The reflectivity of a 9 nm AlAs layer embedded in a GaAs matrix was measured using a near-field scanning optical microscope. Results for freshly cleaved surfaced are understood considering the electrostatic nature of the electric field in the near field of a subwavelength aperture. The optical contrast arises from the different index of refraction of AlAs and GaAs, with topography associated effects playing no role. While keeping the wavelength of the incident radiation and the aperture dimensions fixed,|E|was mapped on planes perpendicular to the tip axis, as a function of the height of the tip above the sample. The results were compared with a finite element calculation for the scattered light. In the analysis, we model the sample with a position dependent dielectric constant. Good agreement between the model and the experimental data was found. ©1997 American Institute of Physics.