Third Harmonic Generation: anomalous behavior in the THG z-response and microscopy applications

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⁽³⁾ and the magnitude of dispersion.