Evanescent field excitation and measurement of dye fluorescence in a metallic probe near-field scanning optical microscope

We introduce a method of dye fluorescence excitation and measurement that utilizes a near-field scanning optical microscope (NSOM). This NSOM uses an apertureless metallic probe, and an optical system that contains a high numerical aperture (NA) objective lens (NA = 1.4). When the area which satisfies NA < 1 is masked, the objective lens allows for the rejection of possible transmitted light (NA < 1) through the sample. In such conditions, the focused spot consists of only the evanescent field. We found that this NSOM system strongly reduces the background of the dye fluorescence and allows for the measurement of the fluorescence intensity below the diffraction limit of the excitation source.