Growth by LHPG, Structure and Spectroscopy of Nd$\text{}^{3+}$-Doped Ba$\text{}_{2}$NaNb$\text{}_{5}$O$\text{}_{15}$ Nonlinear Single-Crystal Fibres

The renewal of the second order nonlinear crystals is very strong in laser materials optics. We are involved in the search for new systems based upon highly nonlinear niobate crystal family. Among these crystals, Ba$\text{}_{2}$NaNb$\text{}_{5}$O$\text{}_{15}$ is characterized by the highest nonlinear parameters but, unfortunately, it is difficult to grow crackless samples. The obtention of good quality and crackless Ba$\text{}_{2}$NaNb$\text{}_{5}$O$\text{}_{15}$ single crystals doped with different concentrations of Nd$\text{}_{2}$O$\text{}_{3}$ is reported. The crystals were grown as monocrystalline fibres by the laser heated pedestal growth technique. A new determination of the crystallographic structure of Ba$\text{}_{2}$NaNb$\text{}_{5}$O$\text{}_{15}$ has been performed. The structure of fibres doped with 1 at.% Nd$\text{}^{3+}$ was found orthorhombic with the new space group Pba2 instead of Cmm2. The well-known twinning of Ba$\text{}_{2}$NaNb$\text{}_{5}$O$\text{}_{15}$ due to the exchange of the a and b axes of the unit cell, which disturbs the optical properties, decreases as the Nd$\text{}^{3+}$ content of the fibre increases. Above 3%Nd$\text{}^{3+}$ ions, such monocrystalline fibres were found to be of tetragonal structure. Low temperature spectroscopy reveals that Nd$\text{}^{3+}$ ions substitute probably both Ba$\text{}^{2+}$ and Na$\text{}^{+}$ ions. The stimulated emission cross-section near 1060 nm of the $\text{}^{4}$F $\text{}_{3}\text{}_{/}\text{}_{2}$ → $\text{}^{4}$I$\text{}_{11}\text{}_{/}\text{}_{2}$ channel and its branching ratios are determined with the help of Judd-Ofelt analysis. This work shows that b-axis grown monocrystalline fibres are potentially self-doubling laser materials.