Scattering-Based Hole Burning in Y3Al5O12:Ce3+ Monoliths with Hierarchical Porous Structures Prepared via the Sol-Gel Route

We have synthesized hierarchically porous Y3Al5O12 (YAG) ceramics doped with trivalent cerium (Ce3+) ions (YAG:Ce) by the sol gel process and examined their optical properties including scattering strength. Starting from the ionic precursors, i.e., AlCl3. 6H(2)O, YCl3 center dot 6H(2)O, CeCl3 center dot 7 H2O, and poly(ethylene oxide) (PEO) dissolved in a mixture of water and ethanol, monolithic wet gels are synthesized using propylene oxide as a geletion initiator. PEO induces phase separation parallel to the gelation in the sol-gel system to form bicontinuous morphologies consisting of gel- and fluid-phases. The wet gels thus obtained are subjected to either evaporative or supercritical drying, and then heat-treated to crystallize the gel skeletons to form YAG phase. The drying process critically affects the nanostructures of YAG skeletons after heat treatment; the supercritical drying results in skeletons consisting of fine grains (<50 nm), while evaporative drying yields skeletons with larger grains. The scattering strength of the heat-treated samples is evaluated by using coherent backscattering measurements and scattering-based hole burning, both of which clarify that the samples prepared via evaporative drying scatter visible light much more strongly than that prepared via supercritical drying.