Applications Of FDTD And Green Function Methods In PSTM Imaging

As photon scanning tunneling microscope (PSTM) reaches a resolution far beyond the diffraction limit, it becomes a powerful technique to research nano scale world. The imaging properties of PSTM remain complicated because of various influences. In order to guide the rapid experimental development and to understand the contrast mechanisms in near field, it is necessary to propose theoretical approaches and simulation techniques. The finite difference time domain (FDTD) method and Green tensor technique are both effective methods. In this paper, both methods above are used to study near field distribution in PSTM.In this paper, FDTD algorithm is used to study the image of the PSTM fiber tip. The near field distribution of metal-coated aperture fiber tip is first simulated. Several parameters, such as polarization of incident wave, distance between tip and sample, thickness of coated film and different metal film, are discussed. Results reveal preferable parameters for setting experimental parameters. On the other hand, a novel fiber tip is proposed in this paper. The novel fiber tip is characterized that nano-composite film is covered on the uncoated fiber tip. By controlling the location of silver particle in film, metallic nano-particle probe can be manufactured. The novel probe tip reaches the resolution of metallic nano-particle probe, but overcomes the shortcomings of frangibility and unsteadiness. The image of this probe is also simulated with FDTD method in order to provide better parameters for manufacturing.Each theoretical method has its drawbacks and strengths, so it only gives proper explanation to some extent. Green tensor technique frees from setting boundary condition of problems during simulating and needn't calculate points outside scattering objects and observing points. In this paper, Green tensor technique is used to simulate near field distribution of PSTM. Precondition conjugate gradient (PCG) technique is proposed to accelerating convergence. The results by Green tensor technique are basically consistent with that by FDTD method. In this paper, the limitation of Green tensor method with PCG technique is also analysised, so the solution and perspective of PCG and fast fourier transform (PCG-FFT) technique is presented. This technique can accelerate the computation and reduce storage considerably. Then extend confinement of green tensor.