基于高次谐波X射线光源的三维纳米相干衍射成像技术

Three-dimensional nano-coherent diffraction imaging technology based on high order harmonic X-ray sources

相干衍射成像是近20年才发展起来的一种高分辨率计算成像技术.其原理是通过采集相干光照明时样品产生的衍射图样,使用相位恢复算法计算实现样品结构的三维(3D)成像.区别于传统成像技术,该技术具有多个显著优势:1)成像分辨率接近于照明光源波长;2)成像系统简单,无需使用成像镜头,成像系统通常由相干光源、样品和CCD组成;3)无相差、色差,极紫外光子利用率高:使用计算成像,避免了引入器件的折射、反射和吸收等效应造成的相差和色差以及光子利用效率下降.自上世纪末,基于大型相干极紫外和X射线光源的相干衍射成像技术发展迅速,已达亚纳米级分辨率.此后,随着飞秒激光高次谐波技术的成熟,相干极紫外和X射线光源的体积和成本大幅度降低,相干衍射成像技术得到进一步发展和推广.发展至今日,基于高次谐波的相干衍射成像技术已经成为一种有巨大应用潜力的纳米成像技术,为半导体材料和器件表面形貌、生物微结构及动态演化、半导体和量子器件的化学成分及浓度分布、物理或化学动态过程以及量子状态等领域的探测成像提供了一种有效的技术方案,并开始在高分辨率半导体检测领域中获得实际应用.相信不久的将来,基于高次谐波相干衍射成像技术将成为纳米量级显微成像技术的杰出代表,成为和现有的原子力、近场光学、X射线、电子以及隧道扫描等显微成像相媲美的主流技术.本文回顾了相干衍射成像及其照明光源技术的发展历程,介绍了相干衍射成像技术现状和发展趋势,然后说明高次谐波光源和相干衍射成像技术原理,最后重点介绍了几种可以利用高次谐波的高相干、短波长、短脉冲及梳状超宽谱特性的衍射成像技术:探针强度约束、反射模式、频闪照相、多模态叠层、单次曝光叠层、时间分辨多路复用叠层、角度扫描相敏成像等技术.

Coherent diffractive imaging(CDI) using ultra-short wavelength light source has become an threedimensional(3D) nanoimaging technique.In CDI,a target sample is first illuminated by a coherent EUV and soft X-ray light,then the diffraction pattern is recorded by using a charge coupled device(CCD),and finally the image of the sample is obtained based on the pattern by using a phase retrieval algorithm.Of the many currently available coherent EUV and soft X-ray light sources,the high-order harmonic generation(HHG) is the simplest in structure,the lowest in cost,and most compact in size.Therefore,it has become the most promising light source for CDI.Through years of development,HHG based CDI technique(HHG-CDI) has become an outstanding 3D nano-imaging technique with the advantages of no aberration,no damage,and no contact either,and it also possesses the extra-capabilities of probing the dynamics,chemical composition and quantum information in various semiconductor and quantum devices.We believe that the HHG-CDI will soon become a generic nano-imaging tool that can complement or even replace the matured nano imaging techniques,such as atomic force,near field,X-ray,electron,or scanning tunneling microscopes.