Nonlinear fluorescence through intermolecular energy transfer and resolution increase in fluorescence microscopy

We investigate a novel concept to efficiently generate multiphoton induced fluorescence from organic molecules. The concept is based on frustrating the energy transfer between a fluores- cent donor and one or more acceptors in conjugated molecules. The nonlinearity is not based on higher order molecular susceptibilities but entirely on their linear properties. Therefore, in contrast to nonresonant multiphoton absorption, this method does not require high local intensities. Like- wise, the production of visible fluorescence does not require an infrared excitation wavelength. Hence, when applied to scanning microscopy this property is predicted to increase spatial resolu- tion. Instead of the10 GW/cm 2 required in non-resonant multiphoton excitation, focal intensities of10 MW/cm 2 are expected to produce an equally strong nonlinear signal. The predicted resolu- tion is up to 30% greater than that of an ideal confocal microscope operating at the same fluores- cence wavelength. The resolution improvement over non-resonant two-photon absorption micro- scopes is about two-fold in all directions.