On contrast parameters and topographic artifacts in near-field infrared microscopy

Abstract

Near-field microscopy overcomes the diffraction limit through the partial conversion of the evanescent fields, formed around the subwavelength sources of light, into propagating waves by interactions between the probe and the sample. Contrast parameters in this imaging technique are quite different from those in conventional (far-field) optics. We study the mechanisms of image formation in the transmission mode of a near-field microscope in the mid-infrared part of the spectrum (6–10 μm). The amount of light propagating from a subwavelength aperture through a flat substrate (“allowed” light) is found to strongly increase as the tip approaches the sample, generating topographic artifacts in near-field images. Such artifacts can be eliminated by flat sample preparation techniques. The transmitted power is strongly influenced by the refraction index of the sample resulting in a substantial difference of the near-field spectrum from the far-field one. A model describing tunneling of light through a subwavelength aperture into the substrate has been developed and is in very good agreement with the experimental data. The model predicts that spectral sensitivity is enhanced with smaller tip diameters.