“…For a small d, by replacing the sine function in (24) with its argument, we obtain (23) up to a phase factor. Let us check for expressions ( 22) and ( 23) the validity of the electrodynamic reciprocity theorem (Section 2.5 [15]), which, as applied to phase gratings, requires that when the wave vectors of the incident and diffracted waves are interchanged, and the sign of the grating vector is simultaneously changed, the diffraction efficiency does not change [12]. The diffraction efficiency η is given by [16] * 1 11…”