Journal article Open Access
Hehlen, Markus P.; Cockroft, Nigel J.; Gosnell, T. R.; Bruce, Allan J.
A spectroscopic investigation of an extensive series of Er3+-doped and Er3+,Yb3+-codoped soda-lime-silicate (SL) and aluminosilicate (AS) glasses is presented. Compared to SL glasses, 4f transitions in AS glasses show higher oscillator strengths, larger inhomogeneous broadening, and smaller crystal-field splittings of the respective excited-state multiplets. The Er3+ excited-state relaxation dynamics is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of 4I13/2, multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 μm 4I13/2→4I15/2 emission by excitation of 4I11/2 at around 980 nm. The absolute 4I13/2 luminescence quantum yield, for low 980-nm excitation density (∼5W/cm2), η, is ∼0.9 at 0.4 mol % Er2O3 and drops to about 0.65 upon increasing Er2O3 to 1.2 mol %, indicating the onset of energy-transfer processes. Samples with high OH− impurity concentration suffer from significantly higher quenching of 4I13/2 luminescence at higher Er3+ concentrations. Energy migration to the minority of Er3+ ions coordinated to OH−, followed by efficient multiphonon relaxation accounts for this effect. At low excitation densities, the strong near-infrared absorption of Yb3+ in combination with efficient Yb→Er energy transfer increases the 4I13/2 population density in Yb3+,Er3+-codoped samples by up to 2 orders of magnitude compared to equivalent samples without Yb3+. The dependence of η on Yb3+ codotation of 0.4 mol % Er2O3-doped samples predicts that a minimum of ∼0.8mol% Yb2O3 is required to achieve efficient sensitization of Er3+ by Yb3+. The relative intensities of upconversion luminescence from 4S3/2 and 2H11/2 are used to analyze internal sample heating in detail. Due to the high absorption cross section of Yb3+, increasing the Yb3+ concentration in Yb3+,Er3+-codoped samples of given length increases the absorbed power and subsequently the total density of multiphonon emission, leading to internal temperatures of up to 572 K in 0.4 mol % Er2O3 samples codoped with 4 mol % Yb2O3 and excited with 51kW/cm2. Multiphonon relaxation from 4I13/2 is shown to be inefficient even at these high internal sample temperatures. From upconversion luminescence spectra of a series of glasses, the thermal conductivity is estimated to be between 3.5×10−2 and 7.7×10−2Wm−1K−1, in good agreement with the known value of 4.8×10−2Wm−1K−1 for soda-lime-silicate glass.