission peak. According to the crystal field theory, the introduction of PO into the glass structure changes the splitting number of sublevels of the D→F (magnetic dipole) transition, confirming the higher polymerization of fabricated glass. The slightly different local environment of Eu centers the results in a number of sites and causes inhomogeneous broadening of spectral lines. It was found that the local asymmetry ratio estimated by the relation of (D→F)/(D→F) transitions also confirms greater changes in local symmetry around Eu ions. Our results indicate that modification of germanate glass by PO allows control of their structural properties in order to functionalize the emissions for application as luminescent light sources and sensors.