Optical characterization of a complex grating profile

Abstract

The grating studied here, produced by interferographic exposure of photoresist, comprises two gratings, one with twice the period of the other, aligned so that their Bragg vectors are separated by an angle of 0.02°. Thus, as one moves along the grooves, these two gratings locally vary their relative phase. Coating this grating with metal allows the optical characterization of this surface by the excitation of surface plasmon polaritons. This paper starts by demonstrating the inability of a profilometer to profile accurately the surface of this bi-grating because of the limitations imposed by the finite width of the stylus on the scale of the grating grooves. Angle-dependent reflectivity data are then used to characterize the metal-coated surface. The size and shape of the surface plasmon resonances in these data, once matched to a rigorous theoretical model, are shown to provide a quantification of the surface profile. This accords with both the profile expected from the limited profilometer study, and more significantly with the surface function predicted using a simple model of the photoresist's response to exposure.