Ultradeep microaxicons in lithium niobate by focused Xe ion beam milling

Abstract

Refractive axicons are conically shaped optical devices that are capable of generating nondiffracting Bessel-like beams over extended depths-of-focus (DOFs). In addition to the substantially longer DOF compared to those produced by parabolic focusing lenses, the axicons can generate beams with better resolution for the same form-factor of the optical element, e.g., its diameter and sag height. These properties make the axicons useful in numerous applications in imaging, particle trapping, and many others. Miniaturized refractive axicons or microaxicons are challenging to realize in hard substrates due to the lack of sufficiently precise and rapid fabrication technologies. Here, we report on the rapid fabrication of ultradeep microaxicons in lithium niobate using high-current focused Xe ion beam milling. Microaxicons with 230-μm diameter with ultradeep sag heights between 21 and 48 μm were milled using 200 nA of beam current. Furthermore, the microaxicons were milled in single-crystal lithium niobate—a material with a high refractive index of >2.2 but which inertness makes it a challenging material in microfabrication. The performance of the lenses was characterized by mapping the transmitted intensity at different positions. The measured spot sizes of the produced beams are in excellent agreement with the theoretical expectations and range from 750 down to 250 nm (∼λ/2) beam spot size for the shallowest and the deepest microaxicons in this study, respectively. The corresponding DOFs are from 500 down to ∼50 μm for the ultradeep microaxicon. The results verify the applicability of high-current milling with a focused Xe ion beam for the fabrication of high-performance optical elements.