Theory of electromagnetic field imaging and spectroscopy in scanning near-field optical microscopy

We derive a general expression for the signal in scanning near-field optical microscopy. This expression, based on the reciprocity theorem of electromagnetism, is an overlapping integral between the local electric field and a function that characterizes the tip. In particular, it includes the dependence on wavelength, illumination conditions, and polarization. To illustrate the potentiality of this theory, we discuss the polarization behavior and the spectral response of the apertureless setup.