Polaron luminescence in iron-doped lithium niobate

Photoluminescence related to the bound polaron Nb_Li⁴⁺ is investigated as a function of temperature and incident light intensity in iron-doped lithium niobate crystals with various iron concentrations. Experiments are done under constant-wave (CW) and pulsed illumination. Its found that the decay time is always monoexponential. The radiative lifetime, the activation energy of the nonradiative lifetime and the quenching temperature are only weakly sensitive to iron concentration. On the other hand, the magnitude of the photoluminescence signal seems strongly correlated to the Fe²⁺ concentration, and the superlinear regime evidenced at low CW illumination definitely confirms that polaron excitation in lithium niobate is a two-step process.