Three and four-photon absorption of a multiphoton absorbing fluorescent probe

The utility of multiphoton excitation processes has been the subject of increased attention due to their potential applications in biophotonics, biology, and medicine through three-dimensional fluorescence imaging and photodynamic therapy. Evidence of this are the multiple applications of two-photon absorption (2PA) in fluorescence spectroscopy and 3D imaging over the last several years because of its large effective Stoke's Shift and high spatial resolution.[l,2] However, because the irradiation penetration depth of 2PA is limited in medical and biological applications due to the unwanted absorption and scattering when two red photons are used, the scientific communib recently started to explore higher order absorption processes at longer wavelengths such as three- (3PA) and four-photon absorption (IPA) that minimize the scattered light losses, and reduce the unwanted linear absorption in the living organism transparency window.