Ceramic phosphors, excited by high radiance pump sources, offer considerable potential for high radiance conversion.
Interestingly, thermodynamic arguments suggest that the radiance of the luminescent spot can even exceed that of the
incoming light source. In practice, however, thermal quenching and (non-thermal) optical saturation limit the maximum
attainable radiance of the luminescent source. We present experimental data for Ce:YAG and Ce:GdYAG ceramics in
which these limits have been investigated. High excitation fluxes are achieved using laser pumping. Optical pumping
intensities exceeding 100W/mm2 have been shown to produce only modest efficiency depreciation at low overall pump
powers because of the short Ce3+ lifetime, although additional limitations exist. When pump powers are higher, heat-transfer
bottlenecks within the ceramic and heat-sink interfaces limit maximum pump intensities. We find that surface
temperatures of these laser-pumped ceramics can reach well over 150°C, causing thermal-quenching losses. We also
find that in some cases, the loss of quantum efficiency with increasing temperature can cause a thermal run-away effect,
resulting in a rapid loss in converted light, possibly over-heating the sample or surrounding structures. While one can
still obtain radiances on the order of many W/mm2/sr, temperature quenching effects ultimately limit converted light
radiance. Finally, we use the diffusion-approximation radiation transport models and rate equation models to simulate
some of these nonlinear optical pumping and heating effects in high-scattering ceramics.