Controlled Tip-Enhanced Raman Imaging through the Evaluation of the Tip Dipole

any intrinsic properties of com-monly used samples are extre-mely sensitive to the polarization of the illumination light. An optical imaging technique based on polarization-controlled illumination can therefore be a valuable tool to study distinct properties of a sample. One of the most prevailing optical techniques is Raman spectroscopy, which can also be used as scanning microscopy for imaging purposes. If the polarization of the illumina-tion light in Raman measurements is pre-cisely controlled, one can selectively excite different Raman modes that are sensitive to the illumination polarization. Therefore, when Raman microscopy is combined with polarization-controlled illumination, it is possible to reveal unique characteristics of samples, such as molecular symmetries, orientations, and intermolecular interac-tions. Such polarization-controlled Raman (p-Raman) microscopy has been used in the past for crystallography,1,2 polymer analysis,3 and observation of biological processes.4 However, due to the diffraction limit of light, the spatial resolution of this imaging technique is restricted at best to about 300 nm in the visible range. Recent development of nanoscale science and technology, where a tiny difference in mo-lecular orientation can be of great impor-tance, is boosting interest in p-Raman imaging at higher resolution. With recent technological advances, the devices are becoming smaller and smaller, whichmakes it even more challenging to characterize these devices at the nanoscale and thereby control their performances. For example, thin films of pentacene molecules are often used to produce organic transistors. Penta-cene is a tall molecule that should ideally attach perpendicularly to the substrate to * Address correspondence t