Theoretical study of an absorbing sample in infrared near-field spectromicroscopy

This paper is devoted to study the near-field spectrometry in the infrared spectral range. To understand the behavior of the infrared light diffracted by an object, numerical calculations have been carried out with Fourier Modale (FM) method within R-matrix algorithm. We consider the case of three-dimensional system including a translational symmetry in one direction, where is included an homogenous layer in which is buried an absorbing object. Using an optical near-field analysis and by calculating the electric field intensity distribution, both of the thickness effect and the lateral size of the absorbing sample are investigated. It is found that the distribution of the intensity related to the electric field is depending on geometry of the absorbing object. Also, we show how the diffraction due to the sample edges has an effect on the field intensity distribution. After that we pay more attention to the spectroscopy mapping description, in particular to the influence of the sample characteristics on detection of an absorbent object in near-field. This technique is also able to detect the doublet of an absorbent object with over-wavelength size but in near-field zone. When the lateral size of the object is a sub-wavelength, the absorption bands are detected with slight distortion but the diffraction effects are present. To diminish the diffraction effects, we reduce the sample thickness which may induce a more important absorption bands distortion. (C) 2004 Elsevier B.V. All rights reserved.