Near-field scanning optical microscopy imaging: theory, simulation and experiment

Near-field scanning optical microscopy (NSOM) is being studied to achieve optical resolution much better than the diffraction limit. Improved resolution is realized when the sample is in the near field of the probe. Strong near-field coupling between sample and probe complicates image analysis. Experiments with well characterized tips and simple samples are needed to produce basic NSOM images. Detailed modeling able to reproduce images and identify essential features in image formation is required. We analyze experimental NSOM transmission images of nanochannel glass arrays and of Au nanoparticles obtained in illumination mode. We use several approaches, including the discrete dipole method and the transfer matrix method, to simulate these images. Experimental and simulated images are compared to identify the contributions of tip-field structure, sample scattering, and the collection process to the images and to provide a clearer interpretation of these NSOM images.