Highly Efficient Lasing Action of Nd 3+ - and Cr 3+ -Doped Yttrium Aluminum Garnet Ceramics Based on Phonon-Assisted Cross-Relaxation Using Solar Light Source

We constructed a theory to explain the mechanism of laser generation with a high optical–optical conversion efficiency for Nd 3+ - and Cr 3+ -doped yttrium aluminum ceramics when sunlight or lamplight sources are used for pumping. As a result, a unique mechanism of laser action was found where the solar or lamp-light power could be converted to laser power with a high efficiency close to 80%, which has not previously been observed. The high conversion efficiency was not only considered to be based on one-to-one photon conversion but on two-photon excitation by a single photon with phonon assistance. Thus, the mechanism of lasing action should include a process where thermal energy is converted to photon energy. The theoretical results we obtained were consistent with those of the experiments.