Direct laser writing of metastable modifications in lithium niobate crystal with ultrashort laser pulses

After the discovery that focused laser pulse is capable to locally change material's refractive index [1] it became possible to integrate various photonic devices or data directly into the volume of transparent material, usually using conventional Direct Laser Writing (DLW) techniques. Over the years this research field had rapidly gained momentum and many different photonic devices, passive and active, integrated in different kinds of material were demonstrated and tested. In majority of cases the change in refractive index comes from rearrangement (or damage) of materials lattice and usually remains permanent. However in particular cases, such as dynamic data recording, it would be beneficial to have possibility to manipulate over recorded information. This could be achieved in photorefractive crystals such as lithium niobate (LiNbO3). Laser-excited free carriers and bulk photovoltaic effect can cause uneven charge distribution in the material and through electro-optical effects this could lead to the refractive index change. Such data recording idea are not new and is used for decades in holographic recording techniques [2]. However, direct recording of metastable modifications with ultrashort laser pulses are quite scarcely investigated: it was demonstrated that it is possible to record rewritable bits in three-dimensional space of LiNbO3 crystal [3], also several integrated permanent photonic devices were demonstrated in such crystal [4,5], but many technological and physical questions still remains open