Volumetric integration of photorefractive micromodifications in lithium niobate with femtosecond laser pulses

After the discovery that focused laser pulse is capable to locally change material's refractive index it became possible to integrate various photonic devices or data directly into the volume of transparent material, usually with conventional Direct Laser Writing (DLW) techniques. Many different photonic devices, passive or active, integrated in different materials were demonstrated. In majority of cased the change in refractive index comes from rearrangement (damage) of materials' lattice and are permanent. Metastable (reversible) modification can be beneficial for some applications and these could be realized in photorefractive crystals such as lithium niobate. While photorefractive data recording is a well studied process in holographic applications, the photorefractive induction via femtosecond laser pulses is scarcely investigated. in this work we demonstrate the possibility to form discrete regions for homogeneously-altered refractive index in bulk of pure and iron doped lithium niobate crystals using femtosecond DLW technique. We shoe that non-linear free charge generation and charge separation caused by the bulk photovoltaic effect are the main contributing factors to the change in refractive index. Moreover, femtosecond pulse induced refractive index change can be by an order of magnitude higher than values reached with longer laser pulses. Femtosecond DLW opens opportunities for precise control of topological charge separation in lithium niobate crystals in volume and in micrometer scale. Various examples as well as strategies to control and manipulate refractive index change is presented and discusse