Anomalous index contrast due to point source illumination in scanning optical microscopy

Abstract

Using a point source formed by the sub-wavelength aperture of a near-field scanning optical microscope tip, we examine anomalies in the image contrast arising from the use of point source illumination as opposed to the more common collimated light geometry. For samples with predominately spatial index variations, the image contrast obtained using a point source is the reverse of what is normally expected from collimated light illumination at normal incidence. The point source image contrast is unusually large and has a strong dependence on the numerical aperture of the collection optics. We compare the measured contrast across two adjoining glasses of slightly different indices with the result of a two-dimensional model, and find that refraction at the index change boundary is the primary reason for the anomalous contrast.