Imaging properties of an extreme ultraviolet microscope objective with reduced Fresnel number
Imaging properties of an extreme ultraviolet microscope objective with reduced Fresnel number
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DOI：
10.1016/j.optcom.2017.08.031
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发表时间：
2017-12
影响因子：
2.4
通讯作者：
M. Toyoda;Kota Kuramitsu;M. Yanagihara
中科院分区：
文献类型：
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作者：
M. Toyoda;Kota Kuramitsu;M. Yanagihara
在全视场极紫外显微镜成像中，由于高放大倍率物镜显着减小了像平面上的数值孔径，所以像平面上的菲涅耳数低于1，而物平面上的菲涅耳数在大多数情况下相对较大。为了理解在这两个平面上菲涅尔数相差很大的高倍物镜的成像，在本研究中，我们通过观察点物体在物侧和像侧的离焦图像来实验证实了成像特性。实验表明，像侧的离焦特性相对于探测器位置是不对称的，而物侧的离焦特性相对于物距是对称的。为了解释这些非常规成像特性，我们提出了一个简单的分析模型，考虑高倍物镜上的两个不同的菲涅耳数。该模型表明，即使像平面在焦深范围内，放大倍率也会发生变化，从而产生不对称的散焦特性。同时，当我们沿光轴移动物体时，散焦像差可以用大菲涅耳数系统的常规方程来表示，这可以很好地解释物侧的对称散焦特性。我们还讨论了修改图像平面上的幅度的附加相位因子的影响。
On imaging for full-field extreme ultraviolet microscopy, the Fresnel number on the image plane falls below unity since a high magnification objective remarkably reduces the numerical aperture on the image plane, while the Fresnel number on the object plane is relatively large in most cases. To understand imaging with the high-magnification objective with far different Fresnel numbers on these two planes, in this study, we experimentally confirmed the imaging properties by observing through-focus images of a point object on both the object and image sides. The experiments showed that, the defocus characteristics on the image side were found to be asymmetric with respect to the detector location, while those on the object side were found to be symmetric with respect to the object distance. To explain these unconventional imaging properties, we proposed a simple analytical model considering the two different Fresnel numbers on the high-magnification objective. The model showed that the magnification would vary even if the image plane was within the focal depth, and this yields the asymmetric defocus characteristics. At the same time, when we moved an object along an optical axis, the defocus aberrations were represented by the conventional equation for a large-Fresnel-number system, which can well explain the symmetric defocus characteristics on the object side. We also discuss the effect of an additional phase factor that modifies the amplitude on the image plane.