Polarization and dielectric function of molecules
Sub monolayer deposition of this porphyrin on HOPG results in the formation of islands in which the molecules adopt two different orientations. In one case the porphyrin ring is oriented parallel to the substrate, in the other it is perpendicular to the surface. The different orientations can be distinguished by the height, as shown in figure 2, which also illustrates the structure of the molecules. The detailed structures of various monolayers are treated in ref. Here the porphyrin islands with the rings parallel to the substrate and a height of 1.5 nm are examined.
The topographic structure and dependence of the s-NSOM signal on the polarization of the incoming light at multiple harmonics is shown in figure. The porphyrin island is 1.5 nm in height, confirming that the molecules align with porphyrin ring parallel to the HOPG surface18. s-NSOM images were obtained for the 4 harmonics of the scattered light. The response functions and therefore the properties of the graphite surface and the porphyrin/graphite complex are compared. In order to quantify the properties, histograms of the optical amplitudes backscattered from areas with and without monolayers are shown in figure b-f. The area marked with the red square corresponds to the substrate (HOPG) and that with the blue square corresponds to the porphyrin monolayer. Clearly when the incident light is polarized perpendicular to the surface there is almost no difference between the backscattered light amplitude at all frequencies. In contrast, the when the light is polarized parallel to the surface the difference in contrast is obvious. Increasing the harmonic order from 1 to 3 results in the increase of the difference in optical amplitude between HOPG surface and porphyrin island because the sensitivity of s-NSOM to dielectric function difference increases with the harmonic order. There is no increase in contrast for the 4th harmonic probably because of the lower signal-to-noise ratio for this signal due to the weakness of the signal.
In the conclusion that current work is the first measurement of polarization dependence of light scattering on the optically active molecules (porphyrin). This is the first time when high spatial resolution measurements (resolution is better then 50 nm) were performed on organic molecules. From this kind of measurements information about lateral distribution of dielectric constant of the sample could be extracted.