Computer simulations: subwavelength resolution with an apertureless SNOM

It has recently been suggested that internal reflection geometry using uncoated optical fiber tips does not deliver subwavelength resolution. Recent experimental results, however, suggest the opposite. Here we model the internal reflection geometry by numerical solution of the full vector Maxwell equations for the electric field. A linearly polarized helium-neon laser is sent down the fiber and reflected from the tip and/or the sample in the near-field back into the liber. We find that detection in the orthogonal linear polarization mode (OP mode) does deliver subwavelength resolution, whereas detection in the incident linear polarization mode does not. We interpret this effect as depolarization in the near field of the tip-sample region. Best lateral resolutions of about 110 nm, i.e. less than a fifth of the wavelength have been obtained so far in the OP mode. We assume that this value can be improved; work is in progress to show this. The simulations also show that other operation modes, such as far-field excitation and local collection, do not deliver subwavelength resolution, which is consistent with established experiments.