基于希腊梯子编码的阵列分束光子筛的逆向设计研究

As the development of fourth-generation synchrotron radiation and x-ray free electron laser, the problem how to get the coherent x-ray source is seemed to be solved. X-ray microscope imaging becomes a research hotspot once again and has given more and more attention at home and abroad. However, refractive lenses are prevented from focusing and imaging in these spectral regions because of strong absorption of solid materials, so the diffraction imaging method is widely adopted. There is no doubt that Fresnel zone plate with one pronounced focus is currently only choice for transmitted x-ray focusing and imaging. Obviously, the scarcity of x-ray focusing and imaging devices is the tightest bottleneck in the historical development of x-ray optical technology. Based on our previous research, in this application project we here propose a new family of diffractive optical element – Greek-ladder nano-photonic device, which can realize three-dimensional array diffraction-limited focusing and imaging. Even more important, the optical ratio which can be defined by the ratio of two focal lengths can completely overlap the mathematical characteristic which is defined by the ratio of two adjacent terms. This property can be considered as a foundation for the reverse design of Greek-ladder photon sieve. Finally, on the basis of focal spot splitting technology presented in our previous work, the generalized design method of x-ray array beam splitting will be presented in the future. The optical functionality mentioned above not only extends a wide variety of x-ray focusing and imaging devices, but also deeply improve the flexibility of optical system design and enhance the development of optical technology in the region of x-ray. In addition, x-ray beam splitter will be also applied to three-dimensional array holography imaging for living cell, optical measurement, array photolithography, material science and space science, and so on.

随着第四代同步辐射和x射线自由电子激光器的出现，相干x射线光源的问题得以有效解决，使得x射线显微成像技术再次成为国内外研究的热点。但材料在该波段的强吸收使得无法使用传统的折反式元件成像，波带片成为目前唯一可用的衍射聚焦成像元件。显然，功能多样的x射线聚焦成像器件的缺乏成为当下制约x射线光学技术发展的瓶颈。基于前期光子筛和波带片的研究，本次项目申请在单焦点光子筛的基础上提出了多焦点的希腊梯子光子筛，用于实现x射线的阵列分束。通过揭示光学焦比与数学特征重合的物理机制，为希腊梯子光子筛的逆向设计奠定数理基础。研究焦斑分裂技术对阵列分束的影响，提出x射线阵列分束的普适性技术方案。本项目的研究不仅扩充了x射线器件的类别和多样性，使得x射线领域有了除波带片以外的其它功能器件的选择，极大地促进x射线光学技术的发展，而且有望对x射线细胞全息成像、光学检测、光刻技术和空间科学等领域起到积极的推动作用。

光学系统的空间分辨率与入射光波长成正比，与数值孔径成反比。然而当波长缩短至极紫外波段及更短时，材料的强吸收导致无法直接折射聚焦成像，x射线分束器的缺失意味着光学段的经典干涉衍射成像技术难以直接应用于短波领域。另一方面，高相干短波光源难以获取，也是限制x射线技术发展的关键问题。随着第四代同步辐射和x射线自由电子激光器的出现，相干x射线光源的问题得以有效解决，聚焦器件对短波成像技术的强烈发展需求显得更为迫切。项目基于课题组前期在波带片和光子筛方面的研究工作，首次发展出了多焦的古希腊梯子光子筛，在光学段设计加工了相应的器件并进行实验验证，实现了单层有两焦斑和十一方形阵列焦斑的三维阵列聚焦，实验值与理论值一致，证明了器件设计的正确性及准确度。在多焦的古希腊梯子光子筛基础上，开展了系列的应用研究工作，尤其是以光学传感与成像为主要研究对象。光束质量直接关联到光学聚焦成像效果，因此光束质量的高精度检测是调控相干光束的前提条件。用超分辨光子筛代替哈特曼波前传感的微透镜，通过高精度的质心定位进一步提升波前测量精度，实现了等数值孔径下的高精度哈特曼波前传感。提出了轴上两焦和三焦光子筛的单次曝光强度传输方程和单次曝光曲率传感方案，促进了波前传感领域的技术发展。针对x射线提出了多焦光子筛的自干涉成像技术，在光学段用鉴别率板实验验证了方案的可行性和有效性，这为x射线的干涉成像提供了新的技术途径。

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