Structure sensitivity in third-harmonic generation microscopy

We characterize experimentally the influence of sample structure and beam focusing on signal level in thirdharmonic generation (THG) microscopy. In the case of a homogeneous spherical sample, the dependence of the signal on the size of the sphere can be controlled by modifying the Rayleigh length of the excitation beam. More generally, the influence of excitation focusing on the signal depends on sample geometry, allowing one to highlight certain structures within a complex system. We illustrate this point by focusing-based contrast modulation in THG images of Drosophila embryos.