Main photorefractive Fe center in KNbO$_{3}$

KNbO$_{3}$ crystals have been attracting a lot of interest in nonlinear optics. In iron-doped samples, a dramatic increase in photorefractive sensitivity and speed can be achieved. Despite the variety of Fe centers known in KNbO$_{3}$:Fe, the Fe center responsible for the photorefractive effect has not been previously identified. Correlated EPR and optical studies of the as-grown and reduced samples shed light on the nature of the main photorefractive center in KNbO$_{3}$:Fe -- it is Fe[Nb]-V$_{\mathrm{O}}$, Fe on the Nb site next to oxygen vacancy. Fe[Nb] centers that are commonly considered as the cause of photorefraction in KNbO$_{3}$:Fe are only of secondary significance. Free electrons are provided through photoionization of Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$ with photon energies higher than 0.85 eV, Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ centers serve as electron traps. Concentration of the Fe[Nb]-V$_{\mathrm{O}}$ centers is quite comparable to that of Fe[Nb], but in the as-grown samples they are only present in the form of Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$. Reduction of the samples takes almost no effect on Fe$^{3+}$[Nb], but it appreciably converts Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ to Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$, which gives rise to the concentration of the photo-electron donors and dramatically improves photorefractive performance. Locations of the Fe$^{2+/3+}$[Nb]-V$_{\mathrm{O}}$ and Fe$^{3+/4+}$[Nb] centers in the bandgap will be presented.