Spectroscopic Investigation of The Optical Properties of Rare Earth Doped Phosphate and Tellurite Glasses

Improving the up-conversion efficiency is the key issue in rare earth doped glasses. The quantum efficiency, optical gap, radiative transition rate and lifetimes of excited states are greatly influenced by the optical properties of the host material, ligand field, multi-phonon relaxation processes, impurities, temperature and concentration of rare earth ions. This presentation gives a panoramic view of the state of art of investigating experimentally the optical properties of rare earth doped phosphate and tellurite glasses with different compositions and preparation techniques that our group has been carried out in the recent past. We have prepared these glasses by using two different methods melt quenching and sol-gel, however in the present paper we only report on the former method. Series of erbium-doped magnesium-phosphate glasses based on (P2O5)0.5–(MgO)0.5-x–(Er2O3 / Nd2O3)x, ytterbium-doped sodium-tellurite glasses based on (80– x)TeO2 –20Na2O-(x)Yb2O3 (x=0.0-2.0 mol%), samarium-doped glasses having composition of (P2O5)50-x–(MgO) 50-(Sm2O3) x (0≤x≤4 mol%), and (50x)P2O5-50MgO-xSm2O3 (x= 0.0-3.0 mol%) have been studies. In addition, ytterbium-doped sodium-tellurite glasses having composition of (80– x)TeO2–20Na2O-(x)Yb2O3 (x=0.0-2.0 mol%) is also presented. The amorphous nature of all these samples is confirmed using the X-ray diffraction technique. The optical properties of the glass have been measured employing Infrared, FTIR and UV-Visible spectroscopy. The vibrational frequencies are attributed predominately due to the OH band, P=O, P-OP, and P-O stretching vibrations respectively. Furthermore, it is found that the absorption cut-off wavelength very much depending on the MgO contents and the integration mode area of absorption band is strongly affected by the phosphate contents. Our systematic spectral analysis revealed that the substitution of P2O5 by a small amount of Sm2O3 has negligible effect on the absorption band and hence on the glass structure. It is further observed that MgO does not affect the P-O bond characters. The optical energy gap (Eg ) and the Urbach energy (∆E) has been estimated from the absorption edge studies. It is found that Eg depend on the concentration of the non bridging oxygen in the glass network. Meanwhile, ∆E is found to be depending on the Yb2O3 concentration. Interestingly, samarium doped magnesium-phosphate glasses have been found to change from colorless to light yellow on increasing the Sm2O3 content. The transmission spectra of ytterbium-doped sodiumtellurite glasses revealed that the predominant absorption peaks are due to the vibration of Te-O-Te and Yb ions. The main results on vibrational frequencies, optical gap, Urbach energy and absorption edge has been found to be quantitatively consistent with other observations. Our detailed systematic spectroscopic studies provide useful information for further development of up-conversion lasers.