Multiphoton Excitation of Fluorescence near Metallic Particles: Enhanced and Localized Excitation.

We examined the effects of silver island films on multiphoton excitation of rhodamine derivatives and biochemical fluorophores. The fluorophores were studied in approximately 1 μm thick samples between two quartz slides coated with silver islands. Intensity and lifetime measurements of rhodamine B demonstrated enhanced two-photon excitation near the metal particles. Time-resolved measurements showed a decreased lifetime of RhB with two-photon excitation as compared to unsilvered quartz slides, which indicates the two-photon excitation was localized near the metal particles. Additionally, rhodamine B showed increased photostability with two-photon excitation near metal particles, which is consistent with the shorter lifetime. For lower quantum yield fluorophores such as rose bengal, eosin, 1-anilinonaphthalene-8-sulfonic acid, and coumarin 152, we found the emission from the micron thick sample was almost completely due to the fluorophores near the silver particles, perhaps to within 300 Å of the metal surfaces. These results suggest the use of metal particles for localized multiphoton excitation of biological macromolecules or cells. Additionally, one can imagine metal colloids with bound fluorophores being selectively excited when placed in highly fluorescent samples such as tissue samples.