NEAR-FIELD, FAR-FIELD AND IMAGING PROPERTIES OF THE 2D APERTURE SNOM

A two-dimensional aperture SNOM is studied computationally, using the Multiple Multipole (MMP) method. The relevant near- and far-fields as well as some imaging properties are determined. Strikingly different results are obtained for the two principal polarizations. For s-polarization the transmission of the aperture is extremely small, the field in the gap region highly confined, and the radiation pattern very sensitive to perturbations of the object surface. The far-field is emitted predominantly in forward direction. s-polarization provides high contrast for amplitude as well as phase objects. For p-polarization these properties turn out to be more or less the opposite; in particular, strong field enhancement occurs at the rims of the slit aperture and the far-field is emitted predominantly at some characteristic angle to the sides, p-polarization has the advantage of a much higher signal level and slightly higher resolving power, however at lower contrast. Line scan simulations for lambda = 488 nm indicate a resolution of two to three times the geometrical slit width. Resolution and contrast can be optimized by appropriate choice of detector orientation and angle of acceptance. For small gap-width a substantial amount of radiation is coupled into the substrate at angles larger than the critical one.