Third Harmonic Generation (THG) from the vicinity of interfaces, using focused laser beams can be obtained virtually
from any inhomogeneous medium. Its sensitivity to the presence and extent of inhomogeneity in the focal volume has
already found a variety of applications ranging from material characterization to label free three-dimensional
microscopy of biological samples. In this presentation, we demonstrate a number of new applications of THG in the
microscopy of food samples and living cells. Also, we report on an anomalous behavior in the THG z-response. So far
the observations and theoretical predictions supported a single peak of THG signal, with the peak position
corresponding to the interface. We have observed an anomalous behavior where a single interface can give rise to two
peaks located across the interface. The simulations, which we carried out using a paraxial theory of THG and
measurements done on typical normally dispersive materials, suggest that this anomalous behavior is due to a particular
combination of χ<sup>(3)</sup> and the magnitude of dispersion.
The general utility of two compact femtosecond laser sources for Third Harmonic Generation (THG) and Multiphoton absorption microscopy is demonstrated. The effect of aberrations due to a dielectric interface on THG microscopy is investigated both from a theoretical and experimental point of view. The significance of these aberration issues in third order nonlinear optical susceptibility (χ<sup>(3)</sup>) measurements using THG microscopy is discussed.