Fluorescence spectroscopy of molecules on surfaces by indirect 2-photon excitation

In recent years, the fluorescence detection of single molecules (SMD) in liquids and on surfaces under ambient conditions has seen a tremendous development. One of the difficulties of SMD is the efficient suppression of background signals, mostly Rayleigh and Raman scattering. Diverse methods were developed to minimize the scattering signals, e.g. usage of highly efficient emission filters; minimization of the detection volume by diffraction limited focusing of the exciting laser beam and confocal imaging of the fluorescence, or using near-infrared dyes employing the fact that the scattering intensity decreases with the fourth power of the wavelength However this strong wavelength dependency of the scattering signal made it, until now, hard to detect single fluorophores in the UV-region. One may use two- or three-photon excitation but the necessary excitation intensities are often so high that they excite also non-linear scattering processes. Here, a new scheme is proposed for efficient fluorescence excitation of surface immobilized molecules via frequency doubling of the exciting laser light within a thin layer of an optically non-linear material, which may be much more efficient then the usual direct two-photon excitation.