New approach to fluorescence spectroscopy of individual molecules on surfaces

A new method for efficient fluorescence excitation of molecules on a surfac e is proposed and theoretically studied. A laser beam is focused into a dif fraction limited spot on a thin layer of an optically nonlinear material. T he frequency doubled light that is generated within that layer is used to e xcite the fluorescence of molecules positioned above the layer. It is shown that such an excitation scheme may be more efficient than two-photon excit ation. This may be of great importance in single molecule fluorescence dete ction at short wavelengths.