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 optica l microscopy. This expression, based on the reciprocity theorem of electrom agnetism, is an overlapping integral between the local electric field and a function that characterizes the tip. In particular, it includes the depend ence on wavelength, illumination conditions, and polarization. To illustrat e the potentiality of this theory, we discuss the polarization behavior and the spectral response of the apertureless setup. (C) 2000 American Institu te of Physics. [S0021-8979(00)02821-8].