Contrast mechanisms in illumination-mode SNOM

We study theoretically and numerically the contrast mechanism in illumination-mode SNOM. We present a model, based on the Born approximation, which shows how the topographic and dielectric content of the sample is encoded in the measured signal. Under certain conditions, the image formation mechanism may be described by an impulse response that we define. Our model is in perfect agreement with numerical simulations of a two-dimensional model SNOM, under s-polarised illumination.