The influence of aberrations in third harmonic generation microscopy

We present an analysis of the effects of aberrations in third harmonic generation (THG) microscopy by considering different specimen geometries. Numerical simulations show the general trend that signal intensity and resolution are reduced as aberrations increase in amplitude. It is also shown that there are certain combinations of specimen structure and focusing position for which the presence of aberrations results in an increase in the image intensity. This occurs, for example, when there are several interfaces near the focal volume. The axial spreading of the excitation focal volume increases the characteristic coherence length for THG signal build-up, resulting in a significant contribution to the image brightness from axial planes near the focal plane. These results have important consequences for the interpretation of THG microscope images and for image based aberration correction in adaptive optics THG microscopy.