Resolution test for apertureless near-field optical microscopy

A test configuration for apertureless near-field optical microscopy permits characterization of near-field signals free from significant topography effects. Using a second tip as the scanned object, a high-contrast dipole-dipole signal is observed when the two tips are closely spaced, by 10 nm or less. A spatial resolution on the order of 5 nm is demonstrated. The measured signal is also material dependent: an inverted contrast is recorded when one of two high-dielectric (silicon) tips is replaced by a metallic (nickel) tip. Measurement results are accounted for by theory, which also confirms the nature of the detected signal: it results from interference between a near-field dipole-dipole field scattered by the end of the tips, and a reference field scattered by the main body of the tips. Different illumination sources are investigated. In all cases, the component of the electric field parallel to the tip axes provides the major source of signal and contrast. (C) 2002 American Institute of Physics.