Fig. 3. The near-field optical probe. (A) An optical fiber is pulled to a final diameter of 20-120 nm and subsequently coated with aluminum. This coating serves to confine the light to the tip region. A subsequent etching step results in a flat and circular endpoint and aperture. The aperture functions as a miniature light source, and its diameter primarily determines the optical resolution of the microscope. (B) The principle of surface-specific excitation. The optical near-field generated at the aperture has significant intensity only in a layer of <100 nm from the aperture; lower lying fluorophores are therefore not excited. Hence, background fluorescence is effectively suppressed. This forms the basis for the high optical detection sensitivity of this technique.