Progress of high-resolution photon scanning tunneling microscopy due to a nanometric fiber probe

The present status of a photon scanning tunneling microscope (PSTM) and its application are reviewed. In order to realize a nanometric apertured fiber probe, a highly reproducible chemical etching process was developed to realize a sharpened fiber with the cone angle and tip diameter as small as 14° and 3 nm, respectively. The possibility of tailoring the shapes of the sharpened fibers was presented. Chemical etching and nanometric photolithography were developed to fabricate a metallic aperture with a diameter of 30 nm (or even smaller) on the sharpened fiber tip. Imaging experiments with biological specimens were carried out by operating the PSTM in the collection mode and illumination mode geometries. Dependencies of these images on the polarization state of the incident light were found, and a resolution of 10 nm or even smaller was achieved. Nondestructive inspection of dielectric optical waveguides with subwavelength resolution was proposed by presenting the diagnosed results of a proton-exchanged LiTaO₃ waveguide. Possibilities of diagnosing nanometric active photonic devices were also demonstrated through imaging experiments of semiconductor quantum dots. Experiments on fluorescence detection from dye-doped nanometric polystyrene spheres confirmed the enhanced efficiency of coupling of the fluorescence to the fiber tip, and this was attributed to the spatially inhomogeneous spontaneous emission due to the short-range electromagnetic interaction between the sphere and probe tip. To demonstrate the possibilities of nanometric fabrication, high density optical storage, especially the photon-mode storage, was demonstrated to realize a stored circular pit of 100 nm diameter on an organic thin film. As an ultimate goal of fabrication to explore the future technology of atomic-level material processing, an atom guide using a hollow fiber and atom trapping by the illumination mode PSTM were proposed to control the thermal motion of freely dying atoms in vacuum. The concept of a virtual photon based on an intuitive modeling of the localized evanescent light was introduced to provide a semiclassical theory of the PSTM. Transfer functions of the PSTM were calculated by using this model, which agreed qualitatively with the experimental results