Résumé
We present the investigation of second harmonic generation in congruent undoped lithium niobate crystal in presence of strong photorefractive and photovoltaic effect. We show that phase matching condition can be efficiently varied employing both the temperature and the electric field tuning. We also perform experiments on second harmonic generation with focused light, showing that the photorefractive effect is responsible for a strong distortion of the beam as well as a local modification of the phase matching condition. Finally, we demonstrate that an external bias can be used in order to switch from optical damage and distortion to self-confinement and guidance, reaching higher conversion efficiency through confinement and good overlap between interacting waves.