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-$\mu$m diameter with ultradeep sag heights between 21 and 48 $\mu$m 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 $>$2.2 but which inertness makes it a challenging material in microfabrication. The performance of the lenses was characterized by mapping the transmitted intensity at different positions. The measured spot sizes of the produced beams are in excellent agreement with the theoretical expectations and range from 750 down to 250 nm ($\sim \lambda /2$) beam spot size for the shallowest and the deepest microaxicons in this study, respectively. The corresponding DOFs are from 500 down to $\sim 50$ $\mu$m 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.

中文翻译：

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通过聚焦氙离子束铣削铌酸锂中的超深微轴锥

折射轴棱镜是圆锥形光学器件，能够在扩展焦深 (DOF) 上产生非衍射类贝塞尔光束。除了与抛物面聚焦透镜产生的自由度相比显着更长的自由度之外，对于相同的光学元件形状因子（例如，其直径和下垂高度），轴棱镜可以产生具有更好分辨率的光束。这些特性使展棱镜可用于成像、粒子捕获和许多其他领域的众多应用中。由于缺乏足够精确和快速的制造技术，在硬基板中实现微型折射轴锥或微轴锥具有挑战性。在这里，我们报告了使用高电流聚焦氙离子束铣削在铌酸锂中快速制造超深微轴锥。具有 230-$\mu$m 直径，超深凹陷高度在 21 到 48 之间 $\mu$m 使用 200 nA 的束流进行铣削。此外，微轴锥在单晶铌酸锂中研磨——一种具有高折射率的材料$>$2.2 但惰性使其成为微加工中具有挑战性的材料。通过映射不同位置的透射强度来表征透镜的性能。所产生光束的测量光斑尺寸与理论预期非常一致，范围从 750 到 250 nm（$～\lambda /2$) 分别用于本研究中最浅和最深微轴锥的束斑尺寸。相应的自由度从 500 到$～50$ $\mu$m 为超深微轴锥。结果验证了使用聚焦氙离子束进行高电流铣削制造高性能光学元件的适用性。