Gratings can be considered as resonant structures in near-field. The enhanc
ement of the intensity recorded by Scanning Near-Field Optical Microscopes
(SNOM) is due to interferences in relation with the ratio of the wavelength
to the product of the optical index by the period of the grating; We discu
ss the effect of this ratio (in the range 0.9, 1.1) on the intensity patter
ns. The influence of the polarization on near-field data is analyzed in bot
h theoretical computations and experimental result.