扫描近场光学显微镜的光耦合偶极子模型

为了研究扫描近场光学显微镜中探针和粗糙样品表面的耦合相互作用，提出了一种光耦合偶极子模型.在该模型中，探针和样品突起都由光极化偶极子表示，在准静态电磁场近似的情况下样品表面的诱导极化效应由影像偶极子表示，应用偶极子辐射理论可以得到系统的自洽场方程.此模型提供了一种直观分析扫描近场光学显微镜中探针和样品相互作用机理的方法.在此基础上，进一步讨论了金属样品的近场成像特点和其特有的局域光学共振现象.数值结果表明：不同于一般的介质样品，金属样品的近场图像与入射光频率直接相关，改变入射光的频率，获得的样品近场图像的形状和对比度都会发生变化.特别是当入射光频率处于样品极化共振范围内时，金属纳米粒子的极化率会出现光极化共振，这样就可以获得样品粒子的最大有效尺寸，为提高系统的分辨率提供了一条重要途径.

Optical Coupled-dipole Model for Scanning Near-field Optical Microscopy

An optical coupled-dipole model was proposed in order to study the coupling between the probe tip and the rough sample in SNOM. In this model both the optical probe tip and the sample protrusions were represented by optical polarizable dipole spheres. The induced polarization effects on the sample surface can be replaced by the image dipoles in the context of quasi-static electromagnetic field approximation. Applying the radiation theory of the dipole, a set of self-consistent equations have been established to describe the field distribution at the sites of the probe tip and the sample protrusions. This method permits us to analyze the physical mechanisms of the interaction between the probe tip and the rough sample surface in SNOM intuitively. On the basis of this approach, the characteristic of the near-field imaging and the localized optical resonance of the metallic sample were discussed. The calculating result indicates that its near-field images were directly related to the incident light frequency, and the shape and the contrast of the images were sensitive to the incident light frequency. Especially when the frequency of the incident light was within the range of the polarized resonance frequency, the polarized resonance of the nanoparticle will appear, and one can obtain the largest effective size of the sample particles. This may open a new way towards the better detection sensitivity in SNOM.