A. Schonle et al., Nonlinear fluorescence through intermolecular energy transfer and resolution increase in fluorescence microscopy, ANN PHYSIK, 8(2), 1999, pp. 115-133
We investigate a novel concept to efficiently generate multiphoton induced
fluorescence from organic molecules. The concept is based on frustrating th
e energy transfer between a fluorescent donor and one or more accepters in
conjugated molecules. The nonlinearity is not based on higher order molecul
ar susceptibilities but entirely on their linear properties. Therefore, in
contrast to nonresonant multiphoton absorption, this method does not requir
e high local intensities. Likewise, the production of visible fluorescence
does not require an infrared excitation wavelength Hence, when applied to s
canning microscopy this property is predicted to increase spatial resolutio
n. Instead of the similar to 10 GW/cm(2) required in non-resonant multiphot
on excitation, focal intensities of similar to 10 MW/cm(2) are expected to
produce an equally strong nonlinear signal. The predicted resolution is up
to 30% greater than that of an ideal confocal microscope operating at the s
ame fluorescence wavelength. The resolution improvement over non-resonant t
wo-photon absorption microscopes is about two-fold in all directions.