Electric field enhancement by a nanometer-scaled conical metal tip in the context of scattering-type near-field optical microscopy

摘要

We present a numerical study of the electric field enhancement in the immediate vicinity of the apex of a conical silver tip and show that an optimal cone angle exists, allowing one to maximize the electric field. This angle depends on the tip length, the wavelength, as well as on the distance from the apex to the observation point. So both the angle and length of the tip can be considered as parameters to adjust the peak enhancement resonant position for a laser source wavelength. At the same time, reducing the cone angle does not ensure a concurrent increase in the electric field enhancement. A simple qualitative interpretation is proposed to explain this phenomenon based on competition of two mechanisms affecting the electric field near the tip apex. The results obtained show that the point-like dipole approximation is invalid for description of the field enhancement of a finite-size metal tip in the case of scattering-type near-field optical microscopy. One more conclusion is that the model of a sharp semi-infinite perfectly conducting tip is also not adequate in our case.