Self-induced waveguides created by second harmonic generation in lithium niobate

In this paper we study second harmonic generation in Lithium Niobate in presence of photorefractive and photovoltaic effect. Our investigation reveals that the application of a strong external bias causes a self focusing effect that efficiently traps both the generated second harmonic and the fundamental beam, resulting in an improved conversion efficiency. The variation of the second harmonic output power during the build up of the space charge field also testifies that the phase relation between the interacting waves is affected by the photorefractive effect, and that the phase matching condition is modified in the part of the crystal shined by the light. Finally, we introduce a numerical model in order to explain the complex dynamic between the photorefractive effect and parametric process. The developed theory is in qualitative agreement with the performed experiments.