Conditions and possibilities for rare-earth doping of KTiOPO4 flux-crown single crystals

The growth of rare-earth (RE = Nd, Tb, Ho, Er, Tm, and Yb) doped KTiOPO4 (KTP) single crystals from high-temperature solutions [K2O-P2O5-TiO2-RE2O3 (or Tb4O7)] was investigated. The concentration regions of crystallization of RE-doped KTP, the maximum (critical) RE concentrations in the solution without losing the KTP phase and the corresponding upper limit of RE ions incorporated into the KTP lattice were obtained experimentally. The crystallization regions of RE-doped KTP are significantly narrower than the region of undoped KTP. Above a certain critical RE concentration in the solution, RE phosphate phases crystallize. The highest concentration of RE in the crystal is in the range 50-600 ppm and strongly depends on the RE ionic radius. Some double substitutions with Nd and a codopant element can significantly change this concentration. In addition, RE-doped KTP single crystals were grown by the top seeded solution growth (TSSG) technique, and their experimental conditions were compared with those of undoped KTP single-crystal growth. Optical absorption spectra for Ho-, Er-, and Tm-doped KTP were obtained and discussed.