Enhancement of the Erbium Concentration in RbTiOPO₄ by Codoping with Niobium

A thorough study of the RbTiOPO 4 ( RTP) crystallization in its self-flux and WO 3 -containing fluxes (10, 20, and 30 mol % WO 3 ) has been performed. The composition regions and isotherms of crystallization were obtained, and most of the crystallized neighboring phases were identified. Afterward, the possibilities of doping and codoping RTP crystals with Er 3+ and Nb 5+ were studied. Adding Nb 2 O 5 substituting TiO 2 in the solution increases the distribution coefficient of Er 3+ but changes the crystal morphology toward thin plates significantly. This means it is difficult to grow crystals of sufficient quality and size for research and applications. To optimize the crystal growth process, the conditions for growing doped and codoped RTP single crystals with Er 3+ and Nb 3+ by the top seeded solution growth technique (TSSG) were studied. For crystal growth from self-flux, stirring the solution with an immersed platinum turbine significantly increased the efficiency of the crystal growth process. These conditions allow achieving 0.65 × 10 20 atom·cm - 3 as an Er 3+ dopant concentration in the crystal. The Judd−Ofelt parameters for Er 3+ in RTP:Nb determined from the 300 K optical absorption spectra are Ω 2 = 5.99 × 10 - 20, Ω 4 = 0.54 × 10 - 20, and Ω 6 = 0.37 × 10 - 20 cm 2 . Finally, the second harmonic generation (SHG) efficiency of RTP:Nb single crystals increased as the concentration of Nb increased up to a 4 atom % of Ti 4+ substitution, after which the SHG efficiency decreased.