Luminescence Anisotropy and Thermal Effect of Magnetic and Electric Dipole Transitions of Cr 3+ Ions in Yb:YAG Transparent Ceramic.

In this article, we present an in-depth optical study on luminescence spectral features and the thermal effect of the magnetic dipole (MD) transitions (e.g., the R lines of 2 E → 4 A2 ) and the associated electric dipole transitions (e.g., phonon-induced sidebands of the R lines) of Cr3+ ions in ytterbium-yttrium aluminum garnet polycrystalline transparent ceramic. The doubly split R lines predominately due to the doublet splitting of the 2 E level of the Cr3+ ion in an octahedral crystal field are found to show a very large anisotropy in both emission intensity and thermal broadening. The large departure from the intensity equality between them could be interpreted in terms of large difference in coupling strength with phonons for the doubly split states of the 2 E level. For the large anisotropy in thermal broadening, very different effective Debye temperatures for the two split states may be responsible for it. Besides the 2 E excited state, the higher excited states, for example, 4 T1 and 4 T2 of the Cr3+ ion, also exhibit a very large inequality in coupling strength with phonons at room temperature. By examining the Stokes phonon sidebands of the MD R lines at low temperatures with the existing ion-phonon coupling theory, we reveal that they indeed carry fundamental information of phonons. For example, their broad background primarily reflects Debye density of states of acoustic phonons. These new results significantly enrich our existing understanding on interesting but challenging luminescence mechanisms of ion-phonon coupling systems.