Abstract: There is provided a chirped diffractive element (20) in the form of a grating (22) configured for supporting a plurality of guided mode resonances (54), which resonances (54) may be considered to comprise a standing wave. Chirping the grating (22) may allow guided mode resonances (54) to be distinguishable in terms of position within a section (34) the grating (22). An incident electromagnetic field may be coupled into at least one of the sections (34) when the electromagnetic field has a wavelength value within a predetermined wavelength range and a sample has a refractive index value within a predetermined index range. The incident electromagnetic field may be reflected by at least one of the sections (34) of the grating (22) exhibiting a guided mode resonance (54).