Near-Field Fluorescence Microscopy Based on Two-Photon Excitation with Metal Tips

We present a new scheme for near-field fluorescence imaging using a metal tip illuminated with femtosecond laser pulses of proper polarization. The strongly enhanced electric field at the metal tip ( $\ensuremath{\approx}15\mathrm{nm}$ end diameter) results in a localized excitation source for molecular fluorescence. Excitation of the sample via two-photon absorption provides good image contrast due to the quadratic intensity dependence. The spatial resolution is shown to be better than that of the conventional aperture technique. We used the technique to image fragments of photosynthetic membranes, as well as $J$-aggregates with spatial resolutions on the order of 20 nm.