Direct confocal lifetime measurements on rare-earth-doped media exhibiting radiation trapping
Yean-Sheng Yong, S. Aravazhi, Sergio Andrés Vázquez-Córdova, Joan Josep Carvajal, Francesc Dı́az, Jennifer L. Herek, Sonia M. García‐Blanco, Markus Pollnau
Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 - 5 × 1021 cm-3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid.
Radiation trapping occurs in rare-earth-doped active media with strong spectral overlap of luminescence and ground-state absorption. It is demonstrated experimentally that a confocal measurement mitigates the influence of radiation trapping on the measured luminescence lifetime, hence allowing for direct extraction of the lifetime from the measured decay curves. The radiation trapping effect is largely suppressed by probing a small sample volume and rejecting the photons reemitted from the unpumped region. This non-destructive measurement method is applied to ytterbium (Yb3+) activated potassium double tungstate crystalline layers with Yb3+ concentrations ranging from 1.2 at.% up to 76 at.% (~8 × 1019 - 5 × 1021 cm-3). The measured lifetime values are comparable to the results reported for Yb3+-doped potassium double tungstate powder diluted in liquid.
Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of potassium gadolinium ytterbium double tungstate with a high Yb content of 57at.%, representing an Yb3+ concentration of ~3.8 × 1021 per cubic centimeter, grown onto an un-doped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, firstly, a fraction of Yb3+ ions not contributing to stimulated emission, secondly, a reduction of population inversion owing to a parasitic energy-transfer process and, thirdly, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ~800 dB/cm, which was achieved without thermal management, is promising for waveguide amplifier operating without active cooling.
Active media with high rare-earth concentrations are essential for small-footprint waveguide amplifiers. When operating at high population inversion, such devices are often affected by undesired energy-transfer processes and thermal effects. In this work, we study a 32-μm-thick epitaxial layer of KGd0.43Yb0.57(WO4)2, representing an Yb3+ concentration of ∼3.8×1021 cm−3, grown on an undoped KY(WO4)2 substrate. The pump absorption, luminescence decay, and small-signal gain are investigated under intense pumping conditions. Spectroscopic signatures of an energy-transfer process and of quenched ions, as well as thermal effects, are observed. We present a gain model which takes into account excessive heat generated due to the abovementioned experimental observations. Based on finite-element calculations, we find that the net gain is significantly reduced due to, first, a fraction of Yb3+ ions not contributing to stimulated emission, second, a reduction of population inversion owing to a parasitic energy-transfer process and, third, degradation of the effective transition cross-sections owing to device heating. Nevertheless, a signal enhancement of 8.1 dB was measured from the sample at 981 nm wavelength when pumping at 932 nm. The corresponding signal net gain of ∼800 dB/cm, which was achieved without thermal management, is promising for a waveguide amplifier operating without active cooling.
Radiation trapping causes significant lengthening of the measured fluorescence lifetimes in solid samples, which leads to overestimating them. Self-absorption inside solid-state samples is considerably enhanced by successive total internal reflections at the sample/air interface. A simple method for quantitative estimation of radiation trapping in solid-state materials within the frame of the Holstein-Biberman equations is presented. This method is based on the assumption that the radiation propagation follows paths with many total internal reflections; it can be applied to many resonant two-level systems such as Er3+-, Yb3+-, or Ho3+-doped materials. The equations are solved for a pulsed excitation localized in a finite volume inside the sample. In our modeling approach, we take into account the initial geometric population distribution and the imaging setup. We derive analytical expressions of the measured decays, which give the deformation of the measured curves compared with the ideal exponential decay. We investigate the effect of the fraction of self-trapped light on the decay modification. We then show that the ratio between the pumping volume, the collected volume, and the sample dimension changes the measured decay significantly.
In this work we expose a recently developed theoretical model for the analysis of the measurements obtained with the so-called pinhole method, for the determination of the upper level lifetime in materials where the superimposition between fluorescence and absorption spectra determines significant radiation trapping effect. Under fairly general conditions the fluorescence decay curve after a pulsed excitation of the sample obtained with this experimental set-up can be conveniently described by a double exponential functional form. The faster of the two decay times is close to the intrinsic fluorescence lifetime, with a difference that can be calculated with geometrical considerations. The theoretical results were tested with a suitably designed experiment, where the upper level lifetime of the laser transition of some Yb doped samples were investigated, obtaining results that are consistent with literature data and in good agreement with the theoretical results as for the temporal dependence of the fluorescence decay.
Nonradiative relaxation and photoluminescence quenching in nanocrystalline powders doped with rare-earth elements are of interest in optical bistability, random laser, and other optoelectronic applications. Here, the luminescence quenching of a one-dimensional random medium made of multilayer nanoparticles (Y2O3) doped with rare-earth elements (Yb3+) is analyzed by considering the transport, transition, and interaction of the fundamental energy carriers. The nonradiative decay and luminescence quenching in random media are enhanced compared to single crystals, due to multiple scattering, enhanced absorption, and low thermal conductivity. The coherent wave treatment is used to calculate the photon absorption, allowing for field enhancement and photon localization. The luminescent and thermal emission is considered as incoherent. The size-dependent absorption coefficient and penetration depth are observed. The nonradiative decay is identified as a multiphonon relaxation process, and is found to be enhanced compared to bulk materials. The luminescence quenching and nonlinear thermal emission, occurring with increasing irradiation intensity, are predicted.
I present a model devoted to linking the properties of a quasi-three-level laser crystal to microscopic parameters. The role of the radiative diffusion is taken into account with a sample-size-dependent parameter, including the total internal reflections, allowing both radiative and nonradiative diffusions to be treated equally. The radiation self-trapping aspect is quantified with the number η′imp of imprisoned photons engendered by one absorbed pump photon. The losses toward the traps that are not always well known are described with the dipole-dipole interaction, giving a microscopic interpretation of the heat dispensed in the sample from the pump. One prediction is that the excited-state decay of the donor subsystem is exponential, in agreement with experiment. The main experimental data, which are the excited-state intrinsic and extrinsic lifetimes τ and τ′ and the extrinsic fractional thermal loading ξ′, can be calculated by adjusting a small number of parameters. This number can be reduced by use of spectroscopy. I have shown that this model can be applied with a reasonable success to Yb3+-doped Y3Al5O12.