Liu Qiang, Liu Tao, Yang Shuming, Li Guoqing, Li Shaobo, He Tao
Opt Express. 2021 Apr 12;29(8):12093-12109. doi: 10.1364/OE.419978.
The axial focus number (the number of focal spots along the axial direction) and focus intensity of a micro-Fresnel zone plate (FZP) are analyzed from deep ultraviolet to infrared using the Fourier decomposition, the vectorial angular spectrum (VAS) theory, and the three-dimensional finite-difference time-domain (FDTD) method. For a low-numerical aperture (NA) micro-FZP (NA<0.1), there are multiple axial high-order foci, and the intensity of each focus decreases slowly. However, the intensity of each high-order focus decreases rapidly with NA increasing. For a relatively high-NA micro-FZP (NA>0.3), the axial high-order foci are suppressed and there is one single focus. A fast, precise, and cost-efficient additive manufacturing method, i.e. two-photon polymerization, is used to fabricate high-NA phase-type micro-FZPs. The experiment has validated the phenomenon of linear negative focal shift of a high-NA micro-FZP. This property can be particularly applied in precise measurement of micro-displacement, film thickness, micro/nano step height, and wavelength.
利用傅里叶分解、矢量角谱（VAS）理论和三维时域有限差分（FDTD）方法，从深紫外到红外波段对微菲涅耳波带片（FZP）的轴向焦点数（沿轴向的焦点数）和焦点强度进行了分析。对于低数值孔径（NA）的微FZP（NA<0.1），存在多个轴向高阶焦点，且每个焦点的强度缓慢下降。然而，随着NA的增加，每个高阶焦点的强度迅速下降。对于相对高NA的微FZP（NA>0.3），轴向高阶焦点受到抑制，只有一个单一焦点。采用一种快速、精确且经济高效的增材制造方法，即双光子聚合，来制造高NA相型微FZP。实验验证了高NA微FZP的线性负焦移现象。该特性可特别应用于微位移、膜厚、微/纳米台阶高度和波长的精确测量。
