Near‐field optical microscopy of localized excitations on rough surfaces: influence of a probe

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Abstract
Starting from the general principles of near-field optical microscopy, I consider the influence of a probe when being used to image localized dipolar excitations and suggest a way of evaluating the perturbation thus introduced. Using the rigorous microscopic (electric) point–dipole description, I calculate the self-consistent field intensity at the site of a probe dipole scanning over resonantly interacting object dipoles and show that the intensity distribution deviates from that existing in the absence of a probe. I demonstrate that this difference increases with an increase in the polarizability of the probe dipole, resulting eventually in a completely different intensity distribution. The calculations also show that the perturbation of the intensity distribution due to the presence of a probe decreases with an increase in the probe–sample distance. In order to evaluate the degree of perturbation, I suggest comparing the images obtained at different probe–sample distances. Finally, I formulate a simple rule of thumb that allows one to roughly estimate the probe–sample coupling when imaging localized excitations.