Design of Yb3+ optical bandwidths by crystallographic modification of disordered calcium niobium gallium laser garnets

Abstract

{Ca□}[NbGa□](GaNb□)O-type cubic Ia3d garnets (CNGG) have been grown by the Czochralski method, either unmodified or incorporating different crystalline modifiers (Li or Mg), and Yb as a laser dopant. From nuclear magnetic resonance, single crystal X-ray and powder neutron diffraction studies it is shown that Li incorporates exclusively into the 24d tetrahedral site of the host, removing tetrahedral-sited Nb and filling cationic vacancies in this site. A comparison of low temperature (6 K) Yb spectroscopy in crystals with different compositions at the tetrahedral sites and a modeling of energy positions of the F(0) and F(0′) Yb levels show that the strongest contribution to the Yb optical absorption/emission bandwidth is associated with the electric charge of cations/vacancies occupying the two tetrahedra at the shortest distance (3.12 Å) from the central 24c dodecahedral Yb. Cationic disorder over the remaining four tetrahedral, octahedral and dodecahedral sites also contributes to the Yb bandwidth but to a lesser extent. To obtain the largest Yb bandwidth, the two nearest tetrahedra must contain cations/vacancies with electric charges as different as possible. Although the decrease in the concentration of vacancies at tetrahedral sites associated with Li incorporation induces some reduction of the Yb optical bandwidth with regard to the unmodified Yb:CNGG crystal, Li incorporation along with the use of high purity precursors yields crystals with less coloration, longer Yb lifetime, and slightly larger thermal conductivity, which favors laser operation performance.