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, opticalgap, radiative transition rate and lifetimes of excited states are greatly influenced by the optical properties of the hostmaterial, ligand field, multi-phonon relaxation processes, impurities, temperature and concentration of rare earth ions. Thispresentation gives a panoramic view of the state of art of investigating experimentally the optical properties of rare earthdoped phosphate and tellurite glasses with different compositions and preparation techniques that our group has been carriedout in the recent past. We have prepared these glasses by using two different methods melt quenching and sol-gel, however inthe 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 (50-x)P2O5-50MgO-xSm2O3 (x= 0.0-3.0 mol%) have been studies. In addition, ytterbium-doped sodium-tellurite glasses havingcomposition of (80– x)TeO2–20Na2O-(x)Yb2O3 (x=0.0-2.0 mol%) is also presented. The amorphous nature of all these samplesis 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 muchdepending on the MgO contents and the integration mode area of absorption band is strongly affected by the phosphatecontents. Our systematic spectral analysis revealed that the substitution of P2O5 by a small amount of Sm2O3 has negligibleeffect on the absorption band and hence on the glass structure. It is further observed that MgO does not affect the P-O bondcharacters. The optical energy gap (Eg ) and the Urbach energy (ΔE) has been estimated from the absorption edge studies. It isfound that Eg depend on the concentration of the non bridging oxygen in the glass network. Meanwhile, ΔE is found to bedepending on the Yb2O3 concentration. Interestingly, samarium doped magnesium-phosphate glasses have been found tochange from colorless to light yellow on increasing the Sm2O3 content. The transmission spectra of ytterbium-doped sodiumtelluriteglasses revealed that the predominant absorption peaks are due to the vibration of Te-O-Te and Yb3+ ions. The mainresults on vibrational frequencies, optical gap, Urbach energy and absorption edge has been found to be quantitativelyconsistent with other observations. Our detailed systematic spectroscopic studies provide useful information for furtherdevelopment of up-conversion lasers.