Conversion of broadband thermal radiation in lithium niobate crystals of various compositions

The conversion of the broadband thermal radiation in stoichiometric ($R$ = 1) lithium niobate single crystals that are grown from melt with 58.6 mol% of LiO$_2$, congruent ($R$ = Li/Nb = 0.946) melt with the K$_2$O flux admixture (4.5 and 6.0 wt%), and congruent melt and in congruent single crystals doped with the Zn$^{2+}$, Gd$^{3+}$, and Er$^{3+}$ cations is studied. It is demonstrated that the conversion efficiency of the stoichiometric crystal that is grown from the melt with 58.6 mol% of LiO$_2$ is less than the conversion efficiency of congruent crystal. In addition, the stoichiometric and almost stoichiometric crystals and the doped congruent crystals exhibit the blue shift of the peak conversion intensity in comparison with a nominally pure congruent crystal. For the congruent crystals, the conversion intensities peak at 520 and 495 nm, respectively.