Investigation of Er doped zinc borate glasses by low-temperature photoluminescence

Er3+ doped (95-x)B2O3-xZnO-5TiO2 (x = 45, 50 and 55 mol%) bulk glasses were prepared by conventional melt quenching method and investigated by transmission and photoluminescence (PL) spectroscopy. Transmission spectra exhibit sharp absorption bands at 364, 377, 406, 441, 448, 488, 521, 541, 651, 792, 974, and 1530 nm, that correspond to absorption on Er3+ ions and they are attributed to the optical transitions from the ground state 4I15/2 to the excited states 4G9/2, 4G11/2,4F9/2, 4F3/2, 4F5/2, 4F7/2, 2H11/2, 4S3/2, 4F9/2, 4I9/2, 4I11/2 and 4I13/2, respectively. The optical gap has been estimated around 3.5 eV with a tendency to increase with increasing ZnO content. Low-temperature PL properties of the Er3+ doped glasses and of the host glass itself were investigated. To achieve this goal, two different wavelengths (325 and 514.5 nm) were used for PL excitation. PL spectra excited by He-Cd laser at 325 nm were dominated by the host glass broad-band luminescence centred at around 570 nm. Simultaneously we observed narrow emission and absorption features due to 4f-4f electronic transitions in Er3+ ions superposed on the broad band host glass luminescence. We argue that these superposed narrow absorption dips represent a direct evidence of the energy transfer from the electronic structure of the host to 4f states of Er3+ ions. Fine structure of Er3+ emission bands at 980 and 1530 nm, corresponding to radiative transitions from the two lowest excited states of Er3+ ions to the ground state manifold have been investigated at room temperature and at 4 K, and a schematic energy diagram of Stark levels of 4I11/2, 4I13/2 and 4I15/2 manifolds has been deduced.