Microstructuring of lithium niobate
This thesis presents the results from an investigation into methods for micron-scale relief structuring of lithium niobate. A wet etch consisting of HF and HNO3 was applied, and directed by 1) patterning the ferroelectric domain structure of the samples and 2) illuminating the crystals with patterned 488nm light. Post-etch treatment of the structures resulted in ridge waveguides and alignment grooves, while pre-etch manipulation achieved an etch-stop. Ablation was investigated as a method of directly structuring the crystal and for patterning photoresist. The etch was found to leave the +z face untouched. The -z face was etched at a rate, k, in µm/hour given by k = e 20.37 - 6300/T where T is the absolute temperature. This differential etch rate reveals a pattern induced in the ferroelectric domain structure by the technique of electric field patterning. The structures had walls with roughness < 5nm. Straight walls were easily achieved aligned along the y-direction at 120o to this. Other directions can result in facetted walls. Ridge waveguide losses <1dBcm-1, fibre alignment grooves and an etch stop were demonstrated using appropriate pre- and post-etch treatments. The etch was found to be affected by illumination with 488nm radiation. In Fe:LiNbO3 complete and partial frustration of the etch was induced on the -z face. Characteristic features of the partial frustration were sub-micron ridges and triangular pillars, separated by gaps as small as 500nm. In LiNbO3 the etch rate was found to increase on the -z face. The etch rate on the +z face was unaffected in both. Direct ablation with an excimer laser produced relief structures. Aspect ratios > 1:1 resulted in a dendritic structure in the ablated area. Direct ablation was suitable for patterning the photoresist. Surface damage was intentionally induced when producing large (>100µm) openings, however, the effect of surface damage on electric field poling could not be conclusively tested. Submicron openings were also created and subsequent poling produced sub-micron domains, revealed by etching.