Abstract
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 Eu3+ 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 Eu3+ ions. Based on the spectroscopic results, a strong influence of P2O5 content was observed on the excitation and luminescence spectra. The luminescence study of the most intense 5D0→7F2 (electric dipole) transition revealed that the increase in the P2O5 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 P2O5 into the glass structure changes the splitting number of sublevels of the 5D0→7F1 (magnetic dipole) transition, confirming the higher polymerization of fabricated glass. The slightly different local environment of Eu3+ 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 (5D0→7F2)/(5D0→7F1) transitions also confirms greater changes in local symmetry around Eu3+ ions. Our results indicate that modification of germanate glass by P2O5 allows control of their structural properties in order to functionalize the emissions for application as luminescent light sources and sensors.
Highlights
Many photonic applications are based on active waveguide systems
The analysis of intensity changes of absorption bands confirmed that the reduction of barium oxide content was evident with a decrease in the intensity of bands related to Ge-O- vibrations and the disappearance of the band ascribed to P-Obond vibration
(7 F0 →5 L6 ) and 464 nm (7 F0 →5 D2 ) showed that replacing BaO with up to 20 mol.% of P2 O5 leads to a decrease in the luminescence asymmetric ratio, which is in good agreement with structural results (XRD, MIR)
Summary
Active hybrid structures are proposed, in which crystalline phases are introduced within the amorphous matrix using various methods, including thermal treatment [1], direct doping [2], chemical vapor deposition (CVD) [3], and 3D printing [4] All of these methods require glasses with modified structural properties, ensuring the conditions for obtaining new luminescent properties (from embedded active structures) while maintaining the stability of the hybrid system [5,6,7]. In the 1990s, one of the popular glass systems—GeO2 -Ga2 O3 -BaO (GGB)—was developed as a candidate for optical fibers doped with rare earth ions [21] Until now, these glasses have been modified by various oxides [22] and fluorides [23], novel luminescent properties can be proposed. Eu3+ is one of the most important lanthanide ions, characterized by strong visible emissions, making it applicable in lighting, biochemical and biomedical sensing, as well as spectral imaging [27,28,29]
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