Hell, S. W. and Stelzer, E. H. K.
Fundamental improvement of resolution with a 4Pi-confocal fluorescence microscope using two-photon excitation. Optics Communications 93: 277-282 (1992).

A new type of confocal fluorescence microscope with an unprecedented resolution is proposed. Calculations demonstrate that an axial resolution on the order of 100 nm can be achieved in a confocal microscope with enlarged aperture when two-photon excitation is applied. This axial resolution is up to four times higher than that of a confocal fluorescence microscope. The proposed microscope yields the highest point resolution ever achieved in far-field light microscopy.