  The optical transmission of resonant guided-mode gratings patterned on
suspended silicon nitride thin films and illuminated at normal incidence with a
Gaussian beam are investigated both experimentally and theoretically. Effects
due to the beam focusing and its finite size are accounted for by a
phenomenological coupled mode model whose predictions are found to be in very
good agreement with the experimentally measured spectra for various grating
structures and beam sizes, and which allow for a detailed analysis of the
respective magnitude of these effects. These results are highly relevant for
the design and optimization of such suspended structured films which are widely
used for photonics, sensing and optomechanics applications.
