2 Optical Characterization Methods for Ultrathin Nanoaggregates H.-G. Rubahn 2.1 Dark Field and Fluorescence Microscopy Light-emitting organic nanofibers are an interesting model system for demon- strating the resolution limit of optical microscopy in the border region between micro- and macrocosmos. In Fig. 2.1 dark field and fluorescence images of the same hexaphenylene nanofiber are shown. A standard optical microscope with dia- (for dark field) and epi-illumination (for fluorescence) has been used. Structures with characteristic dimensions of a few ten nanometers such as breaks in the nanofibers (exemplified by an atomic force microscopy (AFM) image as an insert in Fig. 2.1b) are barely visible even in dark field images since the difference in indices of refraction of semiconducting nanofibers and under- lying dielectric substrate (mica) is small. In dark field microscopy (Fig. 2.1a) one illuminates the sample under nearly grazing incidence, thus enhancing the visibility for structures on the surface that scatter light. Consequently, such structures appear bright on a dark background. The basic idea for this technique stems from 1903 (H. Siedentopf and R. Zsigmondy). Note that the structures shown in Fig. 2.1 have heights of less than 100 nm, i.e., much smaller than the wavelength of the light used for scattering. The evanescent wave condenser maximizes this principle by ensuring that the illumination occurs solely via surface waves, i.e., within the evanescent part of the electromagnetic field. Dark field microscopy is also called “ultramicroscopy” since it allows one to investigate structures with characteristic dimensions smaller than the wavelength of the imaging light (perpendicular plane visibility limit roughly λ/100). Even better contrast and visibility of subwavelength structures is obtained for light-emitting objects via epifluorescence microscopy (Fig. 2.1b). In such a setup UV light irradiates the nanofibers under normal incidence and the resulting luminescence is observed under normal incidence, too. Excitation and luminescence light are separated with the help of a wavelength-selective beam splitter and color filters. At the breaks in the needles, the UV-induced luminescence is scattered into the far field and thus submicron structures