IMAGE-FORMATION IN NEAR-FIELD OPTICS

An overview is presented of the image formation theory in near-field o ptical microscopy. The emphasis is placed on the basic concepts and th e understanding of the images. We briefly recall the general principle s 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 scat tering of electromagnetic waves by inhomogeneous surfaces is then pres ented. For objects much smaller than the wavelength, a closed-form exp ression of the scattered field is derived, which provides a link betwe en the near field and the structure of the sample. The different set-u ps 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 o f the system can be neglected, an impulse response can be defined for the three different types of set-ups : illumination mode, collection m ode and apertureless. The importance of coherence in the near field is studied. Finally, the influence of the different control modes (const ant height, constant intensity, constant tip-sample distance) is analy sed and the existence of artifacts is discussed.