The fundamental features of third-harmonic generation microscopy are examined both theoretically and experimentally, and the technique is applied to the characterization of layered structures. Measurements and model calculations have been performed of the third-harmonic yield generated from homogeneous layers. Model calculations based on the paraxial approximation show good agreement with the experimental results, despite the conditions of high numerical aperture. The method proposed here allows for the determination of (i) the layer’s third-order susceptibility relative to that of the substrate, (ii) its index of refraction at the third-harmonic frequency relative to that at the fundamental frequency, and (iii) its thickness and for the identification of a gradient region between two adjacent layers.
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