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Optical imaging with nanoscale resolution, beyond that possible with conventional diffraction-limited microscopy, may be achieved by scanning a nano-antenna in close proximity to a sample surface. This review will first aim to provide an overview of the basic principles of the technique of scanning near-field optical microscopy, before moving on to consider its most widely implemented form, in which the sample is illuminated through a small aperture held less than 10nm to the sample surface, for optical imaging with a resolution of about 50 nm. The exciting new possibilities for high-resolution optical imaging and spectroscopy promised by “apertureless” near-field optical microscopy are then considered. Such techniques may involve local scattering of light from a sample surface by a tip, local enhancement of an optical signal by a metal tip, or the use of a fluorescent molecule or nanoparticle attached to a tip as a local optical probe of a surface. These new optical nanoprobes offer the promise of optical microscopy with true nanometer spatial resolution.

We study numerically the constant-height image in transmission mode of photon scanning tunneling microscopy (PSTM). The local field inside the probe tip is calculated, which scans over a dielectric rectangular structure. The substrate is illuminated by a s-polarized incident light with oblique angle. The influence by the dielectric functions of the tip and the lateral size of the structure on the optical signal are analyzed. We find out that the transmitted intensity is correlated with the shape of the sample and the assumption of passive probe may be a reasonable approximation when the optical constant of the tip is close to that of air.