Image formation in near-field optics

An overview is presented of the image formation theory in near-field optical microscopy. The emphasis is placed on the basic concepts and the understanding of the images. We briefly recall the general principles used in near-field optics to break the resolution limit. Since some of the concepts widely used in optics become meaningless in near field, a brief critical review of basic concepts is given. A theory of scattering of electromagnetic waves by inhomogeneous surfaces is then presented. For objects much smaller than the wavelength, a closed-form expression of the scattered field is derived, which provides a link between the near field and the structure of the sample. The different set-ups and their imaging capabilities are analysed. A general relationship between the signal and the induced currents in the sample is derived by means of the reciprocity: theorem. The set-ups are compared and an equivalence between illumination and collection mode is proven: It is shown that; when multiple scattering between the sample and the rest of the system can be neglected, an impulse response can be defined for the three different types of set-ups : illumination mode, collection mode and apertureless. The importance of coherence in the near field is studied. Finally, the influence of the different control modes (constant height, constant intensity, constant tip-sample distance) is analysed and the existence of artifacts is discussed.