  We report on the design and experimental characterization of a suspended
silicon nitride subwavelength grating possessing a polarization-independent
guided-mode resonance at oblique incidence. At this resonant wavelength we
observe that the transverse intensity profile of the transmitted beam is
consistent with a first-order spatial differentiation of the incident beam
profile in the direction of the grating periodicity, regardless of the incident
light polarization. These observations are corroborated by full numerical
simulations. The simple one-dimensional and symmetric design, combined with the
thinness and excellent mechanical properties of these essentially loss-free
dieletric films, is attractive for applications in optical processing, sensing
and optomechanics.
