Optically smooth microchannels in the volume of lithium niobate fabricated by selective etching of fs-laser written structures and subsequent annealing

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3D-hollow microstructures with few tens of micrometer in diameter with an optical-quality surface roughness (Ra ≤ 1 nm) have been fabricated in the volume of lithium niobate by selective etching of fs-laser written structures and post-etching annealing. The fs-laser writing parameters and the annealing process have been refined to reduce the average surface roughness and the shape change. Systematically investigating the annealing process, an empirical functional description of the temporal evolution of the surface roughness was found completing the data set of processing parameters for selective etching of fs-laser written structures, allowing to control the fabrication process of the hollow microstructures concerning both shape and surface roughness. Thus, our results represent another milestone within the research towards monolithic micro-(opto)fluidic applications inside the multifunctional crystal lithium niobate.