Generation and imaging with the third harmonic

In this paper we study the spatial distribution and wavefront characteristics of third harmonic generation in relation to some material and interface conditions over the focal region of the fundamental beam. We investigate, mostly from an experimental point of view, the implications the physics of the THG generation process has in situations where THG may be employed for -3D- imaging. Due to the non-linear character of the THG generation process it is inherently suitable for this application. For the first time images of the distribution of the THG radiation, as the interface is moved through focus, are shown. Experiments on closely spaced interfaces or bilayers confirm unambiguously the correctness of the vector model for THG generation (Ward et al. 1969) in uniform media. In view of these and other data the image formation, especially for biological objects, with THG radiation will be discussed.