Scanning apertureless microscopy below the diffraction limit: Comparisons between theory and experiment

The exact nature of the signal in scanning apertureless microscopy techniques is the subject of much debate. We have sought to resolve this controversy by carrying out simulations and experiments on the same structures. Simulations of a model of tip-sample coupling are shown to exhibit features that are in agreement with experimental observations at dimensions below the diffraction limit. The simulation of the optical imaging process is carried out using atomic force microscope data as a topographical template and a tip-sample dipole coupling model as the source of optical signal. The simulations show a number of key fingerprints including a dependence on the polarization of the external laser source, the size of the tip, and index of refraction of the sample being imaged. The experimental results are found to be in agreement with many of the features of the simulations. We conclude that the results of the dipole coupling theory agree qualitatively with experimental data and that apertureless microscopy measures optical properties, not just topography. (C) 1999 American Institute of Physics. [S0003-6951(99)02451-1].