Photon Scanning Tunneling Microscope (PSTM) is a near-field optical microscope that can measure local optical properties with high resolution beyond the diffraction limit and was widely applied in practices in recent years. The resolution of PSTM, which mainly depends on the shape of the taper tip, is an important issue to be discussed in the application. In this paper, the near-field distribution around a new PSTM probe is simulated by the method of 3-D Finite-Difference Time-Domain (FDTD). In this model, a nanometric metallic pyramid is attached at the apex of the metal-coated probe. Considering the interaction between the sample and the probe tip, the near-field distribution in a section at certain height is plotted as a function of the various sample positions. In order to optimize the optical property of this kind of optic fiber probe tip, the influence of the parameters of the taper tip can also be studied. To understand the effect of the probe film and the metal tip, the electromagnetic field distribution in the vicinities of the sample and the fiber probe during the third period is plotted. Thus, these simulated results offer references for the selection of the probe shape in experiments.
The combination of plasmon near-field scanning optical microscopy (PNSOM) and Raman spectroscopy named Near-field Scanning Raman Microscopy (SNRM) provides not only surface topography information but also chemical structural information of sample with nanometer spatial resolution, which are very important for a wide range of applications, such as the study of liquid sample, nanometer film sample, quantum dot, single molecules of biological samples and so on. But Raman scattering cross-section is too small to get Raman signal of nanometer structure, and surface enhancement Raman scattering (SERS) effect is the main technique to solve this problem. Local electric field distribution and the form of the hot spots are evaluated by the FDTD (finite difference time domain) method in SERS with respect to many kinds of models. As a result in this paper, (1) vast Raman enhancement factor of 1015 was obtained around the junction between the Ag ball (tip) and the Ag thin film (sample stage). (2) The enormous large electric field at the hot spots rapidly decays with increasing gap between the Ag ball and Ag thin film. In the process, we use the equivalent incident wave method to deal with the incident evanescent wave problem and a frequency-dependent finite-difference time-domain formulation ((FD)2 TD) to deal with the negative permittivity of Ag, and the validity of these two methods have been approved by references
It is important to analyze the images obtained by photon scanning tunneling microscope (PSTM). Recently some theories have been developed including finite-difference time-domain (FDTD) method applied in near field optics. In this article wc extend a simple FDTD method (equivalent incident wave method) to PSTM system, which has been widely used to detect objects buried underground in geophysics. In PSTM system lighting wave is evanescent wave, that is the equivalent incident wave exciting the samples and probe over the interface. Numerical simulations show that the results ofthis method are in agreement with the results ofthe moment method and perturbation theory.
Our research group has recently developed a new type scanning probe microscope —AF/ PSTM. Using this setup, the optical false image caused by the inclination of sample surface can be eliminated; the optical image and the topography image are separated; and also two optical images (refractive index image, transmittivity image) and two AFM images (topography image, phase image) are obtained during one scanning. As a primary biologic application, this setup is engaged in the imaging of some biologic samples. The primary images ofthese biologic samples are obtained. Due to the advantage of AFIPSTM, four images can be acquired at the same time during one scanning. Consequently more information ofsample is given by comparing these images. This work shows that the AF/PSTM may be improved to be a useful tool in biology research.
Photon Scanning Tunneling Microscope (PSTM) is a newly developed technique. As a near-field optical microscope, it has the high resolution breaking through the diffraction limit and the advantage for easy to prepare the samples. PSTM is a useful tool in the inspection for many kinds of material film as the image containing the sample's information of topography and refractive index. We have developed a PSTM with resolution 5 to approximately 10 nm and scanning range 6 X 6 micrometer. By making use of two lasers, the false image caused by inclination of sample surface can be reduced. This PSTM is engaged in the inspection of Al2O3 optical waveguide film made using ion-beam-enhanced-deposition (IBED) technique at different substrate temperature. The PSTM images of the optical waveguide film are obtained and analyzed. The PSTM images show that as the increasing of the substrate temperature during the deposition, the sample images of refractive index and topography tend to smooth and even, consequently the scattering loss can be decreased. The conclusion is that by properly increasing the temperature of the substrate during the deposition period, the scattering loss can be decreased and the property of Al2O3 optical waveguide can be improved.
A comparison between PSTM and A-SNOM in resolution, contrast and optical efficiency is discussed. The order of magnitude roughly estimated shows that PSTM is must better than commercial A-SNOM.
KEYWORDS: Light sources and illumination, Diffraction, Super resolution, Refractive index, Near field scanning optical microscopy, Scanning tunneling microscopy, Numerical simulations, Near field optics, Photonic microstructures, Optical microscopy
This paper attempts to answer the question of 'how to explain the image of photon scanning tunneling microscope (PSTM)'. To explain the image of PSTM, the essential difficulty is the false image information within the complex image of the topographical image and refractive index image of sample. In this paper, we have derived the PSTM imaging formula and introduced the eliminating false image information method and image separating method, and discussed the key to super-resolution of near-field optical microscopy. Numerical simulation and experiments with the method of perturbation diffraction combined with (pi) - symmetric lighting eliminating false image information are provided.
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