A new method for efficient fluorescence excitation of molecules on a surface is proposed and theoretically studied. A laser beam is focused into a diffraction limited spot on a thin layer of an optically nonlinear material. The frequency doubled light that is generated within that layer is used to excite the fluorescence of molecules positioned above the layer. It is shown that such an excitation scheme may be more efficient than two-photon excitation. This may be of great importance in single molecule fluorescence detection at short wavelengths.

• Received 22 April 1999

DOI:https://doi.org/10.1103/PhysRevLett.83.3804