Polarization-dependent contrast in near-field optical microscopy

This article presents a simple modeling and simulation of experiments with a near-field optical microscope or a photon scanning tunneling microscope (PSTM) in the sub-100-nm range. The simulation employs a semimicroscopic and perturbative approach based on field propagator and linear response theory. A probe tip and sample are approximated as nanometric spheres in order to clarify the behavior of the near-field and far-field signal intensities, I, and the contrast, i.e., visibility, (I-max-I-min)/(I-max+I-min), for s and p polarization of incident light and three scanning methods: constant height, constant intensity, and constant distance. The signal intensity then becomes a function of the taper angle of the fiber probe tip B or the numerical aperture of the collecting lens, in addition to the variables mentioned above. Note that the signal intensity I(B = 90 degrees) corresponds to that for the near-field. The simulated polarization-dependent intensity and contrast are in good qualitative agreement with the experimental results. At the same time, for each polarization and scanning method, there is an optimal angle B for maximizing the contrast and maintaining a high signal intensity. This result indicates that the taper angle and scanning method are very important factors in the polarization-dependent contrast and resolution of near-field optical microscopy. (C) 1997 American Vacuum Society.