Using Fdtd Pstm Imaging Mechanism

Near-field optical microscope, a super resolution optical microscope, has been developed recently. It is beyond the diffraction limits of the conventional optical microscope and can obtains the image of nano-scale resolution of sample.As the development of the near-field optical microscope, the theory of near-field optical has also been developed. To this day, there have been several methods for analysis of the near-field optics such as Green Function Method, Multiple-Multipole Method, Perturbation method and so on. However, it is difficult to deal with complicated inhomogeneous material geometry with these methods. In this paper, a prevalent numerical calculation method, Finite-Difference Time-Domain ( FDTD ), is investigated which is simple, broad applicable, and can be applied directly in time domain electromagnetic field calculation.Photon scanning tunneling microscope (PSTM) is one of the near-field optical microscope. A beam of incidence light illumine with the incident angle bigger than critical angel, so the evanescent wave is give risen to on the surface of the sample holder. When a probe is inserted into the evanescent field, the evanescent wave will be frustrated and be coupled into the fiber probe through the top end of the tip so that the photon tunneling information can be obtained. The currently PSTM can't scan under constant distance mode. Being integrated atom force microscope (AFM) with PSTM, AF/PSTM can scan under constant distance mode and obtain more images.The image obtained by PSTM depends on a various complex factors such as the sample's optical properties, the polarization of the incident field, probe properties and so on. In this paper, FDTD method is applied to study the optical near field around mesoscopic surface structure. The results show that: the near field distribution depends on the topography and refractive index of the sample, and the polarization of the illumination light, hi photon scanning tunneling microscope based on the principle of the photon tunneling, the case of the evanescent wave coupled into optical probe is discussed with FDTD method.The near field distribution and the phenomenon of photon tunneling are discussed, then the interaction between the sample and the probe in mesoscopic scale is taken into account. Some results show that the optical probe interferes the original field distribution and the shape of the probe has great effect on the resolution. At the end of this paper, the scanning images of constant height and constant distance are discussed.