Non-linear effects and role of scattering in multiphoton imaging of thick biological samples

Non linear optical scanning microscopy has became a useful tool for living tissue imaging. Biological tissues are highly scattering media and this leads to an exponential attenuation of the excitation intensity as the light travels into the sample. While performing imaging of biological scattering tissues in non linear excitation regime, the localization of the maximum 2PE intensity was found to shift closer to the surface(1) and the 2PE imaging depth limit appears strongly limited by near surface fluorescence.(2) In this work we computed the illumination and the photobleaching distribution(3) in order to characterize the effects induced by scattering. The simulations have been performed for different scattering coefficients and different focus depth. An experimental test has been carried out by imaging, with 0.9 numerical aperture objective, thick scattering fluorescent immobile sample (polyelectrolyte gel). Results confirm that under these conditions no photobleaching effects due to scattering occur close to the surface.