近场光学显微成像数值模拟研究

【作者】 王晓秋；

【导师】 吴世法； 简国树； 王克逸；

【作者基本信息】 大连理工大学 ， 光学工程， 2004， 博士

【摘要】 近场光学显微镜是近二十年来发展起来的一种新型超高分辨率的光学显微镜。它利用细小的光学探针探测样品表面隐失波的超高频空间信息，超越经典衍射极限的限制，可对样品表面纳米尺度区域的各种光学信息进行扫描成像，在生命科学、化学及材料科学领域中都有着广泛而重要的应用。 本文围绕对介观尺度范围的电磁场分布及近场扫描成像过程进行数值模拟而展开，目的在于能从理论上对近场成像过程有清楚的认识，正确地解释实验图像，达到理论与实验结果可比较的效果，从而为新仪器的开发提供理论依据。 本论文的内容分为三个部分： 一、为解决近场光学理论计算这样一个复杂的难题，将FDTD方法引入近场光学及成像系统中。首次提出和发展了光子扫描隧道显微镜(PSTM)系统中全内反射条件下入射波的设置方法——三波法，解决了激励源的设置问题。研究并实现了完全匹配层(PML)吸收边界条件的应用。实现了FDTD方法在金属薄膜中的应用，给出了等离子体动力学方程与麦克斯韦差分公式。完善了散射场公式在光子扫描隧道显微镜系统中的应用。较好地解决了数值模拟真实近场光学系统成像问题。 二、深入研究光子扫描隧道显微系统的物理成像机制。将微扰近似理论和FDTD方法用于PSTM系统中。将微扰近似理论推广应用到多层结构系统中，结合消假像理论，模拟了PSTM系统周期性样品的成像，解决了样品图像偏移问题，进一步完善了微扰近似理论在PSTM系统中的应用。本文还利用FDTD方法，研究和讨论了p-极化波照射条件下样品的近场分布与样品尺寸、厚度和折射率之间的关系。首次模拟演示了光子隧道效应现象。分别模拟裸光纤尖和镀膜金属尖在真实实验条件下扫描成像过程，讨论了扫描工作模式对光学图像及分辨率的影响，取得了与国外文献和实验一致的结果，并应用经典电磁场像-偶极子模型定性地理解PSTM系统成像并进一步解释获得高分辨率图像的机理。 常规(单光束-非对称照明)光子扫描隧道显微镜(PSTM)对样品的平整度要求很苛刻，仅对折射率均匀、只有形貌起伏的样品或表面很平、只有折射率变化的样品能分别得到形貌和折射率图像，而对于既有形貌起伏又有折射率变化的样品得到的却是二者的混合图像，造成图像解释方面的困难。为了解决这个题，我们介绍了新一代AF／PSTM组合显微镜的原理及AF／PSTM弯光纤尖实现分离形貌和折射率图像的理论依据，给出AF用STM双功能弯光纤尖共振恒振幅成像模式和AF/PSTM系统其框图，运用FDTD方法数值模拟，证明了AF/P STM中处理样品折射率图像的公式是可信的，存在一定的比例关系。数值模拟了玻璃基板上MgF:膜的折射率图像并得到与实验相符的结果，此外，还给出其它样品的一次扫描获得分离的光学与形貌图像。 三、发射模式扫描近场光学显微镜(SNOM)能够实现超衍分辨的技术核心是用亚波长尺度的光源，在SNOM光探针的设计中最具有挑战的任务是实现两个相互矛盾的参数的设计:即在保证足够大的通光效率的同时尽可能地减小光斑尺寸。为了实现更窄的光束斑点和更高的输出效率，我们设计一个由高折射率介质构成的、完全镀薄贵金属膜的、垂直顶角的四面锥探针尖。由于高折射率芯和薄金属膜，光在芯中传播并在金属尖端被转换为表面等离子激元，使输出通光效率加强并得到超小输出光斑。 最后，本文介绍了将垂直腔表面发射激光器(Vertical一cavity surface一Emitting Laser，vCsEL)直接与近场光探针尖祸合，即反射式扫描近场光学显微镜读写头阵列集成(Theintegrated Refleeted Seanning Near-field OPtieal MicroseoPy Reading/Writing Head，RSNOM一RWH)的概念设计。给出了实现几个关键技术的可能，如集成RSNOM一RWH和它的工作原理，探针头的设计与数值模拟。讨论了实现等间距扫描和超高数据转换速率的可能性。 本论文的研究结果表明: 1，利用电磁场理论和先进的数值计算方法—时域有限差分方法研究物体表面近场区域光与物质相互作用现象，如光的散射、反射、衍射、吸收及光谱等，是非常有用的。这对于近场光学基本物理现象的认识，对于正确理解近场光学显微图像所代表的物理本质以及新型近场光学显微镜的开发和应用都具有十分重要的意义。 2.对近场光学显微成像理论和数值的深入研究，有助于有目的地设计和操作实验，并将实验结果与理论进行核实比较。 3.新型光探针的设计为新型仪器的开发与设计打下了坚实的理论基础。更多还原

【Abstract】 Near- field optical microscope is a new type of super-resolution optical microscope developed for the recent twenty years. It obtains the various optical images of the sample’s nanometer surface, transcended the diffraction limit of conventional optical microscope, by using a fine optical-fiber tip probing ultra-high frequency optical information on the sample surface. And it has extensively been applied on life science, chemistry and material science fields.This thesis concentrates on the mesoscopic sample’s electromagnetic field distributions and numerical simulations of scanning imaging process of near-field optics. That aims at clarifying near-field imaging process theoretically, providing the scientific basis for correctly interpretation of near-field optical images and developing new class instruments.The thesis mainly includes the following three parts:1. The finite-difference time-domain method is applied to imaging system of near-field optics, in order to solve the complicated problem about theoretical calculation of near-field optics. The three-wave method under the total internal reflection in PSTM was proposed and developed firstly, resolving the incident source technique problem. The application of Perfectly Matched Layer absorption boundary condition is researched and carried out. It is realized that FDTD method is applied to metal thin film and the dynamics equations for plasma and Maxwell’s discretized formulas are presented. Here the scattered field formulas applied to Photon Scanning Tunneling Microscope are perfected. Numerical simulations of the imaging of system of realistic near-field optics are resolved well.2. The imaging mechanism of PSTM is explored deeply. The perturbation approach method and Finite-Difference Time-Domain method are applied to PSTM system respectively. The perturbation approach method combined eliminating spurious images theory is extended to the multiplayer system of PSTM. We simulated the imaging of a periodic sample for correcting the shift of the images of the sample. The perturbation approach method applied to PSTM is further perfected. The thesis studied and discussed the near-field distribution depending on scale and the thickness of the sample for p-polarized incident light. The photon tunneling effects are showed firstly. We simulated the scanning imaging process in the realistic experimentalconditions for using a bare probe tip and a metallic probe tip respectively. The scanning modes influenced on the images and the resolutions are discussed. The numerical results are consistent with the published articles and the experimental results. The high-resolution PSTM images can be qualitatively understood using dipole model of the classical electromagnetic field.The samples are limited for conventional PSTM imaging. The surface topographical images and the refractive index images can only be obtained for the samples of uniformity of the refractive index and topographical surface or flat surface and varied refractive index. The images of the samples of either varied refractive index or topographical surface show their mix images. In order to solve the problem, this paper introduces the principle of a new generation of AF/PSTM and theoretical basis of separated topography and refractive index images, which a bent optical fiber tip does a dither on constant amplitude and scans in constant mean gap mode in AF/PSTM. The diagram of AF/PSTM system is presented. The formula of refractive index imaging is proved by FDTD method. We simulated the images of MgF2 film with holes on optical glass and the results are consistent with the experimental results. Moreover, the images of separated topography and refractive index are presented for one scanning.3. The key element of scanning near-field optical microscopy (SNOM) breaking the diffraction limit to realize super resolution is the light source of sub-wavelength scale. The most challenge work is the design of optical probe in SNOM to realize two contrary demands: higher throughput and smaller light spot siz更多还原