Engineering and characterization of ferroelectric microstructures for photonic crystal applications

We report on the fabrication and characterization of periodic nanoscale one- and two-dimensional surface structures in congruent 500 µm thick lithium niobate crystal samples. Structures with periods from 2 µm down to around 500 nm, lateral feature sizes down to 200 nm and depths compatible with conventional waveguide fabrication, have been obtained. Such structures are fabricated by applying polarity selective etching to periodically domain reversed samples obtained by electric field poling performed by overpoling regime. Holographic lithography is used to obtain sub-micron periodic insulating gratings to be used for selective ferroelectric domain reversal. The short pitch structures are attractive in a wide range of applications ranging from nonlinear optics, for short-wavelength conversion processes or backward second-harmonic generation, to the field of photonic crystals to fabricate novel tunable photonic crystal devices or electro-optically modulated Bragg gratings. Moreover moire beating effect is used in the photolithographic process to fabricate even more complex structures which could find applications in complicated photonic bandgap devices involving for example micro-ring resonators. In order to investigate the possibility to utilize these structures for photonic crystal applications, accurate topography characterization has been performed by using different techniques. Atomic force microscope provides high-resolution information about the lateral and depth feature size of the structures, while interferometric techniques, based on digital holography, have been used for wide field information about the homogeneity and periodicity of the structures.