  We demonstrate an optical scheme for measuring the thickness of thin
nanolayers with the use of light beam s spatial modes. The novelty in our
scheme is the projection of the beam reflected by the sample onto a
properly-tailored spatial mode. In the experiment described below, we are able
to measure a step height smaller than 10 nm, i.e., one-eightieth (1/80)of the
wavelength with a standard error in the picometer scale. Since our scheme
enhances the signal-to-noise ratio (SNR), which effectively increases the
sensitivity of detection, the extension of this technique to the detection of
subnanometric layer thicknesses is feasible.
