Refractive micro-lenses and micro-axicons in single-crystal lithium niobate.

The high refractive index of lithium niobate crystal (n = 2.2) and the highly transparent range (300-5000 nm), makes it a perfect material for refractive lenses and other types of micro-optical elements. This material already finds extensive use in waveguides and photonic crystals, however, little work has been done on producing refractive optical components in lithium niobate, presumably due to the challenges associated with its fragility and difficulties in three-dimensional micromachining. In this study, we fabricated high-quality refractive micro-lenses and micro-axicons with low surface roughness (< λ / 20), with 220 µm diameters and sag heights up to 22 µm in single-crystal LN using focused Xe beam milling. Xe ion beam milling is a flexible and rapid technique allowing realization of complex three-dimensional surface reliefs directly in lithium niobate. We characterized the optical performance of the fabricated elements showing sub-µm focusing capabilities of both the lenses and axicons.