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A further feature of our sample is the fixed orientation of the molecular dipole moment parallel to the cover glass. To investigate the dipole orientation we utilized a back focal plane imaging technique, which allows us to study the angular emission pattern of single molecules [16]. Cross sections through the back focal plane of an exemplary image along the two orthogonal polarization axes are plotted in Fig. 3 (a). The symmetry of the obtained shape indicates a horizontally aligned molecule. We found that all molecules within an area of a few tens of microns showed similar emission patterns and were out of plane at most by a few degrees. The symbols in Fig. 3 (b) show that the sum of the signals from many molecules was also centered around zero. The solid curve in this figure displays the theoretically expected pattern for a single dipole at the interface. The central part of the angular pattern shows a very good agreement with the experimental results and was fitted. The experimental side lobes miss the fast modulations due to the finite angular resolution of the experiment. They also fall short of the theoretical prediction because the latter did not take into account the exact distance of the molecule from the AC-air interface, which sensitively determines how much light is emitted at angles beyond the critical angle. As a second check for the alignment of the molecules, we performed measurements where the orientation of a polarizer in the detection path was varied. Figure 3 (c) shows that the fluorescence signal of a single DBT molecule could be varied with a visibility of 97 %. We note in passing that the maximum detected fluorescence occurred at similar polarizer positions for molecules in the same field of view, supporting the fact that large crystalline 

